To bond or not to bond. That is the question.


The following insight was given by of one our engineers in Palma.  After losing a seacock during an extended stay in a Superyacht Marina apparently dripping in stray current, the owner of  a large catamaran preparing for a RTW cruise was determined to put the best fittings in place and to understand why the yard specified the boat (like many) to remain un-bonded.

The symptoms? The yachts’ engineer on a routine operational check found a perfectly good looking sea-cock actually crumbled when operated. Divers were called and disaster averted. The zinc that strengthens the metal had all been drawn out by stray power leaving the unit with the rigidity of chocolate.

Groco valves were chosen as the best product available for an acceptable outlay. Groco offer fittings with an enlarged base to add protection and rigidity in case of impact.  These were bolted into tapped holes on a stainless steel plate bonded to the inside of the hull.

Our engineers opinion:

GROCO SEACOCKS:THROUGH-HULL INSTALLATION. Quote from Groco installation manual: “Machined holes are provided in the flange through which the seacock is MECHANICALLY fastened to a backing block or through the vessels hull, making the seacock an INTEGRAL PART OF THE HULL STRUCTURE.”  While this does not make any mention of a stainless ring bonded into the structure, standard engineering practice does NOT provide for permanent , male threaded fasteners which cannot be replaced. Male threads are more likely to be damaged than female, so the permanent thread (such as the one in the stainless ring) is always female. None of the above is personal opinion, and I am not keen on inventing new ways of doing things. The stainless ring may still seem to be overkill, one could tap threads into the grp, however my own feeling is that these bolts should be as strong as possible, as if the strainer should be knocked off the hull, a standard seacock will fall off the inside of the hull. The Groco valve will still be strongly attached, and no water will enter. This may seem a very unlikely tthing to happen, but I have seen this happen a few times, once when a catamaran ran over a cable between two pair-trawlers, and was dragged backwards for quite a while at four knots.I may be oversensitive about this issue, but when I salvaged my yacht in St Maarten after we sank during hurricane Luis, I  found that an external strainer had been “scraped” off the hull. Had Groco valves been installed, I reckon we would have stood a better chance altogether.

GROCO VALVE BONDING: “The seacock base has a bonding screw. Use at least 14 gauge marine grade stranded copper wire to connect the seacock to the vessel bonding system. Bonding may be made with other non-motorised equipment such as strainers.”WARNING : Do not bond motorised equipment such as airconditioner pumps in series with the seacock. Motorised equipment  must have a seperate connection to the vessels A/C or D/C grounding bus, in accordance with ABYC-11,FIG.18”. These instructions from Groco dont leave much room for interpretation, “if you do not agree with the manufacturers installation instructions, dont do the installation”.

We are all aware of the different views on cathodic protection, however they are all based on theories which if they were perfectly understood, would all be in agreement with each other. The reality is, that the mechanisms which actually create a cathodic or anodic cell have not been scientifically explained yet. What all these theories seem to lack is common sense. From a purely practical point of view, we have a huge database with which to work. Apart from peroxide damage to wooden hulls, the very worst damage I have ever seen from a bonding system (with anodes seperate to the electrical earth) was some paint saponification in the anode area on a steel hull. This can only damage paint, not the steel hull or fittings, and in this case the hull was way over-protected. On the other side of  the issue, if an unbonded 12 volt seawater pump developes an earth leak, the powerfull current will discharge to earth through the seawater in the hose. This is NOT theory, it is a fact, because seawater is electrically conductive. You need to choose whether you would prefer the current to exit the boat via the seacock or the anode. It is possible to determine EXACTLY AND SCIENTIFICALLY  how long the seacock (or anode) will last if we know the material, mass, voltage and amperage of the stray current- THIS MAY ONLY BE WEEKS, OR DAYS.

Whenever we  do welding on board any type of vessel, we attach the positive earth of the welding plant to a cable with an anode which we lower into the water, to dissapate any stray current. During the welding, it is amazing to watch this anode fizz, boil and bubble, with occasional big chunks flying off! (You wont read about this in any books, as I developed this earthing independantly to any current literature Ive ever read) Despite this,  all sorts of UNBONDED EQUIPMENT, in particular GPS antennas and engine control units such as D-DECKS, will almost certainly be fried, if they are not disconnected from the vessel.

If one balances the relative risks, then its a no-brainer really. Bonding the shaft and prop (but not the engine) and all the thru-hulls means that everything metalic below the waterline is protected. In the event that  something weird happened, I guess that you could allways disconect the wire at the anode stud. On a professionly installed system, the corrosion meter, or earth leakage device would alert you to this problem. Please note that it is NOT possible to have a corrosion meter installed on unbonded  thru-hulls.

I hope that this helps to clarify the issues, and explain why I am not bothered by the” voodoo” aspect, which I firmly believe stems from way too much theoretical pondering, and not enough hands on experience from installing  cathodic protection systems, and then seeing what the results are over a period of decades.

And what about props?

Maxprops are made out of the identical bronze as Groco valves. There is an anode on every single prop that is supplied, and it it absolute necessity to change it every year. Failure to do so would invalidate any warranty, and more importantly, would actually lead to corrosion. I find it quite amazing that experienced sailors who would NEVER, EVER , dream of relaunching their yacht without the Bruntons or maxprop anode, or indeed the shaft anode, (which you can see on EVERY SINGLE SAILBOAT where it is humanly possible to fit one) object strongly to giving their seacocks the same protection, on the basis of some bullshit THEORY they read somewhere! I am convinced that the propshafts on (catamaran X, 5 years old) were not subjected to pitting corrosion, despite the total lack of reoxygenated water in the sterntube, due to regular shaft anode changes. It has been shown that pitting corrosion does not occur where cathodic protection is present- how this works has not yet been explained scientifically; and I really couldnt care less!!!