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rmf1643 05-17-2001 12:30 PM

Seakindly Boats vs.the rest
As I begin me search to move up from my Catalina 30 (to 38-40'',5-8 yrs old) I get overwhelmed with all the choices. One common theme seems to be if a boat has seakindly motion. Can someone expand on this? I''ll be doing coastal stuff in S.Cal. From what I gather it seems that most of the large "production" boats (HUnter,Catalina) don''t get a seakindly vote. I''d appreciate your input.

RichH 05-17-2001 08:57 PM

Seakindly Boats vs.the rest
for technical discussion, go to:
then follow the links, etc. and go from there.

Jeff_H 05-18-2001 04:49 AM

Seakindly Boats vs.the rest
There seems to be a variety of things happening in yacht design. Some offer an improvement in sailing ability and comfort and some don''t. Many of the current trends in cruising yachts centers around maximizing dockside or at anchor cruising comforts at the expense of seakindliness and sailing ability, but there has been a lot of work on the motion of vessels underway.

One of the most recent frontiers in racing yacht design is the realization that large angles of rotation and quick motions are slow. With the advent of smaller more powerful computers and miniaturized accelerometers full sized boats have been instrumented and motions measured. As a result theories on the factors involved in the dynamics of motion are being better understood.

At the same time there has been a clearer understanding of what makes for seakindliness. If you look at the kinds of motion that wear people down and cause seasickness the primary factors are the speed and the angle of rotation with the resulting accelleration also a major component of comfort at sea or seakindliness. Marchaj''s slightly dated tome ''Seakindliness the Forgotten Factor'' includes a chart that shows a navy study on the causes of seasickness and both rate and angle were equal factors amounts a large population of sailors. A more recent study indicated that individuals have different thresholds for discomfort and that the triggers of discomfort are purely individual as well. In other words, some people are more affected by the speed of the motion more than amount of the motion and other people are affected more by the amount of motion than speed of motion. By implication this means that a boat that rolls through wide angles slowly is no more universally seakindly than a boat that snap rolls through small angles.

The amount of motion a boat experiences is related to the amount of energy that is imparted into the boat, the ability to store energy, and the ability to dampen that stored energy. One thing that has come out of the most recent studies that has been determined that the single most important factor controlling comfort in seakindliness is length. All other things being equal a longer boat will have a more comfortable motion. While sources do not universally agree whether it is waterline length or overall length that is critical, the general agreement seems to be the longer the boat the more comfortable the boat will be. That said a short waterline boat compared to overall length boat will have a greater range of pitching motion and may have a quicker motion as well, because the reserve buoyancy of the ends can suddenly come into play jerking the pitch to a stop and back the other way.

Another factor that comes into play is the issue of the loads imparted into the boat. A heavier boat passes through the waves while a lighter boat tends to move over the waves. Tank testing and some of the early instrumentation testing suggests that heavier boats experience greater impacts. This is somewhat mitigated by the greater momentum of the boat but depending on hull shape those impacts can cause rapid de-accelleration which in itself can be quite uncomfortable. Another factor in motion (especially pitch and surge)is the shape of the bow. A blunt bow feels a much greater impact than finer one.

Next comes form stability as a culprit. Form stability allows a light boat to have a lot of initial stability but at the cost of poor ultimate stability. Form Stability''s has two affects on seakindliness, it allows more energy to be imparted into the boat and form stability results in a lot quicker motion as the boat quickly builds stability to resist the rolling motion. Form stability comes from wide shallow hull forms be they light like the IOR boats of the Fastnet era or the Moorings 38 or heavy like the Island Packets. Of course without some form stability the boat would sail at large angles of heel (look at the English lead mine cutters for example) and that is not very seakindly either.

When we look at the relationship of a boat''s weight and seakindliness we are really talking about the effects of moment of inertia as well (the amount and distribution of weight in a boat; the bigger the weight and the further from the center of motion, the greater the inertia.). The inertia of a boat has several affects. A boat with a large moment of inertia takes more energy to get the boat in motion. Once in motion, it is harder to stop from moving so the kinetic (stored) energy of a boat with a lot of inertia causes a boat to roll or pitch further than a boat that has less inertia. Of course a low inertia boat will accelerate and de-accelerate quicker.

