I plugged some assumed and given numbers into boatdiesel's prop calculator- 8,000 lbs, 27' lwl, 2.1:1 ratio, 3,600 rpm
, 27 hp, 2.1:1 ratio. It calculated 16 hp required to hit hull speed
of 7kts which will be 3,000 rpm
. That's not so bad as it gives you another 11 hp for head
seas. The 2 cyl Yanmar
will be bit too small for that boat.
The 13" prop diameter works but the calculator says 11" pitch
for a 2 blade
. Check your shaft diameter and material, ie Aquamet 22, etc. It may be too small for the higher torque.
Post actual numbers and I will run again for you. Or join boatdiesel for $25 and run your own. Best $25 I ever spent. It is a general marien diesel