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post #1 of 16 Old 09-08-2011 Thread Starter
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Draft vs water depth

My boat is on Lake Texoma and the water level is continuing to drop and to all time lows. When I was at the boat last Monday the depth finder read 8.3 feet. My draft is 5.5 feet. My question is since the transducer is mounted in the hull just below the water line doesn't that mean the clearance below the keel is 2.8 feet? I'm wondering how shallow I should let it get before deciding to move it to deep water.

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post #2 of 16 Old 09-08-2011
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No, you need to factor how much of your hull is in your draft. The draft is the bottom of
the boat, not necessarily the length of the keel. In my case, for instance, I draft a little over 4', but the keel itself is only 2 1/2 foot of that, meaning the hull of the boat is 1 1/2 foot of the draft. So, once you know, and you should be able to tell based on your water line, add that amount to the 2.8 feet and "that's" how much further you can sink. Plus, talk to your dock neighbors, as they may have more experience in this specific area if they've been there any length of time.
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post #3 of 16 Old 09-08-2011
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Quote:
Originally Posted by LakePirate View Post
My draft is 5.5 feet. My question is since the transducer is mounted in the hull just below the water line doesn't that mean the clearance below the keel is 2.8 feet? .
LP,

Maybe so, maybe no.

Double check to see if your depth finder was programmed with any offset -- on many units, you set an offset to either add distance (to offset the difference bewteen the 'ducer and the waterline), or subtract offset (to get a depth reading that reflects actual water under the keel.)
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post #4 of 16 Old 09-08-2011
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Draft versus Depth Reading

PorFin and emoney have given you good information.

More info.

Draft and your Depth reading have little to do with each other.

Your boat draft is somewhat fixed: Distance of waterline to lowest point point in the keel. The measurement is pretty fixed, +/- a few inches depending on the amount of "stuff and people" on the boat. I am not sure if the draft is measured at max loading of the boat. Nevertheless, the draft won't be more than the reported value.

The depth sounder, with no offset, reads the depth of water under its position on the hull, so if the depth sounder is reading 4' and it is 2 feet below water line, the actual water depth is 6 feet. If your boat draft is 5 feet, than you only have 1 foot of clearance under the lowest part of your keel.

Many depth sounders have the ability to input an offset. So some folks input a +/- value to either a) read the depth of water from the water surface (so actual water depth), b) depth of water under the lowest part of the keel, or c) some other value that the boat owner wants to know. If you are new to your boat, you need to understand if there is an off-set programmed into to sounder. An easy way to do this is to get a weighted line and measure the water depth at several places around your boat. Then compare your readings with the depth sounder reading. Don't forget to take into account the location of the sending unit in the hull and how far it is below the waterline. You may have to estimate this. Readings the are close to the water depth readings means the sounder has be calibrated to read the actual water depth. Readings that are different suggest that the sounder is reading a different value (water under keel, water under sounder, etc.)

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post #5 of 16 Old 09-08-2011
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Ours is set up to measure from the base of the keel, 6'. It's pretty accurate.
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post #6 of 16 Old 09-09-2011
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Quote:
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Ours is set up to measure from the base of the keel, 6'. It's pretty accurate.
I lived with ours set with zero offset as it was when we purchased the boat -- hated it, but didn't have an accurate measurements of waterline-> 'ducer/'ducer-> bottom of the keel.

Once I had the correct distances, I set it to show water under the keel. Very good at letting us know how close we were to grounding, but a little confusing when checking against charts -- had to remember do the math in my head.

After a while, I changed it to show actual depth. This made both chart checks and making rode scope calculations faster. That's where it is now. YMMV.
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post #7 of 16 Old 09-09-2011
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So our transducer says 30' for example. +6' is 36 actual. Free board is 3 ft. Then there's the whole highest tide above MLLW predicted for the period at anchor. It's all pretty simple arithmetic I'd much rather know what's under the keel rather than -6 from say, 11 ft.
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post #8 of 16 Old 09-09-2011
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Pirate

We have a gently shelving sand beach in our area. We slowly motor til our keel just touches and the reading on the depth instrument with no offset applied is what we then set as offset - or we then set it to zero. Mud works just as well. Rocks - not so much

Having the reading set to depth under the keel is very comforting. Especially when sailing in skinny water

Mike

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post #9 of 16 Old 09-09-2011
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Stick a pole in the water until it touches bottom. Measure the wet part. Compare with your depth meter reading.



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post #10 of 16 Old 09-09-2011
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I use a lead line to measure. Then I set the depth finder to the bottom of the keels at zero. So I amm measuring what is beneath the keels......i2f

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