[kevlarpirate]

Very interesting read on the carbon masts. Thanks Viking

The very good points he makes are that it improves the ability to carry sail. That is fun.

However, short of doing a FEA, it could result in substantially higher dynamic and impulsive loads on the rigging , chain plates mast step, mast compression and hull. The static solution to the problem by removing mast weight, cannot be
assumed to be equal to the new dynamic forces of higher sail loading. It may be nothing to worry about in the end.

My rigging buddies in Florida tell me rigging failures are not mostly from high winds with deep reefs ,and no.3’s, they are more common using heavy no.1’s, under 20 kts.

Other good points are aluminum masts can develop stress cracks as he mentioned. I have seen this on a couple of production boats which, to save assembly time, employ trick designs, for instance spreaders being held by one big pin drilled through a casting
The casting being very solid produces a stress riser on the thin walled aluminum tubing and guess what …. Cracks. My 30 year old triple crane Sparcraft (both boats) have none of these problems. Personally I would dread ever losing them.
Because I may end up with something I don’t trust.

Another very important point is the paint issue. 6ooo series aluminum is alloyed with copper and is prone to that white bubbling which is very unsightly and it happens to all masts, even the ones well primered with zinc chromate. Sooner or later it happens, and the cost of repainting is very painful as he states. Another reason I am very protective of my masts. And regularly check my rigging. They are anodized and after 30+ years, I have no issues,

The author also states exactly the point I have been making re: moment of inertia.
He states twice the jerky movements. One statement he says he was seasick.
This is exactly the price you pay for the fun factor. It is the comfort factor.
Jerky movements are high accelerations, nothing else.

When I mentioned cutting a wave in half and looking back at the hole we just made
That is my fun factor, I would rather do that than realize another 2/10 of a knot
because I can take my number one up another 4 knots true. But if the extra 2/10 rings your bell, then great , it’s your boat.

When I cut a wave in two , I don’t want to feel the boat pitch up. I want to blast through it. To me it’s kind of like the feeling I would get driving a monster truck over a car and crushing it. Now perhaps some of you are more sophisticated than me, but I like crushing things.

A note I can make about carbon masts is they are not as torsionally stiff as an aluminum mast, so if you have swept back spreaders, you have some considerations to deal with as for standing rigging.

One issue I would disagree with the author is with lightning strikes.

I had my 41 in Florida for 8 years and was blessed by not getting hit, however I have
been close, so I will tell one account here . I had my boat in the yard and got a call that a boat next to me was hit. I raced down to the yard on two wheels around every corner ran to the boat to see the damage. First I can say, I made a couple of grounding spikes and had them pounded in the ground and cables up to the headstay and backstay , so I hoped they worked if I got any of the strike. My shrouds are connected to the keel by ¼ inch copper tubing conductors, so I was not worried about that, also I have one of those fuzzy things on the mast head.

The boat next to me did get hit and on all six jack stands where the pads met the hull
The hull was pierced from the current. The jack stands had all been moved off to the side. Each area was black with soot where the resin was carbonized and missing.
You could take a pencil and push the glass fibers around and push the pencil right through to the inside of the hull. Some holes were worse than others.

The energy of an average strike is in the order of 200 kilojoules, about the energy of a hand grenade. That energy has to be dissipated somewhere. The current also shot
Out the transom somehow and ended up in the boat behind the struck boat , so obviously there was plenty of energy there to do damage.

My point is this: all resins carbonize around 450 deg. F. so one would expect serious damage. Especially along any path where current must run on its way to ground.
This could be between the mast head and the upper shrouds. The resin is a essential stabilizer for the fiber. I don’t know how well the carbon conducts in such an event,
However, I would need much more data to convince me they are safe for a strike.
I would talk to insurance claims people first.

Paul , My statements referencing physics and boat behavior are all consistent back to my first post. If you don’t find any thing of value, then just don’t read my posts and let others judge for themselves.

One note I will make is that I attempted to make a definition of what comfort was
and was not . I proposed acceleration was the most likely agreeable definition.
No one came forward with any other definition. Including you.

The best that could be proposed was that it is different things to different people which gets us no where.

What is obvious is that comfort and motion produced by light weight boats are opposite. You can’t have have comfort on a sport boat or a ULB, in a developed sea unless your definition of comfort includes black and blue marks .

Paulo, did you read the insurance claim article? Humerous too. I don’t doubt
There are better cats than others for passage making, but again when a 76 foot cat
Capsizes in 8-10 foot seas, that scares me , I would guess it was as simple as burying the lee hull , I have done that plenty on the Hobie, and I don’t see any reason you can’t scale that problem up. Gone for the weekend here . cheers, kev

 

[PCP]

Some more so than others. I haven't been following kevlarpirate's arguments much but what I'd look for rather than some "index" is a hull form and weight (inertia) distribution that gives low accelerations in response to disturbing forces and has high lateral damping.

That's the physics of this in a nutshell.

The tradeoff being that such a boat will have a smaller righting moment at small angles of heel and will therefore tend to heel more- but that's life.

Yes, heel more and more...till it capsize. Boats with smaller righting moments, will have a low positive stability curve and the energy needed to capsize them will be small. You can have a soft boat with a slow motion but that boat will be heeled a lot by each wave. That's not my idea of a comfortable motion and it is also dangerous.

A boat with a small righting moment at small angles of heel will not be capable of carrying any significant amount of sail and will be a slow and bad sailboat.

Things are not that simple .

