Props and anode connection

[dunkelly]

My prop is showing some sign of deteoration and my annode doesn't ? Should I get continuity when I check prop to annode ? I have a flexible coupling on my shaft which doesn't appear to have any connection between the 2sides hence no connection to the prop .

 

[VicS]

My prop is showing some sign of deteoration and my annode doesn't ? Should I get continuity when I check prop to annode ? I have a flexible coupling on my shaft which doesn't appear to have any connection between the 2sides hence no connection to the prop .

There must be continuity between an anode and what it is fitted to protect.
If there is a flexible coupling between the two it must be bridged.
Alternatively an "Elecrtro eliminator " could be used to make contact with the shaft in-board of the stern gland

 

[QBhoy]

Thee will surely be metal bolts through the flexible coupling ? Usually anyway. The anode should be conjoined with a few things usually. Is it easy enough to perhaps have a look around the inner hull where the anode comes through the hull ? Then follow the bonding wires to each item or system it is trying to conjoin and protect ? Generally speaking, it will have a bond going to the stern gear in general, to include the rudder stock, P bracket fixings perhaps, Shaft gland etc etc...likely one to a main point further forward that will ensure the shaft is within its contact too. This then will allow continuity to the prop.
All that said, it should be known that you might want to be careful or on the look out for a scenario or circumstance that might just make your prop particularly vulnerable to perhaps even acting as the sacrificial item to something closely that's of a galvanic grade, likely to encourage such a situation. hull fittings, boarding ladders, tabs or even fittings adjoined or holding to the prop perhaps.

Anyway..long story short, If you own or borrow a multi meter...very easy to have a quick check externally. You will likely know how perhaps, but if not...briefly speaking, set it to the ohms setting...put one test lead to one of the bolts securing the anode to hull (get a confirmed good contact with the lead by putting the other lead to the other bolt on the anode, proving the contact with a reading of continuity on the meter), then the other to the rudder (making sure the lead finds its way past any thick paint coating), then to the shaft, P Bracket and prop. The readings shown will tell the story of where to go next. It is very common to find a corroded bonding wire or terminal internally...It may look like it's attached..but often it can be only just holding in place by means of a very green/black and sorry looking strand of wire !
Perhaps if it is at all accessible, it would be better again to do the same tests with the meter inside the boat, if you can get to each item. This will avoid any sporadic readings that you may have read externally..caused by poor contact.
Last of the boring stuff from me...All the above is in the assumption that the anode itself hasn't been painted or anything silly like that, haha..that its of the correct type too..or that it of course even has the required bonding distribution from it at all !

Good luck...

 

[Roberto]

My prop is showing some sign of deteoration and my annode doesn't ? Should I get continuity when I check prop to annode ? I have a flexible coupling on my shaft which doesn't appear to have any connection between the 2sides hence no connection to the prop .

Where is your anode positioned?

 

[Tranona]

Thee will surely be metal bolts through the flexible coupling ? Usually anyway. The anode should be conjoined with a few things usually. Is it easy enough to perhaps have a look around the inner hull where the anode comes through the hull ? Then follow the bonding wires to each item or system it is trying to conjoin and protect ? Generally speaking, it will have a bond going to the stern gear in general, to include the rudder stock, P bracket fixings perhaps, Shaft gland etc etc...likely one to a main point further forward that will ensure the shaft is within its contact too. This then will allow continuity to the prop.
All that said, it should be known that you might want to be careful or on the look out for a scenario or circumstance that might just make your prop particularly vulnerable to perhaps even acting as the sacrificial item to something closely that's of a galvanic grade, likely to encourage such a situation. hull fittings, boarding ladders, tabs or even fittings adjoined or holding to the prop perhaps.


Good luck...

That is all very confusing and misleading. On the vast majority of shaft drive boats there is only a need to bond the anode to the shaft/propeller as it is this combination that creates a galvanic cell. The most common method is to wire the anode to the gearbox mounting bolts so that there is a path through the gearbox coupling to the shaft. However as the OP suspects the flexible coupling (which being flexible does not have continuous bolts connecting the gearbox flange to the shaft flange) should be bridged or as in post#2 an electroeliminator used which runs on the shaft. I have the latter on my boat because the flexible coupling fitted is particularly difficult to bridge reliably.

There is no need to connect the anode to anything else such as P brackets, shaft gland or rudder stocks unless they are clearly constructed of 2 dissimilar metals - and most are not. I find it very difficult to imagine a propeller connected to any other bit of hardware in the way you suggest.

