Cavitation problem, O/B pontoon boat

[DeanPA]

Hi all, <br /><br />I recently bought a good used 1996 Harris 24' pontoon (my third boat) for use on a lake in PA, where we weekend.<br /><br />The boat came with a Force 75HP, Model 75ELPT, which was professionally maintained (former owner was a very straight-up 75 year old gentleman) and obviously not abused. The mechanic near me in PA does not know the guy nor the boat, and said he could find nothing wrong with the boat or motor.<br /><br />Other than an instance where the engine lost prime (mech. told me the Force outboards have a tiny fuel pump, and will occasionally do that after running at constant speed for a long period) it has run perfectly for the 10-15 hours we put on it this beautiful summer at the Lake (Wallenpaupack)----BUT------<br /><br />I think I may have the wrong prop on. Specs call for WOT at 4750-5250 RPM. I am at about 4700, but when I run at any speed above 2500 for any length of time, I get cavitation eventually, especially in a slight chop. Even in fairly calm water, it'll eventually lose its bite. If I lean over the stern and look while running (wife driving!!!) I can see the path of bubbly water trailing astern. Acts like a car with a slipping clutch until I back it down to 1500 or so. Also, the motor has Power Trim, but forget running at any trim other than full down.<br /><br />I was in the water last week, trimmed the motor up all the way and gave the prop a good look. It's mint, there isn't so much as a ding in it, and the lower unit is just as clean.<br /><br />The prop bears a number very close to:<br /><br />48-73133 A40 13P<br /><br />Could have been 73113, but no pencil on board.<br /><br />A friend, retired marine mechanic, who lives in NC and hasn't seen the boat, had told me a cupped prop might be the answer, but from researching the part number, it appears the prop on the boat may already be cupped (I couldn't tell).<br /><br />I am certainly not going to start buying props of different sizes willy-nilly just to "try" them out, and could probably use a little education in this prop selection business.<br /><br />Oh, by the way, the motor's mounted as far down as it will go. (top set of mounting holes).<br /><br />Help. Thanks,<br /><br />Dean

 

[Navigator]

Re: Cavitation problem, O/B pontoon boat

Dean,<br />Is the motor the correct shaft length for the boat?<br />It kind of sounds like you have a long shaft mounted where an Extra-Long shaft is required. <br /><br /> Can you see where the anti-ventilation plate sits when running? Also, does the sound decrease when you lean over the stern (this gives more weight to the rear and pushes the prop deeper) It it was cavitating, you would be hearing exhauset sounds aswell as the regular engine sounds.<br /><br />Nav

 

[DeanPA]

Re: Cavitation problem, O/B pontoon boat

Nav, <br /><br />The ventilation plate sits just about flush with the surface of the water while running---MAYBE an inch or so under, can't remember exactly.<br /><br />The motor is a 75ELPT---I am assuming that means "Extra Long, Power Trim", maybe I'm wrong on that. The motor was original equipment with the boat when new, and that's documented. <br /><br />I realize that new pontoon boats ship without motors, and perhaps dealers throw whatever motor they have a surplus of on the boat, but I find it hard to believe the boat has had this problem since new. In person, I'm a fair judge of character, and this fellow would have told me about it for sure, although he did say that his son did most of the driving while taking the grandchildren out on the lake, so perhaps it was always a problem, and the seller wasn't even aware of it. Maybe they just accepted the problem and didn't think about it.<br /><br />OTOH, this prop looks almost, if not brand new, so I'm wondering if it was a recent replacement.<br />And if so, what did it replace?<br /><br />But more importantly, what now?<br /><br />Dean

 

[AMD Rules]

Re: Cavitation problem, O/B pontoon boat

E = Electric Start<br />L = Long Shaft (20")<br />PT = Power Tilt/Trim<br /><br />Have you examined the weight distribution? (Wondering if it is possible that you have a bow-heavy situation thus lifting the stern)

 

[steelespike]

Re: Cavitation problem, O/B pontoon boat

Im pretty sure most marinas will let you try props to find the right one.I'm no prop expert<br />but cupping should help and I think possibly 4 blades.There is a chance that your newer Force may use a Merc prop.Just speculation I don't really have a clue.Other than they did use more Merc stuff as time went on.<br />Also do check your weight distribution.Moving just one person might solve your problem.Movimg 100 lbs from bow to stern is like a 200 lb change.

