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Old 05-17-2013
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Re: Bob Perry's take on Wolfenzee's dream boat

I apologize up front that this is a very long one (even for me):

There are two questions that I would like to address, the first is Wolf's question about how much faster this boat would be, and the second is the question(s) about seakindliness.

To address Wolf's question, lets just say that if we give Wolf's boat the benefit of the doubt and suspend physics for a moment, and we might assume at best Wolf's boat is 30 seconds a mile faster than something like a 1960's era keel version of a Bristol 29/30, which in itself was a pretty fast boat for its day. That would give him a rating in the 180's.

I would expect the 'My version' boat should be faster than my current boat at 87, or a J-40 at 78, and probably be close to comparable to something like a Beneteau First 40.7 down in the 50's.

In other words, in 10-12 knots of wind, the 'my version' boat starts out a couple minutes a mile faster. That would be the wind range when the speed different is at its least and a 25% increase in actual speed is a huge difference in real speed when cruising. But the speed disparaity would grow even larger in light air, or in heavy air.

In light air, relative to Wolf's boat, the massive increase in sail area relative wetted surface means that the 'my version' boat would sail far better than its rating in comparison, and would hit its higher hull speed much sooner as well.

In heavy air, Wolf is right that his boat is a displacement boat, and therefore pretty much limited to at best sustaining its hull speed of 6.75 knots. A boat like the 'my version' is a semi-displacement boat.

My current boat is primitive compared to this design, but as a point of reference, it's not hard in wind speeds in the mid-teens for Synergy to beat at speeds in the mid-7 knot range and reach at higher speeds. Reaching in apparent winds in the 20 kt range, it is easy to sustain speeds well over 8 knots. My boat has sustained speeds over 9 knots in the right conditions and hit speeds in the high 11's without terribly big waves to surf.

That is the kind of relative speed difference that we are talking about. It means hours of time cut off of a day's passage and perhaps weeks off something like a trans-oceanic passage.

I also want to address Hartley's question about seakindliness and a similar point that Marty made.....Marty's first. Marty, to get a boat to plane, you do not need to make the forward sections flatter, and therefore you would not expect that a boat that can plane, would necessarily pound as much as your boat. The bow sections can be finer and have a fair degree of deadrise, which combined would reduce pounding.

That said, I do not expect this boat to plane except in very extreme conditions. At best I would hope that it would spend a lot of its life in semi-displacement mode. That is achieved by minimizing the size of the bow wave and therefore the drive required to achieve speeds above hull speed. It is the same principle that allows Catamarrans to achieve the speeds that they do.

But to the broad generalities of the seakindiness, I will apologize that I don't have time to write this from scratch and that much of this was written for another purpose and so may not directly apply.

But to touch briefly on this question specific to the two designs, there is a tendancy to 'read' boat drawings in a way that suggests that the boat is standing still. When you try to visualize the behavior of a boat, especially relative to motion comfort, you need to visualize that motion over time as the boat passes through the source of that motion.

Starting with pitch, if we visualize the bows of Wolf's Original boat ('WO' from here on) and 'my version' ('MV' from here on) passing through a wave, the fuller bow on Wolf's boat builds buoyancy very quickly. The 'MV' boat would slice into the wave and more progressively build buoyancy. Considering that these are boats of near equal weight, 'WO' boat's rapid increase means more force being imparted into the bow, a harder collision in effect, and therefore a more rapid rotation and deceleration surge. As the two boats rotate, the 'MV' stern progressively builds buoyancy at a higher rate than the 'WO' and so dampens that pitch more effectively better keeping the boat in phase and resulting in a smaller slower pitch angle.

Hartley, I would respectfully suggest that you are mistaken that the 'MV' boat has hard bilges. If you visualize the 'MV' boat rolling, a number of things happen, first of all, the cross sectional shape is intended to progressively shift buoyancy as the boat rolls, and thereby progressively dampen the roll, and avoid a lurch at the edge of the roll. In reality, this cross section is actually more rounded and shifts buoyancy more progressively than the deep deadrise on 'WO' boat.

