vibration in forward vs reverse

[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]

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]

another idea

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]

thoughts

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

[Marty Levenson]

back in the water

After the splash today the vibration motoring home seemed greatly reduced! Maybe it was the zinc, or the mussels.....but on smooth water motoring at 2000 RPM I could barely feel any vibration on the tiller....and zero on the back stay. She really seemed to fly with the clean hull and prop. Think I'll just say good enough for now, go sailing, and see what transpires.

Many thanks for all the feedback.