 

[DeanPA]

Re: Cavitation problem, O/B pontoon boat

I'm pretty sure weight isn't the problem. Shows the problem the same with two aboard seated in back, and six aboard distributed all around the boat.<br /><br />Yeah, it's a Merc part number on the prop. The motor is labeled "Force---by Mercury Marine", and the lower unit bears a very close resemblance to a Merc, might even be a Merc lower unit.<br /><br />AMD: in regards to the translation of "ELPT"....<br />any ideal if they made an "Extra Long" (longer than 20") shaft model ?<br /><br />The idea of trying different props out is a great one, I'll start asking around my local marinas.<br /><br />Where should the AV plate be while running?

 

[Dunaruna]

Re: Cavitation problem, O/B pontoon boat

I had a similar problem, motor had to go down another 2 inches - problem fixed.<br /><br />It's difficult to find the correct position of the AV plate on a toon boat because there is no hull to use as a reference.<br /><br />Triple check that prop number, the only number that is close (that I can find) is 48-73138a40 and that is a force part number (10 1/4" x 14"), designed for light loads.<br /><br />Aldo

 

[DeanPA]

Re: Cavitation problem, O/B pontoon boat

Aldo, <br /><br />Thank you. I'm kind of afraid of that.<br /><br />The number could possibly be 73138 (I was treading water while I read it) but it ends in "A40 13P". Would that be a 13 pitch?<br /><br />I guess if I need an Extra Long, but only have a Long, I'll have to live with it. Just trying to figure out what I can do to mitigate the problem.<br /><br />How the heck could I lower the motor.....???

 

[Dunaruna]

Re: Cavitation problem, O/B pontoon boat

With mine, I had to modify the mounting beam.<br /><br />How much of the toons are underwater? 50%?

 

[DeanPA]

Re: Cavitation problem, O/B pontoon boat

I would say the pontoons are slightly less than half in the water.<br /><br />I don't know what the terminology is, but the transom is the back of an aluminum structure underneath the stern. The structure looks kind of like the bow of a rowboat, and is bolted to the frame.<br /><br />From a practical viewpoint, I can see how I could unbolt the structure from the frame, put spacers between, and bolt it back up, but putting that structure closer to the waterline (and perhaps IN the water) would no doubt have an adverse effect on my profile in the water. <br /><br />No one responded in regard to whether Force made a longer shaft motor than the one I have. I'd like to know if it's possible to swap lower units. Probably a headache, though..

 

[CharlesW]

Re: Cavitation problem, O/B pontoon boat

Originally posted by DeanPA:<br />The ventilation plate sits just about flush with the surface of the water while running---MAYBE an inch or so under, can't remember exactly.<br /><br />Dean

While I'm certainly no expert, that sounds like the motor is too high.<br />On my pontoon boat, the cavitation plate is about even with the bottom of my pontoons.<br />On my Chris Craft, it is about even with the bottom of the boat.<br />In both cases, this puts the cavitation plate about 6" under the water. Possibly even as much as 10" when at rest.

 

[BillP]

Re: Cavitation problem, O/B pontoon boat

Your're not alone. My pontoon boat does it when running wide open with people in the bow. It's a plague of pontoon boats and why merc invented "Big Foot" motors. Standard props are pitched too steep for the slow speeds (20-25mph at full throttle)and they run on the edge of cavitation. All it takes is a little wave action or loading heavy and the problem gets worse. Running the motor deeper will help but this isn't ventillation so deeper doesn't make a big difference unless the motor is way high in the first place. The best fix is dialing in the engine with a large diameter prop with lower pitch for max rpm.

