SailNet Community banner
  • SailNet is a forum community dedicated to Sailing enthusiasts. Come join the discussion about sailing, modifications, classifieds, troubleshooting, repairs, reviews, maintenance, and more!

Full or fin keel?

191K views 846 replies 107 participants last post by  mstern 
#1 ·
Can somebody pro/con a full vs. fin keel for a newbie (will learn to sail on said boat) and taking it thru the Caribbean? All I can seem to come up with so far is fin keel is better to the wind, and a full keel will protect your rudder.
 
#501 ·
BryceGTX must have missed my point. We all make tradeoffs to suit our needs. The cabin layout and storage space of our Clearwater 35 is less than that of an IP 35/350 (and many other 35 footers) but it is not "extremely" restrictive. Unless you needed the shoal draft, you would likely pass it over for a boat with a more conventional layout. We wish we didn't get shoal draft at the expense of interior space, but that's life. We--that includes my wife--like our boat overall and after 16 years aren't about to trade it in.

Having the boat at our shallow water dock about 150' from my front door is a lot more convenient and a lot less expensive than keeping it at a marina. The reality is that there is limited choice in ocean-capable boats that can come to our dock at a normal low tide (2.5' @ MLW). The Shannon Shoalsailer, the smaller Southerlies, the Seawards might get to our dock most of the time, or maybe some of the smaller catamarans. But, since we get to see our boat every day in season, looks matter. The Clearwater 35 is arguably one of the prettiest shoal draft boats in this size range and that matters to us

Comparing the Clearwater to an IP is apples and oranges. A clear differentiator between our boat and an IP--other than the interior space--is sailing performance. My Clearwater 35, as with most well-designed fin keel boats in this size category, will outpoint the IPs. Coming home to Mystic from Nantucket, Martha's Vineyard, Cuttyhunk, or Newport, this is the difference between sailing and motoring. This is very important to us--particularly my wife, who hates to motor.

Another factor is that anchorages are bigger when your boat can float in 2' of water. Cuttyhunk is one example, where the shallow end of the rental pilings is 4' at low water. Also, there are Cuttyhunk town slips with less than 4' that are the last to be taken on a busy weekend. And don't forget the increased opportunity for gunkholing, like in Hadley's Harbor.

I've seen my share of high winds, but unless we are talking off the wind, I'll stick with my Clearwater, which is much easier to control than the IPs I've sailed. I attribute this to the keel/rudder configurations--with the fin keel providing more responsiveness when needed. When it come to safety below in a seaway, the more confined space in my Clearwater becomes an advantage. When it comes to tropical storm weather, you won't get me out there on any boat, including the Mirabella V.
 
#515 ·
BryceGTX must have missed my point. We all make tradeoffs to suit our needs.
Your boat was designed entirely as a performance shallow water boat. That means at all costs, even salon room was compromised to performance. That huge divider running directly down the center line of the boat from the steps to the forward bulkhead, from deck to ceiling, over a foot wide is a serious compromise.

So now you have a shallow water boat. Lets compare the shallow water performance to the IP. A particular IP has a draft of 4 feet. To put your boat in the same shallow water requires you to place your keel only half down. So now you keel is at 4 feet.

Problem is, only half of your keel is in the water. This means, only half of your lee resistance is provided. And of course only half of your keel weight is where is suppose to be. The other half of the weight is inside your salon uselessly creating a heeling moment.

The IP owner would have to remove his keel and place it on his salon floor to get the same effect as your boat in this configuration. So clearly, when we compare the performance of your boat in the same shalllow water you are at a serious disadvantage.

You complained that the IP is hard to tack because of its dual headsail configuration. However, its inner jib is invariably a self tacking jib. In situations where constant tacks are required why use the outer jib? In that case the IP owner has it all over your boat. He just turns his wheel.

My argument with you is not that your boat does not head to wind just fine. But you thoughtfully left out all the other advantages of the IP over your boat.

