Cavitation, Ventillation or Spun Hub?

[Woodnaut]

Helping out a buddy with his pontoon boat. Here is the the technical stuff I know about:

Boat: 21' Starcraft Pontoon Boat ~1987
Engine: 1981 Johnson 90 HP (2 stroke) outboard
Prop: 3 Blade Alum
Diameter unknown, Pitch 17" (I think), no cup
Tachometer presently inoperative (on the list of stuff to fix)

Symptoms: The boat previously had a 70 Hp 1987 Johnson with 3 blade SST prop (Dia and Pitch unknown). The boat and engine ran great on smooth water at all speeds including WOT. But with heaver loads, and particularly in choppy waters, the engine would rev up indicating that either cavitation or ventillation was occurring. The 70 HP suffered powerhead damage and was recently replaced with a 1981 90 HP Johnson with a 3 blade aluminum prop.

This 90 HP setup is exhibiting the same symptoms except the cavitation/ventillation is occurring even on smooth water when the engine is run at higher RPM.

It seems to me that on a conventional boat, the water coming from under the boat hits the outboard's lower unit at approximately the same place regardless of sea conditions. On a pontoon boat, however, it seems that in choppy seas the "water level" on the lower unit would vary with each passing wave. If this is the case then I could see where ventillation might occur.

I'm also guessing that if the prop pitch is too low, or if it needs some cup added to it, then perhaps cavitation is occurring. Maybe even a spun hub would exhibit some of these symptoms...

Sorry I don't have more particulars on the prop and WOT RPM. Any thoughs and suggestions are welcomed.

 

[Silvertip]

Re: Cavitation, Ventillation or Spun Hub?

Since the problem happened with both motors it would appear to be a ventilation problem but since the 70 had the issue when loaded, that would indicate a load distribution problem. Too much weight up front raises the back of the boat. Most pontoons benefit from the use of a "pontoon-specific" prop. These usually are bit larger in diameter and have totally different blade shapes. Many are four-blade which provides a little more bite due to the large blade area. You might check to make sure the engine is mounted as low as it will go. A spun hub can certainly cause a flare in engine rpm but once it starts it generally doesn't go away. You can check that by making a mark on the prop hub and in line with that mark, place another mark on the prop barrel. Then run the boat until the rpm jumps up. Stop and check the marks. If they are still lined up, you do not have a spun prop. If the blade area and hub slipped, the marks will no longer be in line. I doubt that's the issue in this case.

 

[metriccrescentwrench]

Re: Cavitation, Ventillation or Spun Hub?

I suspect that the prop isn't a very good choice for that boat. I've got a 15 ft fish-n-ski with a '81 Johnson 90, it's got what I believe is the original prop, a 3 blade aluminum 13-1/4 x 17, kinda sounds like the same (or very similar) prop as your friends pontoon. In my case the prop's a pretty good match for my boat which is only about 1300 lbs fully loaded (5600 rpm WOT, 42 mph on gps). For the pontoon I'd bet that prop's got too much pitch rather than too little. And of course Silvertip is right about checking to see if the engine can be mounted lower.