Prop Slipping? Cavitation?

[phipps]

We went camping this weekend at the lake and thus were able to get our pontoon out and really run it around.

We have a 24' pontoon with a 1995 Johnson 28HP motor. I realize that we are underpowered, but that's what came with the boat.

My question is this, when I got several people on the boat and opened it to WOT it would feel like the prop was "slipping" and then grab again. The motor would rev up when it happened so I know it wasn't dieing. If I moved the weight to the front of the boat, the boat would plane up and not "slip".

What is this "slipping"? Is that cavitation?

I'm guessing it is because I am underpowered for the size and load on the boat.

Would changing the trim on the motor help any? I have a manual trim, no fancy electrical trim.

 

[HighTrim]

Re: Prop Slipping? Cavitation?

I personally would probably start by checking if the prop hub is spun, then look into other possibilities.

Draw a line across the hub and prop, run her until you hear it 'rev', then pull it out and check the line. If it is no longer lined up, she is spun and will need to be rebuilt, or you will need a new prop if it is beyond repair.

 

[phipps]

Re: Prop Slipping? Cavitation?

Thanks for the suggestion, but what is a spun prop? I am a boating newbie.

Can cavitation be felt? I read something about prop slip when adjusting trim, maybe?

Are my thoughts totally off the wall?

 

[Ross J]

Re: Prop Slipping? Cavitation?

Cavitation and prop spin can feel much the same. The only real test is the good old one mentioned before of marking the prop and testing it.
Sometimes cavitation can be overcome with correct adjustment of the depth the prop is below water, or by pushing the trim down all the way.
For my 02c worth the spun test of marking is the best. If it shows the prop is spun then get it repaired or conversely if it shows as being Ok then look at cavitation.

Cavitation is caused by numerous things from incorrect prop size/pitch, incorrect setting of depth, hull shape, following very close behind another boat, incorrect positioning of the outboard, boat set-up wrong.
Ross

 

[phipps]

Re: Prop Slipping? Cavitation?

Could someone explain to me what a spun prop is? I thought the prop mounted onto a shaft with splines on it. Is there something between the shaft and the actual propeller?

 

[HighTrim]

Re: Prop Slipping? Cavitation?

There are 2 common occurances with regard to outboard props. Both cavitation and ventilation. They are both kinda hard to diagnose.

Ventilation occurs when surface air or your outboards exhaust gases are drawn into the propeller blades. The load on the propeller is reduced by the mixing of air or exhaust into the water steam causing over revving.

Cavitation is generally the aeration, or bubbling and boiling effect of water caused by creation of a low pressure area around the blades of your prop. Usually caused by a prop with a rough edge on it. The rough edge will not cut smoothly through the water, creating a low pressure area like I mentioned. If the pressure drops below the vapor pressure, a cavitation bubble will form in that region. These bubbles will collapse when they reach the higher pressure region of the blade. This causes a rapid change in pressure and can result in physical erosion. You may notice burns (erosion) at some area on the face of the blade. A propeller exposed to cavitation resembles the surface of the moon, with tiny, scattered craters.

Generally, Ventilation can lead to excessive slippage, and Cavitation can lead to ventilation and/or slipping.

But start by checking the prop hub to start, which is just a likely scenario and go from there. Most propellers have a splined bushing in the hub that attaches the propeller to the propeller shaft. The bushing is mounted to the propeller with flexible rubber. This rubber acts like a shock absorber. If the propeller strikes something hard, the rubber flexes and helps protect the gearcase components from damage. If this rubber becomes worn, it is said to be 'spun', creating the symptoms you describe.

 

[phipps]

Re: Prop Slipping? Cavitation?

Thanks for the detailed explanations of cavitation and ventilation!

Ill get the whole crew up to the lake and try the spun prop test, unfortunately it doesnt do it with just me on the boat. Doesnt do it unless there are 3 or 4 of us on the boat and then it doesnt do it if everyone but me is near the front.

One more question, is there a FAQ on here somewhere that explains how to set up a motor on a boat, ie propellor depth, etc?

 

[HighTrim]

Re: Prop Slipping? Cavitation?

unfortunately it doesnt do it with just me on the boat. Doesnt do it unless there are 3 or 4 of us on the boat and then it doesnt do it if everyone but me is near the front.

One more question, is there a FAQ on here somewhere that explains how to set up a motor on a boat, ie propellor depth, etc?

You may simply have too much weight at the front of the boat, and if the motor is trimmed out too far at the same time, is over revving. Try to avoid this, by distributing the weight to keep the boat level, or by not over trimming.

Regarding the setup, simply post a pic of the stern at eye level. The guys on here will be able to guide you from there.

 

[phipps]

Re: Prop Slipping? Cavitation?

The motor does not do it when I distribute weight towards the front so I may just have a weight distribution issue. There is no furniture so Im rear heavy.

As far as a pic, cant do right now. I dont have a trailer to get it out of the water.

Ill still plan on checking for a spun prop, there is a nick in one of the blades so it has hit something at some point before I had it.

I appreciate all the suggestions and explanations. Ill be a boating expert in no time thanks to the nice people here!!!