vibration when moitor sailing

[jason -and the arguenauts]

my engine is smooth when moroting but vibrates quite noticeably when motor sailing. last night for example, going down wind at 6.5 kn I wanted to hurry up to catch the lock so I started the engine and it virated like a jackhammer. as soon as I dropped sail to enter the approach channel, the vibration stopped.

my suspicion is the folding prop which I suspect isnt opening fully when motorsailing reasonably fast. anyone had the same experience?

 

[Sans Bateau]

I would say that your assumption is about right. At 6.5 knts the engine will have to be near flat out to spin the prop open, it may just be some crud stopping it opening, but try some tests. Spill off some sail power to slow up, then get the boat motoring rather than sailing. Once up to about 6.5knts, sheet in and start to sail again, if you can sail past the motoring speed without vibration then it is likely to be the prop not opening properly.

 

[jason -and the arguenauts]

But given that the blades of the prop are geared together and balanced, why should this cause vibration?

 

[shmoo]

Get enough of that to understand what you are saying but nowhere near "jackhammer".

I am not mechanical but I have always assumed that there where thrust bearings and things in the transmission chain that were meant to run with, well, thrust, and if were none then bad things may happen in the long run.

So we take a bit of vibration when motorsailing as a cue to try it with the engine off in case the wind and boat speed has crept up without us noticing. It's not unknown for the speed to increase when we kill the engine!

<edit> BTW we have a two bladed, geared folding prop. A Gory as I recall.</edit>

 

[AntarcticPilot]

my engine is smooth when moroting but vibrates quite noticeably when motor sailing. last night for example, going down wind at 6.5 kn I wanted to hurry up to catch the lock so I started the engine and it virated like a jackhammer. as soon as I dropped sail to enter the approach channel, the vibration stopped.

my suspicion is the folding prop which I suspect isnt opening fully when motorsailing reasonably fast. anyone had the same experience?

Could it be that you are getting asymmetric flow round the prop? Under sail, you will have a flow of water from leeward to windward (relative to the boat), as the boat makes leeway. So, the leeward blade of the prop will be in a different flow from the weather blade (at any instant). Could this cause cavitation problems? I can't visualize the situation more clearly, but I agree with those who say that a folding prop will always open or close symmetrically, so vibration from that cause seems unlikely.

 

[Sans Bateau]

But if the prop wont open fully, because of cavitation???

 

[PeterGibbs]

When the sail powers the boat beyond the speed at which the prop is turning (freely or under motor) there will be noise of one kind or another from the prop and/or the gearbox. It's a general thing and to be expected.

You either rev the engine to cancel out this effect - if you can - or (in my case) pop the stopped engine into reverse gear and take the small turbulence cost this imposes.
For any period of hard sailing, the prop noise and gearbox whine, are intolerable!

You weren't told this when you spent £700 on a whizzo umbrella prop. Oh dear...

PWG

 

[jason -and the arguenauts]

Prop came with the boat but in any case speed when racing is more important that a vibrating engine!

Think I've worked it out. I was sailing at 6.5kn when I put the engine on and since the boat is a bit undepowered, it will only do that speed under engine alone at 2800 rpm. I was doing 1500 to 2000 so the prop wouldnt have been fully opened. My guess is that it was fluctuating in terms of opening and taking up any slop in the gears that link the two blades ie it wasnt vibrating because it was out of balance. Might even have been between 97% and 100% open and repeatedly banging against the stop.