Optimax engine heights

[Paul E]

My girlfriend's brother has a 19ft Boston Whaler with a 200 optimax. Top speed he's seen is 49 knots, I was getting 47 last summer. The engine is set on the middle holes, with the cavitation plate 1/2" above the v. How high can we safely go, we want 50 knots and this seems to be the easiest solution! Current prop is a 21" Lazer 2, revs to around 5,200-5,300rpm. It likes a lot of trim to see good speed, it's a pretty heavy boat.

Does anyone else have any suggestions to help us break the 50 knot barrier?

Cheers

Paul

[Big Dutch]

Each hole is an inch I reckon move it up one 100rpm more
I wouldnt go too much higher as it might cavitate being a heavy old hull

[andy21]

I've got a 23" laser 2 you could maybe try before mine goes back in the water, see how it goes...

Might know of one for sale too if it suits...

[Renegade]

What is your rpm range? What your getting seems a bit low to me, it may be worth trying a smaller prop if raising the motor don't work. Maybe try a 20 inch prop and one that has lots of natural bow lift so you don't need to trim. Remember that raising the motor will drop the nose even more. Hope that helps. Also try moving some weight about.

[Paul E]

Cheers guys. I'm not sure what the WOT RPM range is, but the maximum RPM stated on the plate is 5,750 - so I think we could go a bit more.

Would lifting it one hole give us a natural RPM increase? Would a prop with more blades be of benefit seeing as it is quite heavy?

[Big Dutch]

The revs you are pulling is a little low.moving it up with the same prop should give you 100 rpm more per inch you go up.
Dont go up too high or you could run out of water and overheat the motor.
Maybe a smaller prop or more modern prop might be a better option Raker/Tempest,try a few.
The laser is quite old tech nology over 20 years at least.
Four bladed prop better for rough and heavy- boat doubt on a 20 footer it would be quicker then a three blader
Its a heavy hull I reckon.

[Paul E]

We'll stick it up a hole then and see how we get on, then maybe start experimenting with props. Thanks

[larby]

Just stop taking Neil out, you'll gain 10mph!!

[Paul E]

Originally Posted by larby

Just stop taking Neil out, you'll gain 10mph!!

hahaha I'll tell him that!! To be fair the 47 I had was just me and Ducky, and he weighs about 8 stone, skinny little sh*t

[scooby_hugh]

I have a 19 p laser 2 kicking about i think if you wanna try it? Funny enough it was on an outrage with a 200 mercury blackmax

[Burty]

each hole is 3/4 inch, the higher you go the more trim you will need to get the nose out of the water so you may find dropping it down a hole and getting a smaller prop with more bow lift will be better

[Paul E]

Looks like we have some experimenting to do then!

First thing to try I reckon will be up one hole, and shift the anchor into one of the rear lockers instead of up in the bow. I may well take you up on the lend of the 19" to get some more revs cheers Hugh! I doubt there's many specialist props over here in 19-21" pitches though - we're desperate to break 50 knots in her but don't want to spend a fortune when we've got other boats to make fast too