Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

Captain Gecko

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Aug 30, 2011
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51
Hi,

Not sure if this is the correct forum, apologies if not but I don't now if this is a boat, engine or prop issue.

My 24' cuddy runs perfectly. 5.7 merc 260 alpha one. Engine starts and runs smoothly. Cruising speed I see around 25 knots at about 3800 rpm. Pushing to 4000 rpm I see about 28-30 knots.

Once I start pushing to WOT (4200 rpm plus) I get a recurring issue. The engine will rev high and the boat loses speed. It's like the prop is losing traction. Ventilation? Not sure. I have to cut the throttle, wait for it to 'grab' traction again and then it's fine.

What I have found is the deeper I have the engine trimmed, the earlier the issue occurs. If I trim all the way down, it occurs very early, far before WOT. If I trim higher, the issue holds off until higher RPM. If I trim even higher (probably about mid-way through my range), I can pretty much go WOT. However, the instant I trim down a little bit at WOT, the issue hits. Very predictable, I can re-create it every time.

The trim tabs also seem to affect the issue. With the trim tabs all the way up, it occurs earlier. With the tabs down, it helps reduce the issue.

The problem is, if I keep the engine trimmed to the position where the issue does not occur and I can hit WOT, the boat porpoises heavily. So I'm obviously trimmed too high at this point.

Other than this, the boat runs very well.

Ideas?? TIA!
 

Maclin

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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

Did this just start happening?

It could be prop ventilation, but the way you describe how you can re-create it seems more like it is related to the load on the prop and not so much the attitude of the prop or speed of the hull. To me that indicates a prop slip problem. If it was the engine coupler you would have described burning rubber smell. That and I don;t think a coupler can hold at all once it has begun slipping. It is odd for a prop also, but I know of instances here and on other sites that the prop was the problem in a case like yours.

It is very easy to check for prop hub slip if you can get at the prop out of the water. Mark the prop from edge to edge across the hub then run it again and see if the marks are still lined up exactly.

It may also be worth just putting on a different prop, same pitch and all, and see if it behaves differently.
 

The Rooster

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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

Almost sounds like you've spun the bushing in the prop hub, but it would be unusual to be able to run near WOT if that had happened.
 

Captain Gecko

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Messages
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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

It has occured since I bought the boat earlier this year. No burning rubber smells. Other than this boat runs perfectly. I have read about marking the hub/prop to check for slippage. Will give that a try, might try another prop as well. Was wondering if I was doing anything wrong operationally or if it was something else obvious. I'm pretty new to boats. Thanks again.
 

etracer68

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Oct 11, 2009
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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

Sounds like it could be a prop/hub issue. I would take the prop off, and mark the spline face, then mark the outside hub inline with the spline mark. Put it back together, and try it out, then check to see if the marks are still inline with each other, if not, you will have to get the prop rehubed.
 

QC

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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

Your combination does seem a little odd, but I am still leaning toward ventilation. Anything at all on the bottom that may be contributing?

The trim tabs also seem to affect the issue. With the trim tabs all the way up, it occurs earlier. With the tabs down, it helps reduce the issue.

The problem is, if I keep the engine trimmed to the position where the issue does not occur and I can hit WOT, the boat porpoises heavily. So I'm obviously trimmed too high at this point.
Can you control the porpoise by adding some down tab? Doesn't fix anything really, but you could run faster without the issue this way.
 

Captain Gecko

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Messages
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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

QC - yes, I can use the tabs to control porpoising, but then I lose some speed. Bottom of the boat is not too bad, there is some slime/algae, it's been in the water (fresh water) all season at the marina.

Sounds like I will need to mark the hub as a starting point.

Just wondering...if you trim down, wouldn't that create more resistance for the engine? i.e. increase drag since you are pushing the bow down? That might be where the increased load causes the slipping point on the prop hub? Hence why trimming up (reducing resistance and load) allows for higher speed before it slips/ventilates? Just theorizing out loud.
 

Bondo

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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

QC - yes, I can use the tabs to control porpoising, but then I lose some speed. Bottom of the boat is not too bad, there is some slime/algae, it's been in the water (fresh water) all season at the marina.

Sounds like I will need to mark the hub as a starting point.

Just wondering...if you trim down, wouldn't that create more resistance for the engine? i.e. increase drag since you are pushing the bow down? That might be where the increased load causes the slipping point on the prop hub? Hence why trimming up (reducing resistance and load) allows for higher speed before it slips/ventilates? Just theorizing out loud.

Ayuh,... That's a problem all by itself,...

Just how beat up is the prop,..??
What is the Prop,..?? size,..?? material,..?? model,..??
 

Captain Gecko

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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

Bond-o - yes, cleaning the hull will be the first thing I do once I get it out of the water. Could that be affecting the prop issue? Does the slime mess with the water flow, creates uneven flow across the hull, increases friction, etc?

I only looked at the prop briefly since the boat has always been in the water, haven't had a close visual inspection. It appears clean, slight wear but no major damage that I can see. Per the literature I *believe* it is an aluminum prop, 14.5"dia x 17" pitch.
 

