Feels like spun hub but isn't

[tech_dog]

I'm hoping for some diagnostic help. Thanks in advance for any suggestions.

I've got a 30 year old Yamaha 175 outboard with an intermittent surge in RPM and loss in power, like I'd expect from a spun hub or ventilation. Reducing RPM stops the slipping, and you can slowly increase power, sometimes back up to full throttle.

It's worse with more weight on the boat. It sometimes goes away for days and then comes back again.

The boat is shifting well, running well, and there are no unusual noises or clunking. Recovery is smooth too.

I marked the hub and prop, went until I triggered a slip, but the marks remained aligned. I don't think it's a spun hub.

This is the same prop, trim, and mount I've used for 30 years. It really cant' be suddenly ventilating.

What other problem can cause smooth intermittent slipping, with no grinding or clunks? I'm at a loss. Any ideas?

Thanks again,

T.

 

[PointedRose]

Any obvious issues with the prop itself? Dings, chips, uneven shape, etc. same with the hull bottom…any barnacle growth ahead of the transom?

 

[Sardinia306Canyon]

This is the same prop, trim, and mount I've used for 30 years. It really cant' be suddenly ventilating

Ventilation problems can come suddenly if you changed something f.e. in weight distribution, installed a transducer or the things PointedRose mentioned above
In this case ventilation occurs mostly when going over the hub when boat's bow rises and the stern plow thru the water before planing speed level.

At what rpm's and/or boat speed your problem occurs?

Other problem could be fuel issues, 30 year old seems to be most likely a engine with carburettors.
What engine do you have?

Chris

 

[seasick]

It doesn't sound like a spun hub. What model is the motor? It probably isn't fuel injected but rather carbed. There are possible causes for both types of motors.
If carbed, it may have a choke issue where the choke enriches the mixture thinking the motor is cold.
If fuel injected, bad TPS's are common faults as are VST issues and o2 sensors if present

 

[SkunkBoat]

"intermittent surge in RPM and loss in power,
Reducing RPM stops the slipping, and you can slowly increase power,
It's worse with more weight on the boat."

Does sound like hub.
Other than hub, I would say bad barnacles on hull preventing planing ...transducer...zincs on tabs spinning...

"It sometimes goes away for days and then comes back again." You got me on that one.

 

[DennisG01]

Sounds like something on the hull causing water flow disturbance. ANY growth, however slight, can cause issues like this. Before you go too far down checking mechincal engine stuff, really check the hull/tabs/etc for damage and/or growth. I can't imagine how something mechanical at the engine end would do this - in order for the RPM's to go up all of a sudden, the propshaft has to spin faster. If it's NOT the hub, the only other thing I can think of could be ventilation.

EDIT: I will somtimes get this with weeds in the water. They wrap around the lower unit and cause ventilation. It only takes a few weeds to do that. Sometimes they fall off by themselves - other times, I back down, pop it in reverse, then continue on.

 

[tech_dog]

Any obvious issues with the prop itself? Dings, chips, uneven shape, etc. same with the hull bottom…any barnacle growth ahead of the transom?

The happened once a few seasons ago, so as part of my restoration I had the old hub replaced and the prop reconditioned last winter. The prop looks good.

The boat lives on a trailer and the hull is clean.

Thanks,

T.

 

[tech_dog]

Ventilation problems can come suddenly if you changed something f.e. in weight distribution, installed a transducer or the things PointedRose mentioned above
In this case ventilation occurs mostly when going over the hub when boat's bow rises and the stern plow thru the water before planing speed level.

At what rpm's and/or boat speed your problem occurs?

Other problem could be fuel issues, 30 year old seems to be most likely a engine with carburettors.
What engine do you have?

Chris

I've made no changes I think could contribute. I've got the same transducer and same prop I've been using for 30 years, now reconditioned.

I'd say the most typical time for this to hit is with 3-4 people on board at about 3600 rpm, maybe 32mph. The boat tops out at 5500rpm and 47mph. I'll be up on plain at a cruise and I'll lose thrust as RPMs go higher.

I redid the entire fuel system over winter and the engine sounds great. When the issue occurs the engine RPMS go higher, so it's not like it's stumbling and slowing. I have to throttle back to maybe 1500RPM to get it to "catch" and then I start throttling up again. A few times I've had to keep RPMS below 2000, cruising at 10mph or so, to avoid the slip.