Regards

Paulo

 

[cormeum]

Yes, heel more and more...till it capsize. Boats with smaller righting moments, will have a low positive stability curve and the energy needed to capsize them will be small. You can have a soft boat with a slow motion but that boat will be heeled a lot by each wave. That's not my idea of a comfortable motion and it is also dangerous.

A boat with a small righting moment at small angles of heel will not be capable of carrying any significant amount of sail and will be a slow and bad sailboat.

Things are not that simple .

Regards

Paulo

Think of a flattened sine curve with the peak at 90 degrees. You don't need a lot of righting force at small angles of heel as long as it builds up when the heel angle increases. It'll be initially tender but will stiffen up fine at larger heel angles. (been there done that)
The added benefit is there'll be no (or a very small) area of inverted stability .
The example you show of a soft boat being repeatedy heeled by each wave had less to do with the righting moment than the wind pressure on the sails (which dominates) or the moment of inertia on that roll axis.
You're right, it's not as simple as it seems, is it.

 

[Jeff_H]

quote kevlarpirate- Jeff you say:

"you would see that even if the boat with the weight added to its mast and the boat with weight added to its keel had equal roll moments of intertia, the boat with the weight up its mast would have a quicker roll rate, through a wider angle and with harsher de-accelerations than than the boat with the weight on its keel. "

This statement is false, it violates moment of inertia.
Total moment is the sum of the individual moments. Total moment
is independent of positions of individual moments. (first year physics)

Kev: You are taking that quote out of context, but also as I see it, the problem with your responce is that you are over-simplifying and ignoring some of the component forces that impact roll rate and roll angle. I suggest that your re-read my post 89 where I explain how this works and perhaps you will understand in more detail the physics behind this quote and why it does not violate moment of inertia. I also suggest that you try to get your hand on a copy of Marchaj's Seaworthiness, The Forgotten Factor and look at the section where he explains all components of physics behind this in more detail than I go into here.

But to summarize, if both boats in my example (one with a smaller weight up its mast vs one with a heavier weight in its keel) had equal roll inducing forces, equal roll moments of inertia and equal roll resisting forces then you are correct that they would have equal roll rates and roll angles.

But where your comment misses the point is that these boats do not have equal roll inducing forces. In the case of the boat with the weight up its mast, as the boat rolls past vertical, that weight adds a roll inducing force. While in the case of the boat with the weight in its keel once the boat gets past vertical, that weight in its keel becomes a roll resisting force. The net differnces in the forces means that the boat with the weight up its mast has a greater overturning force and so would have a faster roll rate. And because it has a greater roll rate, that boat will also have a greater momentum. If both boats have identical hull forms and rigs, they will have equal dampening, and so the boat with the greater momentum will roll through a greater roll angle as well. Lastly because of this greater roll angle, the boat will also be further out of phase with the wave and so experience a greater impact/ deceleration. (First year physic regarding yacht dynamics in waves) - Jeff


Kev quotes Jeff as saying: "If a boat has reduced roll and pitch angles can be reduced, its roll rates slowed, and if it can be designed with softer transitions,"

Kev says Jeff has it backwards: the only way to reduce pitch and roll angles is to lower moment of inertia and mass which will increase accelerations.

Kev: Again, I suggest that you are taking my quote out of context and over-simplifying and also ignoring some of the component forces in your response. Roll moment of inertia and mass are certainly important, but these are not the only factors at play in determining roll and pitch angles and rates or impacting acceleration and deceleration. Just for example two cases which alter pitch and roll independent of moment of inertia and mass would be, 1) dampening, which will slow roll/pitch rates and thereby reduce momentum and thereby reduce pitch and roll angles. And 2) in roll motion, hull forms that progressively shift the center of buoyancy rather than suddenly shifting the center of buoyancy will reduce the severity acceleration and deacelleration by attenuating the distance/time over which the speed is lost. while increasing the amount of roll angle.

Respectfully,
Jeff

 

[PCP]

Think of a flattened sine curve with the peak at 90 degrees. You don't need a lot of righting force at small angles of heel as long as it builds up when the heel angle increases. It'll be initially tender but will stiffen up fine at larger heel angles. (been there done that)
The added benefit is there'll be no (or a very small) area of inverted stability .

The example you show of a soft boat being repeatedy heeled by each wave had less to do with the righting moment than the wind pressure on the sails (which dominates) or the moment of inertia on that roll axis.
You're right, it's not as simple as it seems, is it.

The sail the boat can carry is directly related with the boat's righting moment at small heel angles (till 35º). There is no doubt about this.

If you have a low righting moment at small angles heel, you can only carry a very limited sail area, and you need sail area to sail a sailboat. So your tender boat, that needs sail area to sail, but that has not the riggting moment (at small angles of heel) needed to contra balance the wind force on the sails, will heel slowly but a lot.

If the righting moment curve has small righting moments on small heel angles, it will be a flattened sine curve (as you say), but will not peak at 90º of heel (they never do) but at around 65/70 degree. That's a typical curve of an old designed boat, for instance a long keeler from the 60's.

If the curve has big righting moments on small heel angles, you will have a very vertical curve that peaks high at 50/55 degrees of heel and then comes down in a more vertical way. That's a curve of a fast modern boat.

The area under the positive part of the curve (representing the energy needed to capsize the boat) will be much bigger on the more vertical sinusoidal curve than in the flattened sinusoidal curve. The max righting arm (GZ) of the first curve will be almost the double of the one from the second one. The curves will have typically very different AVS (or LPS) points. The first one will have an AVS between 115 and 135 degrees and the last one, an AVS between 135 and 180 degrees. But, at 90º of heel the bigger curve will have normally a bigger righting moment, compared with the more flattened curve.

