I suppose the best way would be to machine them from oversize castings, then they would have the dimensional accuracy of the machined from solid, but at much lower cost, assuming that enough are made to amortise the cost of the casting tool. I guess there just isn't the volume in any one boat to specifically design a machined prop that absolutely maximises its performance. I agree with the above poster that it does seem to be a black art.

Mikef, i'm sure you're right with your different prop theory, but if the boat was out the water I guess you also had it antifouled and all the stern gear cleaned up, could the 2 knots be due to a different type of a/f?