prop advice

[markymarkP]

hi all

am doing very well with my Yam 90 hp.

just a quick question for my learned fellows

i have fitted a 13.5 x 15 K prop to my boat, and she does 42 knotts

nice,
but when she is heavy laden, she struggles like mad to get on the plane.

if i fitted a 12:5/8x21 K prop instead, would that give me a bit more push, but slightly less top end?

thanks

[Nos4r2]

What's your boat?No boat should be able to reach 42 knots on Yam 90 with a 15" prop without either a minor miracle or blowing up the engine.

21" will make it far harder to get on the plane.

[markymarkP]

17ft Dory, rigid raider type.

fine when unlaiden, but with 4 divers plus kit, she weighs about 1.5 tonnes!

she is fine when its just passengers, but with all the diver gear, that adds another 250kgs!

she made 42 knotts yesterday, - eventually. but for most of the trip she was 20 knotts. and not quite on the plane. but a wind behind the boat just was enough to make thie difference, then she was flying along.

speed confirmed by two GPS. i have a rev counter ready to be fitted next week, so sadly i cannot tell you the revs, but she was not over reved

[Nos4r2]

You sure the gps isn't in mph?

Let me put it this way. Running the calculations of your engine at 6000rpm which is waaaay above where it should be revving to (rpm, gear ratios, prop size etc) gives negative slip for your prop. Your figures are physically impossible.
At redline with your engine and a 15" prop you should reach a MAXIMUM-with NO prop slip at all-of 34 knots.

[markymarkP]

yes it was knotts

i will wait until i have the rev counter in this week, and i am taking her out again friday.
but i have had 40knotts on two occations now without much effort, unless i am full of divers!
perhaps the prop i have quoted is the wrong one. i will check and let you know

thanks