Low WOT rpm?!?

OK, I have been battling this for half a year now. I have a Mercruiser 140 Hp (1978) freshly rebuilt by machine shop, bolted to a 1967 Merc I ez-shift (1.84:1 ratio) All of this inside an aluminum starcraft hull (starchief, 1967)

No matter what I have done my max RPM is 3600 RPM. I have installed a water seperator/ fuel filter; removed the primer bulb that was on there before; rebuilt the carb; tested the fuel pump; inspected the hull; changed the prop from 19p to a 17p (with absolutely no change in RPM) checked and rechecked the throttle cable and throttle plate are both fully open. The timing has been done twice ( including the advance timing).

So I am now dumbfounded. no idea why the rpms stay so low especially when I repitched the prop and still can only get 3600 rpm when the manual is telling me I should be in the 4100-4600 range.

Thanks in advance for your time in trying to help resolve this issue.

Re: Low WOT rpm?!?

Maybe the tach is wrong?

If you can measure your speed with gps then take your speed, RPM, gear ratio and prop pitch you can calculate your prop slip. If your prop slip shows a negative number then you know that one number is wrong. My money would be on the tach.

I would try comparing that tach to a known good one or work out your prop slip as described above.

Once you have the numbers for speed, RPM, gear ratio and prop pitch go to this link and plug in the numbers. http://www.rbbi.com/folders/prop/propcalc.htm

Post back with your results.

Cheers

Re: Low WOT rpm?!?

Had a similar situation with an ali boat here with a 4.3 in it... It ran ok with as 470 in it and when we dropped the V6 in, the increase was only a few knots, regardless of the prop. We concluded that we had reached the maximum speed that the hull would travel at... It wouldn't have mattered if we dropped a 502 in there, the boat was only going to do a certain speed and that was it!

Chris......

Re: Low WOT rpm?!?

achris said:

Had a similar situation with an ali boat here with a 4.3 in it... It ran ok with as 470 in it and when we dropped the V6 in, the increase was only a few knots, regardless of the prop. We concluded that we had reached the maximum speed that the hull would travel at... It wouldn't have mattered if we dropped a 502 in there, the boat was only going to do a certain speed and that was it!
Chris......

Click to expand...

Yes and no, certainly with a displacement hull there is an efficient maximum speed beyond which the power requirement for increased speed is just so uneconomic that it isn't done.
According to Wikipedia and several other sites this speed is about 1.34 x square root of the waterline length
It's why trawler-style cruisers don't have very big diesels, just a nice slow efficient diesel.
With planing hulls the hydrodynamics are so entirely different that power tends to be limited by engine space, and why FB (acronym that I won't spell out) petrol donks are thrown into them.
There is no such formula for planing hulls as they are limited by available torque, prop pitch and the sheer guts and wallet of the driver.

Re: Low WOT rpm?!?

triman said:

Yes and no, certainly with a displacement hull there is an efficient maximum speed beyond which the power requirement for increased speed is just so uneconomic that it isn't done.
According to Wikipedia and several other sites this speed is about 1.34 x square root of the waterline length
It's why trawler-style cruisers don't have very big diesels, just a nice slow efficient diesel.
With planing hulls the hydrodynamics are so entirely different that power tends to be limited by engine space, and why FB (acronym that I won't spell out) petrol donks are thrown into them.
There is no such formula for planing hulls as they are limited by available torque, prop pitch and the sheer guts and wallet of the driver.

Click to expand...

There is a limiting factor with some hulls. The ability to keep at least part of the hull in the water.
When I first started building boats I scored a Merc 502/bravo package (a take out) for little more that the labor (mine) of swapping it out for the guy.
I put that into a v-hull 20 foot IMP Aztec. Replacing my 5L. The boat did 57 MPH with it.
Freaking thing was a rocket right up until the point where there was so little of the hull in the water that the props bite in the water took over and next thing I knew the boat was walking sideways on the prop at 100+ miles an hour. Only blind luck and the deadman switch kept me from barrel rolling that day. Broke 3 ribs and my arm when I hit the side of the boat.
That's when I learned that to skip across the water like a rock, you need sponsons or a boat with more draft or beam. Even with 24 inch Dana tabs that boat was squirly. So I put the 502 in a Daytona and still have the IMP.
But every hull has a point where where it loses controlability and attains lift.

Re: Low WOT rpm?!?

You have an outdrive gear ratio for a V6 (170 - 190 hp) and trying to run a 130 hp engine with it.
Have you also trimmed the drive up when on plane?