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Old 11-30-2007
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Limit of Positive Stability (LPS)

I thought I might begin a thread to discuss this topic. I hope the knowledgeable designers amongst us will chime in with their thoughts and correct any errors I may introduce below:

A recent post inquired about the Limit of Positive Stabilty (LPS) of a particular boat model. LPS, sometimes referred to as the Angle of Vanishing Stability, is a measure of a boat's ultimate stability. The LPS figure, expressed in degrees, is supposed to approximate the point at which a particular boat will heel so much that it cannot right itself. At that point, the boat may continue to roll upside down. In theory the sailboat will eventually self-right, usually by completing the roll through 360 degrees (assuming no downflooding, etc).

The ORC (Offshore Racing Council) recommends a minimum LPS of 120 degrees for off-shore racing. An LPS of 120 degrees would mean that the boat could heel an additional 30 degrees past a perpendicular 90 degree knock down, and still right itself (again, in theory, since LPS figures are static calculations which don't reflect dynamic variables such as sea-state, downflooding, loading, etc).

In the other thread I mentioned, JeffH made the following observation:

Quote:
Originally Posted by Jeff_H View Post
I would say that the angle [for the Bayfield 29] is less than that, somewhere down around 105-110 degrees. You are talking about a beamy, high freeboard, heavy rigging, moderately lightly ballasted, shoal draft boat.
It struck me that JeffH essentially describes the vast majority of modern production boats.

Published stability tables for older production boats, as well as many newer ones, can be hard to come by. When I can find them (not all builders publish the LPS), it generally surprises me how low the LPS figures are. For instance, we are presently considering purchasing a larger sailboat for our family, and I was surprised to learn that the LPS of this popular 42 foot model was only in the range of 114 degrees. Our current 31 footer has an LPS of 139 degrees, which is among the lowest for all the models made by this manufacturer.

Looking at the hull form of the Bayfield 29...



...I would have thought the LPS to be higher than 105-110 -- it just looks like a more stable hull form than many of the modern production boats. So, lacking the LPS tables from a builder, how do we guage the suitability of a design for coastal or off-shore sailing? The figure of 120 degrees or better is considered desirable for off-shore sailing. What is a minimum LPS figure for coastal sailing?
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