Stability and Seakeeping

On http://www.image-ination.com/sailcalc.html you can find a big saling boat database and an online calculator for MCR, hullspeed, etc

Articles

Norman L. Skene's(in SKENE'S ELEMENTS OF YACHT DESIGN) gives 2 methods of judging the sea-keeping and stability of a boat, methods based on statistical stability diagram of several boats.

• Firs method is called Wind pressure Coefficient Method [page 292]
• Second method is called The Dellenbaugh Angle Method [page 296]
• At page 299 there you can find a the flowing table which allows you to judge a boat's stability by analyzing the transverse metacenter GM.
  • Note 1: If G is below M the vessel is stable and if G is above M than the vessel is unstable
  • Note 2: A vessel with a large GM comes to the upright position very suddenly, whereas a vessel with a small one comes to the upright position more slowly and is more comfortable in a seaway

Table of Trans. GM for different Types of Vessels
Harbor vessels, tugs       | GM=0.35-0.45 m
Small power cruisers       | GM=0.60-0.76 m
Shallow-draft river boats  | GM=     3.65 m
Merchant streamers         | GM=0.30-0.91 m
Sailing Yachts             | GM=0.91-1.37 m

Principles of Yacht Design, Larsson & Eliasson (page 52):

• "In general, modern yachts have larger GMs than traditional ones due to their larger beam/draft ration. The scatter is very large, however. in a survey of American IMS yachts around 1990 the lowest value was 0.67m and the highest 2.1m. Contrary to what could be anticipated, there was no definite trend of increasing GMs with length. This is an effect of the reduction in relative beam for larger yachts."
  • Vast majority of yachts had GM: 0.75 - 1.5 m;
  • Several yachts in the survey had positive stability range up to 180°;
  • The average AVS(Angle of Vanishing Stability) was around 122°, which must be considered a relatively low value from a safety point of view.

Dynamic Stability Calculator

From the website of Donald L. Blount and Associates you can download a dynamic stability calculator (see also H:\Library\Naval Architecture\Stability and Seakeeping\Dynamic stability calculator)