I just purchased an 08 18ft Misty Harbor pontoon w/Suzuki 70hp 4stroke.
I've been having problems with what appears to be prop ventilation. Under
full trim and only 2 people boat goes along fine. Put 1 person in front, and
engine revs wide open. 4 peope in boat w/ modest waves @2000 rpm it
happens. I've been back and forth with the dealer and he says "all pontoons do that", but he's willing to try and minimize this problem. I thought that
maybe the motor's set too high or wrong/bad prop. Motor cavitation plate
is at about middle of pontoons. Original prop is 13.75 x 13 running 5200rpm@wot. He let me try a 13.5 x 15 prop. This seemed to be very effective
at reducing the problem, but wot is now 4500 rpm. Is this a good solution or
should we try something else?
well you have a boat that is very touchy about balance, weight to the bow, stern rises, prop out of water. you could lower motor, but it will effect your speed. you need to prop it so that it is turning the proper rpms, for the motor, under normal load. again you are the Captain, people sit where you tell them. think of the center of the boat is a fulcrum, like a see-saw. also too much bow weight, if a wave break over the bow, the bow will submarine, go under, and will not come back up until you stop.
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First you need to get back to the proper rpm.I would think 5,200 would be the minimum.Usually they like to run the 4 strokes at or near their max.
I wouldn't think it is necessary to run full trim.In fact I would think on a pontoon it might even slow you down.I would start trimmed in with a well balanced load then slowly trim out to the apparent best speed without venting.You could try a 4 blade intended for pontoon application.
Also a stainless prop or a prop with a little cup added will resist venting.
You may have drop down in pitch with a cupped prop.
The best mileage for a pontoon is between 6 and 9 mph and it will do better than most planing boats at the same speed.
wi-rob,
Could we get some more information please? With the boat in the water, where is the waterline? Does the back of the sit low in the water? Where is the waterline on the motor? How deep in the A/C plate when the boat is at rest? Where is the AC plate in relation to the bottom of the engine pod? Are there other positions on the motor pod that engine could be mounted in to raise or lower it?
Here are some possibilities that occured to me. Maybe the motor is mounted to high. Since the RPM went too low with the prop that helped the problem, I don't think the prop is the problem, especially since the original prop was spinning at 5200. The motor probably shoud spin a little faster than that, at like 5500 or 6000. Check the operators manual. Another possible solution is adding a Hydrofoil fin to the motor, but I am sceptical in this case because I really thing the problem has something to do with the depth of the A/C plate/prop.
Some more questions; since it's a new boat, has the dealer sold this exact boat with the this exact same motor combination? Any issues with other boats like it? Have you contacted the boat manufacter?
Thanks and good luck with it.
5200 is the lugging point for that motor. I have the Johnson version of the DF70 and with a prop that turned 5200 @ wot I lost speed/rpm quickly with added weight/load on my fishing boat. I dropped down 2 pitches and it turns 6000rpm running empty with one person and handles heavy loads with ease now, I only lose 2mph and 200rpm adding 3 people to the boat. I'm in the process of putting the motor on a 20' pontoon I just bought and will be experimenting with props. If for some reason you have trouble finding 4 blade props you can always go to a Johnson/Evinrude dealer and get the special spacer for the Johnson 4 stroke 70hp motor which will allow you to run Johnson/Evinrude V4 13 spline props on your DF70.
I had the same problem with a 20' PartiKraft and 50hp Johnson. After talking to Godfrey (the mfg) they shipped me a new transom under warranty which set the engine back and down more. That took care of about 90% of the problem. It was still a little prop and turn sensitive but liveable. Use as much pitch as possible and stay in the reccomended rpm range. You could possibly use a jack plate to do the same thing. I now have a 21' Tracker with Merc Bigfoot. Those transoms are really long and low and I have not had any kind of problem.
Thanks for all the responses. Here's some more info.
With the original prop, I could not run at all unless it was at full trim.
No trim gage, so I'd hit the raise button and after 1 second problems occur.
The pontoons centerline (rear) is about 1" above the water with 2 people on board seated in back. The a/c plate is about 2" below waterline at full trim. The motor is fully down on the transom. The dealer seems new to Suzuki motors, and mostly sells smaller or even electric driven pontoons so I doubt that he's sold many like this one. He does imply that I'm the only person that
has made this problem an issue. I still need to contact him about the prop change results. Somebody mentioned talking to the manufacturer, and thats
where I think I need to go. I'll keep you all informed. Hopefully this can be resolved and the info can help someone else.
I'd put a double cupped 14 X 11" on there. It would get the revs up and help with the ventilation problem. RPM range is 5200-5800. Propping toward the top end will help your engine live a long life...
I think trimming and load balance is more of an issue for you than props. I have a 2008 Misty Harbor 1880CF (four corner fish) powered by a Suzy DF40 and I have to add a fair amount of trim to make it ventilate. The engine is mounted full down. With two people up front on the two fishing chairs I can make it ventilate quite easily at high throttle settings. Move those people back and it doesn't. You want to trim so the prop is pushing perfectly parallel with the water. This will result in the nose of the 'toons just slight up.
I have an 11P prop on this rig and it spins the engine at exactly 5800 which is its recommended WOT RPM. Pontoons typically have a great deal of turbulent water at the stern so a large diameter prop with less pitch, or a four blade pontoon prop (elephant ear) may be better in your application. You are lugging your engine so getting it back into its recommended RPM band is essential for best performance.
New pontoon ...need advice
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