 

[QBhoy]

That is all very confusing and misleading. On the vast majority of shaft drive boats there is only a need to bond the anode to the shaft/propeller as it is this combination that creates a galvanic cell. The most common method is to wire the anode to the gearbox mounting bolts so that there is a path through the gearbox coupling to the shaft. However as the OP suspects the flexible coupling (which being flexible does not have continuous bolts connecting the gearbox flange to the shaft flange) should be bridged or as in post#2 an electroeliminator used which runs on the shaft. I have the latter on my boat because the flexible coupling fitted is particularly difficult to bridge reliably.

There is no need to connect the anode to anything else such as P brackets, shaft gland or rudder stocks unless they are clearly constructed of 2 dissimilar metals - and most are not. I find it very difficult to imagine a propeller connected to any other bit of hardware in the way you suggest.

The Rudder stock is definitely better protected, if it has a fed bonding wire from the hull anode.
I might also suggest that a huge number of boats have a flexible coupling type that has a flexible spider (spidex) type part between the gearbox output flange and the shaft coupling flange. With bolts obviously holding the arrangement together and in place.
Hope that clears up any confusion. Things like the P bracket usually have no way of achieving continuity through the shaft, just with them usually having a cutlass bearing in place that would obviously be fairly non conducting.

 

[MoodySabre]

Isn’t it usual to have a shaft anode To protect the prop? I’ve always had one and the bigger MCDuff ones last for a couple of years at least. Last year I had a new prop with its own anode and that has taken the wear, leaving the shaft anode hardly worn. The hull anode is bonded to the P-bracket.

 

[Pye_End]

I might also suggest that a huge number of boats have a flexible coupling type that has a flexible spider (spidex) type part between the gearbox output flange and the shaft coupling flange. With bolts obviously holding the arrangement together and in place.

Regarding the OP, it sounds as though there is no bridging of the flexible coupling.

Not sure we know if a shaft anode is either present or possible, or whether the boat in question has a rudder stock or a P bracket etc etc.

MGDUFF have things to say on bonding here: MGDUFF - Wood & GRP Hull Anode Fitting Instructions if the OP wants more authoritative advice.

 

[Tranona]

The Rudder stock is definitely better protected, if it has a fed bonding wire from the hull anode.
I might also suggest that a huge number of boats have a flexible coupling type that has a flexible spider (spidex) type part between the gearbox output flange and the shaft coupling flange. With bolts obviously holding the arrangement together and in place.
Hope that clears up any confusion. Things like the P bracket usually have no way of achieving continuity through the shaft, just with them usually having a cutlass bearing in place that would obviously be fairly non conducting.

In what way is a rudder stock "better protected"? most sailing boats have either stainless or aluminium stocks going into GRP (or maybe wooden) blades. No dissimilar metals so no need for "protection". Some shaft drive motor boats have stainless stocks and bronze blades so an anode is normal because the 2 metals are in contact with each other underwater. Stern tube housings are also single metal although sometimes attached with stainless fastenings. If these are underwater as in an outer cutless housing then bedding in sealant is sufficient to isolate the metals - although some bronzes are very close in potential to 316. P brackets likewise do not normally need any protection as they are either glassed in or fastened with stainless bedded in sealant. Some P brackets are made of a copper alloy that contains zinc (like many cheap propellers) so can dezincify. An anode won't prevent this, although painting the bracket can reduce it.

Of course couplings have fastenings either side, but if flexible they are not continuous otherwise the shaft could not move independent of the gearbox output. hence the need for a bridge. Read here
randdmarine.com/flexiblesc.asp to avoid any confusion - first line.

The guiding rule of anodic protection is to identify any components that have dissimilar metals in contact underwater and then determine the best way of connecting an anode. Fortunately the vast majority of boats (particularly GRP) have very few such situations so do not need anodes. The most common are shaft/propeller on shaft drives, housing/shaft on saildrives and sterndrives, folding/feathering propellers and thruster shaft/propellers, engine cooling systems.

 

[Farmer Piles]

Surely apart from differential metal skin fittings, the main electrolytic action is going to be between the prop and the S/S shaft. Put an anode on the shaft and that will always take the hit instead of the prop.

 

[michael_w]

A multi-meter is your friend. Check continuity between the anode and everything else.

 

[dunkelly]

Gosh lots to read thankyou to all . Few things to clear up - no p bracket . Shaft directly out of the hull . No connection either side of the flexible coupling . Anode is attached just below. The engine bay and is only bonded to the engine mounts and there is good connection between engine and anode . I'm thinking just to add a copper wire strap across the flexible coupling - would it cause any balance issues or other problems?

 

[dunkelly]

Sorry forgot to ask what is an electro eliminator ?

 

[dunkelly]

Another thought . There is no room to put a shaft anode on the shaft externally but would a shaft anode on the shaft inside the boat work ?

 

[Pye_End]

I'm thinking just to add a copper wire strap across the flexible coupling - would it cause any balance issues or other problems?

