New prop, still problems.

leslie riggan

leslie riggan

Well-known member
I bought a new 4 blade comp prop for my 25 hp merc. I replaced my old 3 blade alluminum prop that was chewed up and causing slippage. The new prop draws so much water I think it is causing another problem. The prop performs fine at low speeds however at half throttle I start getting a "surge" type condition. The rpms go up and down up and down. When the rpms drop the boat pulls well, and then you can feel it "slip" and the rpms jump. Has anyone had these problems before? If so how did you remedy it?
 
I JUST BOUGHT A 4 BLADE SOLAS PROP FROM CABELAS FOR MY SNEAKBOX IT HAS A 15 JOHNSON AND WOW IT DID GREAT
 
Eric, I have the motor unlocked at the lowest pin setting. I did look at the level of the square three hole port on the shaft. I think this was usually under water with the old prop, but it wasn't today.
 
well, I guess I will buy another one. I have had this one for about a year and have no clue where the receipt is. I did however forget to mention (looking back), I stepped down a pitch from 13 to 11. I did this to help get up on top faster. Did I shoot myself in the foot?
 
I should have tached it out but I dont have one. There isn't a boat shop within thirty miles of here. It looks like I will go back with the original prop diameter and pitch 10&3/8x13p 3 blades.
 
ive heard tell of the comprops flexing under heavy loads, just what ive heard i broke mine too soon too tell im not a fan of comprops
 
I had one with a bad hub act like that when the hub slipped. Not a comprop though. Does the comprop have a rubber hub between the splined part and the prop itself? Might be worth looking at? I think any good prop shop can replace them.
 
Charlie, I don't think they have hubs..my 3 blade didn't. Leslie, you may be experiencing over revving. Going from 13 to 11 is a pretty good jump. Try it with a big load in the boat and see if that makes a difference. I used a 12 for most of the time on my Mariner 25 on the BB2 and picked up a 9 1/2 for when I knew I would have a max load. The 9 1/2 would just boil the water with a light load but would work much better with a "hunting "load. I forget what boat you are using it on.
 
I have never had good luck with 4 blade props on smaller hp motors. Every 25 I have ever owned always seemed to run best with a 12 pitch stainless prop when carrying a typical hunting load. You can get away with a 13 pitch running empty or in a light boat but most 25's just can't turn that much prop loaded down.
 
Lee

I too thought maybe he was over revving causing the over-rev protection to kick in and out. BUT, he said it was happening at half throttle which I can't see it occurring there. It's really hard to resolve these issues without tach data, especially when we are not there to see it.
 
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I think it's free wheeling. If you took the prop off and went to half throttle..it would over rev. That's why you aren't supposed to rev it up on muffs, no load. It could be cavitating, but I thought a 4 blade could run better, higher on the transom than a 3 blade. My 9 1/2 pitch would push till it all of the sudden would "slip" and just boil the water.
 
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This is the skinny on the load and boat. the boat is a double wide alluminum bass boat type jon. I cut out the bench seat and floored it with 1/2 treated ply. The 25 merc weighs probably 120 lbs. 2 Batteries one up front for trolling motor, and one starter. I have 8 gallons of gas 6 in the back two up front. I have removable seat boxes weighing about 35 lbs total. There were two people in the boat wieghing about 175 each. The total weight if I guessed would be 900 lbs...? My take was a lower pitch prop would give more low end and lose on the top end speed. I never go wot in this river, too much changes. There is a built in rubber clutch on the compprop just like the factory one. I know I burned the clutch on the previous prop because it would just freespin under heavy load and current. (I hit too many underwater floating logs, they are hard to see at 5 am in the morning.) Where do you get a tach for measuring the rpms ? I have no clue.
 
Leslie

I have a Actron multimeter that has one. Some folks have bought a "tiny tach" and permanently wired them to their boat motors. I'm sure there are other solutions too.
 
Thanks Eric, now I have a starting point. There's nothing worse than a piece of equipmet that doesn't work like it should!
 
That's interesting. My 3 blade Comp Prop didn't have the rubber hub. So when it surges up and down, it isn't sounding like it's screaming up to WOT and then dying down? Or is it the boat lurching? If it isn't severe, it sounds almost as if you have an air leak in your fuel system
 
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