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]

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]

Originally Posted by 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

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 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]

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.

[jbsoukup]

yes. my old (bad coupling) was a solid one so no chance of repositioning the shaft.
I mean I could reposition the coupling to the output, but the problem was in the coupling Inside diameter.

[hanleyclifford]

Originally Posted by jbsoukup

yes. my old (bad coupling) was a solid one so no chance of repositioning the shaft.
I mean I could reposition the coupling to the output, but the problem was in the coupling Inside diameter.

Ideally, whenever you get a new driveshaft you should also get a new coupler, fitted by the machine shop and checked on a lathe so that the mating surface of the coupling is perpendicular to the axis of the shaft. If the mating surface of the gearbox output coupler is likewise made perfectly perpendicular to the gear shaft, and the pilot bearing is new and true, and etc etc all should be nice. But in the real world over time most such factors decay - and you are left to the rotational workaround in the quest for trueness.... but nothing can rectify a coupling that does not fit correctly on the shaft.

[jbsoukup]

Well, the new Moyer solid coupler solved my vibration problem.

[Marty Levenson]

I am still trying to track down the cause of vibration in forward gear.

We are on the hard for a few days (bottom paint + zinc) and I have been thinking about the prop and shaft. Last July I installed a new coupler, shaft and cutlass bearing. When I went to install the prop I discovered that the new shaft was 3/4" longer than the old one. It didn't seem like a problem, as I was still able to get the prop on: there was just barely room to squeeze it past the rudder. The doughnut zinc used to fit snugly between the prop and the bearing, but now there is about 3/4" of shaft showing.

Could the prop's being 3/4" closer to the leading edge of the rudder cause the vibration through changed cavitation? Could that 3/4" change be enough to do that? I do feel the vibration through the tiller when motoring in forward.

Could I test this by running in forward at the dock, with the tiller turned to one side? If the vibration goes away does that confirm the cavitation is the problem?

If so, could I do a work-around by either narrowing the leading edge of the rudder, or grinding out a slight curve in that edge? Or perhaps going back to our old two blade prop?

Haven't ruled out poor engine alignment as the cause yet. I can't see any vibration at the stuffing box, but I can feel it slightly there with my hand on it in forward.

Any thoughts appreciated!

[hanleyclifford]

By all means disconnect that drive shaft and check alignment (should be .003" variation or less). When it comes to misaligned shaft nuts you will feel it before you can see it.

[jbsoukup]

could it be that the key in the prop/shaft joint is not allowing the prop to fully seat in the taper thereby causing it to be off center maybe?

[Dave Neptune]

Marty, are you sure you have an engine vibration? The prop being closer will make really big difference. The props proximity being closer will allow the "prop-wash" to really be exaggerated by being closer. A 2 bladed prop will do more shaking than a 3 would.

It would not be a cavitation causing the vibration although the cavitation should have noise emanating "up" from below and not necessarily vibration.

Dave Neptune

[ndutton]

Marty, I'm doubting you have a problem. You don't see any driveline oscillation and your shaft RPM is maybe 1400 RPM maximum in forward (70% of what we direct drive guys have) virtually eliminating any possibility of cavitation (assumes a clean prop).

I'd do three things just to be sure:

1. See if the vibration changes as you turn the tiller from side to side. If yes, you're dealing with prop wash on the rudder.
2. Align the engine with the boat in the water. There's no downside to doing this.
3. Revert back to the two blade prop for a test. Is it the same pitch and diameter as the three blade? If there's an improvement, take the three blade to a prop shop for analysis (or stay with the two blade - your call).

[Marty Levenson]

Thank you all for your replies. I should add that there was a fair bit of growth on the prop - hoping problem gone with a clean prop!

Hanley - if my in-the-water dock test doesn't point to prop-rudder relationship as the cause I will recheck alignment for sure.

jbsoukup - I think if that were the case the vibration would be extreme...and in both forward and reverse? This vibration is probably acceptable, but more than it used to be with my old engine and shaft. Same prop.

Dave - there is no vibration at idle or reverse, so I don't think it is engine vibration. Thinking prop-wash. I'll have to read up on cavitation before I throw that word around!

Neil - thanks - 1 and 2 are what I'm planning. Probably not #3.

[JOHN COOKSON]

Marty
Just thought here. Is the engine bolted down super tight? The engine might be shifting a little bit when you put the power to it. The alignment and so on would appear to be perfect at rest only to move a bit when running in gear.

TRUE GRIT