This is an email answer to a member question about depth meters in the Tradewinds Fleet. The question was: “Where is the depth instrument mounted on (Specific Boat)?”
I would guess the location of the actual instrument is about 1.5-2’ below the waterline. It really counts as a thru-hull, so it’s a good idea to know where those speed & depth instruments are located before you leave the slip. They are a potential source of flooding like any other thru-hull.
Now I’m going to take a stab at why you asked the question and try to answer it – Electronic depth meters have a setting in them called “Offset”. This can be changed by anyone who pushes the right buttons on the display unit. This is the reason I will never answer the other version of this question: “Is this boat’s depth meter set at the waterline or the bottom of the keel?”. People ask it often and I always say, “I can’t answer that – anyone can change the offset with the touch of a few buttons, so trust your charts & tide tables!”
Offset is used just for that purpose, though – let’s say the actual instrument is at 2’ below waterline and you want your depth meter to read from the bottom of your keel. For a boat with a 6’ draft, you would set the offset to -4. If you wanted it to read at the waterline, you would set the offset to +2.
I would guess that 90% of the fleet boats have an offset of 0 – so they read from where the instrument is mounted. There are a few boat owners who take the time to set them to their preference – bottom of keel or waterline.
One of these days I’m going to get around to painting depth markings on some of the pilings on the docks in the marina, that way when you motor out, you can compare the marks to the reading on your depth meter. In the meantime, if it’s important for you to know (you plan on anchoring in a tricky location, for instance) – get yourself a piece of line of a known length with a good size fishing weight on the end. Mark it every couple of feet with a knot. Drop this in the water before you leave and compare it to your depth instrument!