But the 'MV' boat with its deeper keel and with its bulb centered 7 feet below the roll axis, and its much taller (albiet lighter) mast, also has a much larger roll moment of inertia which should result in a slower roll rate. A higher roll moment of inertia is usually associated with larger roll angles but in this case, the purposeful progressive dampening of the hull shape, and the tremendously greater dampening from the deep keel and rig should mean a smaller roll angle as well.

In reality, that result will vary with wave height, steepness, and period, but as a broad generality the MV boat should roll and pitch a slower rate and through a smaller angle.

The rest of this was exurpted from something that I wrote for another purposes but I added some comments in italics to try to explain how the generalities apply:

Much of the 'common knowledge' concepts about motion and light boats came out Marchaj's book, "Seaworthiness, the Forgotten factor" but that book was written at a time when our understanding of motion and weight was at a very primitive state of study and does not reflect the 30 years of research that has occurred since. Marchaj's book clearly explained most of the dynamics of motion but many if not most of his conclusions were based on studies of light boats of the IOR type form, which tended to have beamy hull forms with pinched ends. small ballast to displacement ratios and high vertical centers of gravity. His conclusions about the causes of problems with this type form were right on target, but the IOR type form was not a very basis for designing light weight boats. Since then a much better understanding of how to design a light weight boat has emerged and so have light weight boats that offer exceptional seakeeping and seakindliness while advancing the speed of these boats as well.

To explain further, in the late 1980's and into the early 1990's, designers of IMS and Volvo type performance boats came to understand that motion was a major un-rated factor in the performance equation because large roll angles and sharp accellerations disrupt the flow over the sails, keel and rudder, creating drag and limiting the production of lift. There was a huge amount of study that went into developing an understanding of motion control and being able to computer model motion. Full sized boats were instrumented and that data was used to calibrate, validate or discredit the various theories floating around.

In the end, the predominant factors that control the faster motions of a boat are:1. minimizing the forces which might generate motion, (roll, surge, and pitch and to a lesser extent surge), 2; to be weight and bouyancy distribution (roll, surge, and pitch), 3. dampening (both static and dynamic) (roll, surge, and pitch), 4. remaining in phase (roll, heave and pitch), The overall weight of the vessel has minimal impact on the roll or pitch speed or angles.

In response, the better designed lighter weight boats (and the 'My version' boat) are purposely designed have dramatically lower and more concentrated vertical centers of gravity, progressive dampening, and hulls modeled to minimize sudden increases in buoyancy. As a result these designs tend to have gentler rotational motions (roll, sway, and pitch) through smaller angles than more traditional heavier weight boats.

Heave, which generally tends to be one slower of the six degrees of motion, is the only direction that modern light boats generally do poorer in but even there it is only in some conditions. And even in this case the greater rate of motion is partially the result of modern bouyancy distribution rather than being simply weight driven.

The current theories on heave is that the relationship between the heave acceleration to the wave configuration is proportional to the weight of the vessel per waterplane area. In other words, the force of a rising wave acts on the waterplane of the boat. The more weight per square foot of waterplane, the slower the boat will accellerate vertically. Since modern designs tend to have a lot of waterplane for their displacement, they tend to be affected more quickly by heave; pretty much following the contour and speed with which the wave surface is rising.

Heavier boats per square foot tend to expeience a kind of delay. It takes longer for them to feel the upward force and change direction, but once they do they store more energy and so momentum takes them higher than the wave surface at the top of the wave. This delay can be very helpful in a short seaway but can be a real liability in steep seas where being out of phase can mean a pretty harder landing. (been there, done that, have the broken toes.) This is a good example of how getting out of phase can be detrimental to motion comfort.


I also want to bring up this issue how light vs. heavy boats seem to be defined, and particularly as it seems to be done on this forum. This goes back to a point that I have raised here before. If we size boats by length then we seem get into an endlessly circular discussion of the merits of light vs. heavy boats.

I suggest that displacement is a much more accurate indicator of the ‘size’ of a boat. This is especially true when you talk about a boat intended for long distance voyaging. In other words, while it is tempting think of a boats size boat solely on a length basis and the need for specific accommodations, the displacement of a particular boat says a lot more about its 'real' size.

