I've tried the "depth below keel" approach. I found that the depth sounder
should show the depth of the water itself. Do the various conversions in
your head. Knowing the true depth helps with setting the anchor, etc.

Doug

"Phil Stanton" wrote in message
...
Does anyone know if there is a depth sounder which shows a negative depth.
OK sounds like a loony question.
I have a lift keel boat and want to set the offset when there is no water
under the keel with the keel down. Same as all sensible people with fixed
keel boats. Easy to do with my Raymarine ST60 Tridata echo sounder. Now if
I raise the keel to go into shallower waters, there isn't 7 ft (2.2m) of
water in a lot of places on the East Coast of England, I still want to
know how much water there is underneath me. Hence I need to find an echo
sounder that shows 0.0m with no water under the keel when down, an
say -0.7m when the depth is 0.7m less than with the keel fully down.
Hence my question. I should have thought with the number of lift keel
boats there would be a reasonable demand, and lets face it, it isn't
rocket science to make a minus sign show up on a LCD display