View Full Version : Your Boat Trailer Lights Unreliable?

Bob Smalser
07-02-2004, 3:37 PM
A lifetime of maintaining farm and heavy equipment out here in the wet…interspersed with over three decades of maintaining Army tactical vehicles used by young soldiers all over the world…have given me some strong opinions about vehicle electrics. These wiring harnesses reflect those opinions…. and the opinion that any small boat liable to be caught out in the ferry lanes in a fog bank needs to have minimal lights, horn and electric pump. Moreover, those lights, horn and pump need to be just as reliable 20 years from now as they are the day they were installed. I’m not a qualified electrician, and my opinions should be weighted accordingly…I’m just a lazy builder who never wants to have to mess with the electrics again once they are installed.


Marine wiring standards call for crimpled connections and protected terminal blocks like the older but professional installation shown above (that needs some maintenance). The books say that proper crimps and terminals have slightly less resistance than properly soldered splices and connections, and I don’t disagree….


…but what do you suppose the resistance is after several decades of user abuse? Add to that the fact that those bus bars are always hot, and invariably hard to get to in a small boat (or the panel the main aesthetic feature of the cuddy), and I’ll go with permanent, sealed connections every time. Crimped, soldered, greased, heat shrunk and armored.


Step One is to understand electricity. There are a few owner-type boat-wiring manuals out there, but the more general commercial manuals often provide the big picture a bit better. Other than the environment, the major difference between automotive and marine wiring is the ground…marine wiring must run a separate wire back to the negative battery terminal, which is the only ground on the boat. Automotive wiring generally grounds each fixture in place to the vehicle frame. Of course, marine harnesses generally require twice as many wires as automotive harnesses.


From the manuals, a plan for the harness is drawn. The amperage loads of each fixture and appliance are computed and the correct wire size selected from the tables in the manual. Then I generally up each circuit one wire size…anticipating somebody adding an appliance one day…. arrange them into circuits and select appropriate-sized switches and circuit breakers.


The necessary supplies are acquired and laid out for the job. I use tinned marine-grade wire almost exclusively…even in trucks and tractors. Ever have to do an emergency repair in the field only to discover the copper is too corroded to solder…even beneath the sheathing? Shown are 16ga, 14ga and 10ga marine wire in red for hot and white for ground. For economy, I generally use labels rather than multi-colored wires. Because I solder and heat shrink all connections, I’m not as fussy about crimp connectors, and use the standard electrical jobber fare (Tacoma Screw, Platt Electric and the like…generally higher quality than NAPA)…but if I were crimping without solder, I’d use all marine grade connectors.



Rather than crawl around for a couple days on bare frames in tight spots, I build the harness outside the boat. All fixtures are laid out in their permanent locations and the wire strung in place, allowing for vertical as well as horizontal runs once installed. I also try to allow a little extra length so the harness can be pulled away from the hull interior some day for repair. When building a boat from scratch, making and installing the basic harness before decking the boat will save a lot of bruises and claustrophobia.


For end-of-circuit wire connections to screw terminals, I generally fuse the strands with a tiny drop of solder to minimize surface area for corrosion to penetrate, and apply heat shrink tape to the ends (here not installed yet) for a good seal. The tip of the wire is lightly greased with proper dielectric grease (it does not conduct electricity…do not use automotive grease), the terminal screwed tight to the wire, and the entry points to the fixture coated with grease…and here protected with a rubber boot.


Bob Smalser
07-02-2004, 3:37 PM

Spade terminals are crimped, then soldered…also with rosin core solder only…absolutely no acid fluxes to promote corrosion….


…and double heat shrunk, also to be bedded in dielectric grease. Before applying the shrink tubing, I use an acid brush to apply a light coat of grease to the bare, soldered wire in all these installations.


Instead of terminal blocks, I use soldered pigtail splices…also protected by heat shrink. I could cap the ends, but choose underground-rated, gel-filled wire nuts to protect the ends. These are removable, and those splices are a convenient place to add an additional appliance if required. I note that…and the amperage limits of each circuit…in the owner’s manual I prepare for the boat….that manual including a complete, “as built” wiring diagram.


Those pigtail splices are also used for middle-of-circuit screw terminal connections like this stern light. For areas where minimal electromagnetic interference is required…like the light to the compass…those pigtails should be tied together with a zip tie several inches before and after the compass…and the like-colored wires twisted around each other fairly tightly to zero out any magnetism.


Once all the circuits are laid out, I go back and permanently mark all the wires at each fixture and each splice.


I armor the harness with spiral wrap before installation – spiral wrap dries out better than other conduits…


…and then install it in the boat as close to the gunwale as I can get it. I use stainless screws for easy removal and am generous with them…and try to avoid routing the harness where it could serve as a handhold.


The completed installation is clean…


Bob Smalser
07-02-2004, 3:38 PM

…and minimal.

Boxes behind the circuit panel and compass for protection? You bet…. but well drained and vented boxes that will dry out. I want protection from bumps, not water…I’ve already protected the circuits from water as best I can.

Battery and charger? Optima Bluetop Marine gel, of course…and a small automatic trickle charger installed in the stern. A hundred and fifty bucks for the Optima, but no acid leaking from a cracked battery box will ever eat a hole in the hull. Moreover, as automotive Optima’s have come down in price over the years, their longer life is beginning to justify the additional cost for all my old iron.

How much solder? Just enough to fuse the connector or splice. More is not better, as it runs down the wire, stiffening it and damaging insulation. The argument against soldering is that it stiffens the wires, making them more prone to cracks from vibration. If you solder, try not to make that argument come true.

Boat trailer lights unreliable? That’s often because the harness isn’t up to the task in salt water. Some I’ve seen aren’t up to the task even in dry air. If it isn’t up to basic marine standards, I’d rip it out and replace it using these techniques…. yes, even the separate ground wires for each light running back to…not the battery like in your boat…. but to a permanently fused double ground.


See the 10ga wire and ring terminals forming a yoke beneath the trailer tongue? The yolk connects the ground wires from the lighting circuit to the trailer plug. There are ring terminals on both sides of that bolt, and the bolt, terminals and tongue are fused together on both sides into one unit with solder. Pick a bolt easy to get at so it’s obvious to a future repairman where the ground is…. pull and clean the bolt…ream and grind both faces of the hole…reassemble as a unit and apply MAPP gas heat, flux and solder to both sides. Then prime and paint. Also use dielectric grease liberally between light bulbs and sockets…and you won’t have to mess with those fussy trailer lights for a long, long time.

Aaron Koehl
07-02-2004, 4:32 PM
A very nicely written and informative article, Bob!

Glenn Clabo
09-17-2009, 5:46 PM
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