Considering boats with a similar mass, the boat with the flattened sinusoidal curve will be a tender boat, much more easily capsized (and also with a better rerighting ability) while the boat with the more vertical curve will be a much more stiff boat, a much harder boat to capsize, but also a boat that will need more time to reright. And, of course, the last will be able to carry probably more than the double of the sail, comparing with the old boat, making it a much better sailing boat.

Please, believe me on this, it is really like that .

That's why the boat designers went on the direction they did on the last 50 years, that’s why modern boats are what they are. The objective is to design faster, safer and better sailing boats.

That does not mean that old designed boats didn’t have some advantages, but all boats are compromises and what Designers do is try to get the best compromise to each boat’s use, from racing to cruising, but never forgetting that the first objective of a sailboat is to sail and to sail well and probably the second objective is to design a boat that sailors can afford.

Regards

Paulo

 

[cormeum]

Paulo, I think we're talking about the same thing in two different ways- Skinny boats having a max righting force between 75 and 90 degrees as we both said will heel easier but as you agree, will also re-right easier. I don't necessarily agree that sailing at heeling angles of 15 degrees is any magic number, the Universal Rule or older RORC rule boats that regularly sailed at twice that angle still cooked along- maybe not quite as fast as a stiffer boat but certainly adequate for cruising speeds. As I said the tradeoff is an easier motion. FWIW, a number of the Universal rule boats had righting moment maxima at right around 90 degrees and AVS's around 180. And yes, they did their sailing on their ears. You also have to realize that the rigs of those days had a lower center of effort than modern rigs which ameliorated the heeling to some extent.

I think we have to agree that the real mover regarding designs has been to win according to the then prevalent rule not necessarily "Better, faster or safer boats". With some notable exceptions.

 

[PCP]

...I think we have to agree that the real mover regarding designs has been to win according to the then prevalent rule not necessarily "Better, faster or safer boats". With some notable exceptions.

Well, I have to agree that like in the auto industry, racing have been the real engine of sailboat design development. You don't win ocean races, Transats or circumnavigation races without better faster and safe boats. Particularly In the last 20 years, great advances have been made having as base the developments needed to allow safe solo ocean sail racing.

These advancements have been successfully integrated on cruising boats, made them safer and more easy to use. And I am not only talking about hull and keel design but also about stronger materials and building processes, furlers, geenaker light furlers, better autopilots, and lots of other things.

Jeff has pointed out the importance of IRC on the development of better boats and I think that today the bigger contribute to cruising sailboats design come from Open boats.

Anyway I don't want to give you the impression that I don't like skinny boats (as you call them). There are some few modern ones around, light hi tech fast boats that I consider very beautiful and interesting. Their stability curves are not exactly like the ones from the old heavy and slow skinny boats, but they have also an easy motion.

I like them so much that some years ago I went to Denmark to test sail one and I can tell you, they are fast....and they heel a lot.

They have advantages and disadvantages over other types of modern boats, I would say more disadvantages, being the most obvious the price. The real problem is that a “skinny” 40ft has the interior space and overall stability of a 36/37ft “normal” modern boat….and costs the same as a 40ft “normal” boat.

I think that is the real reason why they are so marginal on the market.

But as I say, I like them, they are just beautiful.

Regards

Paulo

 

[seabreeze_97]

Jeff, additional weight up the mast does not act to increase the roll speed. Yes, it will possibly net a slightly greater roll but only in as much as it will take the righting effect of the keel to reverse....and that starts as soon as the boat begins to roll. Lemme say that again. As soon as a wave or blast of wind hits the boat, it begins to roll. Both boats' keels immediately begin to counter this, thus slowing the roll, not allowing acceleration because of heavier mast weight. The heavier masted boat will react slower as there is greater mass and leverage to act against the outside force initially. Depending on the magnitude of the forces at work, the heavier-masted boat may actually not roll as much in totality as a shorter or lighter-masted boat since it will initially respond more slowly, and with more resistance to the outside force. At worst, a heavier masted boat will be slightly slower to reverse a roll with a given amount of ballast. Again, slower motion, greater comfort...less jerkiness. Will it roll further? Again, maybe. It will begin the roll more slowly, so the total roll will still be very close to the lighter masted boat. The keel has more time to act to right the boat as it is initially responding more slowly. It has been shown that mast weight is essential to resisting capsize (post-FastNet). The solution is not to use ultralight masts and greater ballast weight. This will only make the boat even more likely to capsize in big seas. If the mast is such a liability, why do ballasted boats become so unstable? No reply necessary, it's a rhetorical question. Since the keel is going to be less than say 8ft down, where the lever of a 40ft+ mast (give or take) is much higher, this would result in much less added weight to resist roll. What's the old adage? 50 lbs aloft requires how much ballast to counter? All else being equal, wouldn't the boat that is 1000lbs (or 2Klbs for that matter) be lighter and faster, at least in lighter conditions? Heavier mast, or even a taller lighter mast dampens motion better, weighs less so acceleration is better, maneuverability is better, and it's more comfortable. Further, taken to the ultimate example, your model would appear to have a super heavy keel bulb 10ft down, hi-tech structure and rig, very light weight. Again, great in calm, specific (racing) conditions, but it'd suck in the real world.

 

[PCP]

Kev has posted a link to what is called “Technical papers” by John Holtrop.