Bryce
 
#503 · (Edited)
Quote from BLT2SKI

"My wife would also prefer one a bit less cramped than what we have. BUT, even a full keeler would not have a lot more room. I was on boar an Eric jr awhile back. Half the room of my boat, with the same length etc. Not sure it would do any better in a blow either."
Eric Jr. LOA= 25'2"; Beam= 7'7"; Draft= 4'

Jeanneau 30 LOA= 29'6"; Beam= 10'5"; Draft= 5'6"
 
#504 ·
To ChucklesR question about a late 80's 3ft' stub keel with a wing: First of all, the Australians created quite a stir with their wing keel in the America's Cup in the 80's and it seemed to create a marketing angle for some boatbuilders. There obviously was some merit, but there are several factors at play when translated to the recreational market.

First of all, the wing keel provides an end plate for the primary keel to reduce tip vortices--for the same reason you see winglets on the later versions of commercial jets--to reduce drag. Also, adding the wing shape produces a fatter (i.e., heavier) "bulb" to provide more of a righting moment for a given keel length.

That said, you may notice that the more efficient racing keels are like glider wings: deep, high aspect foils. The downside is the deep draft required to pursue this approach.

But, getting back to ChucklesR's Irwin, it's all about a real world compromise. A short stub keel is not going to provide the lift of a longer, high aspect foil, but, by adding the wing (for added ballast) it can offer reduced draft.

Oh, by the way, a full keel is the antithesis of an efficient, high aspect keel, for those who care to think about keel compromises. You might look to aircraft design for a reference point. The only aircraft I can think of with low aspect wings are "wing-in-ground" (WIG) craft, like the one built by Merrifield Roberts in Rhode Island about 20 years ago or the huge, "Caspian Sea Monsters" built by the Soviets several decades ago. That said, I am not a hydrodynamicist or naval architect/boatbuilder, so you can take my comments with a grain of salt.
 
#508 ·
Ted Brewer's Motion Comfort Ratio might be germane:
MCR = Disp / (2/3*((7/10 * LWL)+(1/3 *LOA))*Beam4/3 )

Now for me that's a little too much math so I go here The roll acceleration: What´s the best for crossing oceans? - Boat Design Forums
and look up a boat. Unfortunately mine is not listed.

Note the formula does not take in keel shape at all - a surprising lack considering the effect of lift and weight, moment arms and all that other 'stuff' that has kept this thread growing.
Of course it also shows Brewer designs as being seriously comfortable.

I also can't see where LOA comes in - my bow spit adds nothing to comfort - that should be LO deck.
 
#509 ·
(I apologize in advance for the cut and paste of something I wrote a long time ago)

The Capsize Screen Formula and the Motion Comfort Index tells almost nothing about how the reality of a boat's likelihood of capsize or its motion comfort. In fact they provide so little indication of a boat's behavior that to rely on these surrogate forumlas for real information borders on the dangerous.

Both of these formulas were developed at a time when boats were a lot more similar to each other than they are today. These formulas have limited utility in comparing boats other than those which are very similar in weight and buoyancy distribution to each other. Neither formula contains almost any of the real factors that control motion comfort, the likelihood of capsize, or seaworthiness. Neither formula contains such critical factors as the vertical center of gravity or buoyancy, neither contains weight or buoyancy distribution (of the hull both below and above the waterline), the extent to which the beam of the boat is carried fore and aft, and neither contains any data on dampening, all of which really are the major factors that control motion comfort or the likelihood of capsize.

I typically give this example to explain just how useless and dangerously misleading these formulas can be. If we had two boats that were virtually identical except that one had a 500 pound weight at the top of the mast. (Yes, I know that no one would install a 500 lb weight at the top of the mast.) The boat with the weight up its mast would appear to be less prone to capsize under the capsize screen formula, and would appear to be more comfortable under the Motion Comfort ratio. Nothing would be further than the truth.

