Join Date: Mar 2007
Location: New England
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Draft versus Depth Reading
PorFin and emoney have given you good information.
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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