Twin Keel Righting Moment

[SailDesign]

Will,
Not so much that the RM _will_ be the same, but that you would _want_ the same to provide the same power to carry sail for a given displacement. You'd look awfully silly doing 2 knots uphill because yuou couldn't carry enough sail......
Steve "my 2-cents"

[Willallison]

Ah - now that makes sense

[bhnautika]

Will sorry I mis-understood the intent of the original question. There are obviously two parts. CG and volume distribution in RM’s. Yes there would be a small difference in the distribution of volume of the two styles of boats as they heeled but it would be small difference in the GZ (mm), I was assuming a set CG. If we are talking CG then bilge keelers tend to have a higher centre of gravity and a lower RM than deep keelers.
Not seeing the sectional shape I think .76m might be a bit optimistic for a bilge keeler. As it is, you have about 12000Kg\m RM30

[MikeJohns]

There are many hulls that come in two forms Single inline or twin keel. The hull is often identical in both only the keels vary. The COG could easily be made identical and hence your heeled RM.

As Steve pointed RM is your power to carry sail and you do not see reduced rigs in the twin keel versions.

A problem some people have is a perception that one keel of the twin will have more influence when heeled, but of course the COG remains firmly fixed.

[bhnautika]

Mike I am fully aware that the CG is fixed. My point is the centre of gravity of the keels is higher on the bilge-keeled boat than a deep keel boat so it influences the total CG of the boat and so the righting arm. On two identical boats with the two types of keels, yes you could make the cg’s the same but which one would be easier to adjust and why?

[Willallison]

In theory I'd agree with you Mike - though in practice, I found that in order to obtain the appropriate CLP / lead etc the total underwater volume of the twin keels needed to be much greater than would have been the case with a single CL keel.
Which in turn may answer bhnautika's rather cryptic post. The increase volume would allow simpler movement of ballast - and hence the CG. Then again this is really only in the LCG. If you were trying to alter VCG I'd expect a CL keel to make things easier

BTW bh (I hope you don't mind me calling you that... which bit of Oz do you hail from?

[bhnautika]

will sunny vic

[Willallison]

isn't that an oxymoron...

[bhnautika]

Ok sometimes sunny