Cavitation or something else?

[Misty IV]

I have a strange problem with my boat. 2000 Starcraft Islander, 4.3L Mercruiser, Alpha-One drive, 3-blade Black Diamond aluminum prop. The outdrive shifts into forward & reverse fine. When I am in forward and adding throttle to get up on plane to get moving, once the engine gets to a certain RPM (~2500rpm) and the boat is moving around 10mph, it sounds like the engine suddenly is spinning faster, but the boat isn't moving any faster, almost like something is slipping. The boat is still accelerating, but not at the rate it should be. Once the boat gets to a certain speed, maybe 15mph & 3000rpm, it's like the prop "grabs" and the boat starts to accelerate like it should normally. I've tried adjusting the trim of the outdrive, but nothing changes. The prop is on nice and tight, no excessive play in the shaft. This started this year. I have a repaired prop on the boat right now, same size as has been on there. I dinged my main prop last year at the end of the season, so am running the spare for now. Could it be a problem with the prop or the rubber Flo-Torq hub assembly? I don't think this spare prop is a Quicksilver model - no distinctive markings on it to say it is. I'm half tempted to go buy a brand new Quicksilver prop (which I need to do anyway) and try that out.

Any thoughts on this? Thanks!

[GAMBLER]

Sounds like the hub in the prop is going bad. I had the same problem with a 25hp Merc outboard. When the boat was getting on plane, it would do the same thing until it was on plane. Find someone with a spare prop and try that before buying a new prop.

[MCF]

I would agree that it sounds like a prop hub issue.

[ray koziatek]

did ya unhook the trailer from the truck ,or the boat from the trailer ?

if yer not driving around with the trailer under ya ,ya better hope the hub is bad,and not the little spline thingies at the back of the moter .

[Ray]

When the engine is spinning faster, is the temperature (of engine) getting hotter? Hopefully, it is just a prop issue and not a coupler issue.

[Misty IV]

Thanks for the replies. I haven't changed anything about the boat except the prop. Prior to me changing to the spare prop, I never had this issue. Since installing the spare prop, this problem developed. Common sense would tell me it is related to the prop, but wanted to ask and see if there was something else I should look for.

[snupytcb]

did ya unhook the trailer from the truck ,or the boat from the trailer ?

.

i have actually heard of this happening. i doubt it's true but the guy swears it is. the guy who did it could not get the boat to plane off. he had to get the manufactures rep to come to the launch to tell him you are supposed to take the boat off the trailer. as i said it cant be true but he says it is.

[ray koziatek]

Hmm maybe ya got a reverse ((rotating)) prop by mistake also ,,or put it on backwards ..

Maybe this will be the "Post of the Week"

[Misty IV]

Hmmm, Ray, I think you are onto something! That would explain why the boat goes backwards and the water is breaking over the transom! Hehehehe!

But, it appears to be the same prop as the old, dinged main one. Once the prop grabs, the boat takes off and pulls strong. Still tops out around 40mph. It's just that 10-15mph area the engine seems to rev up with no additional acceleration, then once the boat gets to a certain speed, the rev's drop and the boat takes off, like the prop is once again cutting into the water and doing what it is supposed to.

[ray koziatek]

ok,ok maybe it is cavation 40 mph is really bookin ,, cavation actually will cause heat buildup on the backside of the prop and you will see pitting on the surface...gotta be something with the dangle of the angle im justa plummer ..hey just dont go 10 to 15 mph

[W.W.IV.]

Misty,if your dinged prop is still usable put it back on and see if you still have the problem.

[FSNmachine]

Sounds like the prop hub is slipping. Not sure what type you have but is it torqued properly/

[Misty IV]

Misty,if your dinged prop is still usable put it back on and see if you still have the problem.

I'll be up this weekend and was thinking of swapping the dinged one back on just to see if it changes (which I have a strange feeling it will). Thanks for the suggestion.

[Misty IV]

Sounds like the prop hub is slipping. Not sure what type you have but is it torqued properly/

The dinged one is a stock Quicksilver 3-blade aluminum Black Diamond. I am not sure what this spare one is. It came with the boat when I bought it used. It has no distinct markings on it to tell the make or manufacturer; just the size: 14.5x19. Size is the same as the dinged one. I do know this spare one was once repaired, as it still had the repair shop's sticker on one of the blades when I put it on in place of the dinged unit. I'm thinking I'll swap the dinged one on this weekend and see if the problem goes away. I have a feeling it will.

[rolmops]

A "repaired" prop is usually a smaller diameter than it used to be.Although my first thought is with a spun prop,it may just be that the repaired prop is now a 13x19 .This will work fine as long as it does not have to work hard.What you describe is that the engine revving up is exactly at the point where the boat is struggling to get onto plane with the bow high up and the stern down and making a big wave.

It may not be a spun prop but just not enough prop and at the hardest point it just takes off but does not move enough water to push the boat along with it.

[dcso]

I had the same problem. at 3200 rpms i was getting 10mph last fall, new, older boat 86 bayliner trophy/305, The prop had some dings which the vendor fixed, when they called to say it was done I asked if it was spun( this was put in the repair request) the guy said " i don't know" consequently it was spun and has been repaired.it looks and runs great now. just make sure you ask!.

ted

[Misty IV]

A "repaired" prop is usually a smaller diameter than it used to be.Although my first thought is with a spun prop,it may just be that the repaired prop is now a 13x19 .This will work fine as long as it does not have to work hard.What you describe is that the engine revving up is exactly at the point where the boat is struggling to get onto plane with the bow high up and the stern down and making a big wave.

It may not be a spun prop but just not enough prop and at the hardest point it just takes off but does not move enough water to push the boat along with it.

That is about exactly when the engine revs up - when the bow is up and the stern is down, trying to get up on plane. Engine doesn't overheat and runs fine once on plane. I'm going to put my old prop on and see what happens. didn't have any issues with the old prop, so expecting the problem to go away.

[Big Dave]

cavation can blister the paint on the prop. The blades might be the same pitch , but things like cupped blades can make a differance in how well the prop bites into the water. They weld up the ends of a repaired prop to bring it back to "new" condition.

[Gill-T]

Could you have a problem with your carb?

[Misty IV]

Could you have a problem with your carb?

No, definitely not a problem with the carb. The engine runs fine, starts right up, trolls for hours at idle, runs like a dream. It's definitely something with the prop slipping & not pushing the boat forward when I'm trying to get on plane. Once it's on plane, the prop "grabs" and the boat takes off.