Cannot trim motor up - Porpoising

atengnr

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I cannot trim the motor up any, as when I do, the boat immediately begins to porpoise. Boat is definitely a bit stern heavy with this motor. The boat runs about 32 mph currently....

Is there anything I can do to improve this??
 

Dhadley

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Re: Cannot trim motor up - Porpoising

What boat / motor / prop / rpm etc? You mention the stern is heavy, can you rebalance the boat?
 

atengnr

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Re: Cannot trim motor up - Porpoising

Dhadley said:
What boat / motor / prop / rpm etc? You mention the stern is heavy, can you rebalance the boat?

Boat is a 1968 saftmate 15 ft, with 1988 Johnson 60 HP, 133/4x15 prop.

The gas tank and battery sit in the back of the boat, but perhaps I could put the gas tank up front, but this is only 50 lbs.

Can anyone see this picture??
 

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Texasmark

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Re: Cannot trim motor up - Porpoising

What are you griping about. :% I just went to the prop calc and punched in some numbers and 32 with that engine on a 15' boat and 15p prop is right where it's supposed to be at WOT.

Problem with your rig is that if you trim out to where you start porposing, the only way to really get away from it is to run away from it.......that takes more hp than you have otherwise you wouldn't be having the problem and wouldn't be posting on here. d:)

Course, depending on what you do with your boat, (if lightly loaded and no skiing) you might opt for a couple inches more pitch which would move your porposing out a couple of mph.

Relocation of a gas tank and battery aren't the answer; like you said, not that much weight.

Back in the '70's several friends had 60's on their boats and if propped for skiing (15P) they would run out about where you are.........and 32 is a nice little clip. Had a nother couple of friends with 65's and they ran around 35-36. I remember these numbers cause I had a 125 on an 18' and I could walk (not run) away from them at 39.

HTH,

Mark
 

Dhadley

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Re: Cannot trim motor up - Porpoising

OK, there's a couple of things to look at. First, are you sure the diameter is 13 3/4 rather than 13 1/4?

Now, if you move 50 lbs from the stern to the bow, in an equal distance from the fulcrum point, you are actually making a 100 lb shift in balance. IE - making the stern 50 lbs lighter and making the bow 50 lbs heavier. If you move the 50 lbs further forward of the balance point than it was behind, you'll actually make a shift of more than 100 lbs. Not as far forward, less than 100. If you move 50 lbs from the stern to exactly the fulcrum, you've lightened the stern by 50 lbs but added nothing to the bow weight. Obviously, none of these moves effect the total weight.

Is the motor mounted all the way down on the transom? We assume it's bolted to the transom with no jackplate or set back, right?

Lets look at this stuff and then maybe look into some Smart Tabs.
 

atengnr

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Re: Cannot trim motor up - Porpoising

Prop diameter is 133/4, runs about 5800 rpm. It is the prop I used on my 1968 55hp previously (the 60hp spins it much faster).
 

Dhadley

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Re: Cannot trim motor up - Porpoising

How did the boat handle with the 55?

This is a 3 blade prop, right?
 

atengnr

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Re: Cannot trim motor up - Porpoising

Dhadley said:
How did the boat handle with the 55?

This is a 3 blade prop, right?

55 did not have power trim. It went 28 mph. acceleration was significantly less than this 60hp.

The 60 hp is about 40 lbs heavier I believe. The boat squats more in the stern looking at it floating at a dock vs. with the 55.

Yes, 3 blade prop.
 

Dhadley

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Re: Cannot trim motor up - Porpoising

But did it porpoise? Even though it didn't have trim you probably moved the pin out a hole or two to see what happened, right?
 

Dhadley

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Re: Cannot trim motor up - Porpoising

Is the pin all the way in on the motor you have now?
 

atengnr

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Re: Cannot trim motor up - Porpoising

Dhadley said:
But did it porpoise? Even though it didn't have trim you probably moved the pin out a hole or two to see what happened, right?

no, it did not porpoise the few times I tried other pin positions.

Pin is set closest to transom with this motor.
 

Dhadley

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Re: Cannot trim motor up - Porpoising

Interesting. Just for kicks, do you know anybody with a 13 spline (3 cylinder & V4) OMC prop you could borrow to try? Even if it's slightly the wrong pitch it won't matter for what we want. What's your spare like?
 

Texasmark

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Re: Cannot trim motor up - Porpoising

Hey D.

Were the engine's mounting brackets on the 55 Triump the same as the later model 60? Or did they make the later versions deeper to cuddle PTT up inside.

If so, this may be his problem with porposing on this engine and not on the old one.....the "effective" transom angle is greater on the later.......but he said he tried several positions with the 55 and no porposing.....but he has picked up 4 mph.........should have reduced the tendency......confusing for me. :%

Unless the 40 lbs he picked up in engine weight is that much of an issue. If so, then movement of other things in the boat may help.

Mark
 

Dhadley

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Re: Cannot trim motor up - Porpoising

The stern brackets are totally different but should be at, or very near, the same angle.

Atengnr, are the two motors mounted at the same height? IE - is the anti-ventalation plate in the same place in relation to the bottom? If so, where is the plate?
 

atengnr

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482
Re: Cannot trim motor up - Porpoising

Dhadley said:
Interesting. Just for kicks, do you know anybody with a 13 spline (3 cylinder & V4) OMC prop you could borrow to try? Even if it's slightly the wrong pitch it won't matter for what we want. What's your spare like?

I can get a prop from a local boat store without problems....What prop should I try??
 

atengnr

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Re: Cannot trim motor up - Porpoising

Dhadley said:
The stern brackets are totally different but should be at, or very near, the same angle.

Atengnr, are the two motors mounted at the same height? IE - is the anti-ventalation plate in the same place in relation to the bottom? If so, where is the plate?

The two motors were mounted in the same holes on the motor bracket. I will need to check where the plate is relative to the bottom of the hull. I will post ASAP. I will send you a pic as well if that helps.
 

atengnr

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Re: Cannot trim motor up - Porpoising

Dhadley said:

cavitation plate is aprox. 1.5 inch below hull. I will send a pic as soon as Im able...
 
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