Help with Keel Calculation

Discussion in 'Sailboats' started by saildog, Dec 14, 2005.

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saildogJunior Member

Is there a calculation to determine the weight/length of a sailboats keel relative the boats weight/displacement/mast height?
Thanks

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Raggi_ThorNav.arch/Designer/Builder

I think you should calculate the rightning moment for different angles of heel, and also the heeling moment with different sail configurations and wind speed.

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the_sphincter*

try looking at another boat that's the same size/displacement. look at the sail area, and then judge your keel accordingly based on what you want to do with it.

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h_zwakenbergHullDrag/32 programmer

in the minitransat class (6.5 lwl/loa), there's a rather simple rule about stability: with the boat heeled at 90°, stability must be enough to be able to handle a weight of XX kgs from the top of the mast. (I don't know the XX by heart, I thought is was 30 kgs, but I'm not sure about this value) The boat must be able to either right itself from this load or to keep 90° equilibrium.

bye
Hans

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RHoughRetro Dude

Self righting with 45kgs at masthead with masthead touching water, at any ballast position.

The 6.5m mini-Transat box rule is a neat challange.

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Raggi_ThorNav.arch/Designer/Builder

This (mini 6.5) is a simple (and good) rule for small boats where the keel bulb is not contributing much to stability when sailing, but is more of a safety factor.
For narrower hulls or boats without much crew (or water ballast) weight you may want more ballast in the keel, like 30 to 50% of the total displacement.
Remember, some years ago 50% was normal when total displacement meant the boat+crew+stores+everything, while now 25% of the empty boat seem to be quite normal. It depends of course on hull form.

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h_zwakenbergHullDrag/32 programmer

So 45 kgs it is. I have the old rules laying around somewhere, from the times it still was 30 kgs.

45 kgs from the masthead of a mast at a bit over 90° heel, that normally towers 12 meters over LWL and all this on a boat just 6.5 m (appr. 23') long is an amazing amount of stability!

I once soloed a Pogo (which is not a proto) on the Baltic. Great little yachts these are...

cheers,
Hans

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saildogJunior Member

Thanks for all of the help. From what I understand you can get by with less weight in the keel IF you increase the length of the keel. Is there a formula for this?

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tamkvaitissailor/amateur designer

value of wieight multiplied by lenght must stay the same. Personaly I think that you shoul calculate moment of sails(I know it is not the corect name for this moment), and righting moment. tricky thing is to calsulate the forse created by wind going threw the sail (lift), there are several coeficients, which depends from sail shape and air densiti. Wind speed and AofA is very important, so you must decide in what conditions must sail. Formula looks like that:
P*h=G*l
P-lift
h-distance from center of bouyancy to center of sail are
G-gravity force
l-gorizontal distance betveen center of boyiancy to the center of gravity

P=(wind speed^2/16)*sail area*coeficient of sail efectivnes

Sorry for english I have translated these formulas from my russian book

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Raggi_ThorNav.arch/Designer/Builder

It depends on why you have the weight in the keel. On a small dingy-like boat, the bulb is there mainly to righten the boat after a knock down, stability when sailing up right comes mainly from the hull shape and the crew weight. Then it's very simple, look at the boat heeled to 90 degrees and let the rightening moment (M) be the same with differents weights (F) and arms, M=FxA. For larger boats you have to look at the stability at typical sailing angles, say 30 degrees heeling. This is easiest done in a design program like Freeship or simmilar.

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