Originally Posted by Richard B
As I understand it, lowering the jack effectively lowers the revs, and a larger prop effectively lowers the revs, so all this will do is increase the load on the engine.
For your £45 though, a good buy as a "get-you-home prop".
Lowering the foot in the water increases overall drag. Depth of the prop alone should have no direct effect on rpm until it gets really shallow and ventilates.
If you find the boat goes with the 21" prop, take the 24" to a prop shop and have it repitched to a 22". It'll make a good light load prop. Certainly cheap enough.
Congrats, Cod; great looking boat.