Power Loss - 20' Pontoon w/ 50hp

[raydelliott]

There seems to be a power loss (overall speed/pull) in our pontoon motor. I was wondering if a scratched/nicked/scraped prop will cause a decrease in performance or could it be a slipping hub. The prop has been run ashore a few times as well as rope tangled. I am gonna mark the hub to see if it is slipping since we are going out this weekend. The motor is mounted in the uppermost holes on the transom. The motor is also missing the triangular anode just above the prop. I haven't done a compression check. I plan on checking the compression this week.

The boat is a 2002 Bentley/Smokercraft 820 Cruise (20'). We usually average 2 kids and 4 adults with 1 large drink cooler. The motor is a 2003 Mercury 50hp (50ELPTO) classic 2-stroke. The stock prop is a Hustler Aluminum 3-Blade with 10 7/8" diameter with a 11" pitch. The max RPM stamped on the motor is 5500. I believe the range is 5000 to 5500 rpm. The motor barely reaches 5200 rpm, usually 5000 to just under 5200 depending on trim. When pulling a tube (2 kids) and several adults aboard, it seems as if it is creeping along. I can't enough speed to get the rope out of the wake. The performance decrease is also noticed when cruising at WOT. I will try to get a speed using GPS as well this weekend.

Any thoughts or suggestions so far.

Thanks in advance

Ray

 

[lncoop]

Re: Power Loss - 20' Pontoon w/ 50hp

You don't mention what your past experiences have been with the rig, so I'm not sure if you're asking because it's not performing as well as it was or if you've never been happy with the speed. However, I suspect it's the latter. A 50 horse is not going to push a 20' barge all that fast. Prop size, pitch and condition are all factors of course, but that's just not a very fast setup. When you GPS it if you attain anything close to 18 mph at WOT that's all you can expect. If it's below 14-15 mph you may have issues, but I suspect she's doing what she's supposed to.

 

[EGlideRider]

Re: Power Loss - 20' Pontoon w/ 50hp

You didn't mention if this problem has just begun or if it is an on-going problem.

I suspect it is an on-going one because a 20-footer with a 50 hp with 4 adults pulling two kids on a tube is not going to attract much attention nor perform much better than you described.

You don't have a hub problem because your RPM's are too low. In fact, the prop would have to be awful banged up to be the problem. If this is not a new problem, then I'd look at a lower-pitched prop.

A two-stroke is almost bullet proof, so unless you have ran it without oil, I doubt if you have a compression problem.

I haven't been much help with solving your delema so maybe report back with more symptoms and/or history and I'm sure you'll get plenty of advice.

 

[raydelliott]

Re: Power Loss - 20' Pontoon w/ 50hp

Yep, this has never been a "speedster". I wish (hind sight is 20/20) we would have gotten the max motor size - 75hp. The "power loss" seems to have been more noticeable to me the last season and this season. I can remember just after we got the rig (2002) that, with three adults on board, I could whip a college kid on a small round tube beyond the wake and dump them. There were a few years when we didn't use the pontoon for tubing, but now we are using it again and we have a wider bat-wing-type two person tube and I can't get it to break past the wake.

Never, absolutely never, has the motor run out of oil. Always fill prior to each use and carry extra along just in case.

I will try to GPS the speed this weekend if weather holds out.

Thanks again.

Ray

 

[Silvertip]

Re: Power Loss - 20' Pontoon w/ 50hp

Three things need immediate attention: 1) Verify the tach is reading correctly (see #2). 2) An 11P prop is not appropriate for this engine UNLESS it happens to be a BigFoot. If it is the large gearcase it will say "BigFoot" on the engine cowl. If not, this is a standard gearcase engine with 1.83:1 gears (vs 2.33:1 for the BigFoot). So 11P on that large a pontoon would be too much pitch for the standard engine hence the low engine rpm. 3) You need to mount the engine as low on the pan as possible. There is lots of turbulence in that area of a pontoon and the prop needs to run in less disturbed water (as in deeper).

If the boat sits in the water continuously, the pontoons may be covered with algae and crud which will slow it down considerably. If the engine has not been tuned recently it could probably benefit from it.

 

[raydelliott]

Re: Power Loss - 20' Pontoon w/ 50hp

How can you verify the tachometer or check the engine RPM's?

Yes, the Mercury 50hp is a standard "Classic" 2-stroke as opposed to a Bigfoot model. The motor has a stamp on the top side of the cavitation plate showing the gear ratio 1.83 : 1. The same stamp also has 12 x 22 which I am unsure about.

