One Difference Between RVs and Boats

This past week we have vended LEDs at the Northern California Boat Show in Jack London Square, Oakland, CA. Response has been so-so. I have learned a lot about lighting differences between boats (or ships as some would say) and RVs.

It turns out that there is a major difference between the incandescent bulbs used by boaters and by RVers. Most notably, the RV that have bayonet type sockets for using the 1141 and 1156 use the single contact type socket. Boaters on the other hand use double contact sockets for the 1142 and 1152 bulbs.

It is interesting to understand why the difference.

I got an inkling of why when a friend asked me to check out why one of the single contact LED lamps would not work in a sailboat at the show. I took along my DVM to investigate.

Yes, the fixture was for a single contact BA15s bulb, and such a bulb burned nicely in the socket. But when I tested the LED lamp, it would not burn, even though it had worked back at the booth. I used the DVM to measure the voltage, and found that the polarity was reversed from what the LED lamp expected. The center contact was ground and the socket was +12 volts. The fixture had been "mis-wired."

Upon inspection, I noted that the fixture did not use different color wire coming from the socket, so there was a 50% chance the wiring would be reversed. I checked another fixture, and found the ground was attached to the socket as I expected.

The salesman said the problem could be easily fixed, and I agreed. The question was, why had it happened in the first place?

Some people do not realize the dangers of having a hot +12v socket, especially in a marine environment where things can be consistently wet, and a salty environment where even the water becomes a good conductor. It is a good way to develop a short and drain a battery.

Someone in the sailing industry realized this early on, and they decided to use the double contact socket and bulb. In this manner, the socket could not be shorted out to anything, and the current would be limited to the wiring, not to the chassis like in an RV. This practice has continued in most boats, especially in the salt water environs.

This problem did not appear in the beginnings of the RV industry. Using the same techniques as those used with automobile, the common ground throughout the RV is the chassis. This is not the best thing to do, but it works.

In many cases, ground connects in the electrical system are wired to the nearest chassis point, and the chassis is connected to the final ground wire back into the batteries. It saves a lot of wire, but can produce some interesting situations when some ground connect does not make good contact with its chassis point.

Of course, it is not a good idea to submerged the chassis into water. It does strange things to the electrical system.

Moral: be careful about transfering your knowledge of RVs to boats and vice versa. Sometimes, things are different.

sam