Prop for 24 foot pontoon and 40 HP Mercury (have RPM + GPS speed data)

Joined
Jul 14, 2014
Messages
7
Hi All. I bought a 1992 Harris Flotebote Classic CF (first toon with a rear swim ladder), I have dubbed it the "Big Ugly". The interior is terrible but I bought it for the 24" toons and a good running motor which it has. The RPM's wont get to where they need to be though. My goal is best loaded speed and low speed handling. If I could get 15 MPH loaded I would love it.

So Here Goes:
  • Motor: 1992 Mercury Classic 40 HP, 4 Cylinder, 2 Stroke
  • Boat: 1992 Harris 24ft original weight 2100 pounds dry
  • Fuel: 16 Gallon Tank
  • Prop: 14 Pitch Mercury Original Prop no paint on the edges
  • Max GPS Speed Empty: 13-14 MPH
  • Max GPS Speed with 6 Adults and 4 Kids: 10 MPH
  • Max RPM: 4000
So running these through numerous calculators they recommend a 9 pitch prop, one even recommended an 8 pitch. I am looking at a Sola Amita 9 pitch 12 diameter on here for about 85 bucks. Should I consider anything else?

Also, this motor is supposed to be good for 5500 RPM, i am wondering if only a 5 inch pitch will get me 1500 RPM? Motor starts and runs great. I have had a full tank of gas through it already on both inland lakes and rivers.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
A 14 is way too much prop. If you go to a 9" It should put you around 5,000. I think the motor is rated for 4500 to 5500.
Slip with the 14" is about 50% a 9" should produce less slip. If we use 5000 rpm and 25% slip we get a speed of about17.
Pontoon speeds vary widely even for boats that appear the same approximate dimensions.
​ I picked a 9" because it is fairly typical. We could shoot for more rpm but I think the speed may fade as we subtract pitch.
I think the 9 will give us good base line numbers to help determine if more or less pitch is necessary.
 
Joined
Jul 14, 2014
Messages
7
A 14 is way too much prop. If you go to a 9" It should put you around 5,000. I think the motor is rated for 4500 to 5500.
Slip with the 14" is about 50% a 9" should produce less slip. If we use 5000 rpm and 25% slip we get a speed of about17.
Pontoon speeds vary widely even for boats that appear the same approximate dimensions.
​ I picked a 9" because it is fairly typical. We could shoot for more rpm but I think the speed may fade as we subtract pitch.
I think the 9 will give us good base line numbers to help determine if more or less pitch is necessary.


Thanks Steel Spike!

Is there any benefit to going 4 blade? Or would you reccomend to keep it on the cheap until I find a pitch that works?

Another consideration is that the boat is rated for 120 HP. I am considering keeping my eye out for a new outboard. I am not a speed fanatic though. We live .5 miles from the sandbar on our lake and I will buy a Malibu when the kids get older for tubing and wakeboarding.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
A 4 blade may help a little more and may respond better around the dock.
4 blade may need to be a 8" to try to maintain the rpm.
Keep in mind your 40 will use about 4 gallons per hour(GPH) at wot. A 120 about 12 GPH at wot.
and thats whether pushing a runabout or a house boat.
 
Last edited:
Joined
Jul 14, 2014
Messages
7
Thanks for the help everyone! I went with the 9P Solas Amita 3 Blade. I now have 5000RPM and 17-18 MPH, but the best improvement is in low speed handling and also cruising speed. With a bunch of adults on board I can cruise at 11 MPH at 3,000 RPM. That classic 40 inline 4 is quiet enough to talk without yelling at that speed. Perfect.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
It's nice when there is a great success.One reason we were so successful was the original prop was so far out of size.
Thanks for getting back with your results.
 

Fred.Furrer

Recruit
Joined
Aug 27, 2013
Messages
5
I have a Mercury Classic 40 on a pontoon boat also. The other day I ran it hard pulling a tube, and I think I overheated the motor and blew a gasket because the lowest cylinder in now getting water in it (milky spark plug). I haven't pulled the engine apart yet, and would like to hear suggestions. From what I am seeing on the boating forums, this is not an unusual problem. The cause seems to be that the pontoon boat creates foamy water that enters the cooling system and does not cool adequately. Can anyone confirm this? The engine still runs, but only on 2 or 3 cylinders. The lower cylinder does not run, and the plug is milky white when pulled out. What all has to be taken apart and repaired in my situation?
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
You need to start a new post in the Merc section You shouldn't resurrect an old posting.
While almost anything is possible with a pontoon with all that turbulence in the water.
I'd be more inclined to suspect the impeller or thermostat.If you don't know the age of the impeller
just change it as it could fail at anytime.You will get more specific help in the Merc. Forum.
 

Fred.Furrer

Recruit
Joined
Aug 27, 2013
Messages
5
I can not figure out how to start a new post, so I will report here. It turned out that the engine had NOT overheated, and there were no failed gaskets. The compression on all cylinders was 120 psi. I did change the two side cover gaskets (block to baffle plate, baffle plate to cover plate). When I started the motor up, it continued to only run on the top two cylinders, and water appeared on the lowest cylinder spark plug. I finally figured out that the lower carb (which feeds the bottom two cylinders) was not providing fuel to the cylinders. I took that carb apart, blew out all passages, reassembled it and then the engine ran fine on all cylinders. I have no idea what was wrong with the carb, but in my experience, just taking them apart and blowing out the passages will cure most problems. What I think I learned is that water in the lower cylinder can be due to that cylinder not producing exhaust to keep water out. In other words, failure of that cylinder to fire is what happens first -- then water enters the exhaust port making one think that the cylinder is not firing because of the water, but that is not true. It may have been that the float needle was stuck closed. It has a Viton tip, and I am not sure that the wire clip that pulls the float needle down was assembled correctly. The amazing thing is that the engine had been running fine on all four cylinders just before it simply quit running on the lower two cylinders. This is not the first time I had carb problems on this motor. About 10 years ago, it was not running properly and so I cleaned the carbs. I didn't find anything wrong, but whatever I did fixed the problem at that time. This has been an excellent motor for my pontoon, and I expect it will run well for many more years to come.
 

steelespike

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Joined
Apr 26, 2002
Messages
19,069
Glad you got things figured out. Just for reference if you go to the prop forum main page just above the Topic label in a high lighted box there is ...
" create + new topic"
 
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