Some help pitching a prop please.

[GILow]

Hello Cruisers,

I am troubleshooting some performance problems with our engine. We have a 96 HP Nissan ED33, in good condition, 2100 hours, recently overhauled injectors, but I can not get it over 2000 rpm, at which point we are doing around 6 knots. The boat is clean though the prop is slightly fouled.

I have done the calculations and everything tells me that the current 20 x 12 prop is way too big, but I am worried that I am missing something subtle as the person who set the boat up knew his stuff, and even went to the trouble of including a SPARE 20 x 12 prop.

Critical figures are:

Engine - see attached torque/HP curves from Diecon, the people who marinised what was basically a small truck engine.

Boat - LWL approx. 37", beam at WL, 12", hull depth 3", displacement 15 -17 tons.

Gearbox - Borg Warner Velvet drive 1.5:1

Typically these boats get by very well with 75 HP and should certainly go faster than 6 knots with the kind of HP I have available.

As an odd aside, the rated maximum rpm of that engine is 3500 rpm according to everyone except Diecon. I understand that there is a difference between maximum rpm and maximum SUSTAINED rpm, but as you will see from the torque curve, they seem to make the differentiation based on much lower figures. Perhaps they modified something in the engine to make it more suited to marine use, but either way, I should be getting more revs.

What do you think?

Matt