Loose Shaft Anode...

[Mike Fox]

Frisky Fox has the most horrible of noises coming from underneath the boat, especially in reverse gear. It sounds like someone has attached a bag of nails to the prop shaft.

 

When in slow ahead, there's a definite clattering sound at low speeds, but fine at 5-6 knots. In reverse, there's a pause, then a loud rattling sound that is blood-curdling.

 

Checking this carefully in the marina, it's definitely not the gearbox - the noise is further back.

 

The arrangement is simple and conventional:

- Gearbox straight to 1" prop shaft,

- Through hull at about 20

Edited September 9, 2007 by Mike Fox

[Afishionado]

I guess you must be fin keeled. I can't think of one but is there a leaning poat any where around Poole? Actually don't Davis's have one?

 

My suspects in this would be the anode or the P bracket. Is stainless has been used as fasteners this could be the problem. Stainles is only so IF OXYGEN IS present. In sea water there is enough O to activate the self generating passive coat that makes s/s work. BUT if the fixing/item is not subject to a clear flow of water it is VERY susceptible to what is called 'crevice corrosion' The areas that are usually affected are the underside of bolt/nut heads and the part of the thread that they cover. Everything 'looks' fine but if the slightest pressure is put on the nut/bolt head it snaps off. Crevice corrosion is most frequently found on s/s prop shafts, P brackets, and anodes. The anodes should be fastend together with heavily electro-galvanised nuts and bolts.

 

Mad Mike

[TomBettle]

Find a leaning post Mike.

 

If you try your pontoon idea I think that H&S at Davis' may have something to say.

The yard won't be too pleased when several thousand pounds worth of pontoon are mangled and with that amount of force in the wrond direction I suspect they will be mangled.

 

It does sound very much like your shaft anode. I hate shaft anodes and try to simply have an anode on the end of the prop. Shaft anodes always wear loose and rattle, bang and vibrate, not to mention, serve absolutely no purpose whilst they are loose. Tighten it up again and it will be fine until it loosens again. Do it sooner rather than later as that clattering and banging maybe against the gel coat of your hull which won't be doing it too much good.

 

Tom

[Adam F]

Mike,

 

If you want me to dive it, drop me a line.

 

dam

[shytalk]

or a loose prop in forward the thrust would force it up the taper at spped but reverse would do the opposite !!!

[TomBettle]

or a loose prop in forward the thrust would force it up the taper at spped but reverse would do the opposite !!!

Plop!

[Mike Fox]

Thanks for the offer to dive under for me Adam, but I thought it was something I should really do.

 

Popped into Shell Bay this morning about 10.30, mask and snorkel ready, with unflattering (beergut camouflage) swim shorts poised, ready to pop on, when something happened.

 

I realised the noise had stopped. It had rattled like a goodun coming out of the berth, and each time I had stopped the boat ready for a short plaice drift in the harbour on the way to Shell Bay, but now it had gone.

 

The Harbour Master was nearby, and kept staring at us, as we motored around Shell Bay, in forward and reverse, turning the boat every which way, desperately trying to make it rattle...but to no avail.

 

Whatever it was (probably the anode) has gone, dropped off, sunk without trace.

 

You cannot believe how relieved I was. I really did not want to do it, but the threat of my swim shorts must have done the trick!

 

Mike

 

 

[TomBettle]

Hi Mike

 

Just a thought, but the anode will need replacing (assuming it was the anode) otherwise you may find that whatever it was protecting will start rattling too....

[gaffa]

Wise Words Tom

 

It doesn't take long for that galvanic action to dine on your nice shafts ,props and bearings especially in the Cobbs Quay Area .

 

It may take a little longer in the main harbour but Cobbs /Holes Bay is particulary fast acting - From Experience

 

Petge G

[Mike Fox]

Thanks for the advice chaps, but I do have other cathodic protection in the form of a large pear shaped zinc anode, which is bonded internally to key items, including the engine, and hence the shaft and prop.

 

The shaft anode was an "added extra", which I first fitted about 3-4 years ago, before I renewed the main anode bonding etc. I think the underwater metalwork had survived the previous 14 years of its life with no shaft anode at all, so I'm fairly relaxed that a few more months will be fine.

 

Interestingly, someone else in the boatyard has heard of similar this year, and they craned out to fix it. The fixings on the anode had eroded away, but the bolts and main body of the anode were apparently fine. That was from a cheaper batch of anodes (rather that the McMurdo ones which seem good), and I think I might have had the same model - to save