We have a 2019 Crestliner 200 Rally FS (fish) pontoon boat with a 2019 Mercury 115 ELPT Command Thrust outboard. Purchased used in 2023 through a local dealer who sells used watercraft on consignment.

We hit a stump at low speed and had some significant vibration. Took the boat to a different dealer that services Mercury outboards to check out the lower unit, etc. Turns out the lower unit is OK other than needing new seals which I had them replace. I had also asked the dealer to check the damaged prop specs for proper match to the boat as I had not been able to get full RPM/speed with the prop that came with the boat. It was a 14.5x19 pitch. The service department told me we should have a 15 pitch prop for that boat and motor combo. They installed a new BlackMax with 15 pitch.

We picked up the boat and drove around to the nearest launch to us. Put the boat in the water and backed out, so far so good. After I got away from the shallow water near the ramp, I pushed the throttle forward and found I could barely make headway speed. 5.5 MPH at 2800 RPM, 7 MPH at 4000 RPM. I called the dealer and eventually spoke with the service manager. After some discussion around why they selected that prop, I said it's pretty clear there's a gross mismatch. I mean I was running 8 MPH @ 2000 RPM with the bent prop. So I will be taking the boat back, they will drop in the water near the dealership and sort it out.

What I would like to do is have a better idea of what prop they should be installing and point them in that direction.

Specs on the boat:
Length 22'1"
Pontoon length 20' (x2 pontoons)
Pontoon diameter 20" (0.80 gauge)
Dry weight 2,265 lbs
Fuel capacity 28 gallons
Max weight capacity 1918 lbs (doubt we ever exceed 1400 lbs, if that)
Draft up/Draft down 12"/21"

Motor:
Mercury model 1115F531D
Dry weight 359 lbs
Gearcase ratio 2.07

Is it a standard 115 or Pro XS? The rpm range is different between the two. Thats why I'm asking.

Also. With your old prop …what was your wide open throttle with correct trim angle rpm?

Gunny456 said:

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Crestliner should have rigging specs for that boat for all the rated HP engines for it.
They should be able to tell you exactly what prop specs and correct engine height settings.
Call Crestliner customer service and give them you year model and model # and engine year model and HP. They should be able to tell you the prop info immediately.

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Sounds good, thanks!

Gunny456 said:

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Is it a standard 115 or Pro XS? The rpm range is different between the two. Thats why I'm asking.

Also. With your old prop …what was your wide open throttle with correct trim angle rpm?

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I guess the standard, nothing about Pro XS comes up searching with the model number I posted. I guess that would be on the housing as well, right?

The specs for the motor are 5000-6000 RPM, wide open throttle. I never ran it much above 4000 because it felt like it would start slipping/cavitating with the 14.5x19 prop. I would like to get more out of it in case we need to make a run from a storm or something.

I don't know anything about setting up a pontoon but your description of the first prop and the new one both sound like the motor is mounted too high. Lots of slipping as you increase speed.

Col Steve. I'm asking you some questions to try and help you on prop selection for your boat:
You said it's powered by a 2019 Mercury 115 ELPT with a Command Torque lower unit. The Command Torque is usually what's put on pontoons. It allows a larger diameter prop than the standard.
However if it has the Command Torque the gear ratio should be 2:38 not 2:07 which makes a big difference.
My initial thoughts is that if you indeed have the Command Torque model ( Which I would think you do have) you can run a larger diameter prop than the standard 115 gear case.
You also need to make sure the engine is mounted at the correct height setting……
The correct "engine mounting hole" height can be obtained from Crestliner as I said above.
Your cavitation can be caused by propeller design or a transducer mounted in the water flow to the prop or incorrect engine height.
Crestliner is owned and operated by Brunswick Corp (Mercury) so they come from the factory with the engines pre rigged and mounted so it should be at the right height unless the engine had been removed by a dealer at some time for repower or such.
Cavitation or prop slippage is typically a fault of prop/error in selection or design.
I am friends with the regional Crestliner factory rep and can call him Monday for prop specs/ engine height specs for you if you like.
Glad to help if I can.

Im with Gunny that Rev 4 he recommended above is exactly what i was coming here to recommend. Great prop for that application.

