Hi, you need a little more information derived from the sail dimensions you mention.
You need to know the combined area of the sails and where this area is centred. This is called the centre of area! OK so far? Look it up if in doubt it is simple.
This point is measured down to the waterline and the measurement noted, I use inches!
Now put the yacht in the water and on a windless day or better still in the bath and see if you can find the point on the hull where you can push the hull sideways and it does not fall off to left or right but keeps going sideways. I stick some decorators tape on the side of the hull and mark with a pencil. This is very easy do not despair, we are nearly there!
I use pounds as well! With the wind blowing at 20 mph it exerts a force of 1 pound on 1 square foot of sail!
Allow for this as the most you will be sailing in. Reduce the sail area from square inches to square feet and note this number.
Multiply the square foot area by the distance straight down from the centre of effort to the waterline. This number is the key to the weight of the keel.
Supposing the numbers are 12 inches and 4 square feet then this comes to 48. If you place the weight 10 inches down you will need 4.8 pounds in weight for the keel. Another option might be the weight at 8 inches down from the waterline and this needs 6 pounds of weight.
Now with this information you can improve the sailing of your yacht. Measure the waterline length and note the centre of lateral resistance (where you poked with your finger).
The centre of effort of the combined sail are shound be 4% or 1/25 of the waterline length ahead of the finger poke spot.
The above will give you a sweet sailing boat that tacks like a dream.
For keel area water is 800 times more dense than air so use the minimum keel area to stop the yacht going sideways while tacking.
If you have a long keel hull then if building perhaps extend the lines down a bit but that is another story!
Persevere and it will work!
Roy
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