96 Mercury 150 xr6 on a 24ft Bentley 2 log pontoon new prop

Caseyjoe

Seaman
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Jun 19, 2017
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70
Had the high five 5 blade prop. Ended up being a 25 pitch. 6 people and pulling a tube made 20 mph at wot running 3500 rpm max. Mercury told me I need the black max 16x14 pitch 3 blade prop. Same 6 people and 1 on tube got me to 26mph at 4600 rpm. And it cavitates during a somewhat sharp turn. Anyone have any ideas where I should go from here?
 

ahicks

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Sep 16, 2013
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So you're getting closer! Sounds like 2 problems to me, or I guess that's how I'd approach it. First, the prop is still too big, but you've likely figured that out already. Any chance the dealer you got it done will help you out with that? Second, the cavitation deal, is likely best dealt with by lowering the engine. Hopefully, it's not already down as far as it will go?
 

Caseyjoe

Seaman
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Jun 19, 2017
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All the way down Hicks. Any ideas on what prop to try next? How fast should that boat be running at wot?
 

ahicks

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The speed will be whatever you get. This is a pontoon boat! This is more about WOT rpm. If you want max pulling power, you shoot for the top of the suggested power band (5600?). Just occasional pulling better cruise and mileage, something closer to 5000.

So, 6 people pulling a tube and you're able to get 4600. It sounds like that prop might work well as a cruising prop with no more load than what you had at max.

If you want more speed/power, you need to get your rpms up. A 16x13 will likely pick up about 500 rpm. However, this Solas prop is a little smaller diameter (15 1/2") AND less pitch (13"), so it may get you something closer to 700 rpm over what you have now. That should make it pretty close to max rpm when lightly loaded, and have plenty of poop when you're lugging 6 people around pulling a tube..... That's the prop would have my vote if that's the direction you want to go in. Keep in mind though - it's a best guess.

http://boatpropellers.iboats.com/Mer...t_id=473416844

A total WAG on top speed will be something pushing 30. No big surprise if you cracked it when lightly loaded going down hill with a tailwind. -Al
 

clemsonfor

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Sep 19, 2005
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Yep if your sure carbs are opening all the way up timing is advancing, I'm sure this motor is all electronic, I know old motors better. And you hitting on all. Cylinders the same.

Prop is still too big as suggested. Go down in pitch.
 

Caseyjoe

Seaman
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Jun 19, 2017
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So if I get my rpms up will my top end be faster? I know it's turning faster but pitch is also less. Kind of like putting small tires on a car. I would think rpms would go up with the same top speed.
 

HotTommy

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Mar 15, 2013
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Caseyjoe,
Think of this as a balancing act where you are trying to optimize the engines power output while maximizing boat speed and minimizing the slip of the propeller as it screws its way through the water. Speed requires power and the power of an outboard engine typically peaks in the optimum RPM range specified by the manufacturer. A common range is 5,000 -5,500 RPM. If the propeller has too much pitch as it did with your first one, the engine will not have enough power in the mid-RPM range to turn the prop much above 3500 RPM. Even at that speed the prop is trying to screw its way forward much faster than the boat is traveling, so the slip is very high causing cavitation. If there were such a thing as a 5 pitch prop, the engine could turn it easily to 5,500 RPM and there would be very little slip as the prop would not be advancing very far with each turn. Of course the boat would be going very slowly. The optimum combination is the prop that is agressive enough to push the boat forward as much as it can with each turn, while allowing the engine to reach its optimum power range and keeping slip at a reasonable level (e.g., 20%). The common way to find the optimum combination is, as was described above, to use a prop calculator to estimate the right kind of prop and then try it out. If the engine RPM is below optimum and all other possible causes have been remedied, then drop the pitch down a little more and try again.
 

ahicks

Captain
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Sep 16, 2013
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Right now, you're trying to climb a hill with a heavy load in overdrive, and the engine doesn't have the poop to pull the load. To improve that condition, you'll likely select a gear that will let the engine turn up to it's top rated rpm.

We're doing the same thing selecting a prop. Original prop was like being in overdrive. With your new prop, it's like you've downshifted one gear, but it's still not enough! I'm guessing one more downshift will do it.
 

clemsonfor

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Sep 19, 2005
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So if I get my rpms up will my top end be faster? I know it's turning faster but pitch is also less. Kind of like putting small tires on a car. I would think rpms would go up with the same top speed.

Yes if you get rpms up speed will be better and engine will be happier. Your lugging that thing now. Sure the prop is going less distance foward each rotation bit it will be turning faster and putting out more power when it gets up at full rpm where full hp is made
 
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