 

[DeanPA]

Re: Cavitation problem, O/B pontoon boat

Please bear with me, I think a lot better in car terms when it comes to pitch.<br /><br />The last characters on my prop are "13P", is it too big a jump in logic to assume this is a 13 pitch prop?<br /><br />Now---me trying to get some perspective on how pitch is numbered----a lower pitch number is less pitch? ----and less pitch means the prop blade slices less of a profile through the water? ---and this would equate to HIGHER gearing in a car?-----so in car terms, I need to go to higher "gearing", which will give me more top end but cost me some hole shot pull?<br /><br />I think I have something backward...but what?

 

[Dunaruna]

Re: Cavitation problem, O/B pontoon boat

I also think you have a 13" pitch.<br /><br />Your gearing analogy is good but you need to use your WOT RPMs' as a guide, less pitch will increase your maximum RPM's which could be very bad. More pitch will decrease you WOT RPMs' which can cause lugging, also bad.<br /><br />Usually, less pitch will give better holeshot (is that what you want in a toon boat?) and top end will suffer.<br /><br />[Quote by BillP:<br />"The best fix is dialing in the engine with a large diameter prop with lower pitch for max rpm." ]<br /><br />Makes a lot of sense to me. I'm currently running a 14 x 9.<br /><br />Aldo

 

[DeanPA]

Re: Cavitation problem, O/B pontoon boat

OK, so your 14 X 9 has a larger diameter and less pitch than my prop.<br /><br />Any way of knowing if this size prop would fit on my Force O/B ?<br /><br />I'm going up to the Lake this weekend. Have to check with the marina and see if they have any props I can try out.<br /><br />Thanks again.

 

[Dunaruna]

Re: Cavitation problem, O/B pontoon boat

I should explain, my boat is totally different to yours, i also run a different motor. The 14 x 9 suits my package nicely, it may not suit yours.<br /><br />In your case, you need to experiment - beg/borrow props and test them. Some prop shops will lend them to you if you agree to eventually purchase through them. You have a starting reference prop - work from there.<br /><br />I have a 50hp efi bigfoot pushing a 12 ton houseboat.<br /><br />Aldo

 

[DeanPA]

Re: Cavitation problem, O/B pontoon boat

Well, back from the Lake. <br /><br />I figured out the problem. Good News/ Bad News. Solving it will be another story.<br /><br />We went out with 7 people. I seated 5 of them in the stern, and ran it wide open. No problem at all. WOT was about 4600, close enough. I made sharp turns and everything---motor never missed a beat.<br /><br />That's the good news.<br /><br />Now all I have to do is figure out how to get another 2-3 inches of motor into the water.<br /><br />Somebody mentioned an extension kit for lower units---anybody know anything about these?<br /><br />Another idea is to unbolt the transom unit and put spacers between it and the frame. Would that work?

 

[CharlesW]

Re: Cavitation problem, O/B pontoon boat

It seems unlikely that you don't have some adjustment for motor height presently.<br />You have no doubt checked and are sure it can't be lowered any, right?

 

[shebe3]

Re: Cavitation problem, O/B pontoon boat

You and some of the members are calling your problem cavitation. I think your problem is ventilation. If your prop was cavitating badly it would not take to much time and your prop would not look like new. Cavitation generates some distructive forces on the surface of the prop and will remove paint and metal in a short time.

 

[DeanPA]

Re: Cavitation problem, O/B pontoon boat

Charles, <br /><br />The motor is mounted in the top holes in the mounting bracket, and the motor bracket is right on top of the transom.<br /><br />Short of a motor with a longer lower unit, extending my existing lower unit (if possible????)<br />or lowering the transom, I don't know what else I could do.<br /><br />Shebe, thank you for the correction. My prop looks great, so perhaps it is ventilation and not cavitation as I stated.