Marine technicians lament: resistance is futile (and inevitable), corrosion on electric cables on boats.

Electrical resistance, "a measure of the opposition to the flow of current in an electrical circuit "

Electronic resistor

Easter is usually when the starting gun sounds for the boating season to get going in earnest, its also the time of the year that many boats come out of storage and others make their way to the yard, often accompanied by a long list of things (usually electrical) that don´t work, sometimes they haven't worked in a long time, other times its a gradual decline in function.

In my experience a lot of these problems are caused by electrical resistance preventing enough electricity (current, measured in amps) from passing through the circuit to allow said faulty device(s) from working. Please read the opening statement at the top of the page again, the key word in that definition is opposition. By the laws of physics, everything has opposition. Electricity is made up of three principle measurements, voltage, current and resistance.

A very simple way to think of how these three interact is to think of electricity like a tap on a water tank, the water in the tank represents voltage, on a boat, the battery powered circuits are typically 12 or 24 volts, there is a finite amount of water in the tank, unless we top it back up.

Now, lets open the tap all the way, the water flow that runs out is like current, with the tap open fully there is little to stop the water from flowing and, if we keep up like this our tank will empty sooner. Lets now close the tap a few turns to slow down the water flow, in closing the tap we have created resistance, it does not stop the water flowing but opposes it.

Some practical examples

When corrosion builds up around electrical connections it causes resistance, it does this by forming a sort of layer of insulation on electrical connections that reduces their capacity to pass enough current, often seen as green (copper oxide) or white deposits (copper sulfate) or salts on the connections themselves. Imagine being in a noisy environment, so you stick your fingers in your ears, the noise is muffled but you can still hear it, this is how corrosion works, some current will still pass but not enough to allow connected devices to work.

Image 1: Corrosion in a multi pin terminal connector

Image 1: This little gem was causing the bilge pumps not to work amongst other things. These kinds of connectors are not suitable for critical circuits such as bilge pumps, this connect block was located in the bowels of the bilge with what could be an old heat shrink cover. Look at the green corrosion that has developed on the terminals, when connections get this bad they are beyond cleaning. The copper wires inside the cable insulation will "wick up" moisture further exacerbating the problem, it is sometimes just better to renew the cables, especially on critical circuits.

Image 2: Corrosion on engine electronics plug

Image 2: This corrosion formed on a main connector on a large electronically controlled diesel engine, tracing these faults can be a complete nightmare and play havoc with engine management, throwing all sorts of errors as was the case with this motor at first. Fortunately, in this case the plug was clearly visible and so was the green corrosion that had formed, the route cause was a slight water drip from the deck above. Solving the issue was straightforward, using contact cleaner and removing the terminals one by one using terminal extraction tools, the plug was back in working order fairly quickly, the leak from the deck was also fixed.

Image 3: Corrosion in an electronics connector

Image 3: Apologies for the blurry background image, but I think its easy to see the green corrosion, this could have turned ugly, these images show a pair of connectors that formed a joystick control system on a large flybridge yacht, the joystick controlled both motors and the bow and stern thruster. The owner of the boat had been experiencing problems with a non reactive system and a couple of times the system getting stuck in use and then not releasing quickly enough, just what you want, two huge diesel motors and thrusters not playing nicely in a crowded marina! Having electronically controlled motors get stuck in gear has happened to me a few times, its not even vaguely amusing.

The connectors in image 3 were finally located under the flybridge helm station and the problem was fixed, as far as I know the issue has not occurred again.

Image 4: A collection of corrosion causing resistance

Image 4: Pictures say a thousand words, the top photo is of a fuse panel from under a dashboard, looks to me like it was submerged, not even worth the effort of trying to salvage it. Bottom row, left, that is a trim pump for an outdrive, this was salvageable despite what it looks like, this was caused by a hose blowing on the motor and slaughtering the engine room with seawater. The middle two photos on the bottom row, the top one is a busbar in an engine room with all the bonding wires from the various underwater metals joined together, the terminals could be replaced and the busbar cleaned. The middle bottom photo is from a pump in an engine room, the terminal had snapped in half, the blue green corrosion is almost the same colour as the terminal. Bottom right, the back of a battery switch, the switch itself was fine, just needed a clean of the posts, the cables and terminals needed replacing.

Is it me or is it hot?

The main cause of fires on boats is electrical related, a by product of resistance is heat, imagine driving on a multi lane highway during peak times with a high volume of traffic, now lets imagine some lanes get closed for roadworks, the same amount of traffic is now trying to get through a smaller number of lanes, creating resistance, as a result engines get progressively hotter (as well as a few tempers I suspect), exactly the same thing happens in the wiring on a boat, when the electrical current flowing in the cables encounters resistance, heat is produced.

