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Revs go up, prop slows after a long run (not hub issue)

U

uptown dog

New member
Hi Folks,

A newbie here that bought a 24' Smokercraft pontoon with a Mariner, 60 hp 2-stroke Big Foot about 15 years old. Has oil injection. Misplaced the manual, will get the serial # soon. Got out of flying as it was too expensive and bought the pontoon instead. Hoping that was a good decision. Yikes.

Here's the problem. Brought the boat down the lake about 45 minutes just before Christmas, running at 4200 rpm per suggestion by previous owner. Ran perfectly and docked her until this month. Went out the Sunday before last and basically tooled around sight-seeing at around 2500-3000 rpm for about an hour. Decided to take a 20 minute run at 4200. Ran fine. Slowed to a fast idle to sight-see again for 15 minutes. Decided to return to marina at 4200 rpm. Ran fine for a very short time and then it felt like the prop was cavitating, prop was slipping or like putting in the clutch on a running car. RPM went up and prop slowed. I felt a very light vibration from the floor.

Went to around 3,000 rpm and ran OK for awhile, increased the throttle and same thing, revs go up, boat slows down. I finally had to take her down to about 2000 rpm to get her back to the marina with no prop slipping. I revved her up a couple times on the way back, but same thing would happen, revs go up fine, prop not biting, not so fine.

Mechanic at marina was sure it needed a new rubber hub, but when he took it apart, he said it was perfect with no signs of spinning. He's still scratching his head as I write this. He took it out for a 15 minute WOT run and said she ran perfectly. I'm guessing it may be heat related after a longer run. Prior owner did not maintain the craft mechanically. It looks nice though. The only instrument working is the tach. Even the horn won't sound. I plan on having all the instruments replaced/repaired, but for now, they're all defunct.

I drained the lower housing oil. It was as black as black could be. Replaced it with my handy new pump and the proper outboard oil. (I only dropped 1 of the 3 little plugs in the drink). Haven't run it for 2 weeks. Too cold here on Lake Wylie, SC this weekend. Could the worn out, dirty and possibly not enough oil cause prop/gear slippage? No discernible clunking when it happens, just a slight buzz under my feet from the floor....er, I mean deck. Have only put in alky-free gas on 2 fill-ups, one before I brought her home and the past Sunday when this happened. I searched and searched, but couldn't come up with anything. Hopefully someone here can help. Thanks.

P.S. Mechanic said he'd readjust the shift/throttle cables. Don't know if that would help. He thinks that maybe it's not engaging fully into gear. Wouldn't I feel a clunk when it popped out, and could it just get back into gear by itself with only manipulating the throttle?

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Either the motor is mounted too high or the prop is slipping / cavitating.-------Did you or the mechanic mark the hub and the outer portion of the prop to see if the marks stay in the same place ?---Guessing just does not work in this case !---------The clutch dog and forward gear usually lock quite well and rarely do they jump out of gear or slip inside the lower unit.
 
racerone said:
Either the motor is mounted too high or the prop is slipping / cavitating.-------Did you or the mechanic mark the hub and the outer portion of the prop to see if the marks stay in the same place ?---Guessing just does not work in this case !---------The clutch dog and forward gear usually lock quite well and rarely do they jump out of gear or slip inside the lower unit.
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I have to plead some newbie-itis here. I haven't talked to the mechanic since last week. He must have taken the day off today. I looked at the motor on the transom yesterday, and it looks all the way down, plus a Big Foot goes a bit lower. I don't know yet if he marked the hub and checked it. I will certainly ask and make he sure he did or does check it next time I talk to him.

With my limited experience, I would discount the cavitating. I've run it at 4200 and slightly more for at least 2 hours without it doing it. Why would she suddenly cavitate and work itself down to where I had to crawl home at 2,000 rpm? Why didn't it cavitate when he took it for a WOT run? I'm just thinking out loud here. It sure feels like the prop is slipping. There's no clunking or grinding at all as far as I can tell, just that slight floor "buzz" when it happens. I'll make sure the hub is marked and checked for slippage if he hasn't done it yet. Could he have been able to possibly tell that the hub was O.K. just by looking at it?

