Sunday, October 15, 2006

See The Light (Part 1)

Navigation Lights, 10/15/2006

This is a fun one. Powerboats under 12 meters in length are required to display navigation lights that include, as shown in the picture:
  • red and green sidelights
  • either a single all-round white light, or a separate masthead light and stern light

Our white light was a single all-round light like the one on the left, but it was in bad shape (shedding fiberglass -- ouch!) and our surveyor pointed out that it would shine directly in our eyes if we pilot from the flybridge.
So I planned to replace it with separate masthead and stern lights, as shown on the right. One hitch: At anchor, the vessel must display a single all-round light (and nothing else). There are special lights for this, called combination lights, which when wired properly will illuminate just the forward-facing masthead portion for navigation (to be used in conjunction with a stern light), and 360 degrees for anchoring (to be used by itself). I replaced our all-round light with one of these combination lights, and plan to install the stern light over the winter.

The light switch is a 3-position switch: NAV-OFF-ANCHOR. If you give some thought this, you'll soon discover a puzzle: The "nav" position must illuminate the masthead part of the combination light, plus the red, green, and stern light. The "anchor" position must illuminate both portions of the comination light (the masthead and aft-facing portions). So the forward-facing portion of the combination light must illuminate in either switch position. But you can't just connect it to both positions, because that would effectively connect the NAV and ANCHOR positions together, so that everything would turn on regardless of which way you flip the switch.

The solution to this puzzle is a "double-pole, double-throw" switch. Double-throw refers to the two "on" positions: on-off-on (nav-off-anchor). Double-pole refers to the ability to turn on two circuits in either position, rather than just one circuit each. The "nav" position circuits are set up so that the sidelights and stern light are on one circuit, and the masthead is on a separate circuit all by itself. The "anchor" position is set up with the aft-facing portion of the combination light on one circuit. The second circuit is then connected together with the masthead circuit from the "nav" side. In this way, the masthead light is kept on its own separate circuit, isolating it from all other lights, but that circuit is activated no matter which way you throw the switch.

Incidentally, the same situation arises even if you use an all-round light for navigation, rather than separate masthead and stern lights. The all-round circuit must turn on in either switch position, and must be separate from the red and green sidelights.

See? I told you this would be fun.

No comments: