Boat Speed - Final Solution (well almost)

[philfin]

To follow up on my previous posts on this subject for anyone who may be interested.

Problem was: Oceanis 430, Perkins 4108 speed under engine 5.5kts at 2,500 rpm, Max Prop feathering. could rev all the way up to 3,500 and still only make 6kts. General concensus was either under propped, under pitched or both.

Obviously first thing to take a look at was pitch. Having sought advice every where, I deicided to increase pitch by 2deg as this would be sure to have a significant effect.

Turns out the pitch was set at 18 deg (prop 20"dia)so I increased to 20 deg. Back in the water full of expectation.... b....r all differnece.

Boat up again, increased to 24 deg pitch, back in the water and happy to say that now nudging 7kts at 2,400 plus a remarkable amount of increased control/response at lower speeds and from ahead to astern and vice versa.

Engine still easily achieves max revs, so much so that the next time I have her out I will increase a further 2 deg, which will, I,m now sure be the optimum setting.

Thanks to all those who gave me advice.

 

[savageseadog]

Still a little slow at 7 knots, which is why you are increasing the pitch a little more I guess.

 

[Robin]

I would agree that 7kts is still slow, it should be possible to get to or very close to hullspeed ie about 1.34xsqrt LWL in feet, that is over 8kts I would guess. We can reach 8.5kts flat out (just above theoretical hullspeed) in a 41 footer with 44hp Yanmar and Brunton self pitching prop.

We have friends who had similar problems with getting a Maxprop pitched right, they would have saved money in the boatlift with a season ticket but it did get there eventually.

 

[MapisM]

hmm... 24" pitch and 7kts at 2.400 rpm?
I suspect that the reduction ratio on the gearbox is wrong.
Are you sure that it wasn't changed for some reason?
18-20" pitch for such a boat should be fine, with a proper reduction ratio.

 

[Irving]

I am having similar problems with a 19" Maxprop on a Moody 41 with MD22 Volvo, 59hp, 4000rpm. Started with 18 deg pitch, then to 22 deg, Now at 2200 getting 5.7 kn and 7kn at 2800. Would like to run around 2200 because of noise. Is this bad for the engine? Should I try to increase the pitch further to achieve higher speed at the lower rpm? Are there any tests I should do to get further information? Thanks for any help.