pontoon prop cavitation

Guff · #1 June 20th, 2002, 08:10 AM

I have a 20' Voyager pontoon with an Evinrude 88 SPL without power trim or tilt. I have been having some problems with cavitation that I have almost completely cured by lowering the motor two holes on the transom plate and moving the trim pin to the second to last hole (furthest from the transom). When I trim it any lower, the front of the boat drops down once I get up to speed and pulls the motor up, then the prop starts slipping. Should I go ahead and drop the motor to the lowest hole on the transom? I was also thinking about buying a CMC power trim/tilt unit. Anyone have experience with one of these units on a pontoon? Would the 6" set-back of the trim unit help with cavitation? According to the CMC website it would.

Thanks

Down South · #2 June 20th, 2002, 09:49 AM

I'm running a 88 SPL on a 24 foot bardge but I have power trim. I think you need to raise the motor up one more pin hole rather than lowering it to correct your problem. The closer that motor is trimmed in it's going to raise the motor farther out of the water and cause more cavitation.

DJ · #3 June 20th, 2002, 09:07 PM

Guff,

Down South is right. You need to go the other way. The CMC should be just the ticket for a pontoon.

Guff · #4 June 21st, 2002, 05:18 AM

I tried both ways. I moved the whole motor DOWN on the transom, then I trimmed it UP (back) one pin hole, this helped reduced the problem significantly. Trimming the motor closer to the boat (down one pin hole) made the problem worse because the front of the boat would plow through the water and lift the motor further out of the water. Trimming the motor further away from the boat (up one pin hole) helped keep the front from plowing and reduced the ventilation problem.

Now I don't think the CMC unit will work for me because my steering cable would not reach the motor with the 6" set-back of the trim unit. At least not without some serious kinks or modifications. I'm gonna drop the motor all the way down so the top of the transom plate is resting on the transom. Each notch I've dropped the motor on the transom has help the problem and I've got one more to go, so hopefully it fixes the problem all together. I think my next steps will be to find a used evinrude trim unit and verify that I have the correct prop. The prop is painted stainless with no markings except for the numbers 475 161, anybody know what kind of prop this is?

Thanks,

Lance

Down South · #5 June 21st, 2002, 06:19 AM

If you can lower that motor down on the transom it should correct your problem. The prop pitch and size should be stamped on the prop hub on the end. You should be able to read it without removing the prop nut.

The WOT RPM for the 88 SPL is 4500 to 5500.

Guff · #6 June 21st, 2002, 06:45 AM

Thanks Down South, I'll check the prop tonight. Any idea what the "correct" size and pitch would be for my boat/motor combination? It's a 20' Pontoon with U-shaped (23" Wide x 25" Tall) pontoons. I'll probably never pull a skier but will definitely pull one or two tubes.

Thanks again,

Down South · #7 June 21st, 2002, 02:39 PM

Guff,

After you lower the motor on the transom, lake test it to make sure the cavitation problem is cured. Check your tach and see what your rpm is at WOT.

If you are between 4500 & 5500 with a normal load I'd leave well enough alone. If you need to change the prop. I'd shoot for the middle of the road, 5000-5200 rpm @ WOT.

Increasing the pitch, let's say from a 15 pitch to a 17 pitch will will probably bring your rpm down somewhere around 250 to 300 rpm from where it was and vice versa.

Guff · #8 June 25th, 2002, 07:45 AM

DS,

Lowering the motor all the way down completely fixed the problem. The motor runs just a hair under 4500 rpm at WOT with only me and full tank of gas. This concerns me a little but my mechanic says it's fine. The pitch is not marked on my prop anywhere, I pulled it off yesterday and there are no markings anywhere except for the numbers "475 161". My mechanic says it "looks like a 13" so I guess I'll take his word for it. Is it that easy to tell the difference between a couple inches in pitch just by looking?

rickdb1boat · #9 June 26th, 2002, 12:46 PM

Guff, I think you need to find out what pitch prop you are really running, because if your RPM's are below 4500 with no load, you will be lugging it bad when you get 3 or 4 people and all the other stuff that comes with them! I have a 70HP Evinrude on my 20 footer and I use a 14"X13" Prop, which puts me at about 5500 RPM's with just me aboard, in calm water! I tried using a 13/3/4 X 15 prop, but it brought my RPM's down too far when loaded up. My sis-in-law has a 24 footer with the 88HP and she also runs a 14"X13" Prop. Seems to be the most recommended pitch for toons in this range, or so I was told by the parts guy when I got mine. You can always pull the prop and take it into a prop shop and they will tell you what the size is, just so you know what you're running with! Not to say that the 14X13 is the one for you, but I would try to get up on the RPM's a little. Just a thought! Rick