On these papers Holtrop says, about dynamic stability and wave encounter:

"Dynamic Stability controls how much the boat rolls in response to a transient wind gust or violent wave. ...Heavy displacement helps dynamic stability, but the center of gravity is not much of a player .. The most important factor in dynamic stability, however, is the boat’s roll moment of inertia. “

Technical Articles

It seems that for him (as for Kev), the most important factor in preventing capsizing, or in boat stability is inertia and particularly roll moment of inertia.

If this was true, then all the work of the naval architects and naval engineers that have defined the parameters that should be considered to access boat stability and capsizing risk, for the European Community recreational craft directive (that classify the boats accordingly to their stability in several categories, for different uses), would be completely biased and would serve no purpose. They don’t have considered that moment of inertia (that is one of the factors) was the dominant factor in boat stability.

Also, on all papers that I have seen about capsizing and boat stability, that factor is not considered the dominant one, among the several that are normally considered: Static stability; Roll mass moment of inertia; Surface area; Roll damping; Speed.

Someone is wrong, or John Holtrop, or many of the best European and American Naval architects and Engineers, including the ones from the Westlawn school of yacht design and the ones from the Dept. of Naval architecture of the US Naval academy. Nobody considers the Roll moment of inertia as the dominant factor on stability or in capsizing a boat. Never saw anybody saying that, except kev and Holtrop.

Let’s look at Holtrop’s experience in Naval Architecture:

“… My engineering background is rock solid, my artistic ability is what it is…. I have built boats using wood strips, stitch and glue techniques, and molded fiberglass. Some have been better than others, but they all floated without breaking!

…My boating experience, which started 50 years ago with the construction of a modified canvas covered canoe. .. Growing up is southern California let me spend a lot of time "messing around with boats", so I jumped at the chance to apply for admission to the California Maritime Academy. .. Not much theory, but a lot of "hands on" experience. I specialized in Marine Engineering. The highlight of my time at the academy was the design and construction of a model steam engine. After graduation, … I worked in the Merchant Marine for five years, mostly hauling war material to Viet Nam. …went back to school and eventually graduated with a degree in mechanical engineering.

….So why buy plans from some guy on the web that no one ever heard of?

Let´s see what some of the best researchers on waves and capsize with conclusions based on test studies say about Inertia and Roll Moment of Inertia:

“These studies showed that in beam seas the location of the vessel relative to the breaking position of the wave is critical. If the vessel is caught in the curl of a plunging breaker…capsize is possible in waves as small as 1.2 times the beam of the vessel.

The roll moment of inertia is also an important parameter because a vessel with a large value of this parameter will roll to a smaller angle on impact, but expose the deckhouse …to the full impact of the plunging wave jet.”

In “ Capsize resistance and survivality when small vessels encounter extreme waves” by Bruce Johnson, from the US Naval Academy.

Naval Architecture and Ocean Engineering Department at the U.S. Naval Academy

Comparing two similar models with different centers of gravity:

“In the smaller (breaking) waves, the high variant (VCG) was knocked down to a lesser angle than the low VCG variant.” (On bigger waves both versions were rolled).

Comparing two similar models with diferent inertia:

"About Inertia variation (all things equal, different inertia on models): “The inertia of the fin keel parent model (nº1) was reduced by 40% of the original. Tests in cross wave conditions indicated no discernable change in response to the wave. In the downwave conditions the high inertia variant pitch poled on more occasions than the low inertia version.”

“An investigation into the stability of sailing Yachts in large breaking waves”, by A. Claughton and P. Handley from the Dept. of ship science, Southampton University

It seems to me that Kev is giving too much credit to roll moment of inertia, to say the least.

Regards

Paulo

 

[Jeff_H]

Jeff, additional weight up the mast does not act to increase the roll speed. Yes, it will possibly net a slightly greater roll but only in as much as it will take the righting effect of the keel to reverse....and that starts as soon as the boat begins to roll. Lemme say that again. .

I don't care how many times you say it; you would still be mistaken, at least given the example in my earlier post 89. I know that intuitively you disagree with this, so I suggest that you read Marchaj's description of the components forces of roll (roughly page 215 of the edition of "Seaworthiness, the Forgotten Factor" that I referred to).

<OI chose some pretty big numbers for my example in the hopes of making the example more intuitive, but apparently that did not work. So I will try to look at some numbers that are closer to real life.

<OA few years ago an acquaintance had a 36 foot boat that needed a new mast. He had gotten two proposals from the same spar shop, the less expensive one was for a straight spar with a stock masthead fitting and stock stainless steel spreader fittings and the slightly more expensive one was a tapered spar with a welded masthead fitting and spreader fittings, all of which matched what came on the boat. The mast maker had calculated the additional weight of the stock masthead and the non-tapered spar at roughly 8 lbs, and I had calculated that the centroid of that weight was roughly 35 feet above the waterline. The spreader bases were roughly 5 lbs heavier and 20 feet above the waterline. And he was adding radar which added roughly 35 lbs (with the SS support bracket and guard) roughly 20 feet above the waterline. We compared that to pole-mounted radar, which added something like 65 lbs with its centroid roughly 9 feet above the waterline.

