prop exasperation, need advice

[cedgo]

Wow! Heck of a forum. Here's the problem...Brother-in-law has purchased used pontoon (24' Playbouy) with 75 hp Mercury engine. Came with a battered 19 pitch aluminum prop. Replaced with new 19 pitch aluminum and can't get engine to push boat at WOT. Engine has plenty of power and runs fine, but prop looses "traction?" as engine approaches WOT, and then easily over revs. (Would this pitch be too aggressive?) My solution is to go to a smaller pitch prop, maybe a 13 or 15. The local boat shop did not want to sell a smaller prop and suggested lowering the engine, but parked on trailer, the prop would be in the dirt. The position of prop in relation to the pontoons seems to be consistant with other pontoons boats sitting in their dock houses. Besides, engine is just about as low as it can go. Don't want to give in-law any bum advice/exspense, so am pleading with you experts here to steer me in right direction. Thank you very much. cedgo

 

[Boomyal]

Re: prop exasperation, need advice

Prop pitch would not cause the problems you describe.<br /><br />Without seeing it it is hard to tell but I had a houseboat (pontoon) that I originally put a short shaft motor on. It did the same as you describe. Changed it to a longshaft and my problem was solved.<br /><br />The other possibility is that the prop has a slipping hub. You can make a mark on the inner hub and the prop. The next time you run the motor, under load, if those marks do not stay lined up, the hub is slipping.

 

[sangerwaker]

Re: prop exasperation, need advice

Does the motor have power trim or is it manual? <br /><br />If manual, try lowering on hole on the trim setting and see if it is any better.<br /><br />If power, where is it set when this occurs?<br /><br />Good advice from Boomyal as well. Make sure to check the prop hub for slippage.<br /><br />BTW-A big welcome to iboats!

 

[Richard Petersen]

Re: prop exasperation, need advice

Boomyal has it covered.

 

[cedgo]

Re: prop exasperation, need advice

Thanks for the replies. Will check prop for slippage, although it is a new prop, and am not sure, but think the old dog earred prop had the same problem. Has manual trim, and we've tried every hole. So, if there is no prop slippage, do you think that the engine does need to be lower as the local boat guy suggested? This engine was a replacement for the original that came with the boat. <br />And thanks for the welcome, can see there's a lot to learn here. Again, replies much appreciated!<br /><br /> Oh, if we did go to a smaller pitch prop, being that the diameter is smaller, would that in essence be the same as lowering the engine, as the prop would not be as close to the surface of the water? Do you think the set-up with which we are dealing with is sucking air from the surface? Could (and this may seem really stupid) add some water to each pontoon to increase depth?

 

[cedgo]

Re: prop exasperation, need advice

Checked prop for slipping last weekend as suggested, but marks are have not moved. Is there a "proper" distance the prop should be below the surface of water? He is bringing smaller pitch prop down today, we will try that. Said manual reommended either a 13 or 15. Am at wits end. Another stupid question, what isthe differnence in a long shaft or short shaft motor? Thanks again. cedgo

 

[ufm82]

