BONDING THE STERN TUBE .

[ALLAN WARD]

I have a hull anode to which a number of items are bonded to i.e. P-Bracket e.t.c. I have noticed that my S/Steel stern tube is not connected to this system and although it is only the outboard face that is exposed to the saltwater , there would also be contact within the stern tube in the stuffing box and cutlass bearing . There is corrosion of sorts regarding rust as there is a small rust stain below the stuffing box . Should the stern tube be bonded ( connected ) to the hull anode ? My concern is electrolytic corrosion ( pitting ) taking place and with the stern tube glassed into the hull there is no way of checking .

[senormechanico]

BOND IT !!
My stern tube developed a pinhole leak and slowly filled the boat 3" over the floorboards.
Being a multihull, it didn't sink, but made a mess of my starter motor.
That was 12 years ago.
I fixed it with a hose clamp, some 5200 and a piece of rubber mat.
Everything is bonded now.

[Andina Marie]

Interesting question. The tube with the shaft down the center is the ideal mechanical construction for a battery with lots of surface area and short distance through the water (electrolyte).

My thought would be that if the shaft metal and the tube metal are exactly (or very closely matched) for alloy then bonding together would guarantee they are at the same voltage and no electrolytic current would flow between them through the electrolyte.

However if the metals are not matched exactly it would suggest that insulating them would break the positive to negative circuit created by the "battery" and prevent electrolytic current flowing.

[rw58ph]

Technically the tube is bonded if the shaft is bonded as the salt water should conduct a current and bond the metal housing of interior shaft seal/cutless. Being the tube is probable SS it should have about the same galvanic rating as the SS shaft. So directly NO, indirectly YES.

[CaptainRoss76]

From an experience marine engineer and delivery skipper;

As senormechanico says "bond it". The tube is conected to the stuffing box
- Eye terminal under a 2nd nut on one stud of the box
- Job done, mind at rest. Smiley Face
- Unless you have a ceramic face or double lip type seal....
Then you must drill a small shallow hole (the size of the striped back bonding cable)
-drill the hole parallel to and against the tube (aftwards into the glass)
-with a needle file clean the metal of the tube
-put the stripped back cable in the hole with petroleum jelly
-hold the cable in place with a wee 'P' clip...

[senormechanico]

Originally Posted by Andina Marie

However if the metals are not matched exactly it would suggest that insulating them would break the positive to negative circuit created by the "battery" and prevent electrolytic current flowing.

I agree with your statement.
My stern tube is bronze, the shaft is stainless, but the shaft/prop have their own zinc and are insulated from the engine and the rest of the bonding system with a shaft coupling.
No problems since.

[ALLAN WARD]

Many thanks for all your replies . I am definitely going to bond my stern tube . My thinking is that because my stuffing box is mounted on the end of my stern tube with a heavy duty piece of hose clamped to both , there is no metal contact between them so my plan is to run a seperate bonding wire from my hull anode to the exposed section of the stern tube and clamp a crimped terminal on the bonding wire around the stern tube . My propshaft is supported by cutlass bearings within the stern tube and P-Bracket , so there is no metal contact there but both shaft and prop have their own anodes and the P-Bracket is connected internally to the hull anode . You have got me thinking about my engine / propshaft connection via the drive coupling . My Vetus coupling , although it has a rubber element has through bolts creating a connection from the engine to the immersed propshaft. I presume the anodes on the shaft would come into play ?