And it is here that I have the problem with the apparent implication that light boats are inferior for offshore voyaging or that they have poorer motion. If we size the boat by its displacement and compare to equal displacement boats, one being longer for its weight than the other, all other things being approximately equal, the longer boat will offer better motion comfort, be more seaworthy, be easier to handle, have an ability to carry more supplies, and be quite a bit faster. In most cases and for most costs, if the boats are of equal weight they will have a similar cost to buy, and maintain.

Respectfully,
Jeff
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Re: Bob Perry's take on Wolfenzee's dream boat

always thought gyradius had major impact on pitch. thought keeping weight out of the ends allowed boat to stay on surface and nearer it's designed waterline with less boat submersed so less parasitic drag as it made it's way though a seaway. ?Is that wrong?
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Re: Bob Perry's take on Wolfenzee's dream boat

I see them.

Wolf/Out:
Sorry I snapped at both of you They were independant snaps.

I snapped at Wolf because I grew impatient with his resistance to change in what is and has always been just an exploration of design ideas based upon his current boat. There was nothing personal to the choice of Wolf's boat. It was just a boat that had been recieving considerable attention and speculation. I get this feeling that there is a consistant knee jerk defence if the current boat. What Jeff and I were doing is not an attack on the current boat. It was simply our way of pushing that concept up to 2013.

Out:
I snapped at you because I relate to the position Jeff was in at the time. He had a bare bones conceptual drawing, i.e. a "first cut". The details you ask for, which Jeff has now thoughtfuly provided, come along farther into the design spiral and each detail is address at the appropriate time as components of the design are prioritized. I address the "design spiral" in detail in my book.

Not sure "parasitic drag" is what's at play with a boat that hobby horses. The energy that makes the boat pitch is energy subtracted from the energy that should be pushing the boat forward. The shape the water is seeing is changing with every pitch. When the bow goes down and shoots a sheet of water of the forward sections that is slowing the boat down. You are most certainly correct in that weight in the ends slows a boat down. The term used is "polar moment of inertia". You want a low "polar moment of inertia". Although I once read a Porsche technical ad that said thet design for a "high polar moment of inertia." Go figure.

In my old office just outside the Ballard locks in Seattle I would watch the boats exit the canal. Some days the outflow from the locks would meet the incoming tide and a very steep chop would build up. I always paid attention to how the various boats (OK, not always but often) handled that chop. Some of my boats like the Tayana 37 with two anchors on the bow, God knows how much chain forward and a dink in davits hanging off the stern did not take to that chop well. They pretty much bucked their way though it. This can be a sign of a high polar moment of inertia but it can also have to do with a heavy boat with lots of rocker and not enough volume in the ends to dampen the pitching, i.e. the curse of the heavy boat. Of course, the boats will all the weight in the ends are usually the heavy cruising types so they are handicapped on two fronts.
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Re: Bob Perry's take on Wolfenzee's dream boat

Quote:
Originally Posted by outbound View Post
always thought gyradius had major impact on pitch. thought keeping weight out of the ends allowed boat to stay on surface and nearer it's designed waterline with less boat submersed so less parasitic drag as it made it's way though a seaway. ?Is that wrong?
When it comes to pitch, you and I are talking about the same thing, rotational inertia. The more pitching rotational inertia a boat has, the slower it will pitch, but through a larger angle of rotation. This of course is a mixed blessing. You would think that a lot of rotaional inertia would be a good thing because it means a slower motion. But a boat with a lot of rotational inertia is more likely to get out of sync. with a wave train and so hit harder on each wave. That collision can mean a more sudden deacceleration and so a less comfortable ride both in terms of harsher motion and larger angle of rotation.

Both can have negative affects on performance as their flows around the keel and sails get more turbulent and less steady state. I cannot speak to the magnitude of impact from the change in trim on parasitic drag.

Jeff
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Re: Bob Perry's take on Wolfenzee's dream boat

Jeff's comments about boat motions mirror my own experience. Some time back I posted some thoughts about the motions of my old Quarter Tonner. It was a fairly extreme Ganbare era boat - diamond shaped plan, skinny bow, stinger stern, empty forward and aft etc.