<OCumulatively, this would add quite a bit to the moment of inertia of the boat with the stock spar and parts, mast mounted radar as compared to the boat with the tapered welded spar and with pole mounted radar <O</O
In rough terms, the additional roll moment of inertia (assuming that the roll axis is at the waterline as it often was on boats from the 1970's at low heel angles and assuming Ir=mass x L squared) the increase would be as follows:
<O</O
Non- tapered spar 8 x 35 x 35 = 9,800 Lb.Ft. sq'd<O</O
Spreader bases 5 x 20 x 20= 2,000 Lb.Ft. sq'd<O</O
Radar (35 x 20 x 20) - (65 x 9 x 9) = 8,735 Lb.Ft. sq'd<O</O
For a total increase in the roll moment of inertia of 25,800 Lb.Ft. sq'd<O</O
<O</O

If we want to duplicate the same increase in the moment of inertia by adding a bulb to the bottom of the keel with the tapered spar, it would require a 716 lb bulb with its centroid roughly 6 feet below the roll center.

<O</O716.5 x 6 x 6= 25,800.

<OBut of course at this point we need to take this additional weight out of the boat if these boats are going to remain equal in other regards. Here I will propose some places to cut the weight. I'll start with a pet peeve of mine, liners. For some reason, boat builders seem to feel the need to install fiberglass liners on every inch of the hull. Since, for environmental reasons, most hulls and decks are vacuumed bagged in one form or another these days, it would be reasonably practical to eliminate the liner and paint the interior of the hull. By the same token, the combination of more complete interior framing and vacuum-bagging which is the norm today, results in lighter hull and deck than a lower-tech hand-lay-up of years yore. <O</O
<O
I calculated the net weight savings of eliminating the hull and deck liners (but retaining the internal molded frames) and reducing the hull weight/framing by 6% would be approximately 525 lbs of the 716.5 lbs of weight that we are trying to save. <O</O
<O</O

And if we change from the varnished 5/8" teak faced plywood that was so prevalent a few years ago to painted 10mm okome plywood that is getting popular for interior flats and non-major structural bulkheads, by rough calculation we should be able to save not only construction cost, but by a rough calculation a little over 200 lbs in construction weight getting us down to a rough weight parity with the boat with the heavier rigging weight. <O</O
<O</O

At this point, we have two boats with equal moments of inertia at small heel angles. (I would suggest that we ignore that the roll moment of inertia will decrease on the mast weighted boat and increase on the keel weighted boat at big heel angles as the roll axis moves upward as the center of buoyancy moves up the topsides).


And if these boats start out vertical, and they both are hit by the same wave and they both have the same roll moment of inertia they would both initially roll at the same rate.


But as they start to heel, the horizontal offset between the weight up the mast and the center of buoyancy will shift in the direction of the roll and add to the force of the roll (see Marchaj). At the same time, the horizontal offset between the weight in the keel and the center of buoyancy will shift in the direction opposite to the roll and subtracting from the force of the roll, slowing the weight of the roll. At 90 degrees of heel, the mast weight adds 500 ft. Lbs. to the roll moment, and the bulb would reduce the roll moment by 4,300 ft. Lbs. for a net difference in roll moment of 4,800 ft. lbs. It is that difference in roll moment that results in the boat with the weight up the mast having a faster roll rate and rolling through a wider roll angle, dispite having an equal roll moment of interia.


I know you are having trouble visualing this. I will use Kev's example of a wheel to try to explain this. If you visualize a vertically mounted wheel with a large weight bolted to it, that is turned by a small motor barely able to turn the wheel, at all times that wheel would have the same rotational moment of intertia.


But if you visualize the section of the revolution where weight is on the side of the wheel that the motor has to lift that weight vertically from the bottom to the top of the wheel, the motor will have to overcome the force of gravity acting on that weight and the wheel will slow down. But once the weight gets past the top of the wheel, the rotation of the wheel will be assisted by the force of gravity acting on the weight, and so the wheel will turn faster on the way down.


The same thing occurs when you add a weight high in the boat in an effort to slow roll and reduce the likelihood of capsize. While the added roll moment of inertia will initially slow the roll rate of the boat, through out a roll cycle it will increase roll angle and speed, at least relative to trying to increase the roll moment of inertia by adding weight low in the boat as is the common practice on modern designs today.


Respectfully,
Jeff

 

[kevlarpirate]

Paulo ,

Your conclusions about me are totally off base.
It has now become apparent to me that you do not know how to interpret the static curves, I am concluding you have it totally backwards.

re: post #28, to allow me to understand your thinking , could you evaluate the differences between the Hanse 430 and the 36 foot Bavaria? just those two. OK?

Jeff , this post 98 you keep mentioning, not yours
Also, as i have mentioned , you are talking to an engineer, Please don't attempt to humor me. I don't humor.

I have read all your posts and I can conclude not only are you totally new to the concept of moment of inertia , in addition, you are cherry picking date to support a pre disposed preference. Won't work , but I am happy you are now down with the concept of radius( length) squared.


Palo , I read that report years ago. This test was to see differences between hull shapes and a couple of variables , no boats had masts. The inertia they mention was not specified or defined and absolutely cannot be concluded to represent the effect of a mast as part of the total system in any way what so ever.

The other conclusions they were able to make is exactly what I have known for some many years, not specific to this conversation, but still related.
Skinny boats win, fat light weight ones lose.

As for Holtrop and Johnsons comments, they hold true and I agree with them.

I personally don't care what you believe of mine and what you don't.
I am however interested if you have the static thing backwards , so please answer, but if you don't I will consider my conclusion correct.

And again, any reader doing technical due diligence will be able to
sort all this out.

BTW my boat slices waves, like swatting flies. No lightweight boat of the same length will ever be able to do that.

