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It seems about time for me to weigh in on this. In and of itself, weight does nothing good for a boat. In and of itself, weight does not make a boat stronger, it does not make it more seaworthy, it does not add carrying capacity and it certainly does not make a boat have a more comfortable motion. These virtues or liabilities emerge on from how the boat is engineered, how it is constructed, how it is shaped and how weight is distributed.

The public perception that a 'so-called' heavier boat has a better motion comes from the assymetrical argument in how we define a heavy boat. It is possible, and perhaps more typical that a 20,000 lb 38 footer will have a better motion than a 11,000 lb 38 footer and that is where the public opinion originates.

But where this thought process falls short is that a 20,000 lb 44 footer will typically have an extremely improved motion over a 20,000 lb 38 footer. Yet folks would call the 20,000 lb 44 footer very light and therefore expect it to have an uncomfortable motion.

The other issue is waterline length and the distortions implied by that. A 33 foot 1960's era racer-cruiser might have a 22.5 foot water line and a displacement around 11,500 lbs resulting in a D/L in the 430 range. By the same token, a reasonably modern 33 footer could easily have a waterline length around 29 feet, and so while not much lighter, weighing around 10,800 lbs for a D/L down below 200. Based on a comparison of 430 to 200, most people assume the short waterline boat to be much heavier and therefore offer a better motion, which is just not the case. Anyway, what follows below is a more in depth description that I wrote for another purpose probably 10 years ago.

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 20 years of research and 20 years of design evolution that has occurred sincep.

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 good pbasis 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 real predominant factors that control the faster motions of a boat, (roll and pitch), were found to be weight and bouyancy distribution (both static and dynamic), with dampening being a much bigger determinant than previously understood. The overall weight of the vessel has less than zero to do with the roll or pitch speed or angles.

So it is that many of the better designed 'lighter weight' boats, which have dramatically lower and more concentrated vertical centers of gravity, progressive dampening, and hulls modelled to minimize sudden increases in buoyancy, 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 of the slower of the six degrees of motion, is the only direction that modern light boats generally do poorer in, but even with heave, this is true 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 overall area of 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 means a pretty hard landing sometimes. (been there, done that, have the broken toes to prove it)

I also think that it is very much a mistake to say "the heavier boat will have a much more favourable sail area to displacement ratio, sail area to internal volume ratio and will usually outrun a light displacement boat of similar length in light weather. Long range cruisers encounter far more calm or light weather than heavy weather, and a heavy, stable boat with large sail area tends to turn in a high average speed in relation to her waterline length"

That simply is not the case and hasn't been for well over a decade. It may have been true of early ULDB's but modern light weight boats have enormous stability relative to their drag or displacement and carry sail plans that are proportionately larger than any heavy displacement boat that I know of. SA/D's in the 22 to 24 range are extremely common on newer light weight designs and when coupled with the very low drag foils and high efficiency rigs these new boats are much faster in an absolute sense at both the light and heavy ends of the wind speed range.

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 agree with Mr. Welsford when he says, “a boat of a given construction type costs pretty much by the pound or kilogramme. “ But I disagee with him when he goes on to say that heavy for their length are scarce on the market because the are more expensive in ‘a price sensitive market’.

I think that he is mistaken because of how he is choosing to ‘size’ the boats in question. In other words, I think 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. If we size the boat by its displacement and compare two 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.

I believe that the popularity of longer boats for their weight (or lighter boats for their length) is not about people wanting to buy boats of equal length for less money as much as it that people want to buy longer boats for their weight. Most sailing magazines that I get have been talking about the trend in new boats where people are buying longer boats than they used to. I contend that they are buying the same sized boats that they used to (by weight) and that these longer boats are just plain superior designs on all counts to earlier, shorter boats for their weights.

I hope this proves to be food for thought rather than the source of a food fight.

Respectfully,
Jeff
 

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Farr 11.6 (Farr 38)
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What you are saying is very interesting, can you please explain to me the difference underwater profiles makes with regards to motion? I see a lot of posts criticising the 'soap dish' shapes of modern cruisers.

I would like to see more boats with more 'V' shaped underwater profiles but that isn't from understanding of physics, more intuition.

I read about a boat recently called a riptide 55 which I thought was very interesting.
While a case can be made that vee shaped sections near the bow can reduce pounding when pitching, vee'd sections do nothing good for motion beyond that. The current thinking is that an elliptic shaped section progressively build buoyancy with heel angle so that there is less of a jerky motion as the boat rolls and the progressive dampening reduces roll speed and heel angle as well. Vee'd sections have no impact on the five other types of motion.
 
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