Hi there Neil

Is your boat water tight yet? If so, then mark the correct waterline on the hull and float. Add all the bits and pieces,(masts, radio gear, batteries etc) in their intended location, then add enough heavy items untill the boat floats on the water line, level in all directions. Take the boat out of the water and find the fore and aft balance point. Take the extra weights out, (but not the bits that belong to the boat) and weigh them. This will be the weight of your keel. Position the keel fore and aft on the hull centre line untill the model balances at the point you marked. The keel should be just a touch foward of amidships. Should be bang on when finished. The size of the keel is a bit off a juggle, larger if the boat is intended to sail to windward ,smaller if not. The deeper the keel, the more righting power it will have for its weight. Keel area maybe a little less than half the hulls imersed area, as seen in the profile drawing could be a starting point? Draw it up and see how it looks, and play around till it seems about right.