Hi everyone!

I´m building a wooden sailboat from orginal plans. The orginal is just about 4,5 meters, and keel weight of 275 Kg. My model is in scale 1:3.3 and I wonder how to recalcultate the weight needed for the keel...

Regards, the sailing wonder...

It is unlikely that you will be able to directly compute this with a simple mathematical formula, since when you scale down a full size boat, you reduce the sail area by the square root, and the hull volume by the cube root. For instance, a half size model (1:2) will have scale sail area of 1/4 of the full size, but it's displacement at that scale will only be 1/8 of full size. In your case, the sail area at a scale of 1:3.3 will be slightly over 1/11 of full size, while the weight will be slightly more than 1/36 of full size. You can see that the difference between the scale area and scale volume gets greater much faster as the scale ratio of the reduction is increased. If you know the overall weight of the full size boat (including payload) when it is floating at it's designed waterline, you might be able to roughly calculate the weight of the model at a scale waterline, but just reducing the weight of the ballast won't work, because it's unlikely that you would be able to build a model using construction that is of scale weight of the hull before it's ballast is applied.

Good luck,
Bill Nielsen
Oakland Park, FL USA

The overall weight will be in scale, but on a model we can distribute the weight differently to that on the original. Build the hull and everything above the waterline light but strong, and make sure that ballast weight is as low as possible to lower the CoG. If required, you can always extend the depth of the fin mounting the ballast.