And while this example would clearly appear to be so extreme as to be worthy of dismissal, in reality, if you had two boats, one with a very heavy interior, shoal draft, its beam carried towards the ends of the boat near the deck line, a heavy deck and cabin, perhaps with traditional teak decks and bulwarks, a very heavy rig, heavy deck hardware, a hard bottomed dingy stored on its cabin top, and the resultant comparatively small ballast ratio made up of low density ballast. And if we compare that to a boat that is lighter overall, but it has a deep draft keel, with a higher ballast ratio, the bulk of the ballast carried in a bulb, its maximum beam carried to a single point in the deck so that there was less deck area near the maximum beam, a lighter weight hull, deck and interior as well as a lighter, but taller rig, it would be easy to see that the second boat would potentially have less of a likelihood of being capsized, and it is likely that the second boat would roll and pitch through a smaller angle, and would probably have better dampening and so roll and pitch at a similar rate to the heavier boat, in other words offer a better motion comfort....And yet, under the Capsize Screen Formula and the Motion Comfort Index it would appear that the first boat would be less prone to capsize and have a better motion when obviously this would not be the case.

There are some better indicators of a vessel's likelihood of capsize. The EU developed their own stability index called STIX, a series of formulas which considered a wide range of factors and provides a reasonable sense of how a boat might perform in extreme conditions. Unfortunately meaningful results require a lot more information than most folks have access to for any specific design. The Offshore Committee of US Sailing developed the following simplified formula for estimating the Angle of Vanishing Stability (Sometimes referred to as the 'AVS', 'limit of positive stability', 'LPS', or 'Latent Stability Angle' ):
Screening Stability Value ( SSV ) = ( Beam 2 ) / ( BR * HD * DV 1/3 )
Where;
BR: Ballast Ratio ( Keel Weight / Total Weight )
HD: Hull Draft
DV: The Displacement Volume in cubic meters. DV is entered as pounds of displacement on the webpage and converted to cubic meters by the formula:
Displacement Volume in Cubic Meters = ( Weight in Pounds / 64 )*0.0283168
The Beam and Hull Draft in this formula are in meters. These values are entered in feet on the webpage and are converted to meters before SSV calculation.
Angle of Vanishing Stability approximately equals 110 + ( 400 / (SSV-10) )

There is a convenient calculator at http://www.sailingusa.info/formula.htm

It should be noted that the AVS is only one indicator in evaluating the likelihood of capsize, meaning it only predicts the point at which the vessel wants to turn turtle. It does not predict the amount of force that would be required to heel the vessel to that limit, nor does it predict how the shape of the boat might encourage wave action to roll the boat closer to the angle at which it no longer wants to return.
 
#510 ·
The Capsize Screen Formula and the Motion Comfort Index tells almost nothing about how the reality of a boat's likelihood of capsize or its motion comfort. In fact they provide so little indication of a boat's behavior that to rely on these surrogate forumlas for real information borders on the dangerous.
I agree 100%

On the AVS - you still need to add righting moment curves, accurate displacement and center of gravity.
It's still a best quesstimate.

My big bulwark, fully enclosed center cockpit, thin (12.3 beam on a 38ft) 4.5 foot draft, Low density (lead pigs in slurry) stubby ballast should roll over if I stand on a rail. Especially considering the 300 feet of chain and 45 pound anchor, Dinghy on the davits and other weight high up (my big head) etc..
I think adding on a couple square yards of solar panels should do it.

Unfortunately the link doesn't work.
 
#511 ·
people can yap all they want to about different keels and styles o boat. it is going to be a huge investment--time, money and lifestyle--why dont you take a sail on each kind of boat that interests you--as opb cruising or racing--see what YOU like best an buy that????
in order to know what YOU like to sail, you have to sail it .....have fun--is the most funnest part of boat purchasing--research!!!!

btw--we dont need no stinkin numbers--when passion hits, BUY.
 
#512 ·
btw--we dont need no stinkin numbers--when passion hits, BUY.
Well said!! Love your attitude!

