keel offset used for depth sounder?

[FBoykin]

Recently bought a 2000 model year Grand Banks 42. I'm used to a depth sounder where the transducer offset is factored in (how far transducer is located below the waterline) so that the reading on the depth gauge is pretty much what the actual water depth is. I've found this very helpful in using charts - matching gauge water depth to charted depth as another check on my location. I know what my boats draw and it's easy mentally to know if I'm about to touch bottom.

This GB 42 is set up so depth gauge reads exactly what's under the transducer (no offset factored in). Spooked me first time I turned it on and saw 1.5 feet in the slip until I realized what was up.

So my question is - who out there adds in the keel offset (shows depth from surface) and who doesn't (shows depth from transducer location)?

 

[dhays]

Third option, set the offset so the depth sounder shows depth from the bottom of the keel. This is what I do currently.

There are plenty of arguments for different ways of doing it and for a long time I had sounder adjusted to show actual water depth. I finally decided to just have it read water under the keel since that is what I really am interested in. In my area, with large daily tide variations and very infrequent depth updates, there is no way that the 4.5 foot difference is going to help with navigation.

 

[Rebel112r]

Mine are set for water under the keel.

 

[hmason]

Mine is depth under keel.

 

[Tootles]

I agree that is what I have too, water under the keel

 

[psneeld]

I use actual water depth.

The alarm tells me when the bottom is too close....but of course any way it is set up, you still are doing a little math in your head. Whether deciding on navigational depth from the surface, your keel, or your sounder.....so just pick what you like.

 

[Lou_tribal]

+1 water under the keel. Cruising often in very shallow water here and I want to know exactly what is left below my keel.

L.

 

[Insequent]

My old, really old but good, Furuno sounder will only give depth under the transducer.
Its 1m below surface and 0.4m above the keel. But thats fine, on the Nav display on the PC I can adjust and display in large letters depth from surface.

We have, in common with a lot of tropical waters, lots of shallow water. Charts have 5m and 2m contours, so using depth of water makes sense for me.

 

[k9medic]

Under the keel for me too. I know I'm really shallow when the depth finder goes from 1' to 18' or more.

 

[Gdavid]

Recently bought a 2000 model year Grand Banks 42. I'm used to a depth sounder where the transducer offset is factored in (how far transducer is located below the waterline) so that the reading on the depth gauge is pretty much what the actual water depth is. I've found this very helpful in using charts - matching gauge water depth to charted depth as another check on my location. I know what my boats draw and it's easy mentally to know if I'm about to touch bottom.

This GB 42 is set up so depth gauge reads exactly what's under the transducer (no offset factored in). Spooked me first time I turned it on and saw 1.5 feet in the slip until I realized what was up.

So my question is - who out there adds in the keel offset (shows depth from surface) and who doesn't (shows depth from transducer location)?

Seems like setting it up to what you are accustomed to would make the most sense. Why force yourself into doing things differently?

To answer the question, I prefer setting the offset such that actual water depth is displayed for the same reason you mentioned but understand why others do it differently.

 

[kev_rm]

There is also the option of using arithmetic when necessary with an offset of zero.

 

[alormaria]

I think it's best if you set it up the way you are used to it. Less chance of error at just the wrong time.

 

[Capt.Bill11]

Set to show the depth from the lowest point under the boat.

Which may be the keel, rudders or props depending on the boat.

 

[Vashon_Trawler]

I always prefer depth under my keel.

 

[Comodave]

I want to know the depth below the keel.

 

[Capt.Bill11]

There is also the option of using arithmetic when necessary with an offset of zero.

The problem with that is unless you know exactly how deep the transducer is mounted below the boat you don't truly know what the depth is.

And if you're going to go to the trouble of measuring that, you might as well measure your exact draft and either set the correct offset or set it to show true depth and skip the need to do the math in your head.

 

[psneeld]

Ultimately...everyone, no matter what method you use.... makes a conscious decision when considering depth....


