From "Larry Ludwig"
"Weighing a keel while attached to a boat can be done with a high degree of accuracy. You can test this out on a boat with a removeable keel. Lay the boat on it's side with the keelbulb on the scale with the keel parallel to the table. Then take the keel off and weight it and you will be amazed that they are nearly the same, as in within 95% or better."