Bondo

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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

Bond-o - yes, cleaning the hull will be the first thing I do once I get it out of the water. Could that be affecting the prop issue? Does the slime mess with the water flow, creates uneven flow across the hull, increases friction, etc?

Ayuh,... That'd be correct...

It appears clean, slight wear but no major damage that I can see.

'n that could also possibly be the problem, or combined with the dirty bottom, makin' things worse...
 

oops!

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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

i agree with bondo

the causes for increased rpm with no forward movement are

coupler
drive//ujoints
prop hub
ventelation/cavitation.

i think we can eliminate 1,2,and three.

this leaves #4. when you are on the water.....i would jump in with a set of goggles and look at the hull......even tho it seems clean.... there could be growth on the lowest part of the hull that is adding to the ventilation
 

chriscraft254

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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

I'm guessing you have a spun prop. If it is less likely to do it while trimmed up, it is probably because it has less pressure on the prop. I would look at the prop before anything else. Yes, under growth can cause issues but it rarely does in one season. You would have to have alot of growth. Good luck
 

QC

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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

Maybe, but spun hubs RARELY can make speed too. Possible, but not probable. A seasons worth of growth is not significant? Never heard that before . . .
 

chriscraft254

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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

Maybe, but spun hubs RARELY can make speed too. Possible, but not probable. A seasons worth of growth is not significant? Never heard that before . . .

He said its got some algae growth and thats basically it. One season in fresh water usually will not give you alot of growth. Some of this depends on wether the bottom is painted and with what. It also depends on what area you are boating in, because different areas, even different lakes in the same area have different growths that accure. Some areas are worse than others. Most boats slipped get a good amount of growth on the outside edges of the hull before anything will spread to the bottom. The bottom of the hull where it would effect the flow of water to the motor if used pretty often will not get much growth because it is warn off while boating.

The op said the boat slips alot more when tabs are not deployed, this could be because it is taking much more rpms and torque to get the boat to plane. When the tabs are deployed, it takes less rpms to plane, thus less torque on the prop. Or atleast less time to plane because the boats stern is pushed upward by the tabs instead of just by the prop.

It is hard to get on plane with a spun prop though, that is the only thing that is confusing me. I guess it would all depend on how spun it is. Easy way to check, is buy a new prop and hub and retest. If the prop isn't the issue, you will atleast have a spare prop, which you should always carry anyway.
 

QC

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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

It is hard to get on plane with a spun prop though, that is the only thing that is confusing me. I guess it would all depend on how spun it is. Easy way to check, is buy a new prop and hub and retest. If the prop isn't the issue, you will atleast have a spare prop, which you should always carry anyway.
That is NOT how you check for a spun hub. All replacement will tell you is that the new prop is not doing the same thing. Post #2 describes how you properly diagnose a spun hub.
 

TilliamWe

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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

... One season in fresh water usually will not give you alot of growth. Some of this depends on wether the bottom is painted and with what. It also depends on what area you are boating in, because different areas, even different lakes in the same area have different growths that accure. Some areas are worse than others. Most boats slipped get a good amount of growth on the outside edges of the hull before anything will spread to the bottom. The bottom of the hull where it would effect the flow of water to the motor if used pretty often will not get much growth because it is warn off while boating...

Chris, I have so much to argue with you about in this paragraph that I can't even figure out how to begin. Suffice it to say, you're mostly wrong.

That said, growth on the bottom usually causes "lugging" of the engine, not slippage of the prop. The engine will not get close to it's WOT rpm, but it will still operate properly at speeds lower than that. A fouled bottom and lugging the enigne causes a whole different set of issues, trust me, i have been there and done that.
 

chriscraft254

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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

That is NOT how you check for a spun hub. All replacement will tell you is that the new prop is not doing the same thing. Post #2 describes how you properly diagnose a spun hub.

Umm, you said the #2 post was correct, well after he explained the way of checking the hub, he also said,
"It may also be worth just putting on a different prop, same pitch and all, and see if it behaves differently." So correct me if I'm wrong but I think we said the same thing!:p
 

QC

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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

The scribe method is how you check for a spun hub.
 

chriscraft254

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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

I didn't say anything about growth causing slippage. I did say that "one" season of growth is usually not enough to effect your performance if used often. Also said it depends on area as to what type of growth you get and how much. This can all depend on what temperatures the water gets to also.

If I am wrong, please do elaborate! I am always open to learning more about different subjects. ;)
Chris, I have so much to argue with you about in this paragraph that I can't even figure out how to begin. Suffice it to say, you're mostly wrong.

That said, growth on the bottom usually causes "lugging" of the engine, not slippage of the prop. The engine will not get close to it's WOT rpm, but it will still operate properly at speeds lower than that. A fouled bottom and lugging the enigne causes a whole different set of issues, trust me, i have been there and done that.
 

QC

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Re: Boat, prop or engine issue? 24' cuddy loses traction at high speed 5.7 merc a1

I am always open to learning more about different subjects.
Who are you and what have you done with Chris? :p
 
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