I'm trying to calibrate my dephfinder. I need a measurement from the waterline to the top of the keel. Also, a measurement from the top of the keel to the bottom of the keel. I didn't realize I needed this when the boat was dry docked. Thanks! Another question any suggestions on a anchor roller and it's placement?
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Draft
Owners manual lists the draft as 5'2" for the deep keel and 4'0" for the shoal draft keel. I always thought that the listed draft is measured from the waterline to the bottom of the keel. When calibrating my depth sounder, I measured the depth to the waterline with a weighted line and calibrated the sounder accordingly. I also reduced the measured depth by 5 feet in order to sound the alarm hopefully prior to running aground. The way you adjust or set the alarm is a matter of taste. You need to be aware of this when using chart depths when navigating.
Owners manual lists the draft as 5'2" for the deep keel and 4'0" for the shoal draft keel. I always thought that the listed draft is measured from the waterline to the bottom of the keel. When calibrating my depth sounder, I measured the depth to the waterline with a weighted line and calibrated the sounder accordingly. I also reduced the measured depth by 5 feet in order to sound the alarm hopefully prior to running aground. The way you adjust or set the alarm is a matter of taste. You need to be aware of this when using chart depths when navigating.
Calibrating sounder
Easiest way I know to calibrate is measuring how deep it is right at my dock by dropping a leadline to the bottom and measure from the bottom to the surface. I do this as close as possible to area where transducer is installed. You can then go and calibrate the unit to show exact depth from water level to bottom. I generally calibrate with the "pilot's foot" substracted, meaning the unit shows one foot less water than reality. That way, when I pull in an anchorage or sail in shallow water, when I get fidgety, I am still safe. Of course, you've got to know your draft. My former 28.5 was 4' Using my method, if I moved into an area that showed 5 feet on the unit, it meant that I still have 2 feet of clearance below the keel (5 feet shown less 4 feet draft plus "pilot's foot"). That way provides me with a little safety when in an unknown area (or in a panic situation)
Easiest way I know to calibrate is measuring how deep it is right at my dock by dropping a leadline to the bottom and measure from the bottom to the surface. I do this as close as possible to area where transducer is installed. You can then go and calibrate the unit to show exact depth from water level to bottom. I generally calibrate with the "pilot's foot" substracted, meaning the unit shows one foot less water than reality. That way, when I pull in an anchorage or sail in shallow water, when I get fidgety, I am still safe. Of course, you've got to know your draft. My former 28.5 was 4' Using my method, if I moved into an area that showed 5 feet on the unit, it meant that I still have 2 feet of clearance below the keel (5 feet shown less 4 feet draft plus "pilot's foot"). That way provides me with a little safety when in an unknown area (or in a panic situation)
What reading is best?
In the process of calibrating my depth sounder I wondered what is the best measurement. 1. Depth of the water - good for navigation. ( Must subtract draft to know clearance) 2. Depth below transducer (probably not very useful) 3. Depth below bottom of keel. (When it reads zero you are aground) The waterline is a variable based on the weight of loading boat but it probably does not vary much. (35 ft boat in my case). I used a lead weight to determine the water depth and then set the depth sounder to match this number. Actually the transducer is below the water line so there is a built-in safety factor.
In the process of calibrating my depth sounder I wondered what is the best measurement. 1. Depth of the water - good for navigation. ( Must subtract draft to know clearance) 2. Depth below transducer (probably not very useful) 3. Depth below bottom of keel. (When it reads zero you are aground) The waterline is a variable based on the weight of loading boat but it probably does not vary much. (35 ft boat in my case). I used a lead weight to determine the water depth and then set the depth sounder to match this number. Actually the transducer is below the water line so there is a built-in safety factor.
I like to have the depth show below the keel....
No math required... what you see is what you get. This is a personal preference..
No math required... what you see is what you get. This is a personal preference..
I use water depth (As you said, navigation...)
Not much math is required. I just know that (in my case) 5 ft. is about it with a foot to spare. Depth below the keel is definitely useful, but God help you if have been used to reading water depth for many years, and forget that you changed your calibration! Don't worry about the transducer being below the water line, it gives you that extra bit of reaction time since the boat is (generally) moving forward into the shallows and the bow is already ahead of most transducer locations.
Not much math is required. I just know that (in my case) 5 ft. is about it with a foot to spare. Depth below the keel is definitely useful, but God help you if have been used to reading water depth for many years, and forget that you changed your calibration! Don't worry about the transducer being below the water line, it gives you that extra bit of reaction time since the boat is (generally) moving forward into the shallows and the bow is already ahead of most transducer locations.
I didn't even think of "calibrating" mine
I bought a 87H285 last year. Since my sailing waters are the upper Chesapeake, I quickly found out that when the depth meter reads "2.6" the keel is scraping the bottom. I've always assumed that this was the depth from the transducer (bottom of hull) to the bottom of the keel. But, I much better like the idea of "calibrating" the transducer to show water depth, so that 4' means 4', not 5.4' as it does now. Eric Lorgus S/V Explorer
I bought a 87H285 last year. Since my sailing waters are the upper Chesapeake, I quickly found out that when the depth meter reads "2.6" the keel is scraping the bottom. I've always assumed that this was the depth from the transducer (bottom of hull) to the bottom of the keel. But, I much better like the idea of "calibrating" the transducer to show water depth, so that 4' means 4', not 5.4' as it does now. Eric Lorgus S/V Explorer
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