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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.
 
#72 ·
utchuckd,
So there; see what I meant when I said "if you can decide in your own head" full vs fin keels?
There really is now way to simplify this decision but to think hard about the kind of sailing you are going to do and where you are likely to do it and then mentally start building the boat of your dreams. Then comes the hard part of finding that boat without having to commission a naval architect.
And then you come to uderstand that saying "every boat is a compromise."
I think very very few people ever got their last boat with their first boat. So don't kill yourself trying to find that perfect boat right out of the box.
Have fun with this journey.
John
 
#73 ·
Arion is a very pretty boat. I imagine a modern version of that would sell a few copies.
 
#74 · (Edited)
that's a nice sailboat...thanks for posting MiTiemp...My own boat has an 8 foot beam too...but she's only 29-feet long ! This is interesting...Perhaps alot more 40-foot+ boats would be speedsters like this one if folks didn't have that understandably strong desire for roominess....that canoe stern helps her get out of her own way too though....
 
#78 ·
Fin keel or full keel, a boat doesn't have to be fat.:)

The Valiant of course isn't quite as narrow as Arion. But after almost 40 years it helps prove a full keel isn't necessary for a boat to be "offshore" capable. There are also many others that have traveled far and wide with fin keels.

I can't think of many full keel boats with high efficiency keels - just large ones with a great deal more wetted surface than necessary.
 
#107 ·
Fin keel or full keel, a boat doesn't have to be fat
There are two reasons to be fat. First is to get more interior room. Second is to create more righting moment. It is the easiest way to get righting moment. And of course more stiffness.

However fat hulls make for poor handling in rough water without significant keel mass. And therein lies their downfall as a substitute for a full keel boat.
Bryce
 
#79 ·
With in the matter of a year I went from swing keel (Chrysler 22) to fin keel (Chrysler 26) to modifed full keel (Endeavor 32) is my only claim to knowadge.

The swing keel was great for the lake sailing we did and it being our 1st sailboat made learning what running aground meant with the ease of dealing with it. I could see sailing it on the coast in fairer weather. Also great for getting in and out of the water. we stored it on the hard between outtings with mast up.

The fin keel was on our coastal/sound boat that we sailed long weekends on unfamiliar waters. Great for the rougher larger waters but it didn't stop me from thinking how nice if it was also the swing keel version when we ran aground a few times. Fun all around thou I'm never sure the other boats knew we were racing at times.

Now that we are liveaboard cruisiers the modified full keel meets our needs very well.
With the experiance we gained with the other 2 boats, We have been able to cruise the east coast sailing alot on the outside or even sailing the ICW due to its shallow draft. I guess we have learned how to handle it well enough that I have been able to back it out of tight spaces with out much trouble. I know I don't sail as fast as the fin keel boats but I'm in no hurry and it is my home. I don't think I would like it as much if I was only useing it on weekends and I was out to play.

So me being NO expert can only say that it really depends on what type of sailing you do as to what type of keel is better.
 
#80 ·
This is just my 2 cents and all its worth. I must have missed it but did anyone mention the safety factor of being able to heave to? If you have ever been in a third world boat yard you will appreciate the ease and safety of hauling out a full keel vs a fin keel. A full keel boat can be safely careened for a quickie bottom job, try that with a fin keel. If you haven't seen what happens when a fin keel hits a reef at 6 knots you should come to the Pacific North West and visit any boatyard, its pretty scary. Just a few thoughts :)
 
#81 ·
...... If you haven't seen what happens when a fin keel hits a reef at 6 knots you should come to the Pacific North West and visit any boatyard, its pretty scary. Just a few thoughts :)
True enough, but I'd think the premise would be to avoid such encounters.. I know accidents happen but I'd hate to think that ramming a rock would be a frequent enough occurrence to dissuade me from a particular boat type...;)
 
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#83 ·
Barefoot,

Both faster and myself are in the salish sea, we know about the 13-14' tide changes......both of us prefer fin keels! Hell, I've even run my fin keel aground once or twice or.......ok, maybe I'd better shut up on that one.....