Prior to it I had not experienced a boat where minimizing polar moments was a priority and I was astounded by how little it pitched. As the boat hit a wave it would slice into it with the narrow bow, not rising until the wave had reached the beamier midsection of the hull, the boat would then rise vertically and quite level and as the wave passed aft the skinny stern sections would keep the stern from lifting and the boat would descend levelly.

In effect, pitch was exchanged or replaced by a simple heaving motion up, over and down each wave.

Now rolling.......
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Re: Bob Perry's take on Wolfenzee's dream boat

As far as what the boat does in 10kts of wind, main and working jib alone (about 400sqft) have the boat doing 6.5kt (I haven't tested it out enough to see how little wind can maintain that speed).
Making modifications to a displacement hull to make a more efficient type of keel is one thing....changing it to the point where it planes saying it is a variation of the original hull and comparing the two is something different. Yes a planing hull under the right conditions will go faster, so will a trimaran.
As far as sailing faster for a "rating comparison" I am not trying to beat the handicapping "numbers".
I have heard alot about how different variations would behave from people that would probably be able to know how what I already have will behave....I just haven't heard much about how what I have would behave....you have the lines, measurements, disp, sail plan, etc....any honest learned opinions on what I have?

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Re: Bob Perry's take on Wolfenzee's dream boat

Having a bit of a hard time following your argument there Wolf. Jeff and I NEVER idscussed "rating" and we never would. It just has zero application to this excersize. As for "planing"? Forget about it. A boat like this can never plane. It might surge a bit on a wave face with the right wind aft of the beam but it will never, ever plane. Even Jeff's latest iteration would never be a "planing" boat. Maybe a brief surge or surf but not planing.

As for what you have now? I'll stick to the facts:
I'd say you have nice old design, pretty sheerline as drawn, narrow and quite full forward with a lot of deadrise and slightly hollow garboards amidships increasing towards the transom. The point of max beam is quite far forward resulting in a very full bow. The full keel has a nice profile to it with plenty of "drag" angle. It's a nice, a very pleasant design.
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Re: Bob Perry's take on Wolfenzee's dream boat

Er, Actually I mentioned PHRF ratings as a way of trying to answer one of Wolf's earlier questions. Wolf asked, how much faster the new design would be relative to his boat. I tried to quantify that by referring to the relative speeds implied by a general range of ratings.

To respond to Wolf's latest points and questions, it is always hard to guess what a one-off boat would rate under PHRF. My sense is, at best, a Capt. Cicero would rate even with something like the keel version of the 1960's era Halsey Herreshoff designed Bristol 29-30 or a Cape Dory 30. These rate in a range around 207 to 218. My guess is that the Bristol would be faster in light air or and power reaching. My guess is that you would be a pretty even match in the mid-range (12 knots or so) with the Bristol not sailing it's rating. The Bristol should be faster than all three in a stiff breeze and the Cape Dory would also do well as the builds.

But that is all total conjecture.

Marty had mentioned his ideal boat would be able to plane. None of these boats are planning types. At best my version would be semi-displacement when there was enough wind, in other words capable of sailing at a higher speed than its theoretical hull speed.

Jeff
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Re: Bob Perry's take on Wolfenzee's dream boat

Ok, I missed that PHRF reference. I'd start Wolf with a rating of 220.
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Re: Bob Perry's take on Wolfenzee's dream boat

Quote:
Originally Posted by Jeff_H View Post
..
I suggest that displacement is a much more accurate indicator of the ‘size’ of a boat. This is especially true when you talk about a boat intended for long distance voyaging. In other words, while it is tempting think of a boats size boat solely on a length basis and the need for specific accommodations, the displacement of a particular boat says a lot more about its 'real' size.
....
Jeff, while generic agreeing I would point that a better indicator would be boat stability, I mean the total area behind the positive part of a RM curve. As you know that is greatly influenced by displacement since a RM curve is obtained multiplying the arm (GZ) by the displacement.

I would suggest that the GZ curve plays here an important role, not in similar boats but in different types of boats. A narrow 38ft with a relatively high center of gravity can have a a max GZ as low as 0.6 or 0,7 (m) while a deep draft low CG beamier hull can have one has bigger as 0,9 or 1.0 a very considerable difference (33%) that would correspond to a 33% difference in weight assuming the boats had similar GZ curves.

Regards

Paulo
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