Shafts - auto rotate phenomenen?????

[mjf]

Need some advice please.


As many will know it was high springs at the w/e and certainly at Newtown Creek where I was we had a huge range. Then on the ebb on Saturday and again yesterday the current was roughly 4 knots.

During the first ebb I thought I heard a odd noise; on inspection I found both my shafts turning at roughly 60rpm! Never had this before anywhere, checked the speed log and this was showing zero knots.

So why are the shafts turning when they are actually quite tough to rotate manually and the paddle wheel log which is free to turn easily is not recording anything?

Next up - does this auto rotate issue cause any damage? I called Seastart to ask if it would be better to start the engines and put into gear before stopping them in gear and thus providing too much resistance to turn. They said leave it no problem for these to turn. I remember ages ago on here there was a debate about steaming around with one engine and recall it was felt that the idle shaft turning would cause some gearbox damage.


I recon when the auto rotate occurred on Sat I had 2ft under the keel, yesterday at strongest current I think as was lying in deeper water due wind direction it was 3/4ft clearance and rotation was very slow this time and was short lived too.

Be intersting to see what happens today as still on the same mooring.

 

[oldsaltoz]

Shaft spinning would indicate a good flow over the prop/s, no clue why the speed was not working, I assume you had power to it.

Given the choice of letting them or stop them I would put them into astern rather than run the bearings in cold oil for extended periods.

Hope this helps.

Avagoodweekend......

 

[Hurricane]

Have you ever run on just one engine with the other free rotating?

And then gone down in the engine room to look at the free rotating shaft.

Its quite scary how much they turn.

And, as you say, they always seem very diffuicult to turn whe the boat is out of the water. It goes to show how much power we are dealing with.

 

[mjf]

Have you ever run on just one engine with the other free rotating?

And then gone down in the engine room to look at the free rotating shaft.

Its quite scary how much they turn.

And, as you say, they always seem very diffuicult to turn whe the boat is out of the water. It goes to show how much power we are dealing with.

No - but I will now!

The rotation ceased on all following ebb tides - off HW Springs and boat in deeper water as never swung slow close to the shore again. The log was fine and showed the smallest 0.1kn as soon as I slipped and moved out.

Still cannot work out how the props moved and the paddle log did not register.

 

[admillington]

Same thing happened to me anchored off East Head on Saturday!

I didn't count the RPM but both shafts were rotating and couldn't easily be stopped.

 

[jfm]

I would put them into astern

Why astern (as opposed to ahead)?

 

[oldharry]

Yotties find that happening all the time when they are under sail at around 4kts+. Big debate is whether it causes more drag rotating or stopped. Someone even produced a shaft driven alternator to use the power to re-charge batteries on a long distance sailyboat. Not very succesful.

 

[jfm]

Big debate is whether it causes more drag rotating or stopped.

We analysed that at length here on a gludy thread about a year ago and got to the bottom of it. Answer is, it depends. There's less drag if you leave it free-spinning, until the blades start to stall, in which case the drag reduces if you stop it spinning. along those lines anyway - it was analysed quite fully in the thread

 

[papa-alpha]

Remember there is no oil pressure in your transmission when the props are free wheeling (unless it is a large transmission equipped with trailing pumps for this very purpose), so nothing is being lubricated. Most ZF and Twin Disc manuals tell you how long this can go on before it is a problem (or there service people can look it up).

 

[mjf]

Yes that was the result of the enquiry.

My issue is the damage allowing rotation vs locking them in gear - indeed can this lead to issues with the safelock system? Also what with the log not recording flow?

 

[Wiggo]

Mike, I wouldn't worry about the log not doing anything - ours sometimes sticks until there's sufficient flow to free it, often at 6-7 knots, and then it's fine.

Depending on the gearboxes, they may need oil pressure to hold them in gear anyway, so leaving them in gear and stopping the engine may have no effect at all.

 

[Trundlebug]

Remember too that one of the reasons they're so hard to turn when out of the water is that the cutlass bearings are water lubricated - therefore easier to turn when immersed than when dry and sitting on the hard.

I'm not convinced it would really do it any harm rotating, but if it bothers you just put it into gear (fwd or rev - doesn't matter) and it'll lock it up. There's no danger of it overcoming the compression of a diesel engine...

No need to start the engine, put into gear then stop, just put it into gear without starting the engine surely?

 

[mjf]

No I tried putting them in gear without starting the engines but zero result.

I think prehaps Wiggo is right - they need to have oil pressure to have any effect.

Asked a VP man today who did not offer any advice??????


Graham, the log was fine and showed 0.1k as soon as I move off the mooring.

 

[OAF]

what gearboxes are they

 

[omega2]

auto rotate

borg warner's do it all the time, nothing to worry about, the pump is on the engine end. When dry cutlass bearings are hard to move, when wet revolve easily. No danger of "bump" starting.