We are all calculating a tiny bit or concentrating on what to do when whatever number shows up on the depthsounder is less than you want it to be.

is the usual look at that point.....

 

[Heron]

I use actual water depth.

.

Ditto...

 

[twistedtree]

I'm a bit surprise to see how many people report depth below keel. I'm not sure why I'm surprised other than perhaps to find myself in such a minority.

I report depth of water.

 

[LarryM]

Depth of water.

 

[diver dave]

WHat I want is ground distance to prop in 10 seconds if heading is maintained.

What I have is ground distance to transducer 5 seconds ago.

 

[Nightsky]

I would prefer the sounder to display the depth below the deepest part of my vessel (negative offset distance from transducer to deepest point). For me, that is the most important thing to know for safety. Offsetting the sounder to display the depth from the surface to the bottom is far less important to me. This latter method requires you to do the math and subtract the boats draft from the reading being displayed on the sounder to calculate clearance.

 

[psneeld]

I would prefer the sounder to display the depth below the deepest part of my vessel (negative offset distance from transducer to deepest point). For me, that is the most important thing to know for safety. Offsetting the sounder to display the depth from the surface to the bottom is far less important to me. This latter method requires you to do the math and subtract the boats draft from the reading being displayed on the sounder to calculate clearance.

What you are not believing is you are still thinking about it as you approach shallow water...and guessing how fast it is coming up and what to do.

About 100 times more thought than subtracting a number emblazoned in your mind, the draft of your vessel..

It takes milliseconds for me to subtract 4 from the depth finder number, but many seconds to look at shorelines, chartplotter, whatever to figure out what I am going to do next.

So I firmly think that any way you set it up is irrelavent to the actions about to be required from running aground.

As a delivery captain and commercial captain who often has to guess what the depth finder is showing, it is small potatoes in the big scheme of things if you can guess what it is and do one tiny, simple math problem in your head.

If not, write it down in big type and tape it to the depth finder.

So pick what you want, but there is NO right or better way .....not running aground has just a tiny part to do with what the depthfinder reads.

 

[Heron]

I'm a bit surprise to see how many people report depth below keel. I'm not sure why I'm surprised other than perhaps to find myself in such a minority.

I report depth of water.

It tskes milliseconds for me to subtract 4 from the depth finder number, but many seconds to look at shorelines, chartplotter, whatever to figure out what I am going to do next.

.

Also Depth of water, IMO, gives you better situational awareness when referencing against your chartplotter or other references....

Easier to track along a shoal line, verify chart data etc..

 

[dhays]

If not, write it down in big type and tape it to the depth finder.

I now have it set for depth under keel but I spent so many years with it set differently, that I did exactly as you suggest. I created a label that is stuck on the depth indicator that says "Depth under Keel". That is just a reminder to me in those tight situations.

 

[LarryM]

Also Depth of water, IMO, gives you better situational awareness when referencing against your chartplotter or other sources....

+1

 

[O C Diver]

Depth under the keel. When the water is getting skinny, I don't need to be doing math, one more unnecessary task loading in a critical situation.

Ted

 

[gehales]

Depth under keel plus 6 inches (4 ft 7 in draft)

 

[High Wire]

Depth under the transducer only because thats what I'm used to. I know that on this boat at 2.6 ft the back of the keel is about an inch off the bottom. At 2.3 ft, I'm dragging. Both of my CC's are 1.0 ft to contact.
Be warned that in shallow water like that the depthfinder can jump to reading the second echo, giving non-conservative readings.

 

[Lou_tribal]

Is there anyone of you that had issues in water with a lot of water grass? Last year when crossing the Rideau waterway, in some shallow water with a lot of grass the depth sounder was a bit lot and was blinking like "help help cannot read cannot read".
I guess it is normal as the grass reflect the echo but the first time I was like "hell what the f..."! Behavior of a newbie of course!

L.