Image 5: Back of a burnt battery switch

Image 5: This is the back of a battery switch, not a cheap off brand one either. The ring terminal on the end of the red positive cable is tinned copper, sealed with heatshrink, the cable is tinned copper and marine grade, you can just make out the 105 degrees temp rating on the cable insulation. The cable was connected to the engine starter motor, this mess was only discovered as the engine would not start, after making all the logical checks, the search begins, from the front of the battery switch this was hidden, you can see just how hot the plastic got. This corrosion was caused by a water leak that dripped onto the cable terminals.

All circuits on a boat should be protected by some sort of overcurrent protection, a fuse or circuit breaker, something important to know is these protection devices do not trip or open because of high resistance, they break the circuit when too much current runs through them.

Image 6: Corrosion causes heat

Image 6: Ignore the poor quality terminal block in the background of the right hand photo, instead concentrate on the scorch mark, remember corrosion causes heat, in this case enough to start burning the connector block. These photos are from the main connection on an electric toilet, the main fuse protecting the circuit of 20A was intact.

Shorepower

This is where mains electricity is plugged into your boat right on the waters edge or at the dock, sounds great, what could possible go wrong? I would say problems with shorepower systems related to corrosion and thus resistance cause more problems that anything battery related on larger boats. All the ports that I know of in Europe where shorepower is available are protected by residual current devices that trip if they detect an imbalance between the live and neutral cables, sometimes the wiring inside the dockside pedestal leaves a lot to be desired however.

The boat end of this set up though, that's entirely the boatowners responsibility, you really would not believe some of the things I have seen, shorepower plugs left plugged in whilst the boat goes out, big no no, homemade shorepower cords made from whatever cable was lying around, including solid core cable! I genuinely do not believe the greater majority of boaters take care of, or, do any sort of routine inspections on the shorepower system until there is a problem.

Image 7: A typical shorepower male & female plug set

Image 7: A typical shorepower cable set up, very popular on US built boats, you will see the three slots in the yellow cord end, the "L" shaped one is for the earth cable, it means you cannot insert the plug incorrectly to the boat, although people do try, I see a lot of these with the plastic chewed up where people have tried to insert the plug with the wrong slots lined up, this is especially a nuisance when the plug is in the side of the boat and not right at the transom, so seeing the pins is difficult. I am not a huge fan of these, the black, threaded locking rings you can see on the yellow plug go brittle over time and break, so the plug is not held securely in place, the plugs also require a slight twist to lock the pins, its just a lot of steps.

Image 8: Typical Euro style shorepower inlet

Image 8: This is a Euro style shorepower inlet and plug from Ratio Electric in the Netherlands, this one is fitted on a Riva. After having installed a lot of these, I have always been very happy with the quality, I think the price is very good too. I find this kind of plug and cord set causes far less problems, its easier to put the cable plug into the receptacle on the boat and the pins are much larger and easier to see, these plugs also use a threaded locking ring, the plug itself requires no twisting to lock it into place and can only be installed one way. The photos shows a model rated for up to 32 amps.

Image 9: Damaged shorepower cable

Image 9: A damaged shorepower cable, the owner was completely unaware of this and the whole cable needed replacing, you should not put any sort of join into a shorepower cable. This cable is from a small boat with only a battery charger and one plug socket, no large loads, but remember, breakers only take into account amperage passing through the cable, not the cross section of the cable. The photos show the insulation has been breached, some is dirt for sure, but the cable looks to me as though it had been getting hot for a while. Not sure what caused the damage, at the time I though maybe the cable had been damaged by a wheel running over it, a dock trolley perhaps.

Reducing resistance

Let´s get a few things out of the way, firstly, everything will eventually corrode on a boat, it lives in the most hostile of environments and, sometimes not always looked after or maintained the way it deserves to be. The next thing is the term marine grade, you will hear this term banded about a lot, it falls into distinct categories

Marine specific products that are made very specifically for use on boats, typically (but not always) electrical devices, made to a very specific set of standards, some examples I can think of are marine carburetors, alternators, starter motors and a lot of battery chargers. Electrical devices that can be safely installed in engine rooms (especially with gas/petrol motors) are built to standard SAE J1171 & ISO 8846, broadly speaking it means the device has been built to not create sparks during normal operation. More than once people have unknowingly installed something not realizing the potential dangers. What does this have to do with corrosion? Well, I see a lot of products that have been made with better quality materials, better suited to being around salty air, apart from the certification standards. The manufacturers knowing where these things will be installed and what sort of life they could lead.

