David Berry discusses the pros and cons of using various methods of connecting electrical wiring on board a boat
There are various methods of connecting electrical wiring on board, but which is best?
When I installed the anchor windlass on Aderyn Glas (PBO January 2012) I used the built-in magnetic sensor to make myself a chain counter at the same time.
This, I know, is a luxury. My wife, Ann, has painted marks on the chain like everyone else and steadfastly uses her marks whenever she is on the foredeck dropping the anchor. In fact the only time the counter display in the cockpit is routinely used is when we are reversing up to a quayside because then Ann is in the cockpit handling ropes. So when it failed it was not a catastrophe, just a nuisance. But I had a fault to trace.
Having checked the obvious I finally decided to take the unit out for a closer look and as I unscrewed the screw connector the cable disintegrated before my eyes.
I should have known better. After all I had worked for a cable manufacturer that designed and supplied cable to warships. I know how good those cables are: the typical military connecting cable, for example, that simply joins equipment together is silver plated copper strand inside a PTFE sheath. It’s exotic stuff the likes of you and I can’t easily get. And it’s designed that way because the most corrosive natural environment in the designer’s bible is salt laden air. And that’s what we spend most of our lives sailing around in. This episode led me to mistrust much of the cabling on our boat.
So is there anything I could have done to prevent the failure of my anchor counter cable? Well yes there was. The first mistake I made was using a cheap co-axial cable designed to carry TV signals through the house. I thought this was a PVC sheathed cable which, considering the many millions of miles of PVC cable the telephone companies use, should have been up to the job. But I obviously got that wrong. I should have chosen RG58 or something equally rugged. It’s interesting to see that the copper braid shield layer has completely disintegrated, not just at the ends, where you might expect moisture to enter, but all through the cable. This is evidence for a fairly porous sheathing material.
The second mistake was to use a chocolate block. Screw connectors are OK for most things but they don’t seal the
core of the cable so the atmosphere can get in and, over time, corrode the conductors.
So can we prevent this? Yes. The MoD gave up soldering about 30 years ago preferring instead to use calibrated crimps in calibrated tools. This produces a cold weld when used with carefully diametercontrolled cable cores. But I am advocating a return to soldered connections. Soldering can fill the interstitial spaces in a stranded cable and seal it off to the atmosphere, locally making a stranded core into a solid one.
To protect the joint made in this way the best practice was a layer of flexible epoxy followed by a heat shrink sleeve then, to make absolutely sure, another layer over that. Joints made like this are no-kidding waterproof and are perfect for any joint we need to make in the bilge or anchor locker or in any other wet environment.
You can buy lined heat shrink online, which will do for straightforward single wire joints in non-taxing environments but if you want to join complex wires or wires that will live in the bilge then always use unlined heat shrink over an epoxy layer. I use cheap, fast setting epoxy in the (untested) belief that it may be slightly more flexible than the better stuff. When I made joints in service equipment we were obliged to use an epoxy called Raychem S1005 which retained its flexibility forever. You can buy it, but it’s very expensive stuff.
We should also seal wires wherever the cores are exposed so if you attach a wire to a switch, a crimp, or the dreaded chocolate block then we should at least make sure any stranded wire core is blocked with solder.
Solid core wires are better than stranded wires because they completely fill the plastic sheath. But they are stiff. Stranded cores have that interstitial space between each of the individual strands and this allows them to be flexible but also allows the atmosphere to enter the core and over time to degrade or destroy it.
Don't join antenna cables like this. You may have noticed that antenna cables are defined by something called characteristic impedance. For example the RG58 cable on your VHF antenna has a characteristic impedance of 50 ohm while the cable that plugs into the Navtex or the TV in your lounge may be 75 ohm. Every joint should be made with a connector with the same impedance. So a BNC connector of 50 ohm must be used to join a 50 ohm co-axial cable and so on.
The various connection methods have pros and cons and I’ve tried to summarise them in the table below. Chocolate blocks and crimps do have their place but if you want the best possible sealing then epoxy and heatshrink is your only choice.
Then there’s the problem of UV. If you have cables exposed to sunlight, such as those running from solar panels, you’re going to find out that UV is pretty destructive. Cable manufacturers include UV blocking pigments in cable sheaths that are destined to be used outdoors but again whether these are available to the likes of us is rather untested.
I take the pragmatic approach and put all my external cables in flexible split conduit. This protects the cable at the cost of sacrificing the conduit which tends to crumble after a year, two at most. But conduit is far cheaper than the cable and easier to fix.
One other point to watch is the cable ties that fix the conduit or wiring. I believed that black cable ties would contain carbon black as a UV blocking pigment but when they started snapping off my anchor light cable I had to revise that belief. I also had to climb the mast to replace them so, yep, I replaced the cable too... just in case.
Copper braid of this coaxial cable disintegrated in a salt water environment Ð though the core is still in good condition
The best of cables: 19 strands of silver plated copper insulated with PTFE
David and Ann Berry keep their Moody Eclipse Aderyn Glas in the Ionian
This cable had run through the anchor locker and had areas that were clearly swollen. When I cut into it this is what I found: the copper strands were slowly disintegrating into a mass of copper salts
Wires that terminate at switches or tags should be soldered and protected with heatshrink sleeves
Chocolate blocks are quick and easy to use but have built-in long term problems
Conduit is cheaper and easier to replace than cabling