June 16, 2007. An article in this morning’s LA Times motivated me to write something about the virtues of weep holes and ventilation. I discovered I’d already done that a year ago in the article below. So I’ll just add this preface. It seems that the Great city of Tulsa buried a ’57 Plymouth in a “bombproof” time capsule under the courthouse lawn 50 years ago as a publicity stunt. (Bombproof? It was the 50’s.) When they recently dug it up, the car was up to its hubcaps in water and encrusted with rust. Oops.
Water will penetrate any space. If you want to keep something dry, give the water a way to get out again. If you want to avoid condensation, ventilate. There are places where it rains a lot and the humidity is high. So what. You would think ventilating would let moisture in. Yes, but it lets it out again too. Think about it. The sidewalk always dries up after a rain.
Lots of stuff on a boat doesn’t need to be waterproof. The main electrical panel is often sheltered enough that you’d have to sink the boat to get it wet. I have a couple of rules about waterproofing. The first is that some things cannot or should not be waterproofed. It’s a corollary to the law of scuppers. When water cannot be kept out, give it somewhere to leave again.
I see a lot of electric heaters inside boats for the purpose of fighting humidity and condensation. Sometimes I see dehumidifiers too. What a waste of energy. Ventilation will do the job better than either heaters or dehumidifiers. You do NOT want to seal up a boat to fight humidity. Install a solar powered ventilation fan, as I have on my boat. If you have musty compartments, ventilate them. This includes the bilge and the sail lockers. Somehow this is counter intuitive and people resist the idea. Oh well.
Any re-enterable enclosure exposed to the elements is going to get water in it. I don’t care what kind of gasket or how much sealant you use. Be it a navigation light, a radome or an instrument cluster, your only hope is to include a weep hole so the water can get back out again. Enclosed spaces breathe as ambient temperature changes shift the pressure of the air inside. If you wonder how water got inside your gauges and condensed in droplets on the inside of the glass, now you know. It seeped in as water vapor mixed with air through what could be microscopic openings.
Very few displays are hermetically sealed. If you have a problem with moisture condensation there are two solutions. The first is counter intuitive. Drill holes in the gauge case (in back of the panel, obviously) to allow air to circulate and moisture to escape. Moisture wants to get out of the gauge just like puddles always seem to dry up. This assumes the interior of the boat is at least minimally ventilated so that water isn’t condensing on surfaces inside and making everything rusty. VDO relies on it’s illumination lamps to keep the gauge faces dry.
The other way is to heat the gauge a little. Wire the illumination lamps to run at a lower voltage continuously and apply full voltage only when you really want to see them at night. Keeping the inside of the gauge just a few degrees warmer than the ambient will drive water out the same way it got in. You can also glue resistors to the outside of the gauge case as a heat source, but the pilot lamps will do. The only disadvantage of this approach it it uses a very small amount of power continuously. If your batteries don’t get charged every so often, they could slowly run down. Don’t worry about the lamps. Run a little below their rated voltage, incandescent lamps will last essentially forever.
The other thing that people sometimes try to waterproof is a hollow sailboat mast. This is mostly a problem where the mast steps on the cabin top and water in the mast can drip down out of the ceiling. If the mast steps on the keel, the water will run into the bilge and you will never notice it. There are lots of little places water can get inside the mast, particularly if you have halyards running inside. A cabin-stepped mast leaks because wires for the navigation lights and VHF antenna enter the mast through a hole in the bottom. It’s possible to construct a mast where most of the water runs out somewhere else, but usually when a leak shows up people do things like squeezing a whole tube of RTV silicone sealant into the hole where the wires come through. This can be effective in stopping the leak, but it will cost you later.
Eventually you want to add wind instruments or radar and some more wires have to get into the bottom of the mast. I can sometimes break the sealant up with a probe and pull enough out with a hook or even drill past it to open up a passage for new wire or to get old wire out. Or you can pay a boatyard to unstep the mast to get the sealant out. I haven’t damaged any existing wires yet by poking around, but you can be sure I will need to drill a hole in the side of the mast near the base so I can thread the new wires through the mess.
Note July 4, 2008. This next paragraph just shows how much one can learn by working on your own boat. Who wants water in the tabernacle core?
My suggestion to solve the “mast water leaking into the cabin” problem is to yield and admit the water is going to come in. Then fashion something on the cabin ceiling to collect the leakage and route it to the bilge through a small plastic tube. It could look like a decorative cap over the wires coming out of the mast and the tube could probably be routed along with the wires and you’d never notice it. I’ve never gotten to try this yet, but I’ll post a picture and a report as soon as I do.
7/4/06 revised 6/16/07