new guy with prop pitch question

[sivlersnake]

Hello everyone! While my wife and I have had a lot of boats, a week ago we bought our first pontoon boat. It is a 2001 20' Sylvan with a 2001 Mercury 75hp 2 cycle. When we bought the boat it was in good shape. We Still have spent a lot of time over the past week we have did a lot of fixing of a lot of little stuff. The only big issue we have left to figure out is the correct prop. I was hoping to get some good input and opinions on what the best prop and pitch would be. What came on the boat was, what I've been told, is a beat up 19 pitch Michigan wheel. The prop had a dime sized chip out of it and the shape was a little beat up. The boat ran 21mpg at 4600 rpm at WOT. I should be turning around 5200 rpm. The prop I am thinking about is a 15 or 17 pitch Mercury spitfire with 13" diameter. Any thoughts? While I am good with bass boat props, I am out of my element with a pontoon boat.

[Moser]

Welcome sivlersnake, I think I would go with the 15 or maybe even a 13, I think you'll struggle to get into the recommended RPM range with any more pitch than that. On my 1998 Tracker 19 footer with a Mariner 75 ELPTO 2 stroke (same motor as yours) I ran a 13, but I was always running the boat loaded down with gear and bait tanks. I don't think your speed will change a whole lot, but you'll get into that 5000+ RPM range where you want to run that motor.

I ran a 4 blade 13 pitch comprop on that motor for years with excellent performance. If I could get one for my current OMC 50 HP I would.

Welcome again and enjoy your new boat and the pontoon experience.

[Friz]

Welcome aboard! Interesting question. I have a '97 Landau DX 20 with a '97 Yamaha C80 2 stroke. I don't remember what the prop was when I bought it in '97 but quite a few years ago I put on a 14 (13 3/4) x 13 pitch prop. WOT was about 5K. With that prop I had no problem getting my son up on his wake board with the wife aboard. I usually cruised at 3800 - 4000 rpm and getting 15 - 17 MPH depending on load. It was time to replace the prop and I read a bit about what RPMs should be at WOT, around 5600. So, I put on a 14X11 prop. Sure enough WOT RPMs jumped to 5600. Problem was that at 3800 - 4000 speed dropped to 14 - 16 MPH. To get back into the 15 - 17 MPH range I had to crank it up to around 4400 RPM and it burns a noticeable amount of more fuel. Not much of a trade off as the engine worked fine with a 13 pitch prop for over 10 years! Now I understand if the pitch gets to high it puts an undue load on the engine. I am surprised that a 19 pitch prop was on your boat. Does it accelerate or accumulate speed over time? (Sorry, couldn't resist it). Actually 21 MPH is not bad. I have 10 more HP and top out around 23 MPH on my GPS with a 11 pitch prop. I agree with Moser's advice - roll back to a 13 or 15 pitch prop. If you chose a 15, let us know. I may try a 15 next time. The prop I have is a Piranha. It is a composite with replaceable blades. Idea of going to a Piranha from aluminum was that if you damage a blade it takes 15 minutes to replace on scene. And blades are $20 each. A lot cheaper than having a aluminum prop rebuilt. With all that said though, next prop replacement will be to stainless steel and have done with the prop replacement merry go round.

Fair winds and following seas, Friz

[rickdb1]

Welcome. I had an older 20' pontoon with smaller logs (19") with a 70 HP evinrude. I ran a 14 X 13" pitch prop and it was about perfect. Ran about 5800 RPM's (Max recommended was 6000) at around 22-23 MPH. Not sure what your gear ratio is or how it compares to the old motor I had, so you may see different results. But I would for sure go with the 13" pitch for starters. Ask the dealer if you can give it a quick test and see if it works out. Some will let you do that as long as the prop is not damaged at all. And shy away from the Michigan wheels. Go with a Merc aluminum if cost is a factor. They really are decent props..

[Moser]

The lower unit on that motor is exactly the same as the much renowned 60 Bigfoot for the same year. I was looking at the Bigfoot when I bought my boat and when they said "It's the same lower as the 75 and 90." I said "OK then how much more than the BF is the 75?" Dude said "$150" I said "Gimme the 75," It was a 22-23 mph boat, best 'toon I ever had.

[Moser]

Kinda curious here Rick, why shy away from the Michigan Wheels? I never heard that before and since your's is an opinion I value I'd like to know why you don't like them.

[Captain Tony]

Welcome Silversnake: - Opinions are like belly-buttons. Everyone should have one. I've read the replies and all offer good advice. So naturally I thought back to my experience with changing props and after a lot of research here is what I've learned. You should always try to get the engine into the sweet spot of the Manufacturers recommended range. That doesn't always mean you have to run at WOT, (wide open throttle) but you need to let the engine develop its best torque within the RPM's it is designed for. At this point forget horsepower - it is what it is and you really won't be developing more horsepower but the torque is the most important twisting force that will deliver the thrust of pushing your boat to it best ability. Notice I didn't say to its top speed; but when you have 2 people and 1/2 tank of gas plus 150/200 lbs. of gear on board; you want the motor to run efficiently at the speed you want to cruise etc. The same is true when you have 10 people, full tank of gas and 250 lbs. of gear/tube/skis, grill etc. After looking at the specs of your engine and the gearing ratio I would say Rickdb1 has the right idea - aluminum Merc prop is good technology - it is less expensive, is ductile and will give a little, and easier to whack out a few small dings and chips. If you hit something hard with aluminum you can usually save the prop - hit hard with stainless and you may crack the lower unit housing at the plate. I also like Moser's idea of the 4 bladed prop but remember that going from a 3 to 4 will drop RPMS by about 200 RPMs for each 1 inch of pitch. 4 blades will also hook up better on hole shot and much less blow out on turns. I'd go with the 4 blade Spitfire w/15" pitch / 13.5" diameter. Fall back would be drop down to 13" pitch in a 4 blade if you don't get into the 5000 / 5200 RPM range at WOT.

[jdhacker]

I also have a question
Hello all I have looked on here for a good while. Lot of good info on here, and as usual new pontoon owner asking for help. I have had a bass boat for a long time. And know very little about pontoons. About three weeks ago I bought a 1992 25 ft crest, one owner and garage kelp. And feel like I got a great deal. The motor is a 70 hp Johnson with 14 hr on complete rebuild. The prop is a 14x11 and with 7 people on the boat I GPS it at 19 mph at 6000 RPM. But the boat will at full power will hit 6200 trimed all the way down. If I trim it up it hit 6500 and did not lose speed. hit 21 mph. I know the max rpm is 6000. Do I need a new prop, or just keep it below 6000. I have not had the prop off, but it looks like 14 1/4x11 not sure. It did surprise me with how it ran.

[Moser]

Welcome Hacker. if we get into your issue much more than just a couple replies I'll get you to start a new thread. I think I'd look into a 13 pitch prop, you don't want to be running in the 6500 rpm area.

[rickdb1]

I agree. Try a 13" pitch out. Might even pick up a couple of miles per hour too...