volvo md7a replacement with yanmar 1GM10

[marto]

I'm planing to replace the engine on a santana 30 sailboat
it was fited before with a 13 hp two cilinder volvo md7a and 2:1 transmision and folding prop.
Is it a good Idea to replace with a yanmar 9hp one cilinder that runs at the same rpm @ 3600 . normaly from factory they leave the engines runing very lite so I suppose the md7a was not at his limit using that prop.
I was using a force 15 outboard before that push the boat at 6 1/2 nots at 3/4 trotle.
I'm also planing to use the same volvo transmission and build a connection plate to stik them together.
I'm new in the forum, thanks very much.
Martin

[marto]

well another quest waht over power a boat needs to be safe in rough weather, lets have a boat with hull speed of 7'2 knots and I requierd 11 hp for hull speed in normal weather conditions , waht hp to prop would be safe , por example to breake into waves.

[gonzo]

Assuming the propeller was properly sized, it will be too big for the Yanmar. The hull speed of the boat is about 6.6Kt, so the outboard is sized just right. A 9HP in windy conditions may let you do 4Kt or so.

[marto]

whell looks taht I will have sail more, spent more in sails, and go