Soft bilge hull vs. Hard chined. Are both seaworthy?

[Tohbi]

i don't know where to find definitive, empirical evidence about this subject. i know that i prefer my lightweight, shoal water boats for inland sailing because they tend to slide over obstacles, are beachable, and have less impact momentum. but the boat i'm building will be deeper and heavier and will ride better in a seaway.

if you're looking for maximum safety, the coast guard has parameters for their craft, and i know ice breakers are tres heavy. boats in alaskan waters tend to be built of metal for impact resistance. so, that's kind of empirical.

[Willallison]

The reason that there are such varied opinions on these topics is because there is such variation in the shapes of each type of hullform.
Take the (not very well drawn) example below. On the left is a round chine vessel, on the right a hard chined one. Clearly here the round chine vessel will have greater initial stability - it will rock less at anchor but will probably have a more snappy motion underway. Hence my earlier comment that well designed and built - to suit your particular needs - is a more important consideration than hard or soft chine, even heavy or light. Or put another way - once you have truly listed all the requirements you have for the vessel, the decision may well have been made for you

[Portager]

Tom;

Centuries of empirical evidence and basic physics support the conclusion that heavier weight vessels are more comfortable in heavy seas. Experience has shown that people find vertical and transverse acceleration discomforting. Physics tells us that force equals mass times acceleration, therefore acceleration is equal to force divided by mass. The vertical force acting on a boat is due to the increase in buoyancy caused by waves suddenly changing the water level or displacement. For small perturbations the vertical force is proportional to the waterplane area times the height of the wave, so vertical acceleration (AKA heave) is a function of waterplane area divided by mass. Therefore, to reduce heave you want to reduce waterplane area and/or increase mass. Since in equilibrium, mass is equal to displacement, which intern is equal to waterplane area times the average draft, in general better comfort at sea equates to deeper draft, so shoal draft boats are rarely very comfortable in heavy seas.

Now that I have said all that I would like to quote Henry David Thoreau who wrote, “All generalizations are false, including this one.” What I mean by this is a lot can be done with shape and form to improve the comfort of a vessel and a poorly designed deep draft boat can be less comfortable than a well designed shoal draft, however a well designed shoal draft will not be as comfortable and a well designed deep draft, heavy displacement vessel of comparable design (i.e. monohull to monohull or catamaran to catamaran).

On the subject of light displacement craft being safer, I will agree that a light displacement craft can be made safe however it can not be made as comfortable as a heavy displacement craft of comparable design and if construction technique and quality are equal, I doubt the light displacement vessel could be as safe as the heavy displacement (unless you are talking about positive buoyancy).

Although they are really talking about sailboats, I think the following quote from http://www.macnaughtongroup.com/designin.htm states the differences between light and heavy displacement boats very well, “While offshore boats are typically of heavy displacement, it is perfectly possible to design a light-displacement boat that is safe offshore. The chief objection to them is that they have a quicker, jerkier motion, which most people find uncomfortable and tiring. This effect can be held to a minimum if the boat is narrow and relatively V-shaped, with a deep keel, which are all good characteristics for the safety of the vessel as well. Most light-displacement boats are given shapes that encourage surfing, but for offshore use it is important to prevent surfing, which can cause a loss of control. Even so, the light displacement boat is apt to be too fast when running off. Therefore, rudders should be large and mounted on the keel or a large skeg, probably mounted under the hull rather than on the transom, so as to prevent ventilation from the surface. The only advantage to the light displacement type for offshore use is in her ability to carry full foam flotation with minimal loss of interior space. This is so easily accomplished that it is a mystery to us why more light displacement boats don't have it. Since a fast, self-righting, unsinkable boat is possible for those willing to accept the motion, this removes any argument in favor of the offshore multihull, as far as we are concerned.
Heavy displacement is the "natural" choice for offshore use. The comfort level is much greater, and conventional proportions produce the right combination of stability, self-righting, motion damping and non-surfing behavior for maximum average speed and safety without gimmicks.”
If you want the ultimate in safety and comfort in an offshore voyager, then I recommend a long and narrow vessel with high displacement (relative to its waterplane area) and a low center of gravity or metacenter. Some excellent example of vessels of this type include; Tad Roberts Passagemaker Lite http://www.tadroberts.com/ , many of the designs by Michael Kasten http://www.kastenmarine.com/power.htm and Dave Gerr’s Ironheart http://www.gerrmarine.com/power_70.html#ironheart , although Will and I agree that the interior arrangement of Ironheart could be improved. The biggest problem is boats like these are not available as production boats that I know of. You might want to consider a custom design. In the price range your talking about you could have a custom design for less that the brokerage fee on a production boat and, if it were metal to eliminate the cost of building molds, the total cost would be very competitive. My preference is to have the boat designed around my needs and desires rather than the production boat one size fits all approach.

Returning to the original subject of round bilge versus hard chine hull forms. I have see unsupported claims and insinuations that round bilge hulls are more comfortable in heavy seas, but I have never seen any supporting data for this position. The only advantage I know of for a round bilge is efficient at any attitude, where as the hard chine hull is only efficient at one attitude and as roll angle increases powering efficiency degrades. This is very significant for sailboats which operate at heel angles most of the time, however for a power boat the degradation over a passage is generally less than 1%. So why do some manufacturers emphasize their use of a round bilge for powerboats. My theory is that it is because most Trawler owners are ex-sailboat owners who are already convinced that round bilges are better (although they have probably forgotten why). I believe that for a powerboat, the hard chine hull is superior because it provides better roll attenuation.

Regards;
Mike Schooley

[Tohbi]

mike wrote "a long and narrow vessel with high displacement (relative to its waterplane area) and a low center of gravity or metacenter" is the basis for offshore designs. if you'd like to participate in the discussion in the sailboat section under 'ballast vs sail area" you'll see pics of a 12-meter design that pushes this envelope.

i'm listening to all the advice available about developing the model so don't be shy about chiming in. thanks

[gonzo]

I think that one of the strongest arguments for a light displacement boats is that fast boats are fun. Surfing and planing make it all worth it. Also, in the subject of safety, I'll take a light displacement fast boat to get away from a lee shore any time. Heavy boats are slow and clumsy. A lot of their weight is unnecessary and doesn't make them stronger, just heavier. The shape of the boat will make more of an impact on the heaving and pitching than displacement. Also, a heavy mast can control speed of movement.