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post #505 of Old 10-02-2012
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Re: Full or fin keel?

Originally Posted by Jeff_H View Post
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.

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’.

Holy WOW! I hope Smack takes some of Jeff's posts and slips them over to the "Old Salts" thread from this post!

1977 38 foot C&C Mark 1
Hull #92
Sailing the Exuma Islands, Bahamas
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