Re: prop exasperation, need advice

Since you said the prop is "losing traction" at WOT, that tells me that the pitch is too high. A slipping hub will slip under a load anywhere in the RPM range. Plus you said the prop was new, so I don't suspect a hub problem. The pitch can most certainly cause your problem. <br /><br /> What you do have is a prop that shouldn't be on that boat. Most pontoons have a low pitch prop on them as pontoons aren't noted for their high speed capabilities. I would think that with a 75hp engine, the 15 would be a good choice. However, without a tach reading I would be careful that you don't over-rev the engine. Does the boat have gauges? If not, I would definitely install a tach if nothing else. <br /><br /> The engine height could be causing an issue but unless it is abnormally high I can't see it cavitating unless you are trimmed way up for some reason. You aren't getting the boat "up on plane" with that engine and the rear of the logs isn't rising that much with your speed. Is the engine equpipped with T&T? Leave it tucked all the way in if it is. There's almost no reason to trim up on a pontoon boat- it's not like your gonna lift the bow anyway. <br /><br /> I would go with a prop like a 14 X 13 or something in that range, maybe even lower depending on what you want, speed or power. You'll want the larger diameter prop for better thrust at maneuvering speeds and the lower pitch for better bite at WOT- I'm surprised the shop didn't want to sell you a prop. There are a number of prop selectors on the web that can assist you, or better yet, call a Mercury dealer and ask them what they would recommend. A 19 pitch prop is NOT the prop for that boat. I'm thinking the 15 would be on the high end of what you want to run. <br /><br />Oh, forgot something. When the boat is sitting in the water, the anti-cavitation plate should be just underwater. When you are running, it should be skimming just on top of the water. Too deep and you'll lose speed and too high can cause your problems. You may even be able to simply look at a water mark on the lower unit and see where it normally sits. <br /><br />UFM82

 

[Moody Blue]

Re: prop exasperation, need advice

cedgo after reading this thread I would have to agree that the prop is not deep enough in the water. Changing the prop pitch most likely will not fix the problem. The difference in diameter is insignificant as far as depth goes. What you are experiencing is called ventilation. The prop is sucking air down from the surface of the water for some reason. Could be the motor is too high or something else is causing significant water disturbance ahead of the prop. Motor lengths are determined by the distance from the bottom of the powerhead to the anti-ventilation plate. These are typically 15" (short shaft), 20" (long shaft) and 25" (extra long shaft).

 

[CATransplant]

Re: prop exasperation, need advice

"So, if there is no prop slippage, do you think that the engine does need to be lower as the local boat guy suggested?"<br /><br />Since the local guy has looked at the boat, and none of us has, I'd listen to him. Sounds very much like the prop is too high in the water and is cavitating.

 

[ufm82]

Re: prop exasperation, need advice

Where did he say that the local guy looked at it? I didn't see that anywhere. <br /><br /> #1, if a prop is too high, it will cavitate when you first take off. The prop "pulls" a lot harder on the water when the boat is sitting still than when it is moving at 20mph. If it's gonna suck air, it'll do it at take-off. <br /><br /> If the prop will be "in the dirt" with the boat sitting on the trailer if he lowers it, either the engine is in the right spot now or the trailer is very wrong for the boat. Most pontoon trailers I've seen sit up pretty high- especially the ones that crank up. For the prop to be that low, it has to be pretty far down on the mount. AND he said that the engine CAN'T be moved down much further. I'll assume that means it is on the lower mounting holes. <br /><br /> The fact that the manual even tells him that a 13 or 15 prop is recommended tells me that someone replaced the prop and put the wrong one on. Maybe the "local guy" sold him the wrong prop and doesn't want to admit it. <br /><br /> Don't jump on me- I'm just trying to help. <br /><br />UFM82

 

[walleyehed]

Re: prop exasperation, need advice

The boat is higher in the water at faster speeds, the prop is more likely to slip as speed increases....very common.<br />I do believe the prop is over-pitched for that application though, but that's not why it's slipping.

 

[Silvertip]

Re: prop exasperation, need advice

Go to Mercury's web site and look at the information on their Big Foot motors. They show a comparison of water flow at the back of a pontoon VS a boat. While you may have a long shaft motor, it may not be set low enough at high speed on a pontoon. There may also be something under the pontoon that is upsetting the water flow causing the prop to ventilate.

 

[cedgo]

Re: prop exasperation, need advice

Thanks everyone! We put the 15 pitch prop on before the weekend. Solved the problem. Was a 4 blade comp prop. Good out of the hole and cruising. Motor still under a pretty healthy load at WOT, so no over revving. Sure sucks the gas now, but he's happy as a gopher in soft dirt. Again, many thanks.

 

[ufm82]

Re: prop exasperation, need advice

You're welcome. It's nice when someone has some success and gives feedback. Have fun with it. <br /><br />UFM82