Motor height off cause other issues?

[clemsonteg]

So I have a thread in the motor forum concerning low rpm

http://forums.iboats.com/showthread.php?t=498166

I was reading about re propping and found that the anti vent plate should be 1" above the bottom of the transom. I have a 1988 24ft playboy pontoon with a 1978 Johnson 85 hp motor, but the anti vent plate is 5 inches below the transom. The motor is mounted on the top hole so I could go down 2 holes effectively raising it 2 inches but I'm betting that wont be enough. Would a plate like this http://www.iboats.com/T-H-Marine-Hi-Jacker-Fixed-Jacking-Plate/dm/view_id.40130 work or do I need another way of rethinking this?

 

[Lawnmedic]

Re: Motor height off cause other issues?

You cannot measure from the bottom of the transom on a pontoon as it could be built different than other pontoons. You need to make sure the the anti cavitation plate stays under the surface of the water. The closer the the surface, the less drag the lower unit will have. But, too close to the surface and you will ventilate...

 

[5150abf]

Re: Motor height off cause other issues?

Ya that measurement is for a conventional mono hull not a toon, the engine on a toon needs to be a little lower because of all the wash between the tubs underway, if you raise the engine you are like to cavitate.

Every boat is different, what rpm are you turning now and has it always run thet rpm or did it just start doing this?

 

[clemsonteg]

Re: Motor height off cause other issues?

You cannot measure from the bottom of the transom on a pontoon as it could be built different than other pontoons. You need to make sure the the anti cavitation plate stays under the surface of the water. The closer the the surface, the less drag the lower unit will have. But, too close to the surface and you will ventilate...

Maybe my terminology is getting mixed up. What's the difference between the anti cav and anti vent plate or are they the same? And where I read that others want the anti vent plate about 1/2" out of the water I want it slightly submerged on a pontoon so I have to adjust accordingly?

 

[clemsonteg]

Re: Motor height off cause other issues?

Ya that measurement is for a conventional mono hull not a toon, the engine on a toon needs to be a little lower because of all the wash between the tubs underway, if you raise the engine you are like to cavitate.

Every boat is different, what rpm are you turning now and has it always run thet rpm or did it just start doing this?

I just bought the boat but it turns around 3500 rpms with 4 adults. I did all the checks on the motor and it's running well so I'm down to it being over propped and possibly too low in the water. It currently has a 13 3/4 x 15 prop which I was told by the local marine shop is too much pitch for a 24 ft pontoon with smaller toons. They suggested I go with a 13 or maybe even an 11 pitch.

 

[EGlideRider]

Re: Motor height off cause other issues?

Maybe my terminology is getting mixed up. What's the difference between the anti cav and anti vent plate or are they the same? And where I read that others want the anti vent plate about 1/2" out of the water I want it slightly submerged on a pontoon so I have to adjust accordingly?

There is no such thing as an anti-cavitation plate. The plate will prevent ventilation, but will have no effect on cavitation. Few pontoons will ever experience cavitation.

 

[Lawnmedic]

Re: Motor height off cause other issues?

Maybe my terminology is getting mixed up. What's the difference between the anti cav and anti vent plate or are they the same? And where I read that others want the anti vent plate about 1/2" out of the water I want it slightly submerged on a pontoon so I have to adjust accordingly?

The large plate above the prop is what your talking about.... The height measurements your talking about are for V-haul boats, not pontoons. I would listen to the prop experts about being over propped. But I would make sure the engine is good. I would hate to think that somebody sold the boat because the engine was going south.

 

[clemsonteg]

Re: Motor height off cause other issues?

Yeah if you check my thread i posted above you will see the compression is good, I have strong spark, and the color of the plugs was good. I think I had 1 plug that might have been misfiring with higher load combined with just being bogged down from being over propped. I'm hoping to go to the lake today just to test my current changes.

 

[clemsonteg]

Re: Motor height off cause other issues?

Well for a short period of time today she hit 4600 rpm and 21 mph with the 15 pitch and no other changes. Then I stopped to manually adjust the trim since it was venting some and then it went back to being around 3200 rpm. Tried putting the trim back but no dice. So I guess it's a fuel or spark issue even though I can't find one.

 

[katman819]

Re: Motor height off cause other issues?

I'm new to pontoons this year and have also been working through some ventilation/prop issues. I have a 24 Tahoe Vista Elite with a 150 yamaha four stroke. When I bought it the prop was pretty beat up so I replaced it with the same OEM 15 1/4 15m. Consistently ventilated at about 3200. My prop guy said he thought cupping would fix the problem. Put the old prop on and took it to 4000 with no problems. Didn't go any further with the old prop since the blades were pretty bad and I had some vibration. Difference between the props was the old prop was cupped. So I had the new prop cupped. Haven't had a chance to try it out yet. This one will be tested to wide open.

 

[clemsonteg]

Re: Motor height off cause other issues?

Well I got my problem sorted out I believe. Hit 20 mph consistently at 4600 rpm. I would like to go to a 13 pitch instead of the 15 I have now but it concerns me when I turn because it cavitates some. I'm no longer concerned about the motor being too low

 

[Jeep Man]

Re: Motor height off cause other issues?

Well I got my problem sorted out I believe. Hit 20 mph consistently at 4600 rpm. I would like to go to a 13 pitch instead of the 15 I have now but it concerns me when I turn because it cavitates some. I'm no longer concerned about the motor being too low

You now may be a tad too high. BTW, rule of thumb is a change of 200 rpm for each single change in pitch.