The amount of inertia that a boat has is a result of the weight of the boat and the distribution of the weights. A small weight located a long distance from the axis about which the weight is rotating can have an equal inertia to a much heavier object closer to the axis of rotation. Since weight is linear but distance from the roll axis is to the third power, distance from the roll center is far more critical to the overall moment of inertia. If we consider a heavy boat, that has a short heavy rig, a lot of weight in the hull and a lot of low density ballast in a long shallow keel, its roll moment of inertia may actually be same or less than a much lighter weight boat with a deep bulb keel and a tall light rig.

Another factor that had little study in any detail until recently is dampening. Dampening is the ability of a boat to absorb the energy once it has been imparted to it. The two best examples are the affects of a deep keel and a tall rig. If you visualize these rotating in a circular motion the air pushing against the sails and keel far from the hull creates a resistance to rotation and supresses the effects of inertia. In other words, dampening results in less motion and slower motion; a definite win/win situation.

So where does this leave us, in terms of seakindliness and the newer designs? A light weight boat, with a fine bow, and a narrow beam, Vee shaped hull sections forward merging into elliptical hull sections aft, with a deep bulb keel and a tall light rig can have very admirable seakindliness characteristics and would be a much faster boat in all conditions than a heavier displacement full keel boat. This hull form can be seen in boats like the Beneteau first 40.7. Blunter lightweight boats are less comfortable, compare the hull form of a Beneteau 40.7 to the Finot designed Beneteau 31 or to a Catalina 42.

Obviously there are trade-offs being made when you buy a lighter weight boat. Properly designed seakindliness or weatherliness doesn''t need to be one of them. Properly designed, the ability to carry the weight of cruising gear in a light weight boat is not lost either. The price comes with deeper draft and these boats do require greater care in their engineering and greater skill to build. Therein lies the problem with many of the current crop of production lighter weight boats. The only way that light boats can have comfortable motions is if their center of gravity can be located very low in the boat. The current crop of big three high production designs seem to mostly get shipped with shoal draft configurations which hurts their chance to get a seakindly motion.


seabreeze_97 06-17-2006 03:53 PM

Okay, but he's got the length, and at least partially, the weight issue covered. He did say he was moving up from a 30 footer, to at least 38 feet. Skip the lightweights. They respond more abruptly to sea and wind influences. Do not get a boat with an emphasis on flat-bottomed hull for downwind'll pound you. Look for a bow with a fine water entry, not a blunt bow. Generous beam makes for a roomy cabin, but don't take it at the exclusion of displacement. As discussed, heavy boats go through the water, they're less likely to get tossed about. In that comparison, it's a lot like a big old car vs the econo-boxes. The big car cruises like on a cloud, the lightweights get knocked around by the clouds.
If possible, arrange for some demo cruises. Nothing like actually feeling the boat in action.

SailinJay 06-17-2006 07:18 PM

Wonder if he still cares, after over five years, or is still around on this board, since he hasn't posted in over four years.

Curt 06-17-2006 09:12 PM


Originally Posted by SailinJay
Wonder if he still cares, after over five years, or is still around on this board, since he hasn't posted in over four years.

I do not know but I can say I do appreciate the thread getting bumped and the great bit of detail Jeff made in his reply. It helps me in my consideration very much. For my needs it is apparent that a shoal keel is the best however I am very reluctant to go with a centerboard for the cavity and maintenance problems it can present. Still trying to sort this out and the information does help.

Valiente 06-19-2006 09:34 AM


Originally Posted by Jeff_H
There seems to be a variety of things happening in yacht design. Some offer an improvement in sailing ability and comfort and some don''t.

Wow. That was unexpectedly lucid for a forum. Thanks.

kms 06-20-2006 08:35 PM

Marchaj,s book
I am a little curious about the book mentioned by Marchaj. It was stated as "Seakindliness The Forgotten Factor". I can't find a copy of this book. I can find a copy of "Seaworthiness The Forgotten Factor" by the same author. Are there two different names to one book or two different books with two different names or one book with one name?


sailingdog 06-20-2006 09:18 PM

I believe the book is titled, "Seaworthiness: The forgotten factor" and misnamed in the previous post.

captainmidnight 12-20-2008 02:31 AM

Pacific Seacraft are "seakindly" boats, they move well and generally stay dry on deck.

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