What's a metacentic shelf ?

[chrisk]

On the plans for Jenny Wren (http://www.duckworksbbs.com/plans/wf/jennywren/index.htm) it says:

"This hull has a balanced metacentric shelf!"

What is a "metacentic shelf" and why is having a balanced one a good thing?

Chris Kottaridis (chriskot@quietwind.net)

[John B]

Look up Harrison Butler. Its all about a balanced hull shape on all points of sail.

[Roger Long]

It's a fancy and rather incorrect way of saying that the trim doesn't change as the boat heels.
This helps some aspects of performance and hurts others.

There is nothing shelf like about the metacenter and the term reveals a good deal of ignorance about stability on the part of the person who came up with it.

[Andrew Craig-Bennett]

Forgive me if I am wrong, but I fancy that the term "metacentric shelf" is in essence a way of describing the position of the heeled metacentre at each station, plotted along the LWL of the boat in plan view. The idea is that one conceives of the boat in terms of a hull model being lodged on the shelf - will she tend to fall off it?

A "metacentroid" is a boat whose in wedge and out wedge are of equal area for each station, so that no part of her rises or falls as she rolls; these are rare but the Mystery Class, which made the reputation of a young Robert Clark, is one - these boats all have amazingly tiny tillers so as to demonstrate how easy they are to steer.

A boat with a straight metacentric shelf, or with a metacentric shelf that curves equally about the mid-point, is said to have a good metacentric analysis; the chances are that she will continue to balance nicely when heeled under sail. A boat with a "crossed" shelf (so that she would tend to fall off it) probably won't.

A modern boat with a flat afterbody and a V shaped forebody is an example of a crossed shelf - and indeed they steer very oddly when heeled!

My boat has a balanced metacentric shelf; it makes her no faster, indeed, almost certainly slower, but it makes her easy to steer at sea.

Its not the be-all and end-all of small boat design - as Dr Harrison Butler himself said, a barrel has a perfect metacentric analysis, but would be a useless boat!

[ 04-15-2005, 10:27 AM: Message edited by: Andrew Craig-Bennett ]