motor cavitation

[gyeager]

I posted a thread a couple of weeks ago about my pontoon cavitating . I'm wondering if it's because it is a short shaft motor. Is there any way to lower the motor? It seems that if I do there would be a lot of drag on the motor(motor to far in the water).This is my first pontoon and i don't know much about the.I dont know if a new prop would do the trick or not. The problem is I don't know the RPM or the pitch on the prop i now have.I need to get bow of the boat up but cant trim the motor with out cavitation. Also this is my first time blogging so please excuse my ignorance. thanks please HELP.... Gyeager

 

[jimmbo]

Re: motor cavitation

Can you post a picture(s) showing how the motor sits on the boat when the boat on the trailer, back and side veiws, preferably taken at gearcase height. By the way, what you are experienceing is Ventilationn not Cavitiation

 

[Bondo]

Re: motor cavitation

I posted a thread a couple of weeks ago about my pontoon cavitating . I'm wondering if it's because it is a short shaft motor. Is there any way to lower the motor? It seems that if I do there would be a lot of drag on the motor(motor to far in the water).This is my first pontoon and i don't know much about the.I dont know if a new prop would do the trick or not. The problem is I don't know the RPM or the pitch on the prop i now have.I need to get bow of the boat up but cant trim the motor with out cavitation. Also this is my first time blogging so please excuse my ignorance. thanks please HELP.... Gyeager

Ayuh,.... This 1,...

You still haven't told Us what the motor is....

 

[gyeager]

Re: motor cavitation

70 horse johnson sea horse 1977

 

[gyeager]

Re: motor cavitation

bare with me guys remember i'm new at this blogging stuff (computer stupid)

 

[steelespike]

Re: motor cavitation

Unfortunately the best solution is a long shaft motor.You could try a 4 blade prop intended for pontoon operation.
It will have a larger diameter than a typical 4 blade.
You might be able to lower it some with a jack plate (opposite of its intended use) and it may be difficult to set up steering. But might be better to spend the money toward a long shaft motor.

 

[jimmbo]

Re: motor cavitation

Hmm. You say it is a short shaft motor. In 1977 the 70hp was only produced as a 20 inch model, which was a long shaft.
Unlike a regular boat hull, its kind of difficult on a pontoon boat, to determine where the waterline is while on plane. Have you looked over the transom to see how much of the gearcase is under water while on plane?
You said you need to raise the bow but can't trim up. Is the boat loaded in such a way that it is bow heavy?
The boat is on the trailer, you have easy access to the prop to find out the pitch. I can tell from the pic it's not an OMC prop. Mercury Marine has a large selection of props, some I would guess are designed for your app.

 

[Texasmark]

Re: motor cavitation

I'm not a tooner but I will share my thoughts on this.

His engine appears to be plenty low in the water. With the AV plate just above the bottom of the tubes, once underway, it will surely have enough water to keep from ventilating if vertical because the bottom of the tubes surely will be several inches below the surface.

If it is trimmed out excessively then surely it will ventilate. With what I read, I think Mr. Yeager did just that in trying to get his bow up. Possibly he was running in rough water, taking water over the deck, and thought he could raise the bow and prevent it.......not going to happen. Toons aren't designed for rough water.

Toons aren't high speed planing hulls. They are long hulls with very little surface area for hull position, i.e. trimming out to raise the bow.....there's nothing back there for the water to push against to raise the bow and it is too far away anyway. The whales tale is a total waste of time in my opinion.....too little, too late.

If your toon sits level or slightly stern down while at rest with no gear (coolers and heavy stuff) and passengers, then I'd say you are just expecting it to do something it isn't designed to do.

Think about it.

Mark

 

[steelespike]

Re: motor cavitation

So true; trim on a toon is most effective for finding a position with the least venting(cavitation) issues.
weight placement is pretty critical adding a little weight at the stern may reduce venting.
I have seen a few toons that raising the motor helped to lose the venting.There is so much turbulence
that experimentation is about the only way to try to solve it.I don't think the foil is a total loss it does increase the size of the anti vent plate perhaps helping the venting.There are those that believe it is the sole purpose of a foil.

 

[Texasmark]

Re: motor cavitation

So true; I don't think the foil is a total loss it does increase the size of the anti vent plate perhaps helping the venting.There are those that believe it is the sole purpose of a foil.

Good point and may be why it's on his boat.

Mark

 

[gyeager]

Re: motor cavitation

Thanks for all the help. I just want to a little,the whale tail was on the motor already. This motor came off of my 15 1/2 foot tri hull and it would ventilate also when in a hard turn. My toon is an 18 foot ,with 4 adults and average gear you can hear the RPM raise do to the ventilating. Also the prop that you see in the pic has no visible markings. I was told to get a 13x15.6 4 blade. Or a 4 blade 11" pitch Solas prop.I hope this is of some help i just do not have all the info for you guys. Thanks so much for your help any other response would be great......Happy Boating All

 

[QC]

Re: motor cavitation

I was told to get a 13x15.6 4 blade. Or a 4 blade 11" pitch Solas prop.

Pretty far apart there . . .

Can you do any weight adjustments? Is there anything heavy in the bow that you can move aft? Heck have some people stand in the stern with nothing in the bow and see if it makes a positive difference. That alone would give you some good info. That's where I would start. Remember to do one thing at a time. It's the only way you'll know what helped

 

[steelespike]

Re: motor cavitation

I'm thinking a 13 or an 11. I think on a 18 the a Solas 4 blade13 is a good start. If there is a choice of diameter take the larger one.
after you try it out you might consider removing the foil of course depending on your results.
When making changes always one at a time and try.

 

[jimmbo]

Re: motor cavitation

Just out of curosity, have you checked for drain plugs on the pontoons? If any, are they empty? Most pontoons are not one big tube, but compartmented.

 

[gyeager]

Re: motor cavitation

i think there is a plug on the one side . Should i check and what am i looking for?

 

[gyeager]

Re: motor cavitation

i only have two fishing seats in the front and it better with all passengers in the back .

 

[jimmbo]

Re: motor cavitation

i think there is a plug on the one side . Should i check and what am i looking for?

Water

 

[steelespike]

Re: motor cavitation

Looking at the stains it looks like its floating at a good level.If its on a trailer in the morning as the air warms feel the toons
if there is water where the water is will feel cooler and your hand won't warm it as much.
You also may be able to shake it side to side or front to back and listen for sloshing. I don't think it would produce much noise though.

 

[gyeager]

Re: motor cavitation

I guess i'll check for water and then maybe start with a prop change. Wont get to do much this month , so i'll get back after i do a few things with it thanks for all the help. happy boating be safe george

 

[gyeager]

Re: motor cavitation

no water in the toon that i can tell. Should i take out the plugs or leave them alone?