Much as I like science and understanding - art, music and passion are really what matters to me! I've never tried a boat before I bought it, probably because I buy cheap wrecks for a song that need work before launching. Always it's the older traditional styles that grab me when I see them sitting beside the road. But this year I bought a Hunter 23 (a wreck). Partly because of this thread, and partly because it was there. A super ugly stubby wing keeled boat, which I hate to look at and always think "toy boat". I must admit, it sails surprisingly well. And, when I'm on it, I can't see how ugly it looks, I just sail around the beautiful traditional boats and look at them. :D With a theoretical hull speed of something like 5.9 knots, I was surprised when my new (tiny, pocket sized hiking style) first GPS showed I was doing 6.2 MPH (not knots) going against the tide while tacking into the wind. Recently I found that it recorded tracks of where I went, and showed my top speed to be 8.3 MPH. I can only think I must have been surfing down some monster waves going with the wind and tide. (Week long sail/anchor.) Of course, when I'm ripping along, I don't bother changing screens on the GPS to see my speed, too busy sailing! So I've enjoyed my first "modern" boat more than I expected. Except looking at it. :eek:
 
#529 ·
Paulo you are right designers have long since abandoned full keel designs when it comes to production boats that require cheap quick building. They also abandoned the stay sail and bow sprite. I don't know how many of you have been paying attention but they both are back as high performance upgrades. It would appear the older designers got it right the first time. I have owned many fin and full keel boats, after 25 years of sailing I find I sleep better in a full keel than a fin keel so that's my preference. A good friend just lost his half million dollar fin keel to a submerged deadhead, his boat went down in under 3 minutes. If you want to know the difference in strength between full and fin keels, don't ask the designer, don't ask the builder, don.t ask the sailor. Ask the boat yard manager who has to do all the repairs on the boats, or at least the ones that didn't sink. :)
 
#530 · (Edited)
Paulo you are right designers have long since abandoned full keel designs when it comes to production boats that require cheap quick building. ... It would appear the older designers got it right the first time. .. A good friend just lost his half million dollar fin keel to a submerged deadhead, his boat went down in under 3 minutes. If you want to know the difference in strength between full and fin keels, don't ask the designer, don't ask the builder, don.t ask the sailor. Ask the boat yard manager who has to do all the repairs on the boats, or at least the ones that didn't sink. :)
I agree with you that is possible to build a full keel boat stronger than a fin keel boat the same way that is not possible to build a full keel boat that is so efficient sailing as a fin keel boat (both boats being well designed).

The question here is if the strength we can achieve with a fin keel boat is sufficient to make a safe sailboat (and if so we can have better sailing boats) or if fin keel boats are dangerous.

The huge number of fin keel boats without problems and the very low percentage of problems show that modern materials and building techniques can provide safe fin keel boats.

Regarding being cheaper and quicker to build, as Jeff has pointed already here you got it wrong, it is more complicated and expensive to build a fin keel boat than a full keel boat. The structure that has to be built to transmit the efforts to the hull is very expensive to built.

Saying this I can understand that a very small minority prefers to have (new) a full keel boat instead of a fin keel boat the same way that I accept that a guy prefer to drive a big truck instead of a saloon because it is safer.

Regarding the used market I can easily understand that some full keel boats are so strong that can, after some tens of years, offer a better warranty of solidity than a fin keel boat with the same age, but each case is a case.

Regards

Paulo
 
#538 · (Edited)
:DI assume(and hope) you are kidding:)

The stiffness of a boat has not to do with the way the boat is sailed and an Open 60 is one of the stiffest boats you can have. Only because it is so stiff it can be sailed that way and remain steady on that position allowing that Alex to be on the keel of Hugo Boss with a Hugo Boss suit. cool:cool:





Regards

Paulo
 
#548 ·
German Yacht Magazine Article on Full vs. Fin Keel

The 25/26 issue of the German bi-weekly Yacht magazine has a very interesting article comparing a full keel (Vindö 40, 31ft, 1971), a moderate fin keel with skeg (Hallberg Rassy 29, 1981) and a modern fin keel boat (Sun Odyssey 30i, 2008).

