Vibrating prop when motorsailing

[SAWDOC]

We were motorsailing yesterday in a fresh F4 with a moderate sea. the swell was heeling my 26 footer in the normal fashion - one of the crew asked Why does the propellor/ shaft become noisier when the boat is heeled and I struggled to put a sensible answer together - less pressure of water therefore the prop is doing less work and runs on a little was about the best I could come up with - any better explanations out there?
Nothing to worry about surely ?

 

[Cornishman]

No answer - just another question. Why were you motor sailing in a Force 4? I doubt you were gaining any benefit (apart from battery charging) from your motor.

 

[ytd]

you don't say what motor set up you have. If the motor is on flexible mounts then when heeled it could fall down a bit to leeward and put pressure on the prop shaft causing a noise form the stern gland. We have an occasional vibration from our saildrive which I have never been able to track down.

 

[curve]

[ QUOTE ]
No answer - just another question. Why were you motor sailing in a Force 4? I doubt you were gaining any benefit (apart from battery charging) from your motor.

[/ QUOTE ]

And more importantly did you have your motoring cone raised. /forums/images/graemlins/tongue.gif

 

[footsoldier]

I have encountered the same phenomenon from time to time.
I have always understood it to be the prop cavitating - when the boat speed through the water under sail exceeds the speed that the prop is able to do.

 

[AntarcticPilot]

Just a guess, but if you're heeling, there will be water flow past the prop from leeward to windward. That will cause the thrust from the blades on one side to be different from the blades on the other side, I think, and could be the source of your vibration. But only a guess, trying to visualize the water flows!

There is a story about <insert famous physicist of choice> being asked what he'd like to ask God if he ever got the chance. Physicist says, "Well, I'd like to ask about the origin of the universe, and about fluid dynamics". Interviewer asks what the answers might be. Physicist says "I think there's a really good chance I'll get an answer about the origin of the Universe!"

Moral: even high powered physicists think fluid dynamics is hard!

 

[lw395]

It can be when the boat is sailing faster than the prop is pushing the water, the force on the prop etc reverses, or goes light causing the drive train to rattle about a bit.
With a folding prop, the prop will fold slightly as the boat accelerates down any wave or whatever. This can cause wear, so personally I always fold the prop by selecting reverse for a few seconds then putting it in neutral in these conditions. Unless I expected to need the motor in the next 10 minutes or was desperate to charge batteries, I'd switch of and enjoy the peace and quiet!

 

[CodStewart]

I luv vibrating prop shafts.

 

[SAWDOC]

[ QUOTE ]
No answer - just another question. Why were you motor sailing in a Force 4? I doubt you were gaining any benefit (apart from battery charging) from your motor.

[/ QUOTE ]

I disagree. We were on a 48 mile passage and with the short day needed to keep our speed hovering around 5 knots. We were able to do so by sailing for 3 hours. when SOG dropped to 4 kts we furled the genny and motorsailed gaining a knot on the log. our preference is of course to sail.

just to add further to my description of the situation, we have a 3 blade fixed prop, connected via a deep sea seal well able to push her along. If i give her plenty throttle, the vibration sounds louder so of course i ease back a little and everything calms down again. I dont think its cavitation as described above as the noise is loudest when the boat is heeled most and quieter when back level without any great variation in speed. /forums/images/graemlins/confused.gif

 

[dunedin]

If you didn't have the three bladed prop you could probably maintain over the target 5 knots under sail ;-)