Offsetting the depth on a SO42 DS

[suem]

We recently installed a new Raymarine system on our SO42 DS (2006/7). We are trying to calibrate the depth sensor, so need to know the difference between the waterline and the depth of the depth sensor on the bottom of the hull just forward of the keel, so that we know how much to offset the system. Can anyone help, please?

The owner's manual gives the draft/draught as 2.13m, but we can't see any information about the depth of the hull from the waterline and/or length of the keel. 

We appreciate the draft will vary according to how many people are on the boat, if it is fully loaded with water, fuel etc etc, but plan to add a margin of error.

I can't see anything on the Forums, but apologise if someone has already answered this question.

Many thanks.

[bereboot]

With my so39i on the pontoon, i measured the depth from the waterline till the bottom. Then i adjusted the settings on the instrument to that depth.So we have the actual depth on the instrument reading and not the depth under the keel.For us this saves calculations to be made in stressy conditions.(our draft is 2 m, which is easy)

[sailor666]

I have not offset the depth on my 42ds instruments. Calculation and practical experience demonstrates that when the instrument reads 1.7 metres, I am aground. So, that is where I would set the offset if I were so inclined.

[moonshadow]

I always want water depth set to the water surface not under the keel. So I can use charts and my info together. So it’s easy to set the offset. A weighted line in the water abeam the sensor location and when I pull it up I measure the wet length. Then just set the offset so display matches reality.

[Charlie-Bravo]

You could measure from the water to , say, the bottom of a port light or other convenient bit of boat , and then measure from that to the transducer down below, the difference giving the figure you are after.

For sure there will be a bit of measuring inaccuracy, but then .... the seabed is not flat, the waves make us go up and down quicker than the instrument reads out the depth, it gives a reasonable average of depth , so a margin of safety in programming the offset would be prudent.
Our 35 in theory draws 1.85 m, I call it 2 meters and get uneasy if the bottom of the keel is by calculation only a foot clear of the sea bed, and so far haven't got embarrassed getting stuck ( yet ).

Water depth v keel clearance on the display is a choice, either works ...... as long as you remember what you have programmed and let the crew know about it.

CB
( excuse the metric and imperial measurements....... it's an age thing )

[suem]

Many thanks for your replies. It looks like we're going to have to make a bit of an educated guess, which is fine. Charlie-Bravo's suggestion sounds logical and easy to do. I had just assumed the depth of the hull would be fairly standard information, but obviously not.


We do indeed want to set the depth from the waterline and are thinking of sticking a (waterproofed) note on the binnacle to remind us and anyone else skippering the boat!

Thanks again one and all.

[plas50]

From the waterline down to the depth transducer, mine was measured at 600mm, plus the keel depth
Andy

[suem]

Brilliant! Thanks, Andy.