shaft anode question?

[Ian_Edwards]

Eynhallow has a 1.5 inch prop shaft. I need a short shaft anode to fit between the prop and the rope stripper, the gap is only 22mm.

The only short shaft anode close to 1.5 inch (~38mm) I can find on line is for a 40mm shaft, and it's 20mm wide. I've bought one from Solent Anodes.

When I measure the ID of the anode, it measures 40mm across the split and 38mm at right angles to the split, when I push the 2 halfs together, this, I assume, is to ensure that it can be tightened onto a 40mm shaft.

So what do I do?

I could just fit it as is, and tighten it up and hope it fits.

OR

I could fit a 1mm thick collar between the shaft and the anode.

The easiest way I can think making a suitable collar is to use a 1mm sheet of copper and clamp it between the anode and the shaft to ensure that the anode is a tight fit and in electrical contact with the shaft. I'd fashion the collar out of copper salvaged from a bit of copper tube, by cutting it along its length, flattening and then annealing it, before trimming it to a 20mm strip.

Is using copper a good idea in this application?

Can anyone think of a better, readily available, material I could use for a 1mm collar?

Is it better to fit as is and hope it doesn't rattle around on the shaft after a few weeks in the water?

 

[Tranona]

A shaft anode must be exactly the correct size. 1 1/2" is a stock size.

Do not put an anode in that gap. It is there for a reason. First to allow water to flow in and out of the bearing and second to allow for the shaft to move backwards and forwards as you go in and out of gear. The minimum is 15mm, so your current 22mm is quite acceptable. If you reduce it to 2mm the anode will foul the bearing carrier when you go into gear and longer term your cutless bearing will run dry and wear out.

Shaft anodes are only suitable if you have a P bracket stern gear when the anode can be fitted to the exposed shaft forward of the bracket. If you do not have this you can either use a propnut type anode or connect your shaft to a hull anode> the normal way to do this is by attaching the anode to the gearbox, plus a bridge across the coupling if it is the flexible type.

Perhaps should add that if you have a feathering or folding prop it will likely have its own anode so you may not need an additional anode unless you are experiencing rapid wear.

 

[Ian_Edwards]

Hi Tranona, Thanks for the reply.

The boat is fitted with a Python trust bearing and a universal joint, so although the shaft will move forward, it's not by very much, it can only compress the rubber bushes between the thrust bearing and the SS bulkhead which takes the axial load, and these are already torqued up to 120 Nm, so the prop' can only move axially by a 1mm or so. The engine mounts don't carry any axial load, so are not relevant in this case.

The prop is a Max prop' and has an anode, however, this anode is depletes very quickly. The shaft anode is placed hard up against the forward face of the propeller hub. I had the yard fit a shaft anode last year and it didn't cause a problem.

Unfortunately, the yard don't have record of what kind of anode was fitted and the staff member who fitted it no longer works for the yard.

The anode was so depleted at the end of last season that it was impossible to tell what size it was.

I can find 1.5 inch shaft anodes on both Solent's and McDuff's website, but they are too long, The only shaft anode I can find which is short enough is the a 40mm anode.

http://www.solentanodes.co.uk/colle...roducts/zinc-shaft-collar-anode-various-sizes

It turn out that it's actually 18mm thick which give 4mm clearance.

I've also found the following information on the solent website.

Zinc Reliance Shaft Anode manufactured in Canada by Metal Distributors Limited to US military specification A 1800 K. Copper studs on the anode ensure good electrical contact to the prop shaft, giving the best protection to your stern gear.

Which is what gave me the idea of using a copper collar or shell to pack the anode out to the correct size.

Is it really a bad idea to fit a copper shell between the Zinc and the SS shaft?

 

[Tranona]

Even though you do not have to rely on the space for the movement of the shaft, you will still have the problem of short bearing life. Fit a hull anode with a brush connector on the shaft. or if you don't want a permanent anode then use a hanging anode when you are not using the boat. Most of the erosion of anodes happens when the boat is stationary - assuming you are not constantly sailing.

Not sure about the copper but you could check continuity between the anode and the shaft when fitted - if you are prepared to take the risk of non lubrication of your cutless!

 

[Ian_Edwards]

Hi Again, an interesting conversation

The anode isn't up against the cutlass bearing, there's a Ambasador Rope Stripper between the anode and the cutless. ie. Prop', anode in contact with the prop, about a 4mm gap, rope stripper, with the usual stand off arrangement. The boat also has a water feed to the Python drive, taken from the raw water outlet before it goes into the exhaust elbow. This means that the cutless bearing is flushed by a steady flow of water whenever the engine is running, so I don't think I'll get premature wear of the cutless bearing.

