Keel offset for depth finder on 28 TE?

[GSSeattle]

I'll be installing a new Garmin GPS/Depth finder with an existing transducer, and am looking for the approximate keel offset setting - vertical distance from the typical thru-hull transducer mounting to the bottom of the keel (so depth shows as depth under keel, not depth under the transducer face). I know it will be less than the 3' 2" draft, but by how much? I'd measure, but the boat is in the water and won't be hauled for many months. Has anyone on here ever measured? I searched the forums and couldn't find anything.

(for reference, my thru hull transducer is mounted slightly inboard of the helm seat)

28 TE / Engine Box model

Thanks,
Greg

[Norseman]

I also tried to calibrate my Garmin, a 1040XS:
Docked at low tide, lowered a pole into the water until it hit the bottom, then marked the depth and programed the Garmin to the same value.
(To show actual depth, not just under the keel)
Found out later when we ran aground that I was within 1/2 foot of actual depth.

[GSSeattle]

And there is the reason I want to 'idiot proof' it to display depth under the keel Avoid any miscalculations

[Pitou]

Have you tried to figure out the existing offset in your current machine?

[Norseman]

And there is the reason I want to 'idiot proof' it to display depth under the keel Avoid any miscalculations

Aye, no miscalculations if you measure within a few inches.

I always wanted to measure the water depth, not the depth under the keel, but we are all different.

[GSSeattle]

Good suggestion Pitou. I'll look into that this weekend. That machine was installed by the previous owner, so not sure if one was entered or not. Should be pretty easy to tell if so.

[Pitou]

And there is the reason I want to 'idiot proof' it to display depth under the keel Avoid any miscalculations

Aye, no miscalculations if you measure within a few inches.

I always wanted to measure the water depth, not the depth under the keel, but we are all different.

I like using the offset to the keel as well. This makes it so the screen displays the distance to grounding.
If anchoring, I always then add for what the boat draws and pulpit to waterline to determine scope.

[carolmarie]

Distance from face of transducer to bottom of keel is 23”.
Don.

[Jkraft]

I set mine to 2' and I have had it show negative depth once

[GSSeattle]

thanks everyone for the replies! Approximately 23-24" it is. Great reminder to add the number back (plus water to pulpit) when anchoring.