Engineers have a certain language and writing , just like lawyers have their language and style. I don't pretend to be a lawyer , I don't think either yourself of Jeff should try to be engineers.

You are both highly impassioned romantic types and you have a particular boat style
you are very fond of. I don't believe for a second, that boat "style"
is either comfortable or safe or had good manners in a well developed sea, storm or even at anchor. Big broad transoms slam all night long
and all the boats you are fond of pound going up wind even in the smallest of chop.

I have sailed the types of boats you are fond of, and i have been on board them on the mooring cans .. To call a cruising version of these boats you describe is laughable. they can't even survive a kiss from a whale .
they sink!

kev

 

[Jeff_H]

Kev,

You are right about post 98 being someone else's post. I'm sorry, I mistyped. The post that I was referring to was post 89. (For the clarity of someone else reading this thread, I went back and changed that mistype in earlier posts.)

Just for the record;

• I have always used the Radius of gyration squared when calculating rotational moments of inertia.
• On a number of occasions and forums you have accused me of selecting data to support my bias. I suggest that it might be helpful to the conversation, to someone reading this discussion, and to my own knowledge if you actually pointed out one or more of those cases.
• You have suggested that I am new to the physics of motion at sea, and compared to some I certainly am since I only started looking into motion at sea back in the 1960's.

At the time my yacht design teacher worked for S&S. S&S was evaluating Titanium for the 12 meter Intrepid's upper mast with the goal of reducing its weight. I had assumed this was to increase stability and therefore reduce heel angle, but he explained that S&S was far more concerned with pitch and roll, but mostly pitch. That lead to a discussion of the bow shaping on Intrepid, which was modeled in part with the intension of progressive dampening of hobby-horsing, which had been a problem on earlier 12 meters. It was the first time that I began to look into motion at sea. It has been an strong area of interest of mine ever since.​

• You refer to the boat that I own ( I don't know what kind of boat Paulo owns) saying things like, "I don't believe for a second, that boat "style" is either comfortable or safe or had good manners in a well developed sea, storm or even at anchor. Big broad transoms slam all night long and all the boats you are fond of pound going up wind even in the smallest of chop."

All I can say is that you obviously have never sailed on the model boat that I own. Having over a dozen owned boats as varied as a 1939 Stadel Cutter, 1949 Folkboat, half a dozen CCA era boats, a few IOR era boats, and several MORC boats. I once sat down and listed the boats that I had sailed on and came up with having cruised, raced and day sailed on close to 100 different designs over the years, that varied from traditional gaff riggers to modern sportboats.​

I came to purchase the type of boat that I own, not out of some pre-conception, but out of a long evaluation of various boats that I have sailed. I understand that there are some shortcomings to all boats, and mine is no exception, but I would say that you got it very wrong when you talk about her motion in a developed seaway, chop or at anchor. My boat was single-handed into the US from South Africa and prior to purchasing my boaty I discussed the behavior of these boats with a diverse group which included Dudley Dix, a couple who sailed one up the Atlantic from South Africa to Scotland, and a fellow who had single handed on from South Africa to the Carribean the first 8 days of which were in winds that did not drop below 30 knots and hit gusts well into the 50 knot range and seas he decribed as being above the mast. All described these boats as having great offshore behavior.

In terms of sailing in short seas, I would say that one of things that I like about my current boat is its ability to slice through waves when sailing going upwind in a chop; rather than pounding, especially relative to the boats that I have owned and sailed on over the year. As any knowledgeable designer will tell you, pounding is more a more matter bow shape than displacment.
​

And while my boat, kites more on the anchor than I would like, her stern does not slap, at least not as badly as the long over hang CCA era boats that I used to own.​

But more to the point, my boat is a 30 year old design. While a pretty advanced design for its day, it represents a snapshot in time of a time long ago. Based on my experience on board the better newer designs, they have far eclipsed the design thinking of that era in producing seaworthy and motion comfortable boats.​

• I respectfully suggest that you need to spend more time sailing on some of the newer designs before jumping to mistaken opinions about their behavior. Over and over again you keep referring to ULDB's of a design era that is nearly 40 years ago. The newer designs are not ULDB's. In fact they do not weigh all that much less than similar length on deck boats from 30 years ago, but a huge amount has changed in hull modeling and weight distribution since that time which have greatly improved the motion comfort and seaworthiness of these designs. And so, I suggest that if you spent more time sailing on some of the really newer designs would actually experience their motion comfort and sailing ability first hand. I think you would be pleasantly surprised.

Respectfully,
Jeff

 

[cormeum]

Now Boys,
How's this: go to Toys R' Us and get a toy sailboat (Bosun boats are pretty good) and fill a bath tub with water. next tape a weight to the mast and set it oscillating transversely. Time the oscillations. Next take the weight off and time it again.

Nothing like a little experiment

N.B. as the math has no scaling factors, the test will be valid.

You can post the results on You Tube.

 

[blt2ski]

cormeum,

I like that idea. BUT, i have to ask a question, this works IMHO when at rest at anchor. How does this wt effect one when sailing in an X knot breeze? and how does the breeze effect the rolling? While the wt at the top will help the roll stop quicker at anchor, will the wt at the top allow the boat to right itself with wind gust as quick? or allow it to capsize quicker? My gut says the wt at the top of the mast when sailing should not do as well as when not, but could see the at rest being better.

Anyway, better add some kind of fan to the experiment, along with a BIG bath tub, or swimming pool so the fan can add wind equal to the experiment.