The point being, in our lighter winds, a lighter fin style boat will keep you sailing longer than a heavy fin keel that is less SA/disp than a higher AS/disp fin.

I was looking at a march or maybe feb sail magizine the other day, there is an article with the authors dream boat. One thing he mentioned, easy to move so it would sail under light winds. Also made some comments about some areas of the world, Puget Ound was one, taller mast and more SA for the light wind days.

Now does a fin equal an easily driven hull? not sure, but it is easier to move than a full keel.

marty
 
#84 ·


onebetter look at this thread if one feel one can not runa fin keel aground per say and do bottom work. This happens in the UK etc ALL the time! picture if it works, which is in the thread is in northern Wales, boat IIRC is 42', and has a 7' draft.

Marty
 
#85 ·
Very good point and this is where design, skill, and sail quality and inventory come into play. I sail a very small, heavy 22' full keeler that can kill a standard 30' fin on any point of sail in light winds. I also sail engine-less which might be why I purchased a high performance boat. I purchased the boat for blue water so performance and safety were at the top of my list. At the end of the day its all about personal preference. My sailing skills are average at best so I chose a boat that I knew would take care of me. I guess another factor in choice might be price, you can drop 10 grand on an old full keel plastic fantastic and set off to see the world. Fin keel boats have there place as do full keel boats, I've owned both.
 
#87 ·
Even your 3'6" of draft, 2' less than mine would not have gotten you into Fishermans bay on lopez at about 11:30 am on july 1 last summer. Tide was a -2.5, channel is dredged to 5' at 0 tide! While you would not have waited as long to get into it after low tide as I did, you would still be waiting! altho not as long as another fellow with his Cat 420mkII with a bit over 7' of draft! Both his and another were sitting on the ground at the dock when low tide hit in the marina! neither suffered any damage. I bumped going in......

Generally speaking, at least off of Edmonds where I am, my depth guage does not work, as it is usually deeper than 400', so reality is, there are VERY few spots here in the sailish sea one needs to WORRY about draft as compared to the east coast.

Then your boat is smaller in WL, weighs 1500 more lbs than mine, 100# less SA........maybe you can out point me, then again...........I doubt you will sail faster ALL the time. One can get a fin to lie ahule, yeah a bit harder, but one can do it!

At the end of the day, there is not a right or wrong. Only what one thinks will suit them best.
 
#89 ·
Having all too recently run over a mooring line and ended up with the whole thing wrapped up in prop shaft and a nearly submerged tender I can well see one of the arguments for a full keel but overall I'm afraid I'll still stick with fin and try and be a tadge more careful.
 
#92 ·
Fuzzy,

I have some pics of wrapping a crab pot line tween the top of the rudder and the hull. Not fun. Fortunetly it was AFTER we won the race that night to claim 1st place in that race series. Still trying to figure out why a crab pot was out, being as the season did not open for a week.......... I did ge the 2011 bent prop from YC out of that one.......that is when the spouse found out about this misadventure.......double OUCH!.......
 
#98 ·
Marchaj's book was a seminal work when it was written. Like Tony Marchaj's other books it did a great job of gathering existing testing, studies and performing targeted testing and explaining the principles in ways that a moderately technically savy amatuer could understand. For us normal sailors Tony's book really progressed the understanding of the factors involved in producing a seaworthy boat.

But the book was written 30 years ago. The research data was based on the testing available at the time. Most of the basic principles remain 100% valid, but many of the conclusions relate to the specific hull forms, keels, rigs and so on that existed at the time, and did not have the advantage of the ongoing testing which has occured in the 30years since.

In the countervening 30 years, designers, using much more sophisticated testing and design tools, have considered the basic principles that were identified in 'Seaworthiness' and designed boats which purposefully address these issues.