They sailed the three boats together in 5 Bft wind and 3 to 5 feet waves to find out which is the most comfortable to sail under these conditions.

They had 3 crews rotating between the boats and all crews reported that the HR and the Vindö are not only more comfortable but the HR even sailed higher and faster than the Sun Odyssey.

(I'm a little disappointed, I expected a German magazine to use 3D accelerometers and data loggers and software to evaluate the boat movements and not just rely on crew opinion ;))

Let me cite the closing sentences of the article:

"The most common argument used to justify the uncomfortable motion of modern cruising boats is that the typical customer (young families, older couples and charter crews) won't leave the marina in winds over 4 Beaufort anyway. For that, these boats are significantly faster in light winds than they predecessors.

The latter may be true. The first, however, should probably be reversed: maybe the reason people don't sail them in rough water is that these boats are too uncomfortable for that?"


"Häufigstes Argument zur Rechtfertigung des unbequemen Seegangsverhaltens moderner Fahrtenyachten ist, dass sie von der angepeilten Klientel - jungen Familien, älteren Paaren und Chartercrews - bei Bedingungen jenseits der 4 Beaufort ohnehin nicht mehr bewegt würden. Da blieben die meisten lieber im Hafen. Dafür seien sie im unteren Windbereich deutlich schneller als ihre Vorgäger.

Letztgennantes Argument mag stimmen. Auf das andere kann jedoch auch der Umkehrschluss angewandt werden: Wird vielleicht nicht mehr bei Seegang hinausgefahren, weil die Yachten dafĂĽr zu unkomfortabel sind?"
 
#549 ·
Re: German Yacht Magazine Article on Full vs. Fin Keel

The 25/26 issue of the German bi-weekly Yacht magazine has a very interesting article comparing a full keel (Vindö 40, 31ft, 1971), a moderate fin keel with skeg (Hallberg Rassy 29, 1981) and a modern fin keel boat (Sun Odyssey 30i, 2008).

They sailed the three boats together in 5 Bft wind and 3 to 5 feet waves to find out which is the most comfortable to sail under these conditions.

They had 3 crews rotating between the boats and all crews reported that the HR and the Vindö are not only more comfortable but the HR even sailed higher and faster than the Sun Odyssey.

(I'm a little disappointed, I expected a German magazine to use 3D accelerometers and data loggers and software to evaluate the boat movements and not just rely on crew opinion ;))

Let me cite the closing sentences of the article:

"The most common argument used to justify the uncomfortable motion of modern cruising boats is that the typical customer (young families, older couples and charter crews) won't leave the marina in winds over 4 Beaufort anyway. For that, these boats are significantly faster in light winds than they predecessors.

The latter may be true. The first, however, should probably be reversed: maybe the reason people don't sail them in rough water is that these boats are too uncomfortable for that?"


"Häufigstes Argument zur Rechtfertigung des unbequemen Seegangsverhaltens moderner Fahrtenyachten ist, dass sie von der angepeilten Klientel - jungen Familien, älteren Paaren und Chartercrews - bei Bedingungen jenseits der 4 Beaufort ohnehin nicht mehr bewegt würden. Da blieben die meisten lieber im Hafen. Dafür seien sie im unteren Windbereich deutlich schneller als ihre Vorgäger.

Letztgennantes Argument mag stimmen. Auf das andere kann jedoch auch der Umkehrschluss angewandt werden: Wird vielleicht nicht mehr bei Seegang hinausgefahren, weil die Yachten dafĂĽr zu unkomfortabel sind?"
Here is the Video.In der Welle: Langkieler gegen Kurzkieler - Yacht TV - Segel Videos von Europas größtem Yacht Magazin
It is definetly an interresting article, but I am not sure we can make the deductions they made purely on keel shape, because these boats are very differrent in other regards, beam, hullshape, displacement, ballast ratio, etc.. However, one propably can safely assume that the SO 30i and a lot of current production boats are built for space below, rather than sailing.
 