I wanted to keep the distance between the prop and the cutlass bearing as short as possible to prevent any additional vibration being caused by the unsupported length of shaft, but guessed that a 1.5 inch shaft is quite stiff and the prop and engine combination isn't that big for the size of the shaft. I have to admit that I expected to find a 1.5 inch thin shaft anode on one of the manufactures website, and disappointed when I couldn't find one.

The boat has a hull anode, but I don't think it's electrically connected to the shaft, unless there is a connection via the engine, gearbox and Python drive. Next time I'm on the boat I'll put a meter between the engine and the shaft and measure the resistance. If it isn't connect or the resistance is high I'll investigate a bush for the shaft.

I've made the copper shells this afternoon, so next time I'm at the yard I'll fit it and check the continuity, I can't see why that would be a problem, if the anode/copper/shaft interfaces are clean and the anode is bolted up tight, so that the assembly is in compression.

I guess my original post was fishing for information about whether there was a dissimilar metals problem, setting up an electrolytic cell of some sort. I also wondered if anyone else had come across the problem of not being able to get an anode to fit the diameter of their prop shaft.

 

[Tranona]

Is the gap between the cutless and your cutter the recommended 15mm? which is required for the water to flow from bearing? and the gap between the cutter and the prop is the one you want to fill with an anode?

If it were my boat I would still use a hull or a hanging anode with some real meat in it as shaft anodes are only a fraction of the weight. There is no other reason for having a hull anode on a GRP boat so it should be connected to the shaft, otherwise it is doing nothing.

 

[Pete7]

Tranona, isn't the hull anode to protect the p bracket and rudder shaft oh and somewhere to stick the earth?

Ian, are you really going to mix stainless steel, copper and zinc in contact with each other? won't they fizz like an alka seltzer in salt water?

 

[Tranona]

Tranona, isn't the hull anode to protect the p bracket and rudder shaft oh and somewhere to stick the earth?

Ian, are you really going to mix stainless steel, copper and zinc in contact with each other? won't they fizz like an alka seltzer in salt water?

No. A P bracket does not need any protection as it is not in contact with any other metal in sea water. It may be used as a convenient place to mount an anode, but is then connected to the shaft. The main purpose of a hull anode is to protect the prop which is in contact with a stainless shaft and is therefore bonded to the shaft usually through the gearbox. The op has a feathering prop which is itself made from dissimilar metals so has its own anode and should not need a hull anode. However the anode on the prop is small and they tend to have a short life. Therefore he is trying to add an additional anode. If he had a P bracket the obvious place is on the exposed shaft forward of the bracket, but he does not have this arrangement so the alternative is the conventional hull mounted one bonded to the shaft.

 

[VicS]

Ian, are you really going to mix stainless steel, copper and zinc in contact with each other? won't they fizz like an alka seltzer in salt water?

I dont think there will be a problem

Copper is between zinc and stainless steels in the galvanic series , although closer to the stainless steel than to zinc.

No great difference to the mix of metals and alloys found in folding props

 

[Keith-i]

Would the simplest solution not be to buy a 1.5" anode and cut the end down to reduce its overall length?

 

[Tranona]

Would the simplest solution not be to buy a 1.5" anode and cut the end down to reduce its overall length?

Yes - and would think cutting one in half would be possible, but of course you end up with a very small amount of zinc.

 

[VicS]

Yes - and would think cutting one in half would be possible, but of course you end up with a very small amount of zinc.

You'll then probably have difficulty bolting the bits on. Thinning one down in the lathe by skimming each side might be more realistic but you would have to keep an eye on any reinforcing band

 

[Ian_Edwards]

Re: shaft anode question? a couple of clarifications

The prop' shaft exits through a conventional shaft log.

There's no P bracket or length of shaft exposed, the shaft exits the log via the cutless bearing then through the stationary part of the Ambassador rope cutter, which is clamped to the external part of the log, There's a gap between the rope cutter and the log, but I don't think it's as much as 15mm. But as I said in a previous post the cutless bearing is flushed by water, taken from the sea water cooling system and used to ensure that the shaft seal is always wet and lubricated.

The shaft has run like that for 8 years without a problem with the cutless bearing.

I did buy a McDuff 1.5 inch shaft anode, with a view to cutting it down, but it would have needed two saw cuts (I don't have access to a lathe big enough to turn it down) and I didn't feel I could saw it accurately enough and it seemed an awful waste of a good anode. It's now in a box marked might be useful some time.

Thanks VicS for the clarification, it's a long time since I looked at the electrochemical series, but I guessed it would be OK, because it's in an American Naval standard.

The boat goes in the water for the season next Wednesday, so I'll see how it fairs at the end of the season.