The condition that I'm sailing in is exactly the same as that for a boat with an inboard engine and a fixed prop. That is, the majority of cruising boats. And as long as I don't introduce a variable by taking the outboard in and out, then we can get valid figures before and after making a change to the rig. This is the major purpose of the testing. In the case of Calisto, to evaluate any gains and losses due to coverting to JR. In the case of Weaverbird, to evaluate any gains and losses due to changing from hinged battens plus a little camber, to wing sail. In a case of a boat upgrading from a flat sail to cambered panels or hinges, what is gained and what is lost? And then we have the case of, say, comparing Weaverbird and Miranda. Two boats the same length, with different rigs. To get a valid comparison, either both boats, or neither boat should be dragging a prop through the water.
And then of course there's Weaverbird, with the same rig, and with and without the prop drag. A valid comparitive test, to see whether Arne is right about how much speed I'm losing. In all these test procedures, it's important to change only one variable at a time. Then we will get a clear picture of what happens when we make that change. It really doesn't matter to me all that much, most of the time, that I'm sailing a little slower when I'm dragging the prop, for cruising. On the other hand, it would be nice to know how much speed I'm losing, if I were to be considering making a major structural change so as to lift the prop while sailing.
This testing programme is all about recording data that is in some way useful.