I just remembered, he said he has a spare prop that he could try out. The hard part is that it only did it after a few hours of cruising the lake, mostly at slow speed and then when I powered up to only 4200. I hate to go cruising on the non-busy lake and have it go out completely. Oh well, at least it doesn't fall out of the sky if she quits. Thanks for the response.
 
...". RPM went up and prop slowed."

Not possible unless the hub was slipping, which it was not. Sounds like simple cavitation to me.

...." I felt a very light vibration from the floor."

And that confirms it.

Is themotor set a bit too high?

Jeff
 
Jeff,

Confirms what, slipping or cavitation? Motor is low with a Big Foot lower end. If you mean cavitation, why only once in several hours of running at the speed at which it happened (4200)? Wouldn't it do it all the time? Went to 20 minute destination at same power setting, slowed for a short while and did it on the way back under the same conditions and rpm. I was in a straight direction when it happened, not turning or anything and I couldn't get the rpm/prop back up. As mentioned earlier, I had to nurse her back to the marina at 2,000 rpm. Anything faster and she'd do it again (rpm increase, prop less bite).

Mechanic did a disappearing act. No contact after 4 calls over several days. Has very bad feedback on one site I found today. Need to go elsewhere I think and start over. Since I'm new to driving this barge, thought I'd pop for a new 4-blade prop for better slow-speed control. When I go out, kids bring lawn chairs to the marina to watch me trying to get her into the slip when I get back. Real entertainment and laughs.
 
guyjg said:
try another prop
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Working on it. Looking to order a 4-blade Michigan Wheel Vortex 4-blade. Need to call them for the right pitch/size. Only comes in 11.5 x 8, 9 or 10. Sound right or will they recommend something else? I'll find out when I call, I guess.
 
uptown dog said:
Working on it. Looking to order a 4-blade Michigan Wheel Vortex 4-blade. Need to call them for the right pitch/size. Only comes in 11.5 x 8, 9 or 10. Sound right or will they recommend something else? I'll find out when I call, I guess.
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It's an underpowered pontoon boat. Get the stock prop and save your money.
 
Stock is around 85 and the 4-blade was about 20 bucks more. I just thought the 4 blade would make it easier to maneuver into the slip...which I totally suck at. I have to make a hard right to come down this channel with slips on both sides and then make a right turn into mine. Sloppy to say the least until I get a lot more practice.
 
Check out my recent thread headed "weak spring - prop slip". If your motor is not so old and the propshaft spring cannot be weak already, then your spring may have broken.
 
Peter Devil Ears said:
Check out my recent thread headed "weak spring - prop slip". If your motor is not so old and the propshaft spring cannot be weak already, then your spring may have broken.
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O.K. That's some new info for me. 2 local mechanics were going to take a look and do the necessary repair. After 1 or 2 conversations, each did a Houdini...no call backs, no messages after numerous calls by me. How do people like this stay in business?

Called "Wholesale Marine" who lists hundreds of props on their site. On hold for over 10 minutes, give the rep all my info to get "Sorry, we don't have anything that will fit your motor." What!!??


I did a search for Mercury dealers and would you believe there's one about 4 miles from my house? Told him my tale of woe, described boat and motor, and he came up with a 14 x 11, same as someone else did. Will go to the store Friday when he'll have it in. He said to bring the old prop in for him to take a look at it, which I'll do.

Motor is a '97. I suppose it will take a real mechanic to check the prop spring? I'll find your thread and check it out. Thanks for the input.

Later: Just read that yours jumps out of gear. Wouldn't that be a noticeable by a "clunk." I don't have to put her in neutral and then back in gear when mine happens. Throttle just stays in the forward position without having to come back to neutral. Just have to slow down and not rev her up. As mentioned, I had to power back to 2000 rpm to keep her slowly moving in order to limp back to the marina. Anything faster at that point would cause the apparent prop slippage.
 
