I ran your numbers through the prop calculator on boatdiesel. Sign up for $25 and you can do it yourself. But in any case browse the boatdiesel forum for free and you will get more than $25 worth of marine diesel
First a couple of questions. What is the maximum diameter that will work with your boat? Remember Dave Gerr's book. It says that you should be 15% of the prop diameter away from the hull
. Also can you increase the transmission
ratio? That will help.
But plugging in your numbers and a 19", 3 blade
prop gives a pitch of 14", right in the middle of what you are getting. The calculator says that the prop shaft rpm will be high and increasing the transmission ratio will help with prop efficiency. And you will have to increase the pitch as well- props with diameter equal pitch (square) are usually the most efficient.
How do you know if you are underpropped (or overpropped)? If underpropped you will be able to rev beyond 3,000 rpm in gear. If overpropped you will rev to less. Since your boat is heavy for that engine, I would prop right on the money
or a little underpropped.
Although FWIW I have never seen a 33' lwl boat that weighs 13 tonnes. Ten is even high. And it does make a difference in pitch. I think you have more difference in the real weight of your boat than anything else discussed.
But you can repitch up or down if necessary. But why not install a MaxProp and stop worrying about pitch. Some MaxProp models can even be adjusted in the water
. A haulout and repitching job will just about pay for the difference in a MaxProp vs a fixed pitch prop. And you get the advantages of more thrust in reverse.