Prop performance

[danmarine]

Hi Guys
I am new to this site and have been very impressed while browsing through the forum.

I have a question about the theoretical and real performance of propellers.

Firstly are there any formula available online for the power to prop size/pitch.

I have a fiberhull Bertrum? Randell? With twin hino ek100 every where I look I get a different result on the power rating of these, the Hino guys say 275hp and log says 234hp and the captain says they are 330hp each with an optimum 2300 rpm. They are old and unserviced for over 3 years apart from keeping them running.We are missing the alternator, water pump, etc off one engine which means that temp, pressure and rpm gauges are out.
Now the new owner wants to increase the speed or at least the efficiency of the craft. I realize that a good overhaul of the engines by Hino specialist and a thorough clean up of water ingress on the underside will solve a large part of the problem but I am a bit stuck with the prop. I am told it is currently an 18-20” four blade of uncertain pitch. I know I can get props repitched here but I need to know what pitch I require.

The vessel is 15m long and 5m beam, displacement of 31 tonnes, used for dive and day charters.

Any help is much appreciated

Dan.

( I have read with interest about the variable pitch props but don’t think this is a viable option due to the bosses finances)