Can anyone provide the formula to work out Keel length to bulb weight ratio for a Marblehead? What I am after is a way to decide what keel and bulb are required to provide ideal boat heel for given sails.

Hope that makes sense to all.

There is no ‘formula’ however you could try this,

Balast = Displacement - weight of (hull + fin + radio gear + rig)

Righting moment is maximised by placing the heaviest possible balast at the end of the longest possible lever.

In other words, the design you are building has a designed weight to float on its lines. Build the boat as light as possible so that the balast is maximised.

Wetted suface of the fin does not seem to matter as much as the extra righting moment of the longer fin.

Make the fin as deep as the class allows except reduce it to avoid grounding if you sail in a shallow pond. (make the fin removable and have two fins, one for shallow and one for deeper pools)

There is a formula. I will look it up from old: 20 years AMYA mag. It deals with sin and cos of heel angle you want, probably: 30 degrees and approx. center of balance . One third up sail luff etc. I found I could use 74" luff 1250" actual sail area with a 2 1/2 # bulb at 18" on a one meter at 5mph. Didn’t believe it till I tried it. Worked great. Everyone was sitting still and I sailed around the pond.

Thats the formula I remember, but could not locate. Many thanks for looking for it.

“Dellenbaugh angle” comes to mind. page 52 Principles of Yacht design.

Seems to be the quickest way to estimate stability I have read about.

Any luck finding the formula yet?

I have ordered 2 books on design, but as yet they have not arrived.

Really the best book on design is Yacht Design by Larsson and Eliasson. The translation from Swedish is a bit wobbly but otherwise it is by far the best on the market. Currently heavily discounted by Amazon.

Nev -

Hoj is located in Hawaii where there was a recent large earthquake. My emails to him went undelivered - but not sure if it was internet or if it was power issues that caused the delivery problems.

Sorry I didn’t get back. My server didn’t charge my credit card till a week late and I had already agreed to get an upgraded card so I could buy a car but it will take 2 1/2 weeks to get it. I have not got it yet since 9/28. So they cancelled my service. I have two computers. One gets email and the other internet at intervals. MYA mag issue # 79 pg. 23 has the formula submitted by Swede Johnson. V=velocity of the wind=the square root of [ sin of 30 degrees .5 x right arm ( 1/3 luff in feet) x sail area in sq. ft. / cos 30 degrees .8 x heeling arm x displacement ] x1/.004. I have hundreds that I did when I find them I will write one out. This does not work. I did not use it but made up my own will find it. SORRY.

found one. This is for a 5# boat with a 60" luff with 892 sq. in. of sail at a max heel of 30 degrees. .5 which is the sine of 30 degrees x .764’ which is the center of balance I can’t find calc. x 5# total weight of boat is divided by.866 the cosine of 30 x 2’ ( which is the luff 60" / 12" / 2.5 = center of sail area x 3.10’ (sail area 892 / 144x2 ) which =1.91/5.36=.356/.004=89.08 sq. root of this =9.44 mph. .5x.764x5/.866x2x3.1=.356 Martin’s formula V=P/.004

Thanks for the help. These are the 2 books I ordered from Amazon.

Elements of Yacht Design (Seafarer Books) [Paperback] by Skene,

Norman L

Principles of Yacht Design [Hardcover] by Larsson, Lars; Eliasson,

Rolf