I would respectfully suggest that that Pvajko is mistaken when he says:
"The most important reason is that a full keel with its bigger surface area damps the rolling motion better."
"While a fin keel performs much better in ideal conditions (flat water), stormy weather with big seas is a whole different story. "

I would suggest that, in the first quote, Pvajko is in part misinterpreting what was being said, but in both cases, these conclusions were based on the type-forms and limited testing available at the time that the book was written.

Today, seaworthiness is no longer forgotten. Designers have long since learned how to design around the concerns raised in 'Seaworthiness' and have the tools to make both of the quoted statements inaccurate.


Jeff
 
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#103 ·
I would suggest that, in the first quote, Pvajko is in part misinterpreting what was being said, but in both cases, these conclusions were based on the type-forms and limited testing available at the time that the book was written.
Jeff, don't misunderstand me, I'd love to be mistaken on this. I have a fin keel boat which is perfectly safe I guess for where I sail it.

I always had the impression though that for off shore sailing a full keel is safer, based partly on anecdotal evidence and Marchaj's book (which, I'm happy to admit, I may be misinterpreting, or at least oversimplifying it's reasoning).

If this is not true, that's great news for me because it means there is a lot more (and cheaper) boats to choose from in case I wanted to go off shore.

I wish there was a book or study like Marchaj's available for the general public, summarizing the past 30 years progress on this subject, because, you see, the reasoning like PCP's "[Tabarly] was not a theorist but you can be sure he knows what he was talking about", with all due respect, is not very appealing to me.

Today, seaworthiness is no longer forgotten. Designers have long since learned how to design around the concerns raised in 'Seaworthiness' and have the tools to make both of the quoted statements inaccurate.
I'd really be interested to learn what these tools are. Could you give more information on that, or at least point out where to look?

And please, don't get me wrong, I'm not arguing against the fin keel or any other keel or feature, I'm perfectly aware of the fact that my knowledge on this subject is way to limited to be able to argue for or against anything.
 
#108 ·
I would respectfully suggest that that Pvajko is mistaken when he says:
"The most important reason is that a full keel with its bigger surface area damps the rolling motion better."
"While a fin keel performs much better in ideal conditions (flat water), stormy weather with big seas is a whole different story.
There really is nothing wrong with what he says, but it is incomplete. It has been well documented in 100 year old yacht design books that full keels have the advantage in big water because the rotation of the water in the wave causes the full keel boat to heel toward the wave rather than heel with the wave as with other keel designs.
Bryce
 
#117 · (Edited)
I do think that there is something wrong with his statement beyond being incomplete and that you are mistaken when you say that "the rotation of the water in a wave causes the full keel boat to heel toward the wave rather than heel with the wave as with other keel designs."

Perhaps I can explain the basis of my comments and we might be able to reach agreement.

I will start with the first quote by Pvajko:
"The most important reason is that a full keel with its bigger surface area damps the rolling motion better."

Here is the problem with that statement, dampening (the ability of a boat to dynamically to resist rotational motion) is directly proportional to a moment of inertia the amount of which results from the resistive force of the rotation and the distance that resistive force is from the instanteous rotational axis. In calculating a dampening moment, the force is a linear factor, but distance from the center of that force to the instanteous rotational axis is to the third power.

So that when you talk about the amount of dampening moment generated by a specific keel or keel type, the amount of area of the keel is a certainly significant factor, but the distance between the center of its rotational resistance and the instanteous rotational axis can be even more significant.

So, if we talk about the fin keels in the era when 'Seaworthiness' was written, these keels had perhaps a quarter of the surface area of a full keel on a similar length boat (and here I am not talking about the boats with long overhangs, an extreme cut away forefoot and raked rudder posts which had little more area than fin keels with separate rudders).

In the era that Marchaj wrote his book, between the shape of the fin keel, and the vertical height of the instanteous roll axis on fin keel boats of that era, the distance between the center of its rotational resistance and the instanteous roll axis was similar between a fin keel boat and a full keel boat and so the greater area of a full keel meant that there was significantly more dampening generated which is what Marchaj concludes.