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That's how mine started behaving when things went wrong. Also explains why it starts only when warmed up. Also explains vibration you feel. However, make sure the cable adjustment is OK and really putting it all the way in gear since I don't expect your much newer model to have the same failure of a spring that took mine 34 years to fail.
 
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I get to expose my boating ignorance again. I had a total misconception of the design of a boat prop. I now realize that there is really nothing to go wrong with a prop unless it hits something or it's just the wrong prop for a particular boat. It's the stuff inside called the hub assembly that can get messed up.

I won't bore anyone with another story of a mechanic that let me down by saying he was going to do something and after 4 follow-up calls, he just totally ignored me and the things that he said he was going to do.....find a prop, have it delivered and install it.

So, I took the old prop off and took it to West Marine near my house. The guy I talked to on the phone left for the day and another guy followed up. He brought the new prop to the counter and took a look at mine and basically said that I didn't need a new prop, the old one was fine. He took out the hub and examined it. There was some very slight wear at the ends of the splines and it was packed with old, hardened grease, but he thought that they looked good enough not be slipping. Aargh. Didn't want to hear that. He basically killed the sale of a new prop by being honest with me.

He mentioned that replacing the hub was the least expensive way to start, see if it helps, and if not, start working upwards. So I bought the new hub assembly and put it in this evening. Was too late to take her out, and the problem doesn't show up until an hour or 2 of running anyway. The old assembly looked a mess to me, but it could be normal looking. The old grease on the motor's spline and some parts of the hub assembly were caked on and almost rock hard. I had to get it off with a wire brush. Wherever there was a depression, it was packed with hardened grease. I cleaned it off, put on some new marine grease and reassembled the prop back on the motor. As black as the oil was when I drained it and the fact that the previous owner let all the failing instruments (except for the tach) slide, makes me think that the grease was most likely pretty old and hadn't been replaced for quite awhile. The guy didn't know the meaning of maintenance except to keep the boat pretty looking. I found every kind of carpet, vinyl and plastic cleaner under the seats along with several polishes and stain removers...and of course the ever popular Armor-All. Most were empty.

So, I'm still baffled about why my prop seems to slip with no clunking, popping out of gear, cavitating (I couldn't push past 2000 rpm getting back to the marina. Slightly higher and it would "slip.") I guess it's wait and see if she still does it after a few hours on the water. Thanks for all the suggestions and help. Much appreciated.
 
Going to take her for a run Saturday a.m. Will take some close up photos and post them. However, the prop is already mounted. That O.K.? Kinda late for any others. This prop seemed fine at 4200. I don't think I ever ran her wide open. Habits from my flying days. I always flew as slow and legally low as I could. Switched into rotor-craft as they can legally fly 500 ft. lower than fixed wings. Flying and now boating are my ways to unwind, sight-see and generally relax. 18 m.p.h. max on the water and close to shore is just fine with me. 80 m.p.h. in the air was my favorite speed.
 
I think the pitch is 11 as those are the last 2 numbers in the prop number. I'll post the whole number after I write it down. I'll see what she maxes out at on Saturday. I'm guessing that the motor is the one that came with the pontoon, so it should have the correct propeller, but I have no way of knowing for sure. Misplaced the boat and engine manuals. I know they're around here somewhere.
 
Took her out today and froze my tush off. It was about 45 out, but the wind was around 15 m.p.h. Ran her across the lake at 4200 to get some gas at the marina that's open. Filled up and started cruising at 4200 and she sounded sweet, both ways. I fire-walled her and the rpms got to 5700 2 times. Can't swear as to the accuracy of the tach, as it's the only instrument running and was probably neglected the same as all the other instruments which are not working now. But it sounds like my revs are in the ballpark.



