Expert: Jeff Grigg - 2/23/2008

Question
QUESTION: hi ive got a 21' hartley ply boat am running a 1kz-te toyota prada diesel motor with a pre alfa leg 1.98-1 ratio.am getting it up to 22knots @4000rpm with a 15 3/4x15 ss prop which is as fast as i can get it to go.@3000rpm am at 15knots.ive been told i may have the wrong ratio in the leg and possible need to go to 1.5-1.have been told i should get 30knots max with this engine if we have correct ratio.what are your thoughts on this?paul

ANSWER: Paul, you can use props instead of gear ratio's to change the rpm and speed. Try going to a smaller diameter prop to get your rpms up a bit, which should allow a little more top end.
Jeff

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QUESTION: yes have tried that went out this week with a pro pulse adjustable 4 blade prop 14 1/2 dia set it at 18" pitch and got 18knots@3500 rpm and 21" pitch 15knots@3800 rpm

Answer
Paul, I am not familiar with the set up or boat or even weight so I really can't say for sure but 30 does seem a little high.
There is something wrong with the prop deal though. While I can see losing a little rpm with the larger pitch you should pick up not lose speed. Also a four blade prop is good for high torque applications and handling, not top end, See if you can find some three blade props in the  15-17-19 range.
I also don't know your full throttle rpm range of the engine. If indeed it is on the 3500-3800 then you will never see 30.
Do this try and find the weight of the hull, full rpm range of the engine, then using a propeller speed calculator, do a Google search they are all over the Internet, factor a slip of at least 15% and the gear ratio. You should be able to get an idea of what the boat, engine combination "should" be able to do.
Jeff