Marty

 

[cormeum]

cormeum,

I like that idea. BUT, i have to ask a question, this works IMHO when at rest at anchor. How does this wt effect one when sailing in an X knot breeze? and how does the breeze effect the rolling? While the wt at the top will help the roll stop quicker at anchor, will the wt at the top allow the boat to right itself with wind gust as quick? or allow it to capsize quicker? My gut says the wt at the top of the mast when sailing should not do as well as when not, but could see the at rest being better.

Anyway, better add some kind of fan to the experiment, along with a BIG bath tub, or swimming pool so the fan can add wind equal to the experiment.

Marty

In think in this we're just testing roll period. For gusts, an impulse is an impulse whether it's your finger or a fan. A dynamic oscillation test can be done with fans in a pool (or get a kid to handle bellows in the tub)- Good idea for another video. We also could get the kid to do some waves- mom won't complain, it's for Science.

 

[Jeff_H]

Cormeum:

I think that we are all in agreement that whether you add a weight at the top of a mast, or bottom of the keel, it will increase the roll moment of inertia, and that will slow the roll rate of the boat, which is all that your prioposed experiment will show. (As a teen, I actually did a similar experiment with a full sized boat and a plumb bob in a bucket of water for my outside experiment for a high school physics course.) I don't think any of us disagree with the impact of increasing roll moment of inertia.

But I do not believe that is the issue that we are discussing. The discussion point that seems to have triggered this discussion is the usefulness of the Capsize Screen Formula or the Motion Comfort Index. Neither contain any data about weight and buoyancy distribution, or dampening, which are key elements in determining the likelhood of a capsize or the motion comfort of the boat, and which from the current scientific thinking that I have been following would suggest are better predictors than the limited number of variables contained in either of these equations.

Jeff

 

[kevlarpirate]

Jeff, My beef is not your boat, I'm sure your boat is exciting enough for you to justify ownership.

The style of boat I refer to is the latest out there I don't want to be specific, These boats claim cruising ,versions. I call that a joke. More specifically disinformation,
meaning a deliberate attempt to mislead, taking advantage of the inexperienced
or highly emotionalized potential buyers. I have been saying the same thing all along.
I highly suspect these calculated static curves are manipulated. I called a MFG and he waffled when I pressed for details.

I also have a beef with statements about having great advancements after IOR , I say hogwash. Static curves for one got worse
Truth be told, design changes were supported solely by the power of the fact "speed sells" mostly downwind. Reducing weight was key

I have nothing against speed, but the sacrifices; comfort, noise, breakage, expense of repair are there and they are not going away. I consider this latest breed as throwaway boats. Buyer beware.

Boats are sold through testosterone, and if necessary, can I at least convince the wife she will be comfortable...Having both worlds is impossible , you can't make whiskey out of wine.

These boats sail on their leeward chine with the crew gathered on the aft rail 6 feet above the water , with the windward underbody out of water and 6 inches of the rudder exposed, and the bow almost buried. A 2 foot chop , the foredeck is soaking wet .... BTW I was/ am there, when I am not sailing on one of these , I am doing RC or on a chase boat , taking pictures for fun. BTW this in in 10KTS true,
I am out there for almost every race.

My friend has a latest design, water ballasted 35, he is an engineer, he knows exactly what he bought and why. I mentioned XXXX says they have a cruising version of their 40. I said it at the wrong time , he had a drink in his mouth which he almost lost .

It's a joke Jeff. We sail to Catalina and I go onboard many boats on moorings too. I If you believe I am old school , out of touch, you are mistaken.

As a note, I sailed a 40 ft canted keel twin foil, what a joke, a monstrosity
of unnecessary complication. we were bounced around like a toy and never enjoyed anything.

My disagreement with you is your belief that the devil is in the details.. That somewhere in the subtle shaping of hulls here and there a perfect baby pops out.

I agree a baby pops out , but when it comes to both performance and comfort combined, you can't get around the mass thing. Mass (includes moment) wins, peak of the curve.
Buy a big massive powerful boat, as much as you can possibly afford and learn how to take care of it so you don't end up in the poor house.

I mentioned (only once) a 50 foot ULDB going to weather pounding and the hull quivering
every wave I took , my chest hit the wheel. I had to stay an extra half a foot back.
I don't call that fun. At anchor, it was quite noisy and irritating, no improvement in my book.

As for moment of inertia, the article on the carbon mast tells all. Owner states
noticeably jerky upwind movements, You could tell he did not expect this, Personally I would never want that, If I did that to my own boat, I doubt if I could rationalize it considering the only speed gain would be revealed only by the knotmeter. The normal visual input would not reveal an extra 2/10 knot , however the accelerations would be felt.

Yes, I agree the titanium, bow section point you raised, but that was not about comfort
That was about a few hundredths of a knot upwind.

I will end with incident acct. It was the Santa Barbara , King Harbor race a few years back . A 32? foot sport boat blew up In the Channel Island area in what we
would call rugged sailing , gusting above 30 knots. Anyway, the boat launched a couple crew into the water. Big wide cockpits do this quite easily considering there is little to grab on to or lean into. One got back on the other didn't . The boat was dragged away
on it's ear faster than the crew could swim. By the time the crew got the ASO down and the #3up they could not work back upwind. These were Transpac veterans, no dummies
Almost 3 hours later , the crew was retrieved by another boat. Once you drill down into how such an incident can take place, I think we can agree it is a chain of compromises
all pointing toward the boat no longer takes care of the crew. Designs are based on
principally down wind speed, everything in it's path is marginalized.

have to run, kev

 

[cormeum]

Cormeum:



But I do not believe that is the issue that we are discussing. The discussion point that seems to have triggered this discussion is the usefulness of the Capsize Screen Formula or the Motion Comfort Index. Neither contain any data about weight and buoyancy distribution, or dampening, which are key elements in determining the likelhood of a capsize or the motion comfort of the boat, and which from the current scientific thinking that I have been following would suggest are better predictors than the limited number of variables contained in either of these equations.

