I don't know that anybody would want to advise you on something like this, particularly since there could be safety issues involved if you opt to sail a two-person boat singlehanded -- safety for yourself as well as for the other sailors on the course. Other factors are involved besides relative crew weights -- difficulty in righting the boat if it capsizes, unpredictability of Mother Nature, difficulty maneuvering in traffic while trying to handle two sails, whether this regatta is a distance race or buoys races...... My advice would be to sail your boat with crew as usual, and let the Prindle 18 take the Portsmouth hit for sailing solo. If he is sailing it with main and jib, he is going to be additionally handicapped by not having a crew. But it is a decision that only you can make.