prop stops & engine revs at full throttle

[oufishinfoolz]

We just bought a 1998 Voyager pontoon with a 50 HP Force motor (1998). If you take the cover off the motor it looks showroom new! We have had it out just a few times and the last time we were out, I slowly pushed the throttle forward as always (I'm not a speed demon!). As we closed in on full throttle, (the previous owner recommended 5600 RPM as 'optimum') and gained full speed at 5600 RPM, all of the sudden, the prop sounded like it quit turning and the motor really revved up high. I backed off the throttle, thinking we were done for, then slowly gave it throttle & the boat started along just fine. Curiousity got the best of me, so I put it up to full again and after a short time it did it again! So, I backed off & we slowly (around 4500 rpm)made our way back to the ramp. After calling a couple of boat repair shops the general opinion was the seal in the prop had come undone & it was slipping. We bought a new prop (a mere $150.00!!!) & went back & installed it just like the old one had come off. (By the way, both props are 8" pitch). So, we decided to go for a trial run and as I approached full throttle the RPM was closing in on 6000 RPM (The lake was like glass on this day). I thought, 'Is this good or not? Having a little more speed with the new prop?' I wasn't sure as I don't know what is optimum for my motor for sure. But right about the time I was wondering about the 6000 RPM, the prop & motor did the same thing as before!!! So, in disappointment, I headed back for the ramp, having nightmares about having to put it in the shop right about the time the crappie are supposed to spawn!<br />Can you tell me the recommended RPM for the motor & do you have any suggestions as to what the problem might be with the high rpm and the prop seeming to quit turning?<br />Thanks so much!<br />Craig A. Cooper<br />OUfishinfoolz@wmconnect.com

 

[roscoe]

Re: prop stops & engine revs at full throttle

Welcome to iboats.<br /><br />The book says 4750 -5250 rpm for your motor.<br />So you are over revving it a bit to say the least. A 12 pitch prop should put you at about 5200 rpm, and more importantly it will put the propper load or resistance on the internal engine parts.<br /><br />Also, you are trying to push a big pontoon with a motor that uses a small diameter prop. <br /><br />You are right to take it easy on the accelleration or you will spin your prop hub.<br /><br />Possibly you did not spin the prop hub. Is the motor mounted at the correct height so that the prop can't suck in air and ventilate? Is the motor trimmed out too far?<br /><br /><br />PROPELLER VENTILATION<br />Propeller ventilation is caused by air from the surface or exhaust gases being drawn into the rotating propeller blades. This results in the propeller slipping more than usual due to the reduced water load on the blades. The obvious symptoms of this are a sudden increase in engine RPM as well as a possible loss of speed. This commonly occurs when trying to turn the boat at high planing speeds or if the outboard or sterndrive is trimmed out too high.

 

[roscoe]

Re: prop stops & engine revs at full throttle

You can check the prop by scribing or painting a line on the prop, from the center, across the hub, onto the outter housing. Take the boat for a test run, make sure to accellerate hard, a few times. Then look at the line you created. If it is still a straight/continuous line, the prop is ok. If the line is broken, the prop hub is spun.

 

[oufishinfoolz]

Re: prop stops & engine revs at full throttle

Thank you, Roscoe. So am I to understand that I don't need a repair? Just maybe trade the 8"P for a 12"P ? Or, keep the RPM at the range you mentioned & we're ok?<br />(when we bought the bought the boat, the previous owner led me to believe that I should trim it until I reached the highest rpm (5500-5600)& that was where I needed to be.) I just assumed that I could push the throttle all the way forward for top speed & be fine.<br />(Also, the problem was like driving a stick shift car down the highway & while keeping the gas pushed, stepping on the clutch.)<br />If we are safe to drive it as is, I'll just be sure to keep the rpm where you suggested.<br />Thanks!

 

[Silvertip]

Re: prop stops & engine revs at full throttle

It sounds to me like you are trimming the engine way too high. Look at the engine when this problem occurs. If its tilted up instead of sitting vertical, the prop is blowing out (pointing too high) and simply pushing air. Set the motor so it sits vertical. Trimming up will not help a pontoon much anyway.

 

[oufishinfoolz]

Re: prop stops & engine revs at full throttle

I hadn't noticed how tilted it was, but will check this weekend. Again - under the instructions of the previous owner, I just trimmed it until I achieved the highest rpms. (which would have the bow up, causing less resistance by the pontoons, so the motor could very well have been too high). Which I now have learned is not the correct way. I have learned a LOT in less than 24 hrs. and I can't tell you how much I appreciate you guys & your knowledge-sharing! I believe I'll try to trade back the 8P & see about a 12P.

 

[Docdoc]

Re: prop stops & engine revs at full throttle

Just an idea, but does anyone think the clutch dog and/or forward gear may be wron and allowing it to jump ot of gear at high rpm?

 

[Docdoc]

Re: prop stops & engine revs at full throttle

should have said worn instead of wron, sorry

 

[Realgun]

Re: prop stops & engine revs at full throttle

BTW welcome to iBoats. I love to welcome new people. As you stated the pontoon is not a speed demom but is fun non the less.<br /><br />All the suggestion prior will help. You just need to try it out. <br /><br />Have you looked into a hustler prop form iBoats?<br /> Hustler This should work however i don'y know if the price includes the hub portion another 30-35 I think. Then it will be easy to switch props on the boat. say a 12 for speed and a 10 for water toys. Or use it as a backup incase you break a prop.

 

[roscoe]

Re: prop stops & engine revs at full throttle

Correct, you may not need a repair or a new prop. <br />If the prop hub is not spun, you are likely ventilating due to being trimmed to high.<br /><br />With the engine trimmed properly, you need to select the prop that will allow you to run the engine in the recommended rpm range with the throttle at full throttle. This is the way you match the motor to the load being exerted on it. That doesn't mean you actually have to run at those rpm, but that if you do go full throttle that the engine will hit the range.<br /><br />Here is an explanation:<br /><br /> http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=36;t=000015 <br /><br />When the "incident" happpens, do you hear any clunks or loud chattering noise? This would indicate a possible worn clutch dog.

 

[oufishinfoolz]

Re: prop stops & engine revs at full throttle

OK, gentlemen, I was able to get a 10P prop & put it on the pontoon. Took it out on the lake (it was about as calm as before) and I opened it up to appr. 5500 rpm. I didn't get caught up in trimming it for max rpm, just tried to make sure it was vertical. At full throttle going straight and then making a semi-sharp turn on the calm water, I didn't have the 'revving' problem like I did before. I believe you nailed the problem... I'm sure now that I had it trimmed too far up and it was just blow out. Thanks again for ALL of your input!!<br />PS... I didn't find the crappie while on the pontoon that night, but I did catch a catfish around 15 lbs. - on my crappie rod!! FUN FUN!

 

[roscoe]

Re: prop stops & engine revs at full throttle

Sounds like fun. Carry on.