Ray

 

[Jeep Man]

Re: Power Loss - 20' Pontoon w/ 50hp

I can relate to your trouble as I am going through a similar experience. I have a 20' as well with a 60 hp Bigfoot. Last season, I ran a 14x11 prop with substantial damage on it. I could not get the RPMs above 4600. I switched to a new 14x9 and performance improved dramatically. But, now max RPMs is 6200, higher than I would like. I am having the old one rebuilt and am confident that it will max at 5800. What I have noticed while running the 14x9 is that balance front to back has a significant effect on max RPMs. by adding weight to the rear, as little as 50lbs, can drop max RPMs by as much as 500. I find it interesting what Silvertip had to say about motor height. I feel my boat should be doing a little better in the speed department and I am running my motor 2 holes from the lowest. Using the lowest hole causes the steering to bind. Maybe I will lower my motor by one hole.

 

[jasperpso2]

Re: Power Loss - 20' Pontoon w/ 50hp

Three things need immediate attention: 1) Verify the tach is reading correctly (see #2). 2) An 11P prop is not appropriate for this engine UNLESS it happens to be a BigFoot. If it is the large gearcase it will say "BigFoot" on the engine cowl. If not, this is a standard gearcase engine with 1.83:1 gears (vs 2.33:1 for the BigFoot). So 11P on that large a pontoon would be too much pitch for the standard engine hence the low engine rpm. 3) You need to mount the engine as low on the pan as possible. There is lots of turbulence in that area of a pontoon and the prop needs to run in less disturbed water (as in deeper).

If the boat sits in the water continuously, the pontoons may be covered with algae and crud which will slow it down considerably. If the engine has not been tuned recently it could probably benefit from it.

in fact, a 10 7/8 X 11p is exactly the prop i run on my 20' pontoon with a 82 mercury 50 hp... it is what the turning point calculator AND the mercury calculator suggested for my boat.. what makes you say it isnt the correct prop?

 

[Silvertip]

Re: Power Loss - 20' Pontoon w/ 50hp

I say it's the wrong prop because it is. Actually, it is the wrong motor since a 10-3/8 inch prop is too small for a pontoon. After all, that's why Merc provides the Big Foot so the larger 14 inch pontoon props can be used. Granted there are some pontoon shapes that provide a little less drag than others but the standard round tubes account for the vast majority of pontoons so shape is probably not an issue here. That prop on a 1.83 gearcase would turn 27 MPH with 12% slip. You know and I know that's not possible with 50 HP. Better check your wide open throttle RPM with an average load. You did not provide any speed data so the slip # is arbitrary and I'm giving you the benefit of the doubt here. The optimum prop for any boat regardless of hull type is one that allows the engine to operate at the upper end of the engine manufacturers wide open throttle rpm band. If the engine can't reach that rpm, you have too much pitch. If the engine revs higher than that, you have too little. As an example, a standard 16 - 17 foot v-hull with that very same engine will run a 13P prop and turn between 5000 - 5500 rpm. That is a very standard setup. An 11P is simply too much pitch for a 20 foot pontoon. Which is why I said check the accuracy of the tach.

[Edit] The 1982 50 HP engine is a four cylinder and has a different gearset (neither 1.83 or 2.33) than the newer 3 cylinder engines so the 11P prop may be correct for your pontoon.

 

[raydelliott]

Re: Power Loss - 20' Pontoon w/ 50hp

How can the tachometer accuracy be checked?

Also, does a missing anode tab, mounted just above the prop blades like a fin pointed down, affect motor performance. I realize it is for corrosion, but does it serve other purposes?

Thanks

Ray

 

[Silvertip]

Re: Power Loss - 20' Pontoon w/ 50hp

If you are talking about the torque tab then no, it does not affect performance but it would have a tendency for the engine to pull against the steering one way or the other. It is there to compensate for steering torque. To verify the tach accuracy you need a known good tach. But check the idle rpm for your engine. If the tach reads within spec at idle is probably ok. If the indicated idle rpm is very different than what is recommended for the engine than it is probably calibrated wrong. There is a dial on the back of the tach. What is it set to?

 

[lmuss53]

Re: Power Loss - 20' Pontoon w/ 50hp

I certainly don't want to get into a big debate, and I will be the first to concede that the Bigfoot gearcase on the midrange Mercs is the ideal gearcase for those motors on a pontoon. I also agree that we need to run our motors, especially 2 strokes, at the high end of the RPM range when loaded lightly so we don't lug them down when loaded heavy.

