sail area to lateral area ratios

[Ian]

I am designing a sixteen foot cruising dinghy and need to estimate centerboad and rudder areas. I have been getting a 25% spread in the results I have obtained from various suggested ratios, and from measuring published drawings.

I am sure that the average fin keel of a keelboat and the average dagger-board of a performance dinghy are sufficiently different that this would affect the optimum lateral areas. Dinghys generaly sail more upright and frequently have high aspect ratio boards.

Foil design information is very widely available--but there seems little point in designing a foil for minimum drag if it is unecessarily large and has way too much wetted surface.

Does anyone out there have a fairly reliable method of approximating the appropriate lateral area for differing vessels?

[SailDesign]

This probably doesn't help much, but if you can confine your data-gathering to similar vessels to yours, or to one particular designer, you _should_ (important word, there ) get some convergence, or at least be able to see a trend.
Usually, the ratio of SA to LA will vary as the boat gets bigger. so confining yourself to boats between , say, 14 to 18 feet might be a start.
Steve

[mmd]

I think that you are discovering the grey area of the "art of yacht design" as opposed to the science of Naval Architecture. Your 25% data spread is not suprising. The method for deriving appropriate lateral area for your design is a combination of regressional analysis of gathered data and designer's (your) opinion based on experience, intuition, evaluation of anticipated conditions of use, and maybe a wee bit of inspired luck.

This is one of the reasons why bigger boats (with bigger budgets) have tank testing done.

[SailDesign]

mmd - Now you've ruined it! I was trying not to scare him off with words like "opinion, experience, intuition, luck"

[Ian]

It occurs to me that it matters quite a bit whether this boat will be used a great deal in light airs or is only taken out in a decent breeze. I recall a Rhodes motor-sailor that would go to windward with the board up in a good breeze and flat sea--but would barely get to windward at all in very light conditions, even with the board down. I know that windsurfers can change to much smaller board when it is blowing hard.
I guess that that mixture of extremely detailed well researched technical analysis, and very vague rule-of-thumb procedures--is part of the charm of yacht design. But still------foil design is so advanced-----but the size of the foil has to be a guestimate? I don't know. But thanks for the input all!

[Hans Friedel]

Acording to Principles of Yacht Design 3.5% of the sail area is a OK for a fin keel cruiser. And 1.5-2% for a racer.

Modern fast racing dingys have narrover foils than older dingys. But narrower foils tends to stall easyer specially if your dingy is slow.

Hans