Prop size and pitch involves calculations using the engine HP/torque, the transmission ratio, hull, and number of blades. You could expand the pool of people who might be able to answer if you could give values for all the variables.
I'm guessing you didn't read the first message I posted at the start of the thread, but I will recap it for you.
In my initial post at the start of the thread, I stated I have an electric motor. The HP rating for electric and combustion engines is different because of the character differences of the two. I have been told the electric motors are more like the diesel vs a gas, more torque, and lower RPM versus a gas motor. So, trying to us a general formula for a combustion engine I don't think would work. An electric 5 hp rated motor would be close to the out put of 10-15 HP gas, so I have been told.
I don't have a transmission, but my pulleys are 2:1 from the motor to the prop shaft. Would be hard to plug my set up in to the formula.
I also initially posted, I have an Islander 28, I'm guessing since you wanted a hull, I have a 1976, hull #23. I was just trying to get an idea of what the diesel owners of the 28 were running to compare their's to what I have, to see if I'm OK with what I have.
In my first post, I stated I have a 3 blade 12x10.
So, kinda back to my initial question at the first post.
What prop are the owners of Islander 28 with diesel engines using?
Just looking for a general feel from the Islander 28 community
Thanks for wanting to help out, says a lot.