But in the years since, several things have changed. Modern fin keel boat have greater draft, and differently shaped keels so that a greater portion of their area is deeper in the water, and their hull forms are such that their roll centers are slightly higher. That combination means that there can easily be a several time greater lever arm between the center of rotational resistance and the instanteous roll axis. So if we think that a modern keel has perhaps 20% of the area of a full keel but 2 or 3 times greater lever arm taken to the third power (in other words something like 8 to 27 times more leverage) it is easy to see that a modern fin keel boat could easily develop much higher dampening moments and so have better dampening than a full keel boat, making Pvajko statement incorrect that "The most important reason is that a full keel with its bigger surface area damps the rolling motion better."

In terms of Pvajko statement: "While a fin keel performs much better in ideal conditions (flat water), stormy weather with big seas is a whole different story."

I might agree with you that this is in part a true statement. All keels generate more lift in flat water than they do in disturbed conditions, but since fin keels tend to stall out much more quickly than longer chord keels, they lose a larger percentage of their lift, in other words, "stormy weather with big seas is a whole different story" for all keels but especially for fin keels.

But here is where that statement is misleading, in the years since 'Seaworthiness' the better modern fin keel shapes and corss sections have been developed to perform across broader range of conditions while losing a smaller percentage of their performance advantage. The impact of better dampening, the endplate effect of the bulb, foil shapes which more quickly establish flow and respond to it, means that fin keel boats may lose some small amount of their advantage over full keels in heavy going, depending on the course relative to the waves(i.e.beating upwind), but the modern fin keels still retain a significant performance and motion comfort advantage over a traditional full keel of similar length and displacement.

This last sentence is where it gets tough to make an 'apples to apples' comparison. In a broad general sense, full keeled boats tend to be heavier for their length (I know this is a 'duh statement) and have different hull forms than most modern fin keel boat. Because of that disparity it is easy to ascribe attributes to a full or fin keel which have nothing to do with the keel type and everything to do with the boat's design as a system. But even taking that into account, the statement seems to imply that a boat with a full keel will out perform a fin keel boat in heavy conditions, and while that may be true for some fin keels vs. full keels, it is not a universally accurate statement.

Regarding your statement: "It has been well documented in 100 year old yacht design books that full keels have the advantage in big water because the rotation of the water in the wave causes the full keel boat to heel toward the wave rather than heel with the wave as with other keel designs."

I personally don't know of any 100 year old yacht design book that says anything like that, but when I go back and look at my earliest copy of Skene's and Kunhardt, I find no reference of the sort so it might be helpful if you could provide a source for that. But even so, the idea that full keels heel toward a wave while fin keels rolls away flies in the face of what is known about the motion of boats in big waves.

What the science would suggest is that there are a number of factors which determine whether a boat heels into a big wave or away from the wave. First of all there is the rotational force. If you dissect the surface of a large wave, the water at the surface is moving faster than the water deeper in the wave nearer to the wave center. This progressive difference in speed between the surface and the center of the wave, means that the deeper the keel, the greater the sheer in the water speed acting on the boat trying to rotate the boat so that it heels away from the surface of the wave. Similarly, a keel with a greater side area will experience greater rotational force and so will have a greater tendency to heel away from the surface of the wave. But also, fin keels stall at very steep angles of attack, as might be experienced beam to on the side of big wave, thereby reducing the side force per unit area that the deeper keel may experience. This combination of factors means that in any specific case, either a fin keel or a full keel could experience the greater rotational force.

Resisting the roll force are stability and the roll moment of inertia. In the case of the fin keel vs. full keel discussion, modern fin keels, with their deeper drafts and densely concentrated ballast bulbs, generally generate much higher proportional stability than full keels. That was not the case at the time when 'Seaworthiness' was written but since modern designers have paid attention to the lessons of seaworthiness, and modern racing rules do not penalize stability as much as they did back then, it is true on the better modern fin keeled designs of today.