I am hoping that replacing the black, probably not enoughprop 001.jpgprop 002.jpgprop 005.jpg oil and the brand new hub assembly banishes the "spinning prop" syndrome for good. Today's circumstances weren't the same, as she only did it previously after running for around 2 hours. Was too cold too stay out any longer today.

Wow. I'm seeing the pics in the middle of my text. Probably not of much use as the prop was already installed.

The number off my prop is 48-77-330-C1-11
 
Went out today for about 45 minutes with 8 souls aboard. Pulled out of the marina got up to 4500 for 10 minutes and turned into a large cove. Throttled back to 3000 for about 15 minutes, made a u-ey to come back and still at 3000, the motor "burped." I glanced at the tach and it was still at 3000. Lasted about 1 second. After that she was fine. Left the cove and pushed her back to about 4500 for 10 minutes back to the marina. Ran very smoothly.

Seeing evidence of the dismal maintenance that was done before I got her just before Christmas, I'm not putting too much stock into it. Might have hit some weeds or who knows what is passing through the gas lines from the tank. When I filled up yesterday, I put in a whole bottle of............marine fuel conditioner? I forget. Comes in a smaller bottle and is blue. Wonder if that may have dislodged some crud. If the filthy lower oil is any indication, there's probably some gunk that needs to be flushed out. Once it warms up a little, I'll start doing the things that should be done and probably haven't been done in many years. The spline grease was almost rock hard when I pulled the prop. Can't believe that it was redone annually. Don't know how anyone can watch all the instruments go belly-up until only the tach is working.
 
kimcrwbr1 said:
First off service the fuel system. Clean/rebuild the carb/s and fuel pump, pull all electrical connectors apart and clean with electrical cleaner and use dielectric grease to put them back together. Pull all the grounds apart and sand metal to metal look for any cracket broken wires and repair as necessary. Drain the gearoil and pressure test it at max 8psi and soap test the seals. If it holds pressure for 45mins change the impeller and top it off with gearoil. Any water or metal fragments in the gearoil it will need to be broke down inspected/resealed. You getting better on parking the boat ag good way to perform maint on the fuel system is drain the float bowls and fill the carb/s with seafoam. let sit over night and connect the fuel line up the next day and run it around 3000 rpms until the smoke stops. That will dissolve the varnish in the fuel system from ethanol in the fuel replacing the fuel lines wouldnt hurt either so you know how old they are same with the impeller. A good service manual and some PM it sounds like your on the right track.
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Thanks. Really appreciate all that information.
 
Re: Revs go up, prop slows after a long run (not hub issue) (Oh, I think it was)

It was around 60 today, so my stepson and I took her out and spun around the lake for a little over 2 hours. 1/2 was slow-speed sight-seeing and the other half was upwards of 4200, mostly 4800. Haulin' butt for a pontoon boat with 60 rompin', stompin' horses. Ran as smoothly as could be the whole time, not even a single burp.

It appears that replacing the prop hub and/or cleaning the gunked up grease off the spline and re-greasing it and/or draining out the totally black oil from the shaft and replacing it did it. Myself, I strongly suspect that the extremely hardened grease on the spline and in the grooves made it almost cylindrical in shape, making it susceptible to spinning in the hub. I had to use a screwdriver and wire brush to get it off. The old rubber hub was probably pretty old and not in that good a shape to be able to hold onto an almost smooth spline, even though to the eye, it looked O.K. Dat's my 2 cents anyway.

Now I have to get around to fixing all the instruments and doing the proper maintenance that the motor requires. I feel uneasy without a temp gauge especially. Thanks to all contributors to this thread. I appreciate the info and learned a lot.
 
Sounds like its ok now just some questions.
1) when this slipping started on you ride had anyone changed position in the boat ( ex, move from rear seats to front seats)
2) Could you have picked up some weeds on the pontoons, gearcase ect or old boat cover, large piece of plastic this would cause this issue and the vibration you felt and then when you stop float away.

Just some things to look for if it happens again!
 
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