Jeff

They're pretty useless then. We agree on that.

Taking a boat on a sea trial in crappy weather will give you better data than trying to solve a series of coupled differential equations-certainly better than looking at a screen that accounts for only minimal inputs.

No model quite like reality.

 

[PCP]

You refer to the boat that I own ... saying things like, "I don't believe for a second, that boat "style" is either comfortable or safe or had good manners in a well developed sea, storm or even at anchor. Big broad transoms slam all night long …and all the boats you are fond of pound going up wind even in the smallest of chop."

All I can say is that you obviously have never sailed on the model boat that I own. Having over a dozen owned boats as varied as a 1939 Stadel Cutter, 1949 Folkboat, half a dozen CCA era boats, a few IOR era boats, and several MORC boats. I once sat down and listed the boats that I had sailed on and came up with having cruised, raced and day sailed on close to 100 different designs over the years, that varied from traditional gaff riggers to modern sportboats.

I came to purchase the type of boat that I own, not out of some pre-conception, but out of a long evaluation of various boats that I have sailed. I understand that there are some shortcomings to all boats, and mine is no exception, but I would say that you got it very wrong when you talk about her motion in a developed seaway, chop or at anchor. My boat was single-handed into the US from South Africa and prior to purchasing my boaty I discussed the behavior of these boats with a diverse group which included Dudley Dix, a couple who sailed one up the Atlantic from South Africa to Scotland, and a fellow who had single handed on from South Africa to the Carribean the first 8 days of which were in winds that did not drop below 30 knots and hit gusts well into the 50 knot range and seas he decribed as being above the mast. All described these boats as having great offshore behavior.

In terms of sailing in short seas, I would say that one of things that I like about my current boat is its ability to slice through waves when sailing going upwind in a chop; rather than pounding, especially relative to the boats that I have owned and sailed on over the year.....

…..But more to the point, my boat is a 30 year old design. While a pretty advanced design for its day, it represents a snapshot in time of a time long ago. Based on my experience on board the better newer designs, they have far eclipsed the design thinking of that era in producing seaworthy and motion comfortable boats.

As any knowledgeable designer will tell you, pounding is more a more matter bow shape than displacment.

Jeff

I have to say that you seem to have some experience, in what regards extensive "test sailing" different kind of boats

Regarding bows and pounding, of course, you are right. Let me show you the moment I started to consider very large transoms boats on a different basis, regarding sea motion and pounding. I believe it was in 2005, at the Paris boat show and I was looking at a 40class boat (I have taken some photos). I keep going to the front of the boat, looking at the bow shape and to its fine entries, and going to the back, and looking to the huge transom and keep thinking that it made sense: Huge stability downwind, limited pounding upwind.

Looking at the bow and at the transom, separately, hardly you would think that they belong to the same boat.

Of course, this is not a cruising boat, it is a racing boat, but in the last years this principle has been applied to modern cruising boats.

That has allowed cruising boats to be optimized for sailing at 17º/20º degrees of heel, boats that can plan and go faster and safely downwind ( faster 4 to 8K) and that have an acceptable, almost comparable motion upwind. They will point a little less than the typical cruiser/racer (less than 5º) but are faster at 40/45º, making the same way, in a less stressing and more comfortable sailing position (being at that position, probably more comfortable than a more traditional modern designed boat, hard at the wind).

My wife, that knows one or two things about boat comfort and motion (sailing with me for many thousand miles), after test sailing the small beam option, ( I like both kinds of boats, for different reasons : Small beam and big transom large beam) declared that she wanted the other option, a big transom beamy boat with fine entries. The main reason is the 10º difference in heeling sailing position.

Only the ones that didn’t try to cook (or sleep) in a boat heeled 30º would not understand her choice.

And while my boat, kites more on the anchor than I would like, ..

Jeff, my previous boat had that problem, probably in a worst way than yours. With winds over 15k mine swung a lot at anchor. That was uncomfortable and on very strong winds that also put lateral stress on the anchor.

I have designed a stabilizing sail that has reduced that motion to a fraction, certainly less than half of what the boat used to do. On very strong wind it has the added advantage to take strain from the anchor (with strong winds, on each side of the small swing, the chain is not making almost any force because the boat “sails” a bit forward).

I have been approached by many cruisers that, being at the same anchorage and after seeing the results, have asked me permission to copy the model. It takes less than 5 minutes for just one person to rig it.

Regards

Paulo

 

[kevlarpirate]

Jeff, in reading Paulo's reprint you state that I "obviously" have not sailed a boat similar to yours.
Please tell me which boats of these (35-40 ft range) are closest to yours I have also raced hot dogs mostly, offshore on a couple too

J35, Express 37, C&C 37R, Schock 40 TFCC, Farr 40, Santana 35, Creekmore 33 I think, J40 (too heavy) Farr 36 one ton 1980? , J-105

Having sailed and raced these, tell me if you think I am qualified to forward
any findings of facts?

Paulo, are you still working on the answer to the static curve question? (chuckle)