Will prop change help on 50hp Mercury

16.5ft Alumacraft Prowler with a 2017 Mercury 50hp. Boat ran 35mph wide open when I first got it but since adding another battery for the Ultrex, Spyder Lok rod holders and transport rack it now maxes out at 31.6 to 32.0. I'm not trying to win any races but it feels really lazy. Current prop is a 11.4 12P 3 blade and max RPM is 5900. If I went to a 11P would the RPM be too high and would it even help the top end MPH? Current prop has a little damage and I feel some vibration just above idle so I want to replace it no matter what. Any suggestions are appreciated.

Thanks..

At 5,900 rpm at wot...I would stay with same size and pitch prop.

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Originally Posted by "G"

At 5,900 rpm at wot...I would stay with same size and pitch prop.

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Thank you sir...

You are probably just sitting too low in the water, switching to a 4 blade will give you more stern lift.

Where is your cavitation plate in relation to the bottom of the boat? Like G said, 5900 is pretty close to right, dropping down a pitch is going to likely put you all over the rev limiter, and reduce your speed even more. You could go up a pitch and possibly gain a little speed, but your hole shot will suffer.

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Originally Posted by DHC

Where is your cavitation plate in relation to the bottom of the boat? Like G said, 5900 is pretty close to right, dropping down a pitch is going to likely put you all over the rev limiter, and reduce your speed even more. You could go up a pitch and possibly gain a little speed, but your hole shot will suffer.

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Plate is right at 2.5" below bottom of the boat. I have experimented with a Stingray hydrafoil because it takes a while to plane out. Seems it runs about the same speed with or without.

Thanks for the responses guys.

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Originally Posted by need2befishin

Plate is right at 2.5" below bottom of the boat. I have experimented with a Stingray hydrafoil because it takes a while to plane out. Seems it runs about the same speed with or without.

Thanks for the responses guys.

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The anti-cavitation plate should be even with or just above the bottom of the boat. Google it and see what I am talking about.

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Originally Posted by Barnacle Bill

The anti-cavitation plate should be even with or just above the bottom of the boat. Google it and see what I am talking about.

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What he said

Well, the motor is mounted as low as it can go. I have several holes I can use to move it up on the transom. Looks like all the control cables and wires have plenty of slack to support a move upward. Shouldn't be that hard to move it. Any idea what this engine would weigh?

Thanks again for the info guys..

Attachment 338450

Attachment 338452

320lb plus oil and prop. Alumacrafts like the motor high, id go 2nd from the top.

Move it up one bolt hole at a time. Easy way if you don’t have a floor jack. Let motor down. Crank trailer dolly down, put a block or 2 of wood under scag, loosen top 2 bolts and remove bottom 2. Then crank tounge of trailer up an inch or 2 and see where your bottom bolt holes are. May have to play with it just a little but you will get the aligned. Then snug them up and go try it. May have to go up another hole but I’m thinking 1 should do it. Watch your RPM, you may well have to go up a size on prop.

Move it up one bolt hole at a time. Easy way if you don’t have a floor jack. Let motor down. Crank trailer dolly down, put a block or 2 of wood under scab, loosen top 2 bolts and remove bottom 2. Then crank younger of trailer up an inch or 2 and see where your bottom bolt holes are. May have to play with it just a little but you will get the aligned. Then snug them up and go try it. May have to go up another hole but I’m thinking 1 should do it.

Sincere "thank you" to all you guys for helping me out. I will make suggested changes and report back asap.

Each hole is 1/2" I believe. You are 2.5" low so at least higher by 2 holes would be a good start. One hole won't do much for you IMO. You don't have to be exactly level with the bottom of the boat, that's just a starting point. You can be one hole one way or the other and improve performance.. its sort of a trial and error thing. 2.5" below the transom is definitely way too low though. You are also going to go up in RPM and you will not have to trim as much to raise the bow, so be aware of this... watch that tach so you don't over-rev that motor... it'll log when you over-rev it and merc doesn't like that! Your rev limiter won't kick in until 6300 ish, but you want that motor in the 5800 range.

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Originally Posted by Kevin22

Each hole is 1/2" I believe. You are 2.5" low so at least higher by 2 holes would be a good start. One hole won't do much for you IMO. You don't have to be exactly level with the bottom of the boat, that's just a starting point. You can be one hole one way or the other and improve performance.. its sort of a trial and error thing. 2.5" below the transom is definitely way too low though. You are also going to go up in RPM and you will not have to trim as much to raise the bow, so be aware of this... watch that tach so you don't over-rev that motor... it'll log when you over-rev it and merc doesn't like that! Your rev limiter won't kick in until 6300 ish, but you want that motor in the 5800 range.

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Thank you :ThumbsUp

The other guys beat me to it, but yes, that motor absolutely needs to come up. You will likely see a pretty significant rise in rpm’s, and might even be able to go up a pitch in prop and gain some more speed. I would go up at least two holes and go run it and see where you’re at, but I bet you may end up needing to go up 4 holes if there is a 1/2” gap between holes that the other poster mentioned (that sounds right in my foggy memory). That would get you a 1/2” low on the transom. I would guess that the motor will end up liking a 13p prop, and possibly even a 14p depending on RPM’s. I have a 14p Yamaha black stainless on my 1556 and it does well.