If you want to check your correct engine height yourself to see if your engine is at least close to where it should be set ……there is a simple old school way of doing it.
I am attaching a you tube that shows a simple way of checking your engine height.
You want to start by leveling your cavitation plate with your power trim and then measure from the AT REST water line on
the pontoons down to your cavitation plate.
If the engine is set correctly that measurement should be, as a rule of thumb, between 3.5" - 4.0". You can use a straight edge of any kind to reach from the port pontoon to the starboard pontoon to measure from.
The video explains it.
Here ya go:

Thanks to all for the feedback, much appreciated!

Gunny, I dug a little deeper and agree the Command Thrust outboard should have the 2.38 gear ratio.

Thanks for the input on prop selection. Curious why you mentioned only stainless props. I am guessing the boat would have been equipped with an aluminum prop from the factory. I looked up some comparisons of aluminum vs stainless. One thing I read about aluminum is that it can help protect the drive shaft by absorbing the impact from stumps, rocks, etc. vs the more rigid stainless which can conduct more of the force to the drive shaft and cause more extensive damage. We're on Lake Livingston which is pretty "stumpy". I have learned many areas to stay away from but there are still "surprises" to be found. I won't be dropping this boat in saltwater ever, so not really worried about corrosion.

If you could make the call to the Crestliner factory rep to confirm what would have been installed by them, it would be greatly appreciated.

We'll be pulling the boat out and getting it on the trailer tomorrow afternoon. I will see about verifying the engine mounting height ahead of taking it to the dealer on Tuesday.

Col. Steve Austin said:

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..........Curious why you mentioned only stainless props. I am guessing the boat would have been equipped with an aluminum prop from the factory. I looked up some comparisons of aluminum vs stainless. One thing I read about aluminum is that it can help protect the drive shaft by absorbing the impact from stumps, rocks, etc. vs the more rigid stainless which can conduct more of the force to the drive shaft and cause more extensive damage. We're on Lake Livingston which is pretty "stumpy". I have learned many areas to stay away from but there are still "surprises" to be found. I won't be dropping this boat in saltwater ever, so not really worried about corrosion.

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Just a few thoughts,

Do you currently have aluminum? Aluminum will flex more. The bigger and heavier the boat and the more torque and power applied, the more it's going to flex. If you have aluminum, this could be causing some of your issues. A lot of people say to drop two numbers on pitch if going aluminum. So a 17 stainless would equal 15 aluminum.

The aluminum can help to protect the motor. Unless you think you might hit something at speed, I dont know if it's necessary. Ive hit thousands of stumps at Toledo Bend at idle speed and never damaged a motor. Lots of years of duck hunting, idling through dense stump fields. Not saying it wont happen but they're pretty tough as long as you're at low RPMs. Lots of high dollar bass boats idle through those same stump fields every day with stainless props and most never have a problem.

That is a myth about aluminum props. In the days when outboards had shear pins aluminum props would break easier than stainless steel so it was felt that the aluminum props would be safer for the prop shaft and driveshaft.
Modern props, both aluminum and stainless, now have expandable rubber or neoprene hubs that protect the driveline from shock and damage. If the prop is installed to the proper torque specifications it will really make no difference if you have an aluminum or stainless prop if you hit underwater objects as the hub will slip the same for either prop material.
Given the above, if you hit a stump with aluminum props it is basically a given that you are very likely to break a prop blade or damage the prop beyond repair.
However with a stainless prop you may have no damage from hitting that same stump.
Secondly, stainless props allow for much greater tooling and design technology. Stainless props can have design geometrics built into the prop blades ( Like leading and trailing edge cup, greater rake angles, etc.) that give the stainless prop much greater hydro dynamics to "hold" the water better, which enhances hole shot, greater turning and handling ability, greater top end speed, and just a much greater level of performance than what can be obtained from cast aluminum propellers.
In today's level of propeller technology, other than initial cost, there is no benefit of running an aluminum propeller. The benefits of the performance enhancement of stainless props far outweighs the cost difference.

I love it!!!

Thanks for the explanation Gunny!