Marine grade products also made to a standard that meets or exceeds the minimum requirements by law to be used on boats, a good example is cable with insulation made to withstand a certain minimum temperature (105 degrees typically), battery switches made to pass high currents, fuse holders (some are marine specific too), the list goes on. Products for use on boats, usually have the CE mark in Europe, the UL (United Laboratories) mark in the USA and very often both. I know it is tempting, the marine catalogue price is so much higher than elsewhere, but, I can assure you, its more expensive for a reason, do not be tempted to cut corners and install products that are not suitable, you could be compromising safety and also causing insurance problems.

Internet and forum advice

I am a member of quite a few groups and forums that are primarily based on boat and marine electrical and electronics. Time and time again the same basic advice gets repeated, sometimes posters get deflated after spending time, money & effort on an electrical project only to have people tell them what they have done wrong or what improvements could be made.

A typical problem goes "my (insert electrical device) is not working, help". The usual first pieces of advice are, check the fuse, check the connections to make sure they are tight and clean, the last two pieces of advice relate to resistance, tight & clean connections offer less opposition, this leads me onto the another piece of advice and a secret third marine specification, I like to call it marine preferable, a brief explanation of this secret spec known only to a select few (not really, its common knowledge) relates to products commonly used on boats that are made of materials that make them considerably better than alternatives.

Some examples of marine preferable are:

• Tinned, stranded copper cable, tinned means that the individual copper wires had a tin coating on them, this makes them much better at resisting corrosion, tin used to be used in antifouling bottom paint to stop growth. The wires must be copper, not any other kind of material and, the cable must be stranded, not solid core cable, solid core cable is never used on boats unlike a house where solid core is common, boats move too much and solid core fails too easily.

• Heatshrink terminals, the terminals that connect cables to other cables or buss bars or terminal strips, instead of being normal automotive nylon insulated terminals, they have a plastic insulation that, when heated contracts and an adhesive inside the insulation melts, this melted adhesive oozes out of the terminal and forms a watertight barrier, no moisture means no corrosion, no opposition, no resistance, the heatshrink contracts to form a perfect seal around the cable, offering mechanical strength. The terminals themselves should also be made of tinned copper.

Image 10: Heatshrink ring terminals

Image 10: Heatshrink ring terminals, you can see where the terminal has contracted after heating, the black cable insulation showing through the red heatshrink, note the adhesive showing out of the bottom of the terminal.

• Dielectric, you may have heard the term before, a dielectric is an insulator that does not conduct electricity. It might come in the form of a spray or grease, its put on after (not before) terminals are put in place, it reduces the rate of corrosion by reducing the oxidizing effects (reaction to oxygen in air and water), see image 10.

Image 11: Terminal strip with dielectric grease coating

Image 11: This terminal strip has a coating of protective dielectric grease on it, that gold sort of colour on the screw heads, you can just make out some of the overspray. Probably done at the factory, this boat at the time was already quite a few years old, this terminal strip is located underneath a dashboard of a sports cruiser, protected directly from water but still exterior to the boat. I would not use these terminals however.

Shoppers beware, caveat emptor

Online shopping is just so easy, click and bingo! your orders arrive without you having to leave the comfort of your bunk, but, please be aware there is a lot of junk out there, products that make all sorts of spurious claims, false advertising and, even counterfeit goods. Please make sure what you are buying is the real deal and/or fit for purpose.

This is another topic that gets hotly debated, I from product "X" on (insert site here), will this work? floods of no replies com in, get this one instead, what about this one? NO, we have told you what to buy, some people are either trolling or just too stubborn to take the correct advice even when told by experts why their chosen part will not be correct.

I would like to remind any readers that anything I recommend or any links I make to products are the same ones I use myself, I do not endorse anything I do not have firsthand knowledge of.

Conclusion

It is inevitable that at some point you will find corrosion in at least part of the wiring on a boat. Take the right steps to repair it, be mindful of the products you are using and take precautions to slow the process down. This problem can sometimes be a real head scratcher and occasionally very difficult to find where the problem is. I have seen many boats where new cables have been run as it was easier than trying to fix the existing installation, I do understand this approach, but please, do the technician a huge favour, either label the dead cables or try and pull them out. Please check your shorepower cable regularly and if you are in any doubt, consult a qualified person. Safe boating.

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