Any boat propeller experts on here? [update #3 Pg4]

Is it a stainless or aluminum prop?

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When you give it wide open throttle the RPMs jump up to 7k and you hit the rev limiter and the boat doesn't move as the prop totally loses it's bite

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At around 60% throttle we're at 5.5k rpms and any more throttle input and the prop will lose it's bite and start spinning.

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Sounds like a spun hub to me. Did you take the prop off to check? A lot of times they'll put boats in the water to test drive.

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Motor probably needs to be lowered on the transom

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This would be my first guess.

Also check the weight ratio, make sure you not bow heavy.

Could be a few things.....

1. Motor is to high
2. Not enough cup in the prop
3. You are trimmed up to high
4. Spun Hub

If I was running a 90 on a 22' pontoon I would run a 4 blade prop. You'll have more hole shot, load carrying capability and control.

An easy check is whether or not the cavitation plate is above the water line. If it is, you probably got a short shaft engine on a boat that needs a long shaft, or something along those lines. Got a slime line on it from yesterday you can check?

I don't have all the facts for your situation to properly troubleshoot. 60% throttle at 5,500 rpm! During launch? During cruise? ?? At first glance, yes - you are significantly under-propped. Still, I'm a retired engineer & past 60 MPH+ bass boats' person with prop experience. I will add my 2 cents.

1. Prop (specifically, anti cavitation plate) must be totally below water level during and slightly after "launch or takeoff". If not, your stock engine will over-rev and SD to protect itself (like revving in air). See DownshiftAndFloorit post. Verify that plate & prop are well below water at rest. If not, address this issue.

2. If you checked off on #1 above, you are then significantly under-propped (by at least 2" pitch, IMO). Black painted per you means you have a black painted aluminum prop, not SS regular cupped or cleaver type prop (regardless if 3 or 4 blade). Aluminum props are relatively cheap to obtain. I am like others and more or less dismiss hub slippage as your problem (I may be wrong). Regardless, get a similar (3 blade per your posts) prop 2" to 3" higher pitch to fit your motor/lower unit and put this prop on. Pitch of your existing prop should be pretty prominently engraved on hub of your existing black painted aluminum prop. Purchase new size accordingly. Cost of new prop is pretty insignificant (IMO) compared to overall cost of your rig. I mean, you are not trying to buy a $500+ SS prop or props. Run your boat. RPMs now? Should be in range of 5,000 to 5,500 rpm (5,500 - 6,000 rpm max.).

If you could get propped to put your engine rpm in the (non cutout) range, you could actually calculate pretty much the pitch of your new required prop to achieve about 5,500 or so max. rpm. Would be simply a ratio of (actual experienced engine rpm/desired engine rpm x the " pitch of existing prop).

For example, if your existing prop is 21" and your max. speed is 6,800 rpm: get a 5,500 rpm/6,800 rpm x 21" = about a 17" pitch prop (in your case 3 blade aluminum prop).

Edited next day. I am so sorry to OP if I misled with the example in the above paragraph. Why did anyone not correct me? I got (only) above paragraph bass ackwards. The paragraph above beginning "If you could..." is totally correct.

My actual recommendation should read: "For example, if your existing prop is only 17" pitch (and over revving): get a 6,800 rpm/5500 rpm x 17" pitch prop = about a 21" pitch prop. Higher pitch new choice actually slows down (bogs down) the engine rpm. Understanding this is important. Again, sorry to wrongly mislead. I feel so bad about this wrong example. Not trying to "bump" this thread.

Hope I helped.

Been too busy traveling for work to deal with the boat so the FiL was handling this for me while I was out of town. Dealer tells him we have a spun prop and normally Mercury doesn't cover that under warranty but he'll put a new prop on it for us for free. And since they are putting a new prop on it for us they are going to put a larger diameter prop on it at the same time to give it more "bite". Now I think they are either liars or incompetent and just trying to cover their asses or maybe a little of both. I debate calling them out on their bullshite but at this point I just want the prop on and back in the water and not have to deal with these guys ever again. FiL and BiL pick the boat up and get it back in the water. Say it's driving much better now.

Spent the weekend on the lake. Took the boat out by myself to measure the prop/engine performance at WOT and the stats are:

25mph
5200rpm

RPMs seem low to me for a completely unloaded boat. It's going to be way below the 5k-6k recommended rpm range once the pontoon is loaded up. Take a look at the Prop and they put a 15P on there instead of a 13P like it originally had.

Loaded 4 adults, 1 kid, couple of coolers, bimini top up towable shoved between the tow bar and rails acting as a parachute I came up with these numbers.

PROPELLER PITCH
15

GEAR RATIO
2.38

ENGINE RPM
4700

ACTUAL SPEED (MPH)
21

PROPELLER SLIP
25%

The speed seemed plenty fine. Would like to get the RPMs up into the correct range. If I add another 2 souls on board I would think the 13P would be over propped as well? Can I add RPM without significantly reducing the top speed? Mercury sells "XC" versions of their props which are supposed to be Pontoon specific with more cup but will adding cup just negate reduction in pitch so I'm back at the same RPM range? 4 blade reduce my top speed too much? Slip seem OK?