vibration in forward vs reverse

[Marty Levenson]

I am finding that there is almost no vibration in neutral or reverse, but it's slightly noticeable in forward. Haul out next week, so will look for zinc or prop anomalies. Is this likely a symptom of bad alignment?

Thanks!

Marty

[hanleyclifford]

Could be, but it could also be a bent shaft. But before looking at that make sure the prop is clean and not bent. At haul out grab the shaft and check for lateral movement; more than 1/16" is a problem developing. Also check your 4 motor mount points for close to even weight distribution. If two diagonal mounts are carrying most of the weight the engine can vibrate. At haul out take picture of prop if you cannot clean it in the water. We will refrain from discussing pilot bearings.

[Marty Levenson]

new shaft

Thanks Hanley,

It is a brand new shaft and cutlass bearing, so not likely....but always possible. Went through all that middle of last summer. I'm thinking uneven weight distribution might make sense. Hoping the zinc is awry. BTW it is 2:1 reduction, not V-drive....if that matters.

[Wisakedjack]

I am finding that there is almost no vibration in neutral or reverse, but it's slightly noticeable in forward. Haul out next week, so will look for zinc or prop anomalies. Is this likely a symptom of bad alignment?

Thanks!

Marty

For whatever its worth... I was finding a lot of vibration prior to finding an issue with the coupling http://www.moyermarine.com/forums/sh...ht=wisakedjack. Happy to report that vibration went away when the fix was done. Another possible source of vibration is the engine misalignment.

[JOHN COOKSON]

Maybe you are running on 3 cylinders. Any lack of power?
Feel the spark plugs to see if one is cooler than the other three after the engine has been running for awhile.

TRUE GRIT

[tenders]

I'd suggest ruling out prop fouling or zinc weirdness before you spend too much time worrying about more complex causes.

[Marty Levenson]

thanks

Thanks to all for the ideas. Yes - I will check the zinc and prop first.

I think I didn't state my question well. The vibration is not bad, just more noticeable than it used to be with our previous A4. My question really is what can make it worse in forward? it is has zero apparent vibration in reverse.

[hanleyclifford]

Once you have eliminated the prop the next step is to check alignment for greater than .003" out.

[sastanley]

Marty, Keep in mind that the engine to prop rotation speed is probably not the same in reverse as compared to forward (without looking it up, if I remember on my direct drive it is 1.4:1 in reverse.)

I don't know the reverse ratio for a 2:1 fwd gear, but I would not expect it to be 2:1 in reverse also...but maybe someone can correct me.

Additionally, prop harmonics are different in reverse too, unless your prop blades are symmetrical in both directions of rotation...but I am pretty sure they aren't.

I don't know if I'd notice much vibration in reverse, because all I hear in is the gears screaming because the brake band has clamped the (I think) planetary gear on the outside and the sun gear is now spinning the other way..but the roar of the engine at 2,600+RPM sounds happy too, because I cannot get it that high in forward.

I am not arguing with you, just contemplating, and typing what my little brain is thinking on this fine spring Saturday evening!

[Al Schober]

Marty,
A slight vibration going forward may be completely normal. In neutral, the shaft and prop aren't turning. In reverse, the flow into the prop is coming from aft with nothing to disturb it (unless your prop is in an aperture). Going ahead, the hull is in the way of the flow into the propeller and the flow is far from uniform. Big ships spend a LOT of money on multi-blade propellers with a lot of skew to the blades to reduce blade rate vibration.

[ndutton]

Marty, do you observe any shaft or packing gland oscillation when in forward, even a little bit?

[sastanley]

Yeah..good thinking Neil. I have been able to feel vibration when I put my hand around my stuffing box...I couldn't see it, but I could feel it. Perhaps try the same exercise in reverse and maybe we can shed some light on the situation!

Now that is a little easier on my boat, since the stuffing box is at the bottom of the stairs in the middle of the galley floor.

[ndutton]

As you know Shawn, the entire drive line - bearing system - alignment dynamic is very low tech and as drive lines go, very low RPM so we can get by with low tech. I expect vibration is more or less a fact of life, the goal is to minimize it the best we can.

[jbsoukup]

If you feel like throwing money at the problem, try a new coupler.
Loosening the set screws might make the vibration go away (just don't run in reverse!) and if it does, the coupler is bad.

[hanleyclifford]

Another observation

Over the years I have found that it can make a difference how two halves of a coupler are joined. A three bolt pair will have three possibilities and a four bolt will have four. When I am aligning an engine I always make sure both mating surfaces are clean and oiled lightly with 3 in 1 machine oil. You might be surprised how rotating the shaft coupling can change the variation by .001" or even more in some cases. Try testing all possible positions of the couplings with respect to each other. You will find there is one that yields the least variance. Mark that position so that the pair can always be assembled the same way. Marty could try rotating one bolt position and re testing.