Prop gurus help!!pontoon cavitates.

[ckmate225]

Ive got a 24 ft pontoon w/ a 40 hp merc classic 2 stroke.
When you first head out it does everything a 24footer should do with a 40 hp, 21mph no cavitation/airration /prop slip or whatever. After crusing for a while at idle speed then you try to give it throttle it will cavitate like crazy the motor will tack like there is no prop on it. I will back off the trottle and have to cruse home slowly. it happens quicker with a heavy load, but it happens everytime we go out for a longer ride?
I 've tried at least 5 different props. but its really aggrivating.
Anyone have any ideas?? I notice on some of the newer boats they have the engine set way back, is that because of this issue??
I hope not! cause if anythings getting any setback its not gonna be a pontoon with a 40 on it!!
Thanks!

 

[Chris]

Is it possible the hub in the prop is slipping and it's not actually cavitating?

Or perhaps the clutch dog is jumping out and the gears are slipping?

It sounds like something is up, since it sounds like it only does it after it's been running awhile.

-Chris

 

[ckmate225]

Thats why I cant wrap my feeble brain around whats happening its only when its been running for a while. I dont think it would be a dog cause its already in gear I guess but it could be popping. Ive tried so many differnt props so I don't think its the hub,But I am no expert, so I'm hoping someone can give ma a real good starting point, its been happening since the motor was new in 2001. Ive just been dealing with it, the marina I bought it from went out of bussiness literally 3 days after I bought it so I didn't have much recourse with them.
Maybe I should get a steeper pitch prop, I am running about a 10 now should I go 13-15??

 

[Chris]

Well you can check to see if the hub is slipping pretty easily.

Just mark across the hub with lines and see if it moves.

How deep is the motor in the water?

Does it have power-trim or is it manual?

-Chris

 

[Big Red]

I agree with Chris thats not a prop problem brother. If it was the prop it would do it 24/7 not just once in awhile ,, What ever prop you stuck on it to get 21 mph is the best prop. 21 mph on a 24 foot pontoon with a forty horse is haulin the mail.

 

[ckmate225]

Well you can check to see if the hub is slipping pretty easily.

Just mark across the hub with lines and see if it moves.

How deep is the motor in the water?

Does it have power-trim or is it manual?

-Chris

It's got power trim, I have the motor burried as deep as she will run+negitive trim. I ve tried just about every position from negitive-nutral - positive.
What I am hoping for is somone that knows magic to wave a wand over it and make it perform perfectly. That would take certainly the guess work out of it.
I think I hate intermittent problems,
Is there anything that could be effected by heat that could cause that to slip? I am going to pull the lower this spring and replace the water pump maybe I will see something. With my luck the splines on the shaft will be shot or the females on the crank and when she warms they expand at dirrerent rates and slip like bizach.
But I have a few weeks til I can get to it so any other suggestions are more than welcome.

 

[hotdaddy27]

I am no pontoon expert, but if you tried 5 different props and she still does it I would count out the hub.

When it warms up and does it, has the load in the boat changed or been shifted forward? If it has not, I think you have bigger problems.

 

[john lamon]

This might be a long shot, just a thought but because it works good when you first head out, do you boat on a very weedy lake? I know if I cruise at a idle speed for a while with my pontoon it will act like that on take off because of a build up of weeds around the lower unit, throw it in reverse for a second and it's good to go. might not be the problem and you probably checked that but I thought I would suggest it just in case.

 

[ckmate225]

This might be a long shot, just a thought but because it works good when you first head out, do you boat on a very weedy lake? I know if I cruise at a idle speed for a while with my pontoon it will act like that on take off because of a build up of weeds around the lower unit, throw it in reverse for a second and it's good to go. might not be the problem and you probably checked that but I thought I would suggest it just in case.

Hmmm... back when I was younger the lake was practically weed free. But in the past 10 years or so we have had increasing weeds to the point where they have been putting togeather an abatement plan. There will be big pods of weeds floating around I try to avoid them but I am sure I can't miss some of the smaller ones. On this boat the motor is kind of burried under the back and you have to crawl out over the side to see the prop. So I Havent been able to check the prop till we got back to the dock. (its almost always run at night,we run the mate during the day), of course I would have hit reverse to dock it and that may have cleared whatever was on the prop.
Maybe John knows magic!!?? and It may be wishful thinking on my part but, I will have to check it out. I can only hope its a weed thing. I would certainly feel like a jacka-- if thats was all it was but I'd rather feel a like a complete idiot than have to replace a lower.
Thanks all, and if anyone has any more ideas keep'm comming

 

[Chris]

Weeds can definitely do that. Once you get out of the weedy area, run it in reverse for a bit - that will get them off.

The only other things that came to mind....

Does the attitude of the boat change after you run for a bit? As an example, is it taking on some water and that changes the depth at which you're running? Is it possible one of the pontoons has a leak and is filling up with water? Do you maybe burn up fuel and it's a bit lighter and then higher up? Do you change the trim angle after running?

Could there be some connection with the engine warming up and there being more turbulance brought on by something to do with the through prop exhaust gases? Maybe that and the combination of a vented prop?

Is the throttle and shift linkage adjusted properly? Is there a chance the shift linkage is not adjusted properly so that maybe after it heats up it's jumping partially out of gear.

Personally, as I eluded to earlier, I'd be looking in your gearcase as a first step. I had a motor that would do something similar to what you were describing and it was a gearcase issue. When I opened it up I found the bearing carrier assembly was cracked and that led to other issues. So basically that was causing it to jump in and out of gear.

I realize some of those are kinda off the wall ideas, but I thought I'd throw them out there and see if they help.

-Chris

 

[ckmate225]

I am pulling the lower to replace the impeller in the next week or two. I will check everything out then. There is a posability that the cable adjustments are off, I did not install this engine then marina I bought it from did the work so there is a chance they fouled something up. A friend of mine is a Merc master tech, he's just gone on his own and opened up a shop maybe I should be one of his first customers. Only worry I have would be towing the boat there. I have one of those scissors trailers and there is a curve on one section of road that is kinda funky, its pitched the wrong way and a 2 pontoons have ended up laying over in the middle of the road.SCARY! Not a real good way to start the boating season! But at any rate I will post what I find.

 

[ezstriper]

when it does it next time, stop, reverse pretty hard for a few seconds, then see what happens...if weeds are the issue should clear them..Rob

 

[john lamon]

Did you ever figure anything out with this?

 

[ckmate225]

Hey John,
I think you were right!!, I just whack it in reverse for a couple of seconds and theres no issue. Runs like a top.
Just goes to show you, even after boating for 30 yrs sometimes the simple stuff gets by you when you try to over think it. I gotta remember to check the eazy stuff first.
Thats why I love this forum.
Thanks again!

 

[john lamon]

Awsome, that was an easy fix, the only reason I knew about that is because I was racking my brain for a while trying to figure out why our pontoon would also do that, and over thinking it instead of thinking of the basics.