Having said that I must also say that there are a TON of midrange Mercs with the small gearcase pushing pontoons around with about that prop on them, not to mention the old Force and OMC motors. I have a friend with a 2008 Bentley 20 footer with that 50 ELPTO and an 11P prop that runs high teens on the MPH with a GPS at 5500 rpm. The motor is all the way down on the transom. In the early '90's I had a 24' Manitou with 19 inch logs and a 50 ELPTO that ran in the high teens with an 11p, no tach on this one. I just pulled a small gearcase 50 OMC off of my 19 foot boat with a Michigan Wheel 11p on it that ran low 20's at 5700 on a good day. It did run out of steam when I added just a little weight, like one or 2 peolple, and ran like a stone if I tried to pull a tube which I only did once or twice. Any pontoon will slow down with the addition of just a little weight with this motor.

I think this man's motor is working about like it should be expected to, considering what he's asking from it. If this prop is taking a good bite on the water and hanging on I would say it is doing what it should.

One point that Silvertip made that could be very important here is does this boat have an accumulation of growth on the logs from being left in the water? That will slow you down tremendously if it is there. If it is there I would pull the boat and clean the logs (have fun) and I bet you will see the return of your old performance levels.

We tend to put things on the boat that don't come off, lights, tools, extra this and extra that. All of these things will take away from the performance of a small outboard. If your logs are clean I would go to a 9 P if one is available for your motor, or strip the boat down to the very minimum in weight and see how it reacts.

EDIT:

Turning point recommends a 10 7/8x11 prop for a 2002 20 foot Bennington with a Merc 50hp "classic", I used Bennington because Bentley's not listed anymore and I could not think of the new name. They list bigger props for that motor in 12x10.5 and 12X8, if you decide to change you might try the bigger prop. Considering where you're at now and what you want to do with your boat I would try the 12"X8P. This is after you make sure your logs are clean, that is the first thing I would be sure of.

 

[Silvertip]

Re: Power Loss - 20' Pontoon w/ 50hp

I by no means suggest that you run out and trade a standard leg Merc for a BigFoot. Any engine will push a pontoon. But Merc took two separate approaches with the 40/50/60 HP engines which power everything RIBs to work barges. For fast hulls, the small gearcase and small diameter, and therefore high RPM props work fine. On workboats, pontoons, and barges, the BigFoot with a much larger diameter prop, more blade area, deeper gears makes a much better combination. If you have the opportunity sometime, try a reversing manuever with a 10 inch prop and then on the BigFoot. If you have a small gearcase engine - prop it and run it. But the fact remains, it will never be as good a setup as a BigFoot. Speaking of props -- if the prop is wrong at WOT it is wrong across the entire RPM band. Overpropping (the most common issue) causes engine lugging. So if the engine lugs at WOT it is lugging at 3000 rpm or any other rpm. That requires more throttle opening than necessary which burns fuel. The issue here is that many folks don't know or understand that they have an option with these engines and thnk that if it looks like an outboard it is "best" for that application. Any dealer that installs a small gearcase Merc on a pontoon should be flogged. They are doing it strictly to get the price point and are not doing the customer a favor. At the very least, they should explain that a BigFoot is available and what the advantages are. More money yes. But options on anything are not free.

 

[raydelliott]

Re: Power Loss - 20' Pontoon w/ 50hp

We were able to get on the water this past weekend. I took a GPS and the WOT cruising speed topped out at 15 to 16 mph. The tachometer maxed at just under 5200rpm. The water was a little choppy and we had 4 adults, 2 teens and 2 kids. I wasn't able to check the WOT rpm with minimal load of 2 adults and 1 kid (my family only). The pontoon is always removed from water after use, therefore only water-staining on the tubes (no gunk build-up).

I was wondering if a 4 blade aluminum 11" or 12" diameter x 8"p prop would increase my RPM and give a better bite or would it be too much RPM increase? The current prop is 10 7/8" x 11"p 3-blade Hustler aluminum.

The Tach appears fairly accurate as the Idle RPM (in gear) was just under 800 rpm.

Compression (cold condition) came in at 105 psi to 110 psi on all 3 cylinders using a compression gauge from local chain auto store.

The motor is mounted in the lowest position.

I guess the ideal prop would allow the motor to operate at Max RPM with the lightest possible load - say driver, fuel, life preserver, fire extinguisher, etc.

Will it hurt to have a prop that slightly exceeds max WOT RPM since the RPM can be lessened by throttle?

I don't usually go WOT except during tubing or racing the thunderstorm. I like to cruise about 50% to 75% throttle for comfort.

 

[lncoop]

Re: Power Loss - 20' Pontoon w/ 50hp

Can't imagine doing much better than 15-16 with that boat/motor configuration. You might pick up a couple MPH with some prop experimentation, but it sounds like your rig is performing as intended.

 

[jasperpso2]

Re: Power Loss - 20' Pontoon w/ 50hp

i would agree, 5200 rpm at 15-16mph with 8 people on board - you are getting allot out of your 50 hp.. my 50 doesnt do that well..