[sailaway21]

I believe Jeff to be mistaken in some of his basic assumptions. That being said, I'll confess to following his posts closely as he is one of the more well versed members of sailnet. If you're not reading him, you're missing out.

Jeff's illustration of the two similar boats and their statical stability curves being similar is patently wrong. Hull form itself effects two things only in those curves; location of center of buoyancy and form stability. Jeff refers to it further down, but inadequately, that the greater weight and the resultant placement of G, center of gravity have a large effect on stability. where he misleads is in implying that G is not part of the statical stability curve. G and B's relative postions are part and parcel of the statical stability curve and it is the reliance on form stability, by definition occuring at lower angles of inclination, that has gotten us into this mess.

For purposes of illustration, let us eliminate the effects of sails. We cannot in practise, but the point is easier made without them and, if necessary, we can discuss their effect afterwards. The stiff, light boat that Jeff prefers has a deficiency. While it has a large GZ, or righting arm, it does not necessarily have a large righting moment, GZ x displacement. This boat will have a short rolling period. This may seem desirable, and it is in certain types of boats, but it will lead to a very quick and violent motion in a seaway. Boats of this ilk, commonly with a bulbed fin keel, are highly dependant on headway to dampen their roll by making use of the flow over their keel. Reducing GZ, or righting arm, by moving weight upwards results in a slower period. But this is rarely done in a sailboat on it's own without a concomittant increase in displacement. The heavier boat can carry a smaller GZ and still have a good righting moment. She will roll much more gently, if deeply, and will exhibit more inertia. The reason I have disallowed the consideration of sails is that we are really talking about seakeeping abilities here and the conditions encountered will be ones where minimal, if any, sail is carried.

This is important because, unlike ships, the boat rides over the waves. the stiff boat is much more likely to get into a pattern of synchronous rolling, where the period of the roll and the wave period coincide. That's dangerous.

In mine and Jeff's defense, as well as yacht designers everywhere, these are difficult matters to divine accurately. The yacht architect does not have the resources and capability to test these things in the way that the ship's naval architect does. And the yacht hull form is much more complex.

Larsson and Eliasson in Principles of Yacht Design,3rd ed., 2007 discuss these matters relating to "STIX".

Regarding length they state:
The size of the yacht is the single most important parameter, when assessing safety at sea, since it defines a scale with which to measure the waves. The larger the yacht the smaller the relative size of the waves. In this approach the size is simply taken as a weighted average of the length overall and the waterline length. Whether or not this is a valid assumption can be discussed. To consider the waterline length twice as important as the overall length , as done in the formula, is reasonable when it comes to the sailing performance of a yacht. But in this context, with the yacht often heeled to 90 degrees or more, the overall length should be more important than the waterline length. As the formula stands now it penalizes yachts with overhangs (old ones) and encourages yachts with square ends (modern ones).

In discussing displacement:

...Penalizing light yachts may seem unfair considering the experiences from races under very rough conditions, such as the Fastnet race in 1979 and the Sydney-Hobart race in 1998, where light yachts indeed survived well. However, these yachts were handled by full racing crews. Yachts covered by the ISO standard are supposed to travel the same waters with a minimum crew, perhaps rather inexperienced. Then a very fast responsive yacht is not a good asset......based on research carried out both in England (Wolfson Unit, Southhampton) and the USA (SNAME) after the Fastnet disaster, it has been concluded that large beam in combination with light displacement accentuates the risk of wave capsize.


The authors go on to discuss hull form and keels with a preference for a traditional long keel for seakeeping abilities. They describe quite well the inhetrent efficiency of the long keel versus the fin keel at resistence to rolling. Much of the fin keeled boats resistence to rolling is produced by her making way. That's a bit off topic other than being a common characteristic of boats with long overhangs.

To overhangs themselves the author's state:
A large forward overhang is likely to increase pitching, since large pitching moments are created when a wave hits this part of the hull far from the center of gravity. Aft overhangs may, of course, have a similar effect in following seas, but the frequency of encounter is then much lower so the problem is small. On the contrary, the stern overhang may be beneficial since it may damp the pitching motion in head seas. A high freeboard forward, and a flared one in particular, prevents green water on deck, and spray hitting the cockpit is effectively avoided also. The hull thus gets much dryer......

....Most modern designers strive for small gyradii, a light hull, large stability and a small keel. All these features tend to increase the accellerations on board the yacht, thus making it less seakind. For a cruising yacht this is unlikely to be the optimum solution.


The author's conclusions seem to mirror those of CA Marchaj in Seaworthiness: the forgotten factor regarding offshore hull design. Both of these books have been quite interesting to read but I'd encourage anyone not familiar with basic styability and trim to read LaDage's Stability and trim for the Ship's Officer which will not make you a naval architect, but is an excellent two hundred page primer on stability, profusely illustrated, and extremely well written given it's professional nature. (that is to say, anyone from any background can read it and readily understand in short order what keeps a ship from tipping over!)

I'd like to thank everyone for their postsm in these design threads; they're not your typical two sentence subject and surrounded with controversy, some passionate. That the discourse is so civil and friendly is refreshing and a delight. And it's a lot of typing for no tangible gain either!

Remember Giu's adage, different boats for different jobs.