CS78 said:

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Col. Steve Austin said:

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..........Curious why you mentioned only stainless props. I am guessing the boat would have been equipped with an aluminum prop from the factory. I looked up some comparisons of aluminum vs stainless. One thing I read about aluminum is that it can help protect the drive shaft by absorbing the impact from stumps, rocks, etc. vs the more rigid stainless which can conduct more of the force to the drive shaft and cause more extensive damage. We're on Lake Livingston which is pretty "stumpy". I have learned many areas to stay away from but there are still "surprises" to be found. I won't be dropping this boat in saltwater ever, so not really worried about corrosion.

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Just a few thoughts,

Do you currently have aluminum? Aluminum will flex more. The bigger and heavier the boat and the more torque and power applied, the more it's going to flex. If you have aluminum, this could be causing some of your issues. A lot of people say to drop two numbers on pitch if going aluminum. So a 17 stainless would equal 15 aluminum.

The aluminum can help to protect the motor. Unless you think you might hit something at speed, I dont know if it's necessary. Ive hit thousands of stumps at Toledo Bend at idle speed and never damaged a motor. Lots of years of duck hunting, idling through dense stump fields. Not saying it wont happen but they're pretty tough as long as you're at low RPMs. Lots of high dollar bass boats idle through those same stump fields every day with stainless props and most never have a problem.

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The damaged prop was aluminum, the current replacement prop is also aluminum but seems to be pretty obviously a complete mismatch for the boat & motor.

Col. Steve. Left you a PM with more information I have found out for you. From what info you have given me your motor is on the lowest setting so the engine height is not causing your issues.
I have sent you the recommendations on prop information in both SS and Aluminum types and diameter/sizes by PM.
The Command Thrust gear case is the same as on the 150 and is recommended for pontoon/tritoon use because of the gear ratio and the ability to handle larger prop diameters than the standard 115 gear case.
This makes the prop selection (diameter and pitch) different than what you would typically use on a standard gear case 115 Mercury.
Let me know if I can be of any further assistance.
Just fyi if you see this before your pm.
Take care.

Yes sir, got the message and prop recs. Very much appreciated!

The most important part to consider of properly propping your pontoon boat is the Command Thrust lower unit on your 115. It allows you to run the props that are used on V-6 outboards, which means you can use large diameter props. This is the key as pontoon boats, unlike regular performance planing boat hulls, can greatly benefit from using large diameter props with low pitch …..as this gives you more torque for control and handling and good fuel economy.
Many dealers do not understand the Command Thrust option on the Mercury 115 and therefore recommend the smaller diameter higher pitch props ( like the 14.75 x 19 you had on the boat originally)…..which is not the right combination for your boat.
Crestliner is owned by Mercury (Brunswick), so last night I made a phone call to a colleague who is the rep for Mercury's Quicksilver product line which includes their props.
He sent me a couple of screenshots out of an interview Boating Magazine did with their propeller product manager that confirms what we have been talking about. I am attaching those for your reading.
It confirms that your boat, with the Command Thrust lower unit, does indeed need the larger diameter, lower pitch prop to perform correctly…..backs up the prop recommendation info I sent you yesterday from the dealer I told you about.
Hope this helps.

Gunny, I'm going with the aluminum prop of the two recommended by your dealer buddy. That's the Mercury Black Max XC Pontoon 16x13. After reading about the design features specific to use with pontoon boats, it sounds like a perfect match. Also saw where it is specifically recommended for use with Mercury 90 & 115 HP Command Thrust outboards. I called the dealer and he got it on order and will credit for the prop previously installed. The new prop should be available for installation Tuesday/Wed next week. I will report results after I get the boat back in the water. At the much lower price point of the aluminum prop, I can easily afford to buy a backup to keep on the boat "just in case". Didn't have one previously.

Just for info to others following the thread, I verified the motor is installed at the lowest setting. Gunny's contact said it should be there or the next hole. I was going to check the height of the cavitation plate in relation to the "normal waterline" on the pontoons. But there's no definitive waterline showing on the toons. I guess it doesn't sit in the water enough to establish that reference point.

Pretty much a given that heavy slower boats tend to use lower pitch bigger diameter props. Higher Rpm that tries to move too little water too fast relative to hull speed will cavitate. It does help to be aware the gear ratio is higher which also generates less prop shaft turns per crankshaft revolution which makes a lower pitch bigger diameter even more important. A 4th blade for more effective moved water surface area also helps at low to mid rPM applications.

But you gotta have the lower unit that can handle the large diameter too.

