Depth finder

[Tony2367]

A few weeks ago we were returning up the St. Mark's River and ran aground. I knew the tide was low but since I was in the channel I did not pay attention. I always keep an eye on the instrument since there are a lot of low spots near the channel edge. However, I was shocked when we came to a halt and the depth gauge read 6 ft. At first I was at a loss to what was happening thinking that the were dead in the water since the wind had gone to zero. But when I started the motor and we did not move I had a big hint at what wass the problem. It was confirmed when my son pointed out that he could see the sandy bay bottom.

Since I draw 3'-3", I was shocked to say the least but we were hard aground. So, where is the transducer located? In a way I suppose I've now calibrated the thing and know to get into deeper water when the indicator gets close to 6 ft.

 

[Ron20324]

Like I said in a previous post, I ran my B23 up to a soft beach until we touched bottom, then calibrated the depth meter to my draft. Works, doesn't it Tony???

 

[Stu Jackson]

Calibrating depth sounders

1. You didn't say how you knew how it worked before you set out.

2. I agree with Ron. There have been exceedingly weird discussions here about "calibrating" the depth sounder to
a. the keel depth
b. the waterline
c. the depth of the rudder below the keel (only for wing keels)
d. the height of the masthead less the waterline divided by the beam
e. the height of the cirtters on the bottom who reach up at you

Guys & Ladies, the best way to do it is to find a soft mud spot and find out what YOUR depth sounder reads on YOUR boat.

For those of you in Maine, sorry, rocks don't work and I apologize completely for this concept.

Our boat: 4.4 hits bottom. I care nothing about any other numbers. That's ALL I ever need to know, and I don't' have to do any math, ever.

 

[Guest]

Depth meter

Our boat came equipped with an Autohelm ST50 depth meter. Our boat draws 4' 11". The ST50 has an offset and alarm feature, which I have set at eight feet beneath the keel. So, when the alarm goes off I know that I have that much water before the keel touches bottom. It's pretty close, less than six inches, between what the meter displays and the keel actually touching bottom.

Our marina, and the Swinomish Channel, which the marina connects to, are both notorious for run-a-grounds, especially during minus tides. In 2006, as I was departing for our two month cruise during a minus tide, our meter showed 2.6 feet beneath the keel at the slip. Moving at dead slow speed toward the marina entrance I noticed the meter displayed two feet, then one foot, one half foot, then zero, before the boat came to a gentle and soft stop in the mud. No harm done, but that was before I knew about the shoal.

Now I know when it is safe to traverse the marina fairway (no less than a -1.5 feet). If you have those alarm and offset features I recommend that you calibrate your depth meter and then trust what they tell you.

Terry Cox

 

[Benny]

It works everytime. It takes a couple of groundings to be able to calibrate your depth meter. I believe in a straight readout with no offset. The alarm is set at 7 ft which leaves a couple of feet of water below our 4'4" wing keel draft.

 

[petersea]

Run aground ? ? Assuming you know your boats draft just use a weighted line to find you true water depth and subtract the difference and factor in where your transducer is and set off set to however you want it to read (waterline, water under keel etc) Take measurements in several different water depths to check accuracy of your depth finder in "real world" conditions. I suppose your "run aground" method may work for a smaller boat but I just don't see this working for a larger boat and I know someone, somewhere will check their depth meter with an outgoing tide and stay just a few minutes too long ! ! !

 

[Stu Jackson]

Run aground ? ? ...just use a weighted line to find you true water depth

Doesn't everyone have a lead line?

 

[sailaway78]

Finding the bottom

Seems to me that running aground is the hard way to calibrate a depth finder. Use a lead line while sitting in your slip, and find the exact depth of the water, opposite where the transducer is located. Most modern depth finders have an adjustable offset, so you can adjust to water under the keel, or actual depth of the water, and an alarm. Mine is set to actual water depth, with the alarm set for six feet. I draw 4'6", and is very easy to subtract that from whatever reading I get. Most prefer the alarm set for a bigger safety factor, but we have lots of shallow water around, and I don't like the alarm going off all the time.