Depth Offset B36 2002

[Sitesurfer]

Hi folks, me again (sorry).

The boat is now in the water and the depth is working (yay!) and I going to have a go at the 10k Ohm fix on the log.
 
Meanwhile, the ST60 Tri Data reports no offset programmed in, and I'd like to have water under the keel, has anyone with a B36 of this era have the offset which I can plug into the head unit, please? Clearly the sounders were installed in the same position so I assume someone must have done it at some stage

I think right now it is showing depth of water from its installed point, and I prefer water under the keel as a measurement.

[Sootydog]

Get a weighted rope, lower it over the side next to the keel.  When it touches bottom, pull it up and measure the wet bit. The wet bit minus your draft equals water under the keel approx. Now adjust the offset on the ST60 to suit. I added a bit on just for good measure so when it says zero I have a bit of water left to play with.

[Salty]

Keep in mind that your transducer is already in the wet bit, so if you think it’s already measuring depth below the transducer, then the off set to apply in order to read depth below keel is the depth of your keel below the bottom of the hull. If you simply subtract the draft from the depth measured on your weighted rope, you will already have added on a safety margin equivalent to the depth of your hull below the waterline to where your transducer is located.

[Sitesurfer]

Keep in mind that your transducer is already in the wet bit, so if you think it’s already measuring depth below the transducer, then the off set to apply in order to read depth below keel is the depth of your keel below the bottom of the hull. If you simply subtract the draft from the depth measured on your weighted rope, you will already have added on a safety margin equivalent to the depth of your hull below the waterline to where your transducer is located.

Yup! Exactly that.

Hence why I was hoping someone else had done it as the measurement from the transducer to waterline and transducer to keel was the one that I forgot to do when she was out of the water - despite having it on the list of things to do!

[Rampage]

Sailboat data website suggests a draft of 1.85m for the 36.  Set to offset to 2m: that’ll give you a little bit of wriggle room if you’re creeping in somewhere on the edge of sensible.  That’s how my 38 is set with the same draft as yours.

[Sitesurfer]

We have the shoal draft version (1.6) but that makes sense to me, simply give an offset that's more than it can be and the result will always be pessimistic - which is ideal for what I want.

I reckon setting the offset to 1.5m would be about right.

Following on from the post above about dropping a line over, its still possible to calculate (albeit roughly) with that method I think:

a) The depth of water from waterline to riverbed (minus) the depth showing on the echo sounder = the offset to waterline on the boat.
b) The depth of water from waterline to riverbed (minus) the known draught = water under the keel

If I then : known draught (minus) a = offset from transducer to bottom of keel : checked by b figure allowing a tolerance

[Salty]

Keep in mind that your transducer is already in the wet bit, so if you think it’s already measuring depth below the transducer, then the off set to apply in order to read depth below keel is the depth of your keel below the bottom of the hull. If you simply subtract the draft from the depth measured on your weighted rope, you will already have added on a safety margin equivalent to the depth of your hull below the waterline to where your transducer is located.

Yup! Exactly that.

Hence why I was hoping someone else had done it as the measurement from the transducer to waterline and transducer to keel was the one that I forgot to do when she was out of the water - despite having it on the list of things to do!

Okay, I’ve got a B36(2002) with a 1.65 metre draft. I’ll measure the depth of the keel below the hull this weekend and let you know.

[Sitesurfer]

STAR!!!

[Sootydog]

I know very little about boats and I'm still learning......... but. Maths I'm ok with.

Take wet string measurement. Take away your draft this equals water under the keel.
Now look at the sonar reading. If the wet string is 2.3 meters and the sonar reads 1.9 meters then the sonar is 0.4 meters below the surface. 
If you want to measure water depth, add 0.4 meters to the reading by adding an offset.
If you want to measure depth under the keel subtract 0.4 meters from your draft and subtract this number as a new offset.

It's been a long day and I've just got home from Gatwick, so I hope this is right and I don't end up looking like a complete chump.

[Sootydog]

Oh dear, it has been a long day!  Sitesurfer has already posted this answer.  Chump alert!!

[Salty]

Keel depth below hull 1.11m or 43.75" on a Bavaria 36 (2002)  as measured today. Note, on my boat the aft end of the keel was slightly deeper than the forward  end by about a centimetre, and the measurements quoted above are for the deepest or aft part of the keel. Hope that helps.

[Sitesurfer]

Awesome - thanks! I have added your dimensions to my drawing for clarity below.

I did my calculations in East Cowes off the pontoon on Sunday and came back with some weirdness.
Bearing in mind that the instrument for the depth shows 'OFF' or (zero) as the offsets for both up and down - my calculations are as follows:

The depth of water measured to the floor of the river from waterline: 5.1m
Assumed correct draught of boat: 1.6m from boot top line.

Sounder at that point showing: 3.2m - correct water under keel figure: 3.5m (approx)

With what the sounder is showing, it's virtually the water under the Keel figure with a 300mm safety margin.
Can you imagine my dilemma now! Leave it alone, or put the offset in again - I just hope that I am looking in the right place (I am, pretty sure I am).

Just can't wrap head around how with no offsets - it's reading as if there was.
Did some sanity checks and worked out what it should be and it's about there agreeing with my diagram.

[Sitesurfer]

OH!!

I wonder if the Tri-Data is failing to show the offsets because it thinks it's in slave mode - an Issue that I have yet to sort with the log (involving a resistor).
The manual does state that this functionality is not applicable when the device is in slave mode - which is what happens when the thermister in the log fails.