Re: metacentric height-technical question

Richard A. Miller

Willard 40 Stabilizers

My two active Naiad stabiliser fins are each 6.5 sq. ft. I think.  They work very well but rotate up and down; they are not fixed bilge keels or fins.

Richard Miller, W 40 Adria

On Apr 28, 2020, at 10:41 PM, Rick E <rick@...> wrote:

Metacentric height is a length, not a location.  Commonly referred to as GM, it is the distance between the center of Gravity and the Metacenter.  The location of the Metacenter, M, is a function of the hull geometry and the waterline immersion. But it is the GM that is needed to size stabilizers. So in addition to the hull geometry and the immersed draft, you need to know the height of the center of gravity. G, too. That is best determined from an inclining test. For small boats, inclining tests are a bit impractical and not usually necessary. But the GM can be estimated based on the Period of Roll, which can be timed after inducing unrestrained roll in calm conditions. The thing to understand is that the center of gravity, and by extension the GM, of two W40’s may be significantly different, depending on equipment installed (engines, generators, batteries, masts, tenders stowed above, etc.) It also varies according to fuel and water tank levels.

Stabilizer vendors will ask for an estimated GM, but they can generally make a good recommendation for a small boat based on size and style of boat (full displacement hull in the case of Willards.) The other thing to realize is that the more inherent stability a boat has (such as the ballasted full displacement Willards), the larger the active fins needed. Also, the slower the boat speed, the larger the fins needed.  So for Willards, the bigger the better!
Rick E., P.E., N.A.

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