Transformational Technologies 6 A Better Mousetrap

I learned to sail on a wooden boat with a traditional stuffing box that always had a steady drip. Checking and manually pumping the bilge on each watch was a part of the ship’s routine.

Ever since the introduction of fiberglass, boats have been more-or-less watertight. With the later introduction of mechanical and lip-type propeller shaft seals, and the elimination of traditional stuffing boxes, even that drip has been shut off. It is not unusual for modern boats to have completely dry bilges with dust in them.

Although manual bilge pumps are a requirement of the ISO and other standards-writing organizations, almost all modern boats also have electric bilge pumps with automatic switches. Given a combination of dry boats and automatic pumps, on many boats, including ours at times, checking the bilge on each watch has become a thing of the past. Whereas the constant small leaks on older boats enforced this routine, it takes a degree of discipline to maintain it on a modern boat where you may go for the life of the boat without finding a problem in the bilge.

Given the lack of attention so many sailors pay to their bilges, it becomes that much more important that pumps turn ‘on’ if needed. Unfortunately, switch failures are one of the more common causes of bilge pump failure (including the time ours got jammed with Mardi Gras beads…).

Over the years, the industry has experimented with a number of switch designs, including electronic switches (measuring conductivity), air pressure switches, and a variety of float switches. By far the most popular has always been the type of float switch that is triggered by a swinging arm. Of these, the most popular has consistently been the mercury-activated switch, in which the switch wires are brought to two terminals connected to a glass vial inside the float itself. The glass vial contains a drop of mercury, a conductive liquid metal. When the float is raised, the mercury runs down the vial to immerse the two wire terminals, completing the circuit. When the float drops, the mercury runs the other way, breaking the circuit.

Unfortunately, mercury is a hazardous substance. A number of efforts have been made to recreate this type of switch without the mercury (e.g. ‘rolling ball’ switches), but for one reason or another, these switches have not been as successful as the mercury switches (in particular, it has proved next to impossible to get the same life expectancy out of conventional switch points as is got from the use of mercury). Faced with increasingly tough mercury legislation in the USA and Europe, Rule, a division of ITT, has taken another stab at a non-mercury float switch.

The new switch looks very similar to a traditional Rule float switch (and costs the same) but internally it is very different. In particular, none of the electrical components are inside the float. Instead, the float turns a shaft that operates a ‘snap action’ switch inside the main housing. This switch has rugged terminals that have been tested to a million cycles at full rated load (20 amps for the ‘super’ switch).

Placing the switch in the main housing eliminates the principal failure mechanisms of the old mercury switches which are wire failures from the flexing of the wires as the float moves up and down, and seal failures where the wires enter a float. Once water gets into a traditional float, current tracks from one terminal to the other, resulting in stray current corrosion that rapidly eats through the positive terminal, rendering the switch inoperative.

With the new design, the wires don’t move. This has enabled Rule to use a stiffer ‘blocked’ wire that has silicone sealant inside the wire insulation so that even if the insulation gets damaged water will still not wick down the wire strands into the switch. On the outside of the wire, the wire entry into the switch housing is sealed with the same rugged cable seal used on Rule’s submersible bilge pumps. The only other potential point of water ingress into the switch is where the snap action switch shaft exits the switch housing. This shaft is sealed with the same time-tested seal used on the drive shafts for millions of Rule’s centrifugal pumps.

The new switch comes with a detachable base that is fastened to the boat. If the switch should get fouled with debris, or needs to be replaced, it will be easy to do.

You would not think there was this much science in such a simple device, would you? It is an accumulation of these kinds of small incremental changes in technology that have taken us from the days of hand-pumped bilges in leaky wooden boats to today’s dry and comfortable cruising homes.

By Ocean Navigator