This greater stability means that a modern design would generate proportionately greater force to keep them upright and therefore greater force trying to heel the deck back toward the wave face.

The other factor, roll moment of inertia is similar to the discussion on dampening. The two factors impacting the amount of roll moment of inertia is weight and the distance between that weight and the instantaneous roll axis. While modern fin keeled boats tend to be lighter, they also tend to be deeper and taller so that due to their weigh distributions, they develop a disproportionately large roll moments of inertia.

In big waves, a large roll moment of inertia does two things, at the top of the wave, it delays the rotation of the boat relative to the rotational force. A good thing, but at the bottom of the wave, its greater stored kinetic energy, tends to cause it to get out of phase with angle of the wave face and continue to roll as the bottom of the wave flattens out so that there is a greater danger of dipping a spar in the water (never a good thing).

But to look at your statement fairly, we might also look at factors that have nothing to do with keel type. Modern designs tend to have greater form stability. Greater form stability tries to keep the waterline of the boat parallel to the wave face. At the top and middle of the wave, that would tend to roll the deck of the boat away from the face of the wave, the behavior that you describe in your quote. But that has nothing to do with the keel type. Two boats of equal form stability, similar draft and ballast stability, and roll moment of inertia would have the same angle of heel whether the boat had a full or fin keel.

And lastly, at the bottom of the wave, the boat with greater form stability would generate more righting force, remaining in sync with the wave surface and so would be less likely to dip a deck or spar and keep rolling.

What all of this suggests is that the specifics of the boat design and the conditions will determine whether it heels relative to the wave surface, but that the use of a fin keel or full keel is but one minor factor.

BreakAwayFL said:
I was under the impression that a full keel also puts more weight lower in the water, which would make it more forgiving if you happen to have too much sail out...
Strictly speaking that is not always or even usually correct as it is written. While it is easier to keep the weight lower in a longer keel of an equal draft. But modern fin keels generally are deeper and have a bulb which makes it easier for them to carry their ballast with its vertical center lower than most full keels. But also there are a lot of factors that make a boat 'forgiving'. A modern fin keel boats relatively greater stability, lighter helm loads, more forgiving rig and sail handling gear, and more easily driven hull form might work in its favor 'forgivingness' wise. The typically better directional stability and lower vertical center of effort work in the favor of a typical full keel boats 'forgivingness'.

Respectfully,
Jeff
 
#157 ·
it is easy to see that a modern fin keel boat could easily develop much higher dampening moments and so have better dampening than a full keel boat, making Pvajko statement incorrect that "The most important reason is that a full keel with its bigger surface area damps the rolling motion better."
BTW Jeff, doesn't this imply that Paolo's argument with full keel boats "tripping over the keel" vs. fin keels moving easily sideways is incorrect?

It appears that the two statements contradict each other. I know you are talking about high dampening moments but doesn't that mean also higher resistance sideways?
 
#99 · (Edited)
Thank you Jeff. I wish I could say that I understand this stuff, but I have only a vague grasp and can be readily influenced by logical sounding information.

Perhaps I don't need to worry too much, as I can't really afford a boat less than 30 years old! :D

I've spent most of my time on wooden sailboats with full keels, so I have no comparison. I am however quite interested in the fin keel concept.

In the non-consecutive pages of the book that I was allowed to look at, I had the impression that it was not the surface area of the keel that was so helpful. It was the linear length (bow to stern), which when running downwind put the different parts of the keel in different (and differing) parts of the wave motion, thus no one part has such a great effect. Plus the old style V hull went deeper, divers know that just a slight increase in depth greatly lowers the wave motion effect.

Of course, I can't help thinking that very deep fin keels would also reach down into less violent motion.

Have you any suggestions on a good place to look for newer information? And are computer models a big part of the new design? I'm a bit suspicious of computer modeling at the moment. We just had a long winter of totally failed computer modeling of atmospheric motion, which gave very poor weather predictions, the worst I've ever seen actually.
 