Moved motor up 2 holes and got to try it this morning. Gained almost 2mph (1.7) RPM was 6150/6200. Seemed to bite good at takeoff and had no issues turning sharply under power. What would you guys suggest for a new prop? Go to 13p and keep same diameter? 14p??

Thanks guys

I think you will for sure be fine with a 13p, and it might could even turn a 14. Your holeshot with be a tad worse going to a 14 but theoretically your top end should be better as long as it can get to the right RPM’s. Best option would be able to test and try both if you know anyone with some props you could borrow to try. I would keep the same diameter.

If your motor was indeed 2.5” low, you could probably still come up even more and gain some more RPM’s, which would allow you to run the 14p prop more effectively. Like I said in my previous post I’ve got a 14p on my 1556, although I would guess it’s a little lighter than your boat.

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Originally Posted by DHC

I think you will for sure be fine with a 13p, and it might could even turn a 14. Your holeshot with be a tad worse going to a 14 but theoretically your top end should be better as long as it can get to the right RPM’s. Best option would be able to test and try both if you know anyone with some props you could borrow to try. I would keep the same diameter.

If your motor was indeed 2.5” low, you could probably still come up even more and gain some more RPM’s, which would allow you to run the 14p prop more effectively. Like I said in my previous post I’ve got a 14p on my 1556, although I would guess it’s a little lighter than your boat.

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Thank you sir. I'll provide feedback once I get to try the props you suggested.

Figure 200 rpm per inch of pitch so the 13 should put you right at top but then as you increase load those are going to drop. 13 over all should be best for hole shot and may even gain another 1-2 mph. Like DHC said, ideal solution is if you can try a couple of different props.

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Originally Posted by Cray

Figure 200 rpm per inch of pitch so the 13 should put you right at top but then as you increase load those are going to drop. 13 over all should be best for hole shot and may even gain another 1-2 mph. Like DHC said, ideal solution is if you can try a couple of different props.

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I still wonder if going up the other two holes and getting his cav plate level would allow him to turn a 14 plenty well.

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Originally Posted by need2befishin

Moved motor up 2 holes and got to try it this morning. Gained almost 2mph (1.7) RPM was 6150/6200. Seemed to bite good at takeoff and had no issues turning sharply under power. What would you guys suggest for a new prop? Go to 13p and keep same diameter? 14p??

Thanks guys

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A suggestion is try piranha props, they are composite props with replaceable blades. They will work with you to get the performance you want and changing blades is free to get the setup you want. Just an idea

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Originally Posted by RobAnderson

A suggestion is try piranha props, they are composite props with replaceable blades. They will work with you to get the performance you want and changing blades is free to get the setup you want. Just an idea

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I’m not saying they don’t work, I’ve never tried them or even heard of them until you just mentioned them, but that sounds like a nightmare if you get into an underwater obstruction of any kind with the prop spinning. Plus, wouldn’t they have a lot of flex/slip?

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Originally Posted by DHC

I’m not saying they don’t work, I’ve never tried them or even heard of them until you just mentioned them, but that sounds like a nightmare if you get into an underwater obstruction of any kind with the prop spinning. Plus, wouldn’t they have a lot of flex/slip?

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Surprisingly not, they have the performance and durability of stainless but not the cost. I ran them on my ski/wakeboard boats when the kids were growing up. That has been the first thing I change when getting a boat since I found out about them. Never had one slip on the hub. It easier and cheaper to replace 1 blade when /if you ever mess up

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I would go up one more hole and see what happens before buying any props. My guess is 14P will be what you end up with, Like I said you want that motor running 5800rpm standard so when you have a light load you are touching 5900-6000 and heavy weight you are still above 5500. They like to be at the top end of the WOT range but you don't want to be running over 6K, and it's a bit of a pain to have to watch the rpm every time you take off and trim up. I run two props on my 90 4s, a 19 and a 17. 19 for light loads, 17 for heavy loads (family trips and my catfish setup that adds a lot of extra weight). I get lazy a lot and just leave the 17 on it then have to watch RPM or she will jump to 6400 before you can blink when I touch that trim button. It's a bit of a pain.

I had a 17'5" Alumacraft Pro 175 with the 2 stroke merc 50. Loaded with everything, 2 people and water in livewell it would run 32-35. Different style boat, but I had a 3 blade stainless from Powertech Props. I ran just under 6000 during summer and just over 6000 in spring with cool air. 1 person, no water in livewell and cold temps it would push 40, based off of spedo on boat.

Well, went out this morning dodging thunderstorms for a test run with a 13p. Left the motor in the same position and almost hit 36mph at 5850rpm. (35.7 to be exact) Holeshot is a little slower but very pleased with top-end speed and handling. If I find someone with a 14p i will give that a whirl but for right now I'm really happy with the result.

CDC never disappoints...Thanks to everyone that offered me knowledge, wisdom and advice.