280 EC - Volvo Penta 8.1 - High RPM

mvgvb

Hi All,

We were out a few days ago and veered slightly out of the channel to avoid an unpowered boat without realizing it. I shut down to neutral immediately, but not until after feeling a slight "tug" on the bottom for maybe 0.25 seconds. We were going 5 mph and it was mid-tide and nothing comes above water at low tide... so if we did hit anything, it had to have been close clearance-wise. If we did hit bottom, no idea if it was the hull skimming the bottom as we moved forward into shallower waters, or the prop/outdrive hitting something while trimmed down. The outdrive didn't pop up, no detectable noise, engine/prop didn't cut out, boat didn't lurch to a stop. Not even positive the "tug" wasn't just in my head after realizing we were out of the channel, as none of the passengers noticed it.

Immediately jumped to neutral and trimmed up, and the current took us back into the channel. We didn't get caught on anything, or have to re-distribute weight. Engine went right back into gear in the channel and seemed fine. Doesn't appear to be water coming in anywhere, and I did manage to dive down with goggles shortly after and it doesn't look like any nicks in the prop, although horrible water visibility up here. Boat also felt smooth - no vibrations, no issue accelerating or getting on-plane.

That being said, I noticed our RPM appeared significantly higher when we went out after this incident. I have a Volvo Penta 8.1 Gi (375 hp), and the weekend prior we'd been getting on plane at about ~4200 RPM, and staying on plane down to about ~3200 RPM. RPMs were a bit higher than MPH based on the weekend prior, that was at 1/2 tank gas and 4 passengers. 

Immediately before this incident we did full up the tank (so additional 50 gallons), and had about ~150lbs more of passengers. But once out, I noticed we were at 4800-5000 RPM to get on plane, and once on plane RPM would continue to pick up above 5000 until throttling back. But I couldn't drop below about ~4000 RPM or would fall off plane, and down at low 4000s RPM was only running about ~27 mph.

Everything otherwise still felt normal, same level of throttle to get up to speed and on plane, no issues, no vibrations, so we did get up into the high 5000's RPM before I even noticed the tachometer difference. No temp issues, and didn't appear to be burning oil, at least not noticeable amounts. RPMs we hit seem to be well higher than factory max recommended.

Unfortunately I hadn't been monitoring RPM/MPH performance for long, and only have the one weekend prior as a comparison. Is it possible that 50gal of gas and 150lbs of passenger is causing the entire difference? Or given everything else felt normal, is it possible it's just a gauge issue? Anything specific I could check to confirm the RPM issue, or anything else to confirm given the possibility we hit bottom?

Thanks!

Aqua_Aura

Possibly only one prop is spinning?

mvgvb

Could be, would that cause high engine rpm though? 

I would have thought that rpm would stay the same and and mph would just decrease in that scenario. 

Any way to test? Even trimmed up I’m not sure the water will be clear enough to see.

YYZRC

50 gallons of gas would weigh over 300lbs.  Not insignificant!  My boat jumps up on plane much quicker at 1/2 tank.

Aqua_Aura

Without the load of another prop the RPM would most likely jump up. Just a theory. Odd issue so I'm just tossing ideas. 

Make sure the RPM gauge is correct which in your case try tapping the gauge. Mine is jumpy and has done this but when I hit the gauge it goes back to normal. I have digital engine readings now so I can really see how inaccurate the analog gauges are. 


If you manually move a prop the other should move opposite. I think it needs to be in gear. ENGINE NOT RUNNING!!!! 

Spyderweb

Adding 450 lbs. (gas + people) is significant.  Also, hard to imagine a bump so light that it leaves no mark on the prop would cause your symptoms.  And my experience is same as @Aqua Aura, analog gages can be very inconsistent.  Try tapping method.

LaRea

Refresh my memory.  Volvo Duoprop ... do your props have hub bushings?  Maybe you have a bushing that's slipping when you reach higher speeds.  Get a grease pencil and mark the props, then check to see if they moved relative to each other.  

Dream_Inn

LaRea said:

Refresh my memory.  Volvo Duoprop ... do your props have hub bushings?  Maybe you have a bushing that's slipping when you reach higher speeds.  Get a grease pencil and mark the props, then check to see if they moved relative to each other.  

Yes, I learned a couple weeks ago that duoprop does have hub bushings.  My neighbors 390 has duoprop and blew out those hubs.  I never thought the two prop setup would have hubs.  Do the Bravo 3 props have hubs?

LaRea

DI -- I thought I remembered you saying that.  (No hubs on Bravo 3.)  @mvgvb that has to be it.  The hubs are 10+ years old, you tweaked one or both hubs when you grounded, and now it or they can't handle the full torque of that big-block engine.  

If confirmed, you should probably get both props reconditioned -- or upgrade to a set of Hill Marine 4x4 props.

mvgvb

@LaRea @Dream_Inn

If the hub, is it urgent? Or safe to finish the season if keeping rpm’s low? We’ve got a backup on everything here… pulling now likely means end of the season.

thanks! 

LaRea

You'll be fine as long as you don't over-rev the engine.  The slipping might get worse over time. 

Dream_Inn

I agree with LaRea.  If you had a spare set of props, they can be changed out while in the water.  Either way, the hub is done and may just slip more as time goes by.  For single props, a quick hard reverse will sometimes help get the prop slightly more on the hub to help you get home.  Not sure it works for duoprops.