#101 ·
Steve Dashew's "Offshore Cruising Encyclopedia" has a good explanation of keel, rudder, and hull design and its effect in real offshore conditions as well as showing the advantages of computer programs for comparing design changes.
 
#111 ·
At the end of the day, it really only matters "IF" the boat design works for the type of sailing you wish to do, where you wish to do it etc. Be it a fin, full or something in between. if a one design, you will not have a lot of choice. If cruising, or ocean racing, look at the designs thru the years, in the past, lots of full keel's, more recent, fin's......

Not sure which is best. but from sailing my post fastnet IOR halfton, vs my step dads Bill Garden modern design plywood "Sea Bird" yawl. About the same wt, length, WL etc...... no way would I want to be sailing that thing in ANYTHING but calm water. It does not back up, does not go forward well either......then again, maybe it is the actual design of the boat!

At the end of the day, does it do what you want? if not, then it is the wrong design boat for you. If yes, then it is the right boat for you!

marty
 
#115 ·
Way too many factors involved for that to be a true general statement. Some full keel boats have little draft and poor righting moment - others have adequate draft but use low density ballast like concrete.... and there are plenty of fin keel boats that are 'stiffer' than some full keelers.
 
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#118 ·
i love my full keel, i have raced many of my friends and true, i am slower... but i always win cause i dont have the hour and half around the kelp beds lol
also on that note, i like to free/scuba dive and having the option to go to there is nice too. have we mentioned the dumb crab pots? hehe
ok beat the not getting hung up on stuff... i am willing to sacrifice the little speed for ease of mind and comfort, she rides well in most weather. im happy... thats what matters right?
 
#121 ·
Here is the problem with that statement, dampening (the ability of a boat to dynamically to resist rotational motion) is directly proportional to a moment of inertia the amount of which results from the resistive force of the rotation and the distance that resistive force is from the instanteous rotational axis. In calculating a dampening moment, the force is a linear factor, but distance from the center of that force to the instanteous rotational axis is to the third power.
Hello Jeff, I pretty much stopped reading after this paragraph. Damping has nothing to do with inertia. Inertia is related to the square of the distance and the force is a function of acceleration. Damping is related to area and the moment is proportional to distance. The force is a function of velocity.

Bryce
 
#126 · (Edited)
Bryce,

You apparently are mistaking simple 'moments' and simple 'inertia' for 'moments of inertia' the term that I used in the quote from my explanation.

As I am using the term 'Moment of Interia' I am using the term defined as, "In classical mechanics, moment of inertia, also called mass moment of inertia, rotational inertia, polar moment of inertia of mass, or the angular mass, is a measure of an object's resistance to changes to its rotation. It is the inertia of a rotating body with respect to its rotation. The moment of inertia plays much the same role in rotational dynamics as mass does in linear dynamics, describing the relationship between angular momentum and angular velocity, torque and angular acceleration, and several other quantities."

When talking about dampening, the resistance to changes in rotation is not just the inertial mass of the object, but the resistive forces of the side force of water against the keel, and the side force of the wind against the sails, times the distance to the axis squared. But since the unit force is proportionate to the radius of the rotation, the formula for calculating roll moment of interia due to the keel, the distance between the center of the force and roll axis is to the third power.
 
#125 ·
To use water analogies: things are "getting deep" in this discussion, and I find more and more that it's "over my head"! :eek:

My copy of the book "Seaworthiness" has been shipped, perhaps if I can digest some of that, I'll at least have a handle on the 30 yr. old side of the discussion. :eek:
Why to pay or wait for it? As I have said it is an old book and has many old books it can be downloaded for free:

