Technical » Stability Speed - SSpd

Release Date: 4/10/2006

About the Stability Speed Index (SSpd)

John Shuttleworth offers a formula for the wind speed in miles per hour at which hull lifting will occur in his design booklet "Multihull Designs". Conditions are wind on the beam, sails sheeted in and no center of gravity offset by crew or ballast. His formula is SF = 9.48((0.5*Bcl*Disp)/(SA*Hce))^0.5 with inputs in US units and results in statute miles per hour of wind speed. For results in knots on the Multihull Dynamics, Inc. website, the constant 9.48 is changed to 8.238. Shuttleworth did not provide a name for SF, so I have named it Stability Speed (SSpd) on the site.

Shuttleworth states "Typical values for SF can vary between 12 mph (10.42 knots) for a Formula 40 racing catamaran, to over 40 mph (34.8 knots) for cruising multihulls. Modern light cruiser-racers would be in the range of 24-30mph (21-26 knots)." ie a light cruiser-racer, with lower displacement and greater sail area will have a lower Stability Speed.

The boats in the Multihull Dynamics Inc. database fall in the 10-20 knot range. An investigation into the formula and other similar formulas by other designers is, at best, confusing. Derek Harvey in his book "Multihulls for Cruising and Racing" gives the formula Ws = 4.4758*((Boa*Disp)/(SA*Hce))^0.5, with inputs in metric units and results in knots.

Comparisons of specific boats using Shuttleworth's and Harvey's equations give results that are not quite the same. Shuttleworth states that his formula considers a 40% gust factor, i.e., a gust from a steady 10 mph to 14 mph, or four mph additional. An evaluation of a boat using his formula gives a comparable result to one using Harvey's, but the discrepancy between knots and mph is not resolved.

We have chosen to continue with Shuttleworth's formula, with the results converted to knots, with the understanding that the formula gives conservative answer that includes a 40% gust factor. It may also be conservative since we do not include genoas or overlapping jibs in our data "only mainsail and 100% fore-triangle" area for jibs. Although the results of this calculation may not be real-world values, they are valid for comparing one boat against another.

Both authors recognize that sail can be reduced, course altered, crew and cargo moved to provide ballast when hull lifting is imminent - unless you like sailing like that!

Calvin H. Markwood
Engineering Analyst
Multihull Dynamics, Inc.

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