Here are the instructions on how to modify stock bulbs off-the-shelf for yourself, as well as what to look for in a compatible bulb. These are handy instructions to keep on board, especially when cruising. Without the modification, you lose the dual-power (high/low) function, and get excess flickering that will over stress the ballast.
Cut off the very end of the plastic base of the bulb. It’s most easily done with something like a hack saw. Inside you should see two splices (either crimp-on sleeves or solder or welds) with three wires on each splice. Cut off both splices — the entire splice, all three wires on each splice. Basically you can cut any wires that you see. Try not to pull on them while cutting them.
Turn the bulb over and shake out the two components (a capacitor and a glass-bulb starter). All that will be left inside the base will be two fine wires. Be sure that they aren’t touching each other, and squirt a little silicone seal into the base to make sure they don’t move and contact each other. We also secure our bulbs with a couple of drops of silicone at the base to protect it from vibrations of the boat after the new bulb is installed in the socket, so you may need to make slits at the base with a sharp knife or razor blade to loosen the old bulb when changing it for a new one.
There is no consistency between brands on bulb numbers, so take an old bulb with you when you shop for bulbs. If it’s the same length, and has the same base configuration, it’s compatible. Somewhere within the part number will usually be the number ‘27’ or ‘41’. That indicates the color, with ‘27’ being the warm color that we use, and ‘41’ being a cooler color.
The first choice of commonly available brands is Philips or Sylvania. There are many other brands that can work, but some don’t tolerate the modification well, and some have the splices at the other end of the base where you can’t reach them. If you can only find a different brand, try one or two before buying a bunch.