I have a 1994 Yamaha 175TXRT carbureted, with fresh carb rebuids, new ignition coils, recent timing adjust, and I can still hit 47mph like the day it was new. I'm getting 3 1/2 mpg, also same as new.

Thanks,

T.

 

[tech_dog]

"intermittent surge in RPM and loss in power,
Reducing RPM stops the slipping, and you can slowly increase power,
It's worse with more weight on the boat."

Does sound like hub.
Other than hub, I would say bad barnacles on hull preventing planing ...transducer...zincs on tabs spinning...

"It sometimes goes away for days and then comes back again." You got me on that one.

This is a trailered boat and the hull is shiny white. The trim zinc was replaced this year, but it's identical to the old one.

Thanks,

T.

 

[tech_dog]

Sounds like something on the hull causing water flow disturbance. ANY growth, however slight, can cause issues like this. Before you go too far down checking mechincal engine stuff, really check the hull/tabs/etc for damage and/or growth. I can't imagine how something mechanical at the engine end would do this - in order for the RPM's to go up all of a sudden, the propshaft has to spin faster. If it's NOT the hub, the only other thing I can think of could be ventilation.

EDIT: I will somtimes get this with weeds in the water. They wrap around the lower unit and cause ventilation. It only takes a few weeds to do that. Sometimes they fall off by themselves - other times, I back down, pop it in reverse, then continue on.

"for RPM's to go up all of a sudden, the propshaft has to spin faster" This is my dead end. I THINK that a problem in the lower unit or drive shaft would make grinding or clunking noises and it would NOT reengage smoothly.

The boat has zero growth on it, shiny white, the config is the same as it's been for 30 years. We do the same with weeds, and recognize when it happens.

I agree that since it's not prob/hub slip, unless a 1/1000 coincidence made it slip exactly back to the mark, ventilation seems the obvious choice, but I can't imagine how a 30 year configuration suddenly goes bad.

I'm going to check my trim hydraulics and sensor just in case the engine isn't where I think it is, and give he hub another chance to slip, and see if I get any new info.

Any other ideas?

Thanks,

T.

 

[PointedRose]

I could be way off of course but personally I think this is a ventilation/prop issue. I’d look very closely at the prop again - it wouldn’t take much for your symptoms, maybe a hardly noticeable ding or prop blade slightly misaligned. Makes sense that it happens only under heavier load, other times it’s good enough to work fine, but when under a load, and like Dennis said, maybe hit seaweed or something and it’s enough to cause the ventilation symptom periodically. Unfortunately it might persist until you find the source.

Edit : maybe whoever worked on your prop has a spare lying around (or maybe you do) to swap it out and try to replicate the issue. Could rule out the prop, or confirm it might be the prop

 

[tech_dog]

I could be way off of course but personally I think this is a ventilation/prop issue. I’d look very closely at the prop again - it wouldn’t take much for your symptoms, maybe a hardly noticeable ding or prop blade slightly misaligned. Makes sense that it happens only under heavier load, other times it’s good enough to work fine, but when under a load, and like Dennis said, maybe hit seaweed or something and it’s enough to cause the ventilation symptom periodically. Unfortunately it might persist until you find the source.

Edit : maybe whoever worked on your prop has a spare lying around (or maybe you do) to swap it out and try to replicate the issue. Could rule out the prop, or confirm it might be the prop

I have yet to see someone say "dog clutch" or some other drivetrain component could be failing smoothly, so I'm still stuck on hub or ventilation.

I'm going to run some more and recheck for a spun hub, I'm going to check and make sure my hydraulics and trim sensors are working properly, and then I'm going to try a different prop as you suggested. Current prop looks good and the guy who did it is the recommended guy all over San Diego, but as you say there's nothing else that fits the symptoms.

Thanks,

T.

 

[DennisG01]

I THINK that a problem in the lower unit or drive shaft would make grinding or clunking noises and it would NOT reengage smoothly.

Yes. If there was a problem with the drivetrain there would be very horrible noises being made and it absolutely wouldn't "fix" itself over and over

A spinning hub will also generally show itself during hard acceleration out of the hole - which it seems like isn't happening.

 

[tech_dog]

Yes. If there was a problem with the drivetrain there would be very horrible noises being made and it absolutely wouldn't "fix" itself over and over

A spinning hub will also generally show itself during hard acceleration out of the hole - which it seems like isn't happening.

I took the boat out by myself, marked the hub/prop with a line, and did full throttle hole shots over and over and could not cause the problem.

I then added another person and 40 gallons of fuel, and the problem triggered as I was heading out our inlet over 2-3 foot swells.

Right now I'm wondering if a hydraulic trim issue is causing incorrect trim, leading to ventilation. It seems like everything else is all but ruled out.

Thanks,

T

 

[Halfhitch]

The next time it starts over revving, resist the urge to back down the throttle, the engine has a rev limiter and won't hurt itself, and trim the motor down instead and see if the prop hooks up again. The trim gauge may be showing you bad info as far as where you are trimmed. The rheostat that sends the signal to the gauge can be adjusted so the rheostat may have slipped or is going bad and you are not trimmed like you think you are.

I always calibrate my trim sender so when the anti-ventilation plate is parallel with the keel, the gauge is about center of the range on the gauge face.

 

[DennisG01]

Agree with what half-tied just mentioned about resisting and trim down to check for hook-up.

I forgot to mention this above... my boat will sometimes gain a few hundred RPM's in choppy conditions and it usually takes a bit for it to bite again if I just let it go. Maybe 30 seconds to a minute or two.
Still another possiblity is a thru-hull installed at a bad location that occasionally seems to cause disturbance depending on the other conditions.

Either way, it absolutely doesn't sound like an engine issue.

 

[Sardinia306Canyon]

I have yet to see someone say "dog clutch" or some other drivetrain component could be failing smoothly, so I'm still stuck on hub or ventilation.

A failing dog clutch jumps out of gear and make horrible noises and you will be in neutral and thats not your case.

I agree that you have a ventilation problem as the symptoms are just the ones you have.
i had same on my Venture 34 and it took me a while and many propeller swaps to find the right one and add weight on the bow to get rid of it.
We had a 25ft RIB in the shipyard with the same ventilation problem and it took us days to figure out that the painter left some big drops just on the angle of the transom in front of the engine and that led to the ventilation problem of this boat. We sanded the drops flat and problem was gone.

The most simple test would be swapping the propeller and try again

and same prop I've been using for 30 years, now reconditioned

Hmmm, may it be that the reconditioning went wrong?

I then added another person and 40 gallons of fuel, and the problem triggered as I was heading out our inlet over 2-3 foot swells.

Again, trust me and add for test purposes some weight on the bow (one two people under safe conditions are ok) and try again.
What you wrote indicate me that weight is one of the problems and that combined with a non perfect prop is a receipt for disaster, same i had with the Venture 34, but same as you, when bow was high and going over the hump.

Chris

 

[tech_dog]

To summarize, I think:

1) No drive train issue could give me this mix of symptoms.
2) If it's not hub slip it's got to be ventilation.
3) If nothing changed on the boat, it's got to be a trim issue or a prop issue.
4) I'm going to try, from easy to hard:

• Pay close attention to my trim, looking for unexpected trim up.
• Next occurrence I'll trim full down as it happens
• If that's not it, I'll get the prop shop to loan me a suitable comparison prop

If it's a prop issue, it means the original prop was proven for 27 years, but then gained a defect that wasn't detected by a highly rated, high volume prop shop during recondition. The prop came back shiny and smooth and sharp, so it clearly got some TLC. I put about 135 miles on the reconditioned prop before the problem re-appeared from a few seasons ago, the last time the boat was run.

By process of elimination, my bet is I've got a trim hydraulics issue and the trim is bouncing up in the swell. If this happens when the boat is loaded, I get my ventilation.

I'll test on Monday when there's less traffic on the water.

Thanks! This was very helpful.

T.

 

[Beyond A Wake]

If you run in waters with any bottom weed growth or floating algy or like. The stuff turtles etc like. Then the ventilation with rpm increase happens. I used to have this issue all the time with small (up to 20hp) 2-strokes when a kid. We ran in shallow waters and all the time the weeds got us.
Otherwise I would take the prop to be tested for slip unless you can score a spare prop to try with. Even one of the cheap composite ones could be used for testing and spare.

when it happens you should be able to look back or have someone see what the water behind the motor looks like, a slipping hub doesn't stir up the water.

H

 

[Beyond A Wake]

If the boat also is 30 years old (or more) is it possible that the bottom flexes or has become a little concave on the trailer??? Just a thought.