No. It will be fine. Keep it tight though or it will fall to bits.

Sorry forgot to ask what is an electro eliminator ?

Electro Eliminators For Prop Shaft Bonding Acts on the shaft so bypasses the need to bridge the coupling.

Another thought . There is no room to put a shaft anode on the shaft externally but would a shaft anode on the shaft inside the boat work ?

No.

 

[Bilgediver]

Big ship sailors use the words siip ring instead of eliminator however some boats just use a carbon brush on the shaft.

 

[Tranona]

Surely apart from differential metal skin fittings, the main electrolytic action is going to be between the prop and the S/S shaft. Put an anode on the shaft and that will always take the hit instead of the prop.

skin fittings do not need any protection because they are single metals, although some may be brass which has zinc in the alloy so prone to dezincification and connecting to an anode won't stop that. Shaft anodes are fine if your boat has exposed shafts, but many like the OPs do not so a hull anode close to the prop bonded to the shaft is normal.

 

[VicS]

skin fittings do not need any protection because they are single metals, although some may be brass which has zinc in the alloy so prone to dezincification and connecting to an anode won't stop that. Shaft anodes are fine if your boat has exposed shafts, but many like the OPs do not so a hull anode close to the prop bonded to the shaft is normal.

This video explains the positioning of a hull anode.

 

[QBhoy]

In what way is a rudder stock "better protected"? most sailing boats have either stainless or aluminium stocks going into GRP (or maybe wooden) blades. No dissimilar metals so no need for "protection". Some shaft drive motor boats have stainless stocks and bronze blades so an anode is normal because the 2 metals are in contact with each other underwater. Stern tube housings are also single metal although sometimes attached with stainless fastenings. If these are underwater as in an outer cutless housing then bedding in sealant is sufficient to isolate the metals - although some bronzes are very close in potential to 316. P brackets likewise do not normally need any protection as they are either glassed in or fastened with stainless bedded in sealant. Some P brackets are made of a copper alloy that contains zinc (like many cheap propellers) so can dezincify. An anode won't prevent this, although painting the bracket can reduce it.

Of course couplings have fastenings either side, but if flexible they are not continuous otherwise the shaft could not move independent of the gearbox output. hence the need for a bridge. Read here
randdmarine.com/flexiblesc.asp to avoid any confusion - first line.

The guiding rule of anodic protection is to identify any components that have dissimilar metals in contact underwater and then determine the best way of connecting an anode. Fortunately the vast majority of boats (particularly GRP) have very few such situations so do not need anodes. The most common are shaft/propeller on shaft drives, housing/shaft on saildrives and sterndrives, folding/feathering propellers and thruster shaft/propellers, engine cooling systems.

I’m not even sure if you are agreeing with what I’ve said or not. Confusing…but anyway..
Preferred might be if things are conjoined conventionally, to the anode itself. Brass, bronze etc etc will corrode very quickly on their own, when exposed to salt water and air environments. Accelerated quicker again, should they be in the vicinity of metals the other end of the scale, as you’ll know.
The thought of dissimilar metals, which are in physical contact to each other, being the only thing that needs protected, is the wrong thought.
If you can, please have a look at most or just about any decently built shaft driven motor boat, with a hull mounted anodes. You’ll find there is certainly a bonding between the anode and the rudder stock.
If you don’t have access to one…I’ll send you a picture of the set up on a princess later on.
All the best.

 

[Tranona]

If you can, please have a look at most or just about any decently built shaft driven motor boat, with a hull mounted anodes. You’ll find there is certainly a bonding between the anode and the rudder stock.
If you don’t have access to one…I’ll send you a picture of the set up on a princess later on.
All the best.

I know that is done - but look at post#9 - I suspect that the rudders are stainless stocks and bronze blades so an anode is appropriate. Marine Projects were one of the builders which festooned boats with anodes and bonding where it was completely unnecessary, continuing bonding rudders when they moved to GRP blades for example. There is a lot of misunderstanding about the use of anodes, much of it arising from practices 40 or 50 years ago which were wrong.

Bronze does not corrode on its own, but brass does (post#17). There are of course many different alloys called "bronze" and while they don't contain zinc, they often have lower potential than 316 so (like the rudders) need anodes. My current boat built in 1979 had no anodes and do corrosion issues until last year when I changed the propeller to a Featherstream which is bronze hub with 316 blades and gears mounted on a 316 shaft. The prop has its own anode, but life is short if left in the water all year round. Last October I added a hull anode bonded to the shaft with an Electro eliminator and painted the propeller to reduce the area of bronze exposed to seawater. The anode on the prop will still wear preferentially because it is in direct contact but the hull anode I hope will provide some back up. Time will tell.