SailNet Community - View Single Post - Limit of Positive Stability (LPS)
View Single Post
post #18 of Old 12-12-2007
Jeff_H's Avatar
Join Date: Feb 2000
Location: Annapolis, Md
Posts: 7,450
Thanks: 11
Thanked 238 Times in 189 Posts
Rep Power: 10
I see your point about maximum righting force. Certainly in the case of the Catalina 42 which is loosely an IOR hull form with a shallow keel depth, it would appear that maximum righting moment occurs near the point that the rail hits the water. This would seemingly be true for any boat where the center of gravity is near or above the waterline.

I somewhat disagree with the implication of your comments that "a traditional full keel boat may have its maximum righting moment at 90 degrees", "the traditional full keel boat will be almost impossible to keep inverted" and "modern designs....(read IOR)". The problem that I have with the two full keel statements is that keel type has little to nothing to do with the limit of positive stability, or the point at which maximum stability is achieved. Since the angles of positive stability and maximum stability are solely dependent on the location of the center of gravity and hull form, keels of almost all designs can achieve very large angles of positive stability and maximum stability. In fact as most 20th century full keel boats are actually constructed, they tend to have comparatively very heavy construction, heavy rigs, shoaler draft, and comparatively low ballast to displacement ratios, all of which would suggest a very high vertical center of gravity relative to the truly modern IMS/IRC derived designs, which tend to have deeper draft, light hulls and rigs, little flare amidships and no tumblehome and carry their ballast in bulbs deep below the rail. As a result, while traditional hull forms (which coincidently also tended to have full keels) typically had greater stability than IOR boats (which by no stretch of the imagination could still be called modern since the rule peaked in popularity over 25 years ago and died altogether roughly 20 years) these boats do poorly in terms of the angle of heel at which they achieve maximum positive righting moment or limit of positive stability as compared to truly modern IMS/IRC derived designs.

Historically, capsizes were very common in traditional full keeled vessels. Even discounting for the pre-20th century use of internal ballast, by any reasonable metric one could not assume that a full keel boat is any less likely to capsize, and once capsized that it will remain inverted for any less time than a properly designed fin keel boat.

Jeff_H is offline  
Quote Share with Facebook
For the best viewing experience please update your browser to Google Chrome