Doesn't rev out!!!

[202_ski]

I converted a 15ft fiberglass outboard runabout to mid mount inboard using a 202ci 6 cylinder holden (GM) motor with the prop shaft connected to the balancer end of the crank with a universal joint as straight as possible. My shaft angle is approx 18 degrees from the line of the keel. Prop is a 12 by 11. Motor has roughly 100HP. I cannot get the boat to rev past 2400rpm, I have run out of ideas. This motor should rev too 5000.

[waikikin]

202, google up, Porters props or Austral or Lundberg for some repropping advice. All the best from jeff.

[Jango]

If you have 100HP, 11" Pitch is correct. Assuming displacement of 1600 lb, you should get about 37 mph @ 4000 rpm with a 11 x 11 prop. 12 dia prop -probably 8 - 10% less rpm and speed. Problem has to be with motor and/or perhaps CG location - too far forward causing plowing.

[PAR]

Check to see you're getting ignition advance at the proper RPM's.

[202_ski]

If you have 100HP, 11" Pitch is correct. Assuming displacement of 1600 lb, you should get about 37 mph @ 4000 rpm with a 11 x 11 prop. 12 dia prop -probably 8 - 10% less rpm and speed. Problem has to be with motor and/or perhaps CG location - too far forward causing plowing.
I am allready getting 30 mph at 2400rpm. Does that mean i should get about 50 mph at 4000 rpm? The motor is quite far foward but it feels like it gets up on a plane well. Could it be riding up to high? Would that load the motor up too much?

[Jango]

You might want to check the accuracy of your speedometer and/or Tach. With a 11" pitch @ 2400 rpm, 11 - 12% slip, your speed would be about 22mph.
Also check your Prop. If it was repitched and is alot more than 11" pitch, it could also account for the lack of revs. Prop has an effective pitch of about 15" @ 30 mph and 2400 rpm with 12% slip. This could be your problem assuming your motor is putting out 100hp with no issues.
Riding higher or more out of the water, should "unload" the motor and increase speed with increased revs and lower slippage.

[lazeyjack]

that is so strange, she is well up on plane, direct drive, how long does it take to come onto plane and at that speed does she trim flat or bow well up?

[202_ski]

that is so strange, she is well up on plane, direct drive, how long does it take to come onto plane and at that speed does she trim flat or bow well up?

It gets up on a plane easily and fast. Probably 2 seconds from idle. It revs up fast enough but just cuts at 2400.

[202_ski]

You might want to check the accuracy of your speedometer and/or Tach. With a 11" pitch @ 2400 rpm, 11 - 12% slip, your speed would be about 22mph.
Also check your Prop. If it was repitched and is alot more than 11" pitch, it could also account for the lack of revs. Prop has an effective pitch of about 15" @ 30 mph and 2400 rpm with 12% slip. This could be your problem assuming your motor is putting out 100hp with no issues.
Riding higher or more out of the water, should "unload" the motor and increase speed with increased revs and lower slippage.

I have read that a traditional pitot speedo cannot give a speed reading HIGHER than actual, but only LOWER. The prop is brand new, and i had the pitch measured because I also doubted it. I first tried a 10 and 3/4 by 14 which gave me 25 mph at 1900 rpm.

[Jango]

Most Tachs, have a 4,6 or 8 Cyl. switch. If so, is yours set correctly? Also if the motor "cuts out" abruptly, it sounds like an ignition advance problem.
An 8 cyl Tach connected to a 6 cyl motor indicating 2400 rpm is actually turning 3200 rpm. Prop with 11"P turning 3200 rpm and 12% slip equals 30 mph.

[lazeyjack]

jango, these are very very basic engines, but the tach advice sound spot on, try reading revs at high idle, thats a diesel term, but means full rpm with no load

[Jango]

WOW, Full rpm, with NO load. Must be some Engine? Is it a Diesel? I,m assuming Gas. Don't like to run gas engines at Full Throttle with NO load.

[lazeyjack]

WOW, Full rpm, with NO load. Must be some Engine? Is it a Diesel? I,m assuming Gas. Don't like to run gas engines at Full Throttle with NO load.
i just mean blip the throttle, not hold the pedal down!!!

[Jango]

Thanks for the clarification. Yes, bliping the throttle might tell something about rev capabilities. Good idea.

[202_ski]

It revs to 4000 easy with no load. I had a friend watch the way the boat rides from outside the boat and it doesn't get up on a plane properly, it just pushes through the water. Im gonna try trim tabs/ cavitation plates.

[Jango]

ANYTHING you add to the back of the Boat will most likely "Push the Bow Down" and make it WORSE - more plowing. Tipping trim plates up, most likely will cause the boat to bounce or do nothing. Cavatation plates will only work, IF it enables you to move the Prop and Motor Back.
You need to MOVE THE CG AFT. If your seating and controls are forward of the motor, move aft of the motor if posible. Good Luck

[lazeyjack]

It revs to 4000 easy with no load. I had a friend watch the way the boat rides from outside the boat and it doesn't get up on a plane properly, it just pushes through the water. Im gonna try trim tabs/ cavitation plates.
you could extend the lower hull section for 2 feet, like a pod
then you would also have a platform for land fish

[kobus]

can it be that u need a gearbox so that u take the load of the motor and make the prop spin faster ea more revs

[202_ski]

ANYTHING you add to the back of the Boat will most likely "Push the Bow Down" and make it WORSE - more plowing. Tipping trim plates up, most likely will cause the boat to bounce or do nothing. Cavatation plates will only work, IF it enables you to move the Prop and Motor Back.
You need to MOVE THE CG AFT. If your seating and controls are forward of the motor, move aft of the motor if posible. Good Luck

The cav plates i put on covered the whole transom, extended out 30 cm and fixed a 12 degrees up, didn't seem to have effect. Same revs, same speed. When i have 2 or 3 people in the back seat and just me in the front it porpoises really bad continously. Does that mean it has no hope with the motor in its position?

[Jango]

The 12 deg up angle on the cavitation plate IS causing the Porpoising - same as a rocker shaped bottom.
A Flat cavitation plate (NO up or Down angle) will ONLY work if you move the CG back. Otherwise it will push the bow down more. Use cavitation plate ONLY if you move prop (and motor) back.
Moving the motor back is the BEST sollution. However,If you want to leave the motor in it's present location, you need to take weight away from, in front of the motor and add weight behind. - again, move CG aft.

[202_ski]

ANYTHING you add to the back of the Boat will most likely "Push the Bow Down" and make it WORSE - more plowing. Tipping trim plates up, most likely will cause the boat to bounce or do nothing. Cavatation plates will only work, IF it enables you to move the Prop and Motor Back.
You need to MOVE THE CG AFT. If your seating and controls are forward of the motor, move aft of the motor if posible. Good Luck

Im going to move the motor fully aft and use a 4 by 4 transfer case as a V drive. But because standard transfer cases are not high ratio, i was thinkin of turning the transfer case upside down so the prop turns faster than the motor. Have you ever seen transfer cases used in boats?

[Jango]

Not real familiar with transfer cases, but you need to be careful with ratios. If the prop is turning real slow (relative to the motor speed), The Prop will need to be Huge, 16"+ Dia. (with a 2:1 gear). Turning the Prop very fast, It needs to be very small, 7" Dia. (with the same 2:1 turned upside down).

Either of these conditions are not good, especially at lower speeds. Best if ratio is as close to 1:1 as possible, probably 1.5 :1 max or 1:1.5 (upside down)