need year of mfg
The answer is- it depends- You know it is 12 volts. This will be the same answer as your steaming light question. Depending on the age of your boat, and depending on previous owners changing the original fixtures--For older models- two prong twist-in. The prongs come in a high or low position or high-low offset. Buy one of each. The only other possibility is a very shallow bulb with very little of a post.For newer models you have a bulb very much like your dome light in the car. These snap-in between two posts and this bulb size depends on the size of the fixture. Your best bet is to wait, remove the bulb and match at a Marine store such as Boat US or West Marine. Auto stores work but rust quickly.You did not ask about the deck light as you may want to consider this type of light at this time plus the condition of your windex and your VHF attena.If the boat is more than fifteen years old, you may want to consider pulling new wire for all your lights while the mast is down. You also will want to take a hard look at the spreader boots and your halyard sheefes at the top of the mast. This is also a great time to run your bare hands over the standing rigging. If you cut your hand, you have problems you will want to deal with NOW.When the stick is down, Its time to look at all the components.Good LuckIan