Finding Correct Prop

[Blue Crab]

My Cal 25 (4500# fin keel/spade rudder) has had a horizontal 12 hp Yanmar retrofitted. It has an approx 13" 3 blade fixed prop. Many onlookers have made no comment but the sailmaker was aghast: he thought it would slow me down too much under sail.

I haven't had it in the water yet so no idea of current performance but I'd give up some speed to have some real power when I need it in tidal currents but how would I even begin to calculate the correctly sized prop?

[SkiprJohn]

Aloha tg etc.,

Try proper prop size in the search engine after my signature. It truly depends on propeller speed, engine hp and waterline length. If you have the cash then a folding prop would not slow you down much but reverse will be dodgey.
kind regards,