Keep offset-why isn't my brain getting it!

If you want depth below the keel, offset will be negative like -0.3
If you want depth below transducer, offset = 0
If you want waterdepth, offset will be distance waterlevel - transducer like +0.3

Above is example I posted on other forum (used cm). So in your case +2'

 

I set my depth sounder offset at -3.0ft. That way I always know what I have under my props.

 

In your case it would be -2' , I have mine set the same way on the chartplotter & fish finder with an alarm to sound at 6'. An easy way to check it's reading correct is use a 12' PVC pipe with a mark around it I used red tape, find an area where your at 10' and check it with the PVC pipe.

This is out of my Garmin manual.

The Keel Offset function adjusts the digital depth readout to display depth readings from either the waterline or from the keel (lowest point) of the boat, instead of from the location of the transducer which is usually somewhere in between. This permits optimum transducer location and depth readouts suited to your needs.

To determine the value to enter into the keel offset setting, first, decide whether depth from the waterline or depth from the keel is desired. Measurements will need to be made for the desired location.

For depth from the keel of the boat, accurately measure the vertical distance between the face of the transducer and the keel of the boat. This measurement will then be entered into the keel offset function as a positive (+) number.

For depth measurements from the waterline, accurately measure the vertical distance between the face of the transducer and the waterline of the boat. This measurement will then be entered into the keel offset function as a negative (-) number.

 

What brand Chartplotter? Some may be different than Garmin but I'm not sure.

 

Im assuming he means Smartcraft. It is a positive value you need to get to waterline.

PO of my boat had a -3 offset with a 10' alarm so the depth read 0 in 5' of water and would alarm in 15'. Drove me nuts.

 

I have mine set so that when I read zero, I have 5' below my rudders.

 

So does it give you a negative value if you were to have 3' below your rudders? That was my hang up with the offset-I was not sure and did not want to find out the hard way. I did not want it to continue to read 0 as the bottom crept up. The alarm made me crazy as it would replace the depth readout with the word "ALARM" exactly when you wanted to know the depth.

 

In my case we have never been close enough to set off the alarm, if it were me where it sounds like you have shallow areas I would first tie up to a dock where you can keep the boat in one position then set it to the waterline & test it with the PVC pipe of whatever length you need marked at 1' increments then set it with an offset 3' below the waterline then change your alarm setting to different depths. This should tell you how your transducer and alarms work. I doubt it will show you a -3' after it's had an offset programmed in it it will calculate your grounded & only give you an alarm.

I'm now going to re-check mine to make sure it works the way I want it to.

Thanks for starting this thread we'll be around some unfamiliar islands next week at if we hit them at low tide we'll only have 3-4' of water under us.

 

You do not have to set an alarm if you don't want to.

 

Good point mine is usually only turned on when we're at anchor or unfamiliar shallower water. You should always have an eye on your depth out of habit we watch it in 1000' of water.

 

The minimum alarm for smartcraft is 2'-don't think you can turn it off. I have mine set for actual water depth & just know I need 3'. I use an analog depth alarm-when the boat stops I've run out of water.