Vibration. Any Opinions?

When my boat is on plane between 2800-3200 RPM, it gets this intermittent vibration. It's deep, can't say you can really feel it but you can hear it, almost like a bass drum. Kinda like a HMMMMM...

I just had a diver go under and check things out. He says that on each prop one blade is out maybe 1 CM. The shafts feel tight with no play, nothing was tangled on the shafts. He did say that they spun easily but he could feel some type of metallic sound coming from them much like a tire on the road.

My boat has 7.4 gas motors with V-drives. ZF Hurths Model: HSW630V1-2.5.

Any ideas / suggestions would be greatly appreaciated.

Dave

beachbum29 said:

When my boat is on plane between 2800-3200 RPM, it gets this intermittent vibration. It's deep, can't say you can really feel it but you can hear it, almost like a bass drum. Kinda like a HMMMMM...

I just had a diver go under and check things out. He says that on each prop one blade is out maybe 1 CM. The shafts feel tight with no play, nothing was tangled on the shafts. He did say that they spun easily but he could feel some type of metallic sound coming from them much like a tire on the road.

My boat has 7.4 gas motors with V-drives. ZF Hurths Model: HSW630V1-2.5.

Any ideas / suggestions would be greatly appreaciated.

Dave

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When I used to have twins I would get what you are describing when the engines were not synchronized. Have you tried adjusting engine speeds?? If your using an engine synchronizer maybe it is out of calibration. Just a thought.

Possibly cavitation. Heard that occasionally on our previous boat, sort of a long drawn out growl from time to time.

LaBomba said:

When I used to have twins I would get what you are describing when the engines were not synchronized. Have you tried adjusting engine speeds?? If your using an engine synchronizer maybe it is out of calibration. Just a thought.

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I use the synchronizer gauge and keep the needle in the middle.

Conrad said:

Possibly cavitation. Heard that occasionally on our previous boat, sort of a long drawn out growl from time to time.

Click to expand...

It's more like a repeating noise.

Listen to LaBomba

RickB said:

Listen to LaBomba

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Thanks.

Can the OEM gauge be recalibrated?

Thinking about it, the engine RPM's vary about 100-150, on the tachs, at those speeds with the gauge dead center on the synchronization gauge.

I have to say, Thank You!

Boat on a plane at 2800???? Once, I hit 15.5 going downhill, wide open with a stiff wind behind me, never been close to getting up on a plane. Sarchasm aside, my vibration was a cutlass bearing but it was pretty constant above idle.

How was the diver able to.measure the props to determine that each prop was out by one c.?

The first thing I think when I feel a vibration is the propeller.

skipperdude said:

How was the diver able to.measure the props to determine that each prop was out by one c.?

The first thing I think when I feel a vibration is the propeller.

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He attached a yardstick to the rudder and to the outer edge of one of the blades, he then rotated the prop to measure any difference in the blades.

This can be very difficult, or something simple. How long has it been happening? Just developed over time or always been there? How intermittent is it?

Best guess is it's props or bearings, have props balanced and replace bearings.

First thing is try different throttle settings on each engine while at speed. Sound remains the same, or changes, in what way? Then try opening a cockpit sole hatch and sticking your head down there while the boat is at speed. Is the noise more or less obvious? Put your hand on structural parts (bottom stringers and panels) to feel for vibration. Check the rudder stocks with your hand while the boat is at speed. It could be something like a broken engine mount bolt (hard to check) or a loose fuel tank strap or broken baffle in the fuel tank.....

First check the alignment of the drive trains. V-Drives are tricky.

Then haul the boat, pull the props and send them to a shop that can tune them to ISO Class I.

Then while the prop is off put a dial indicator on the shaft and rotate it to check runout. Should only be a few thousandths of an inch.

With the shafts straight and the props true that should solve your vibration problems. If not then look at mounting and alignment of the V Drives.

David

Sure sounds like a sync issue...ignore the sync "meter" and try and match with just your ear...if it gets better and worse...then have someone shoot the engines with a photo tach or electronic tach...if still an issue, get it close then see if one engine is producing the vibes more than the other just by going down and feeling them. Start pulling props and anything else is a waste of time till you know if it IS a vibe issue and then whether it's one or both engine/drive trains

Psneedle said to do the cheep and easy things first.
Always the best idea.

djmarchand. I think is right on if none of those things work.

Tad has some great ideas.

I would do all of the above.

Then do this

Keep the ideas coming guys something has got to work.

SD

psneeld said:

Sure sounds like a sync issue...ignore the sync "meter" and try and match with just your ear...if it gets better and worse...then have someone shoot the engines with a photo tach or electronic tach...if still an issue, get it close then see if one engine is producing the vibes more than the other just by going down and feeling them.

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Exactly. It sounds like you have a harmonic beat. The physics are a bit complicated, but when you combine two frequencies you will get the two originals plus the difference plus the sum. So if you combine a 2940 RPM engine and a 3000 RPM engine, your ear will hear a one-second pulsing (60 RPM = 1 second period). It sounds like in your case you may be out by only 10 or 20 RPM.

You can also test for this by unsynchronizing the engines, just set one 200 or 300 RPM above or below. If the sound goes away it's a harmonic.

Now with a single you never have this problem... time to resurrect the Great Single vs. Twin debate!

Scott Welch
Island Eagle

Both prop shafts were just removed, one replaced and the other one cleaned up. Cutlass bearings were inspected and seemed to be good.

I'm thinking the syrno gauge is off and I need to adjust engine rpm by ear.

I will fill you in after the next outing if the rpm adjustments dont work out.

Dave