Yep, bigger gearing. That's why I love my Yamaha F70. It has the same lower unit as the 90 and 115. Same one that used on my old C60, essentially. Less hydrodynamic but it is big and tough relative to the outboard.

The merc 115 with the higher gear ratio sould have effectively more torque at the prop at a given RPM to make using it with a bigger diameter prop pushing a larger cross section of water more manageable. The lower shaft rpm will help prevent cavitation if the prop grabs enough water surface area and has less slip.

That's why I recommended the XC prop. XC is for extra blade cup to help hold the water better but that additional blade cup on the trailing edge also creates lift which help the run and handle better.
His complaint was cavitation at higher RPM's. Prop blade design and diameter is paramount in preventing that. However, as you know, outboards develop their horsepower with RPMs. That 115 of his doesn't make the 115 HP till it's at 5500-6000 rpm. He needed a prop that would accomplish that at WOT and not cavitate.

Col. Steve Austin. In one of my post I mentioned that prop cavitation can also be caused by something disrupting clean water flow to the prop. This could be a transducer or a structural piece of aluminum brace for the pontoon or motor pod.
Just saying to check for that as well.
Lots of dealers mount transducers on the right side….directly in the path of the right side of the prop….which is notorious for causing prop cavitation.
Only one prop blade is actually creating thrust as it rotates and that is during the downward rotation on the right side of the gear case. That is what typically causes steering torque pulling to the right. So the right or starboard side of your gear case is the area that is susceptible to any disruption of water flow….like a transducer or such stuck down in the water there.
Hope this makes sense. Just check that for the hell of it.
Would hate for you to spend the money on the new prop to still have it cavitate because somebody mistakenly installed a transducer or such right in the path of water flow to your prop.

Yeah, unless you have a power Jack plate, fine tuning this can be a pain. I'm still trying to work out if I am getting a bit of cavitation at higher rpm on my skiff as the rpm goes up a tiny bit if I have it where I mount it now but I hear a bit of a crunchy whine at high speed but if I go down one hole, it has just about the same top speed, a tiny bit lower max rpm at WOT, but I don't notice that sound as much. I'm almost always running solo so I can't go watch what it is going back there at higher speeds to see how the cav plate is sitting in the water flow off the hull. With a hydraulic Jack plate I could fine tune it but I can't afford that extra weight on a small skiff.

Bobs and TH marine make lightweight jackplates for smaller horsepower that do well on a skiff. I have used 4" setbacks as i dont want the motor cantilever back and I want it tucked in to the tunnel as much as possible. I like the Bob's then mount the pump as far forward as possible and the minimal added weight is negated. And worth it for the adjustability.

What Grem says. The guides up here on the White River have very small hydraulic jack plates for their small ouboards and trout fishing boats. Think they are made locally. Extremely light weight and self contained….no external pumps to mount.

Gunny456 said:

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Col. Steve Austin. In one of my post I mentioned that prop cavitation can also be caused by something disrupting clean water flow to the prop. This could be a transducer or a structural piece of aluminum brace for the pontoon or motor pod.
Just saying to check for that as well.
Lots of dealers mount transducers on the right side….directly in the path of the right side of the prop….which is notorious for causing prop cavitation.
Only one prop blade is actually creating thrust as it rotates and that is during the downward rotation on the right side of the gear case. That is what typically causes steering torque pulling to the right. So the right or starboard side of your gear case is the area that is susceptible to any disruption of water flow….like a transducer or such stuck down in the water there.
Hope this makes sense. Just check that for the hell of it.
Would hate for you to spend the money on the new prop to still have it cavitate because somebody mistakenly installed a transducer or such right in the path of water flow to your prop.

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The transducer is mounted behind the left pontoon. So, should be fine I guess.

Yes sir. Just look forward of your prop under the boat and make sure there isn't anything hanging down that could disrupt smooth water flow. Something as small as a temperature probe mounted too low can cause it.

Gunny456 said:

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Yes sir. Just look forward of your prop under the boat and make sure there isn't anything hanging down that could disrupt smooth water flow. Something as small as a temperature probe mounted too low can cause it.

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Okay, I will take a look when I get the boat back.

Gunny, no luck with the new prop. Sent you a PM with details.

Got your PM. Replied by PM.