Islander engine height

[Greg B]

The Suzuki 300 is installed and running a 19" Rev4. Right now I'm way low on rpms - 5500 and about 31.5 knots but with the engine trimmed all the way down. I'll re-run with it trimmed better but the installer tried to match the Yamaha height. I'm wondering if it should be up a notch.

Any help would be greatly appreciated.
Thanks
Greg

 

[ocnslr]

My "sources" indicate that you should be up at the top end of the specified RPM range with the engine(s) trimmed up, but certainly short of ventilating.

If you are getting 5500 tucked under, then I think you should be up at the top end of the range, with more boat speed, when you trim up.

Four-blade props can usually handle a higher mounting height.

Your RPM and speed figures give about 24% prop slip. This is very high, and close to what we had on the single Yamaha 250HP OX66, before we went with the PT OFS4 prop (as I mentioned to you in those PMs).

http://www.go-fast.com/Prop_Slip_Calculator.htm

 

[Greg B]

I know you did a lot more research on this than I have, what are you using for weight - I used 7000 lbs and the Suzuki gear ratio is 2.08:1. I think I either have more slip that that or I'm doing something wrong. I bought a Suzuki 16x18.5 and I'll see how that does.

 

[ocnslr]

I know you did a lot more research on this than I have, what are you using for weight - I used 7000 lbs and the Suzuki gear ratio is 2.08:1. I think I either have more slip that that or I'm doing something wrong. I bought a Suzuki 16x18.5 and I'll see how that does.

Not sure what you mean by using weight.

The prop calc uses RPM, gear ratio and pitch to calculate theoretical speed. That is simply based on each revolution of the prop moving the boat forward a distance equal to the pitch in inches.

The actual speed is then used to calculate the slip.

If you used the prop calc that I provided a link to, did you forget to convert your speed of 31.5kts to MPH? That would be 36.27 in this case.

Using 5500RPM, 19" pitch, 2.08 gear ratio, and 31.5kts, gave 24% slip.

The 7000# is probably pretty close for our weight, with fuel, water, batteries and engines, but with none of our gear onboard.

Brian