Originally Posted by
tictac80
The Skandia is 8.75 pounds. I would think the keel would have to be weighted to at least half that ... maybe 5 or 6 lbs.
I sometimes balance an RC sailboat by laying it down horizontally on a dowel. It should balance when the dowel is near the vertical center of the keel.
This will depend on hull shape and sail area, so it's only a rough estimation.
If possible, add some ballast, then test it in the water. Add more ballast till it floats at its waterline level.
Here is a good explanation that I found online:
"General rule of thumb is that the bulb should be some where between 60-70% of the total weight of the boat for good righting moment, the lower the bulb weight ratio the more boat will be prone to heel too far. Having said that, most commercially made RC yachts are not designed to carry high bulb weight ratios, so you may then wish to keep adding weight until you get the hull sailing on its lines correctly, too much weight will sink the hull in the water and increased wetted surface area and slow the boat down (noticeably in light wind)".
A bit of extra to the above - the added weight, if external to the fin, needs to be a good shape. I did see a few years ago a guy who had added weight down below that was an unshaped block. The yacht sat up right nicely, but was reluctant to actually move. A nicely shaped end cap that added weight and increased fin length worked well.
Area of the fin provides damping against the tendency of the boat to flick over, the weight at the end provides the righting moment. Any weight more than half way up the fin is not really helping.
"Real" boats like dinghys that have a relatively unweighted dagger board work because they have moving counterbalance ballast to try to keep them upright. Also called a "crew".