#555 ·
Once the wind speed gets to the point where comfort would be an issue my boat is doing hull speed (9kts I am doing hull speed,6.5kt, under main and working jib). So unless you are talking about racing fin keels that can plane, a fin keel would not be any faster. 30' at 8ton it is smooth in nasty seas
 
#556 ·
.So unless you are talking about racing fin keels that can plane, a fin keel would not be any faster. 30' at 8ton it is smooth in nasty seas
there are no racing fin keels. Fact is that most modern performance cruisers and even some mainstream cruising mass production boats use similar keels to the ones that are used in racers just with a smaller draft.

Regards

Paulo
 
#560 ·
My previous generation by 3 or 4 Jeanneau, has 2400 to 6500 lbs of boat. With a 5.5' draft.

At the end of the day, if the boat works for your purpose, keel design and where the ballast is set at works, away you go! enjoy your boat! I'll still take my Jeanneau over an equal Catalina with a lead keel, I may heel a bit more, but I am faster in all but light winds!

Marty
 
#563 ·
A light weight disp fin keel will do better than my boat in light air (<5kt), but not by much, alot of heavy disp full keels can't get out of their own way in light air, my boat has always surprised me in light air. It doesn't take much for me to get to hull speed, 10kt wind w/main and working jib = 6.5kt. My boat starts to excel over the lighter fin keels as the wind kicks up, and when things get dicey I would have anything else. My "obsolete/antiquated" rig works differently and behaves differently with performance weeks over the years is quite satisfying, aside from performance, she has a style and grace that just doesn't happen in production boats. I respect "to each his own" as should everyone.
 
#561 ·
Wolf,

Curious, is your B/D only using the "lead" in you keel? if so, you probably have a higher % than that, as one "should" IMHO include the rest of the keel as ballast per say. A swag would then be 4500lbs/15K = 30%. As you really have what is a precursor to todays T keels, but do to materials and knowledge of some material strength, many things were designs heavier than need be. With what is probably a 5'long, 6"wide, 1'high chunk of lead at the bottom of your keel, it behaves like a heavier fin keel from a stability point of view.

my 01 on the subject.

Marty
 
#569 · (Edited)
Hull construction does have alot to do with it. My hull is 1" thick mahagony strip plank, glued and top nailed with 3" nails, screwed onto 2x3" double frames (which are bolted together with 3/8" through bolts) and 1/4" ceiling....to finish it off the builder also covered the boat with two layers of fiberglass. The weight is distributed alot more evenly.
B/D ratio by itself doesn't mean alot whole lot, it is simply "An indication of a boats stiffness or it's ability to resist heeling", some boats actually perform better when heeled (mine does best at 25-30 degrees and it doesn't take much to get it there) and some of weekend cruisers get nervous if there boat goes beyond 10 degrees so there are boats designed accordingly.
The height of the mast, sail area and beam all are necessary calculations here is a performance calculator for cruising boats: SloopIT - Boat performance calculator
 
#572 · (Edited)
I've owned full keeled boats with encapsulated ballast, one off fin keeled with keel bolts, modified fin with bolts . It's been mentioned several times in this thread full keel boats have encapsulated ballast (not always true) which runs the risk of complete ruin in a hard grounding and bolt on keels do not. Given I just spent my kids inheritance on a high aspect bulbed fin keel boat with encapsulated ballast (yes they exist) is there any data to support these statements.?Is this just ancedotal. Would think just like a properly engineered spade rudder can be stronger than a skeg hung one -a properly engineered fin with encapsulated ballast should be as strong or stronger than keel bolted.To the point that the grounding would have to produce whole structure failure of the vessel regardless of encapsulated or bolted. ?Are Shannons and Outbounds lousy boats.
P.S.- right on Paulo -having lived with both beasts unless thin water precludes it's a fin that's a better choice- just like we don't see many gaff rigged schooners being made these days unless we are in Lundenburg and see more solents modern fin keel boats can tract better, sail better and are as safe or safer in bad weather if properly designed and constructed. One post was kind enough to have a link that allowed comparisions.
please keep this thread going
Tx,
 
#574 · (Edited)
"Suhali" was the name of Robin Knox Johnson's boat that won the first solo nonstop round the world race.