Seaworthiness: The Forgotten Factor - Free eBooks Download

Regards

Paulo
 
#128 · (Edited)
I certainly understand the theoretical advantages of the modern design but I question whether those advantages apply in at least one very common scenario. If this boat is 32 feet and used for very long distance cruising, ie a dinghy or two, 3 or 4 anchors and rodes, 40 days of food for 2, etc.etc, is it still going to be just as stable and fast as when it was test sailed by the owner? Personally, in my opinion, this modern design is going to prove to be slower and MUCH less stable than had it been designed a little heavier in the first place and with a little more boat in the water.
I once counted the rolls of 2 very different designs as they were sailing DW off Baja. One, a full keel 32, the other a modern 36. The full keel 32 rolled exactly 30% less than the 36. That is, the 36 rolled 100 times during the 32’s 70 times. Both boats rolled at what seemed the same angle. The 32 was sailing about 1/4k faster. Are future cruisers going to have to give up comfort for the modern design or go much longer in WL to get reasonable comfort?
 
#129 ·
I certainly understand the theoretical advantages of the modern design but I question whether those advantages apply in at least one very common scenario. If this boat is 32 feet and used for very long distance cruising, ie a dinghy or two, 3 or 4 anchors and rodes, 40 days of food for 2, etc.etc, is it still going to be just as stable and fast as when it was test sailed by the owner? Personally, in my opinion, this modern design is going to prove to be slower and MUCH less stable than had it been designed a little heavier in the first place and with a little more boat in the water.
I completely agree with what you are saying if we compare boats based solely on length on deck. If the comparison is two equal length boats that are equally loaded, one a traditional long waterline relative to LOA, heavy L/D boat, whether full keel or some partially cut away keel form, and the other a more modern equal length boat, the newer boat will not be able carry as much weight in gear and consumables without seriously compromising performance and seaworthiness. But also, since design loads on the hull and rig are proportionate to the displacement of the boat, when equally loading these two boats a lighter modern design will be loaded proportionately more as compared to its design displacement and so would end up being a comparatively less robust boat when loaded to the max loading of the equal length heavy d/l cruiser.

It is for this reason that I have always suggested that a better way of comparing offshore boats is to compare boats of equal empty displacement rather than boats of equal length on deck or waterline. In that comparison, within reason, the longer boat of equal displacement will generally offer better performance across a wide windspeed range, offer better motion comfort and seaworthiness, be easier to handle, have better carrying capacity without impacting handling or seaworthiness and have roughly similar operating costs.

I once counted the rolls of 2 very different designs as they were sailing DW off Baja. One, a full keel 32, the other a modern 36. The full keel 32 rolled exactly 30% less than the 36. That is, the 36 rolled 100 times during the 32's 70 times. Both boats rolled at what seemed the same angle. The 32 was sailing about 1/4k faster. Are future cruisers going to have to give up comfort for the modern design or go much longer in WL to get reasonable comfort?
Anecdotally I can imagine how this would be true more often than not. Similar to the discussion above, a heavier (D/L) displacement boat will typically offer a slower roll rate, which can be a very good thing in a comparative short chop or in long wave length ocean conditions.

Depending on the wave frequency, and the weight and bouyancy distribution of the two boats, the heavier boat may also roll through a narrower roll angle as well.

But if we compare boats of equal displacement, one longer than the other, the longer boat will generally offer a similar roll rate, but if the boat is a modern design which has been properly designed for motion, the roll angle should be smaller and the edges of the roll more gentle. But also the collisions with waves when beating and close reaching should also produce less hobby-horsing and a softer collision with each wave.

On the other hand, depending on the wave frequency, and relatve speeds of the boats, the longer boat for the same displacement may not fare so well with regards to heave. Typically a longer boat with a smaller D/L, when encountering a single large wave or a widely spaced wave train, will heave at closer to the speed of the wave than a boat with a heavier D/L giving the heavier boat a much more comfortable heave motion in those conditions.

On the other hand, the heavier D/L advantage can get lost in a repetitive, closer spaced wave train, where the inertia of the heavier boat can take it our of sync with the wave train and so cause it to hit harder at the trough and peak of the wave resulting in a less comfortable motion in a narrow range of conditions.

Respectfully,
Jeff
 
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