It was a heavy, old-fashioned, full-keeled, Colin Archer design that survived the race where all others were eliminated due to material failures or insanity in the case of Donald Crowhurst. It averaged 4 knots per hour - the classic 4ksb.
 
#579 ·
A friend of mine decided to compare his Calalina 30 to my boat, same LOA, same LWL....lots of other differance....the final results surprised him
Captian Cicero
Ballast to weight ratio 28%
Max hull speed 6.7
Displacement length ratio 428.57
Sail area disp' ratio 13.15
Capsize screening ratio 1.42
Motion comfort ratio 46.71

Catalina 30
Ballast to weight ratio 41.18%
Max hull speed 6.7
Displacement length ratio 291.43
Sail area disp' ratio 15.17
Capsize screening ratio 1.99
Motion comfort ratio 23.99

I used SloopIT - Boat performance calculator
 
#580 ·
Do not remember which number, be it capsize or comfort ratio, but was not one of them "hood" a naval architect that came up with that ratio. Later said it was good, but at the end of the day with some of the newer designs not as useful as with past designs. Not saying that that ratio should be thrown out per say, as it favors longer/skinnier designs vs some of the fatter hulled designs of today. Rather apparent that hull design will potentially make or break a design depending upon useage as to if it will or will not work for the end user. Not just fin vs full vs bilge vs CB or some combo there of!

The more I type, I believe it is the motion comfort ratio number. Even short will come out on the lower side of things than width. As a 30'L 10' wide boat will come out with a worst number than a 60' x 20', even tho the length.width ratio is equal. I am also recalling disp being part, maybe that needs to be equally doubled to get the same ratio, where is going up double in length, usually (typically) quadruples or equal the disp of the boat. That would be an interesting plug in numbers to see what or if one can get different equal length and beam to equal....

Marty
 
#581 · (Edited)
Do not remember which number, be it capsize or comfort ratio, but was not one of them "hood" a naval architect that came up with that ratio. Later said it was good, but at the end of the day with some of the newer designs not as useful as with past designs. Not saying that that ratio should be thrown out per say, as it favors longer/skinnier designs vs some of the fatter hulled designs of today. Rather apparent that hull design will potentially make or break a design depending upon useage as to if it will or will not work for the end user. Not just fin vs full vs bilge vs CB or some combo there of!

The more I type, I believe it is the motion comfort ratio number. Even short will come out on the lower side of things than width. As a 30'L 10' wide boat will come out with a worst number than a 60' x 20', even tho the length.width ratio is equal. I am also recalling disp being part, maybe that needs to be equally doubled to get the same ratio, where is going up double in length, usually (typically) quadruples or equal the disp of the boat. That would be an interesting plug in numbers to see what or if one can get different equal length and beam to equal....

Marty
Hi Marty,

Many years ago, having already an interest in boat design I bought a more complete and complicated performance boat calculator the kind wolfenzee used to compare those boats and also many years ago I trough it to garbage.

That type of calculators only works with old boats and even so they have to have a similar hull shape and type of keel. The Capsize ratio ratio is particularly misleading and the comfort ratio is pretty meaningless. Jeff posted a thread about motion comfort in modern boats and posted in it also a very good article about that.

http://www.sailnet.com/forums/sailb...n/37548-modern-hull-forms-motion-comfort.html

Regarding motion capsize ratio probably the best discussion about it is on other forum but the results from the discussion are quite clear. I do not even feel the necessity to discuss that on sailnet. I guess that almost all know that the ratio does not make any sense when applied to modern hulls and by modern I am talking about 35 year's old hulls, at least the good ones.

Capsize Ratio's - Cruisers & Sailing Forums

Regards

Paulo
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top