Does anyone know the 30+ depth sounds transducer offset to the bottom of the keel?

[light24bulbs]

I estimated 3 feet but I haven't got a clue and I didn't measure last haul out. I assume it's good to get this one right.

Existing transducer hole was reused, the more vertical one in the middle of the boat. The angled one that had the speed transducer I simply plugged since my new airmar 800 is combined speed+depth.


Attached is my new raymarine i70s kit install. $1000 of parts and $600 labor for the new transducers, plus me doing all the wiring.

A lot of work to see those little numbers but hopefully it's worth it.

 

[paul culver]

According to the specification sheet in the resources tab of this site the 30+ draft is 5'10" standard or 4'0" optional. If you berth at a slip you can drop a plumb line to measure the bottom as a reference for calibrations.

 

[Bolo]

This is an interesting question for me. I recently changed from the old Datamarine depth gauge and transducer to a new Raymarine set of electronics. So new that I haven’t had to time, and not been near my boat, to throughly look over the set of Raymarine owner’s manuals. It’ll take some time since I upgraded everything.

While using my old Datamarine depth gauge, which I never adjust for offset or even knew if that was possible, it seemed to me that it gave a fairly accute reading as to how much water was under the HULL not under the keel. How do I know this? By the experiences of ”soft” groundings I made a few times into the Chesapeake Bay mud. The shoal keel on my E32-3 is about 4’-6” so when I got to that depth, while nosing around a gunk hole, or less then 4’-6” I’d find the bottom. The Datamarine did have an alarm setting for 4 and 8 feet. I mostly kept it on the 8 foot selection because that gave me some wiggle room In thin waters.

Now my point is, and actually my question, wouldn’t it be useful to have ”0” offset on the transducer with (in my case) a 6 foot alarm that would indicate a distance from the hull (where the transducer is mounted) and 1’-6” under the keel? Offset or not it pays to be cautious in thin water. Maybe this is all just a personal preference? Is setting an offset to the bottom of your keel a rule written in stone? I’d like to see what others write about this question.

 

[Loren Beach]

Being more of a Luddite, i just install the transducer and note how far below the boat's WL it is. Usually around 1 to 1.5 feet. Then just remember that the the transducer itself is showing a true reading of depth and that we just subtract our keel depth from whatever it tells us....
We have done that for two different boats in the last four decades.
Presently, when the readout gets down to about "5 feet" we are starting to scrape the sand or mud. A simple solution for a simple mind, I guess.
After initial install, I do a simple check at he dock with a weight on a string; rest it on the bottom and then pull it in and measure the depth to the waterline. I have always been impressed with the out-of-the-box-new accuracy of these devices (across three different brands).

This way we do not have to remember or announce to anyone the amount of "offset" we have set it for. And then have to explain what 'offset' is. Most visitors seem to understand that the device is just reading from the transducer face and a fixed number is kept in mind for bottom of the keel.

Our method is certainly not a Better way to do this, but it is simple and workable. I do have techie friends that fret about whether their digitally altered keel offset ## display is exactly right..... (sigh).

 

[bigd14]

My notes say 4.5 feet from the bottom of the keel to the transducer (6 foot keel).

I use 2-foot offset on the instrument to give me a tiny margin. I know I draw 6 feet so I just want to know the real depth and I try to avoid anything under 10 around here.

 

[bgary]

I do something different. I don't care how about the distance between the surface and the bottom, or between the transducer and the bottom... I care about how much distance there is between the *keel* and the bottom.

I don't want to have to do math in real-time to figure out if I have enough water in a shallow place, I want the display to be idiot-simple with the number I care about, so I have my offset configured to show "0" in the display when I've still got 2 feet of water under the keel. (which works out to about 8 feet of "real" water).

I do have to remember to add that 8 feet back in when figuring out anchor scope...

 

[light24bulbs]

According to the specification sheet in the resources tab of this site the 30+ draft is 5'10" standard or 4'0" optional. If you berth at a slip you can drop a plumb line to measure the bottom as a reference for calibrations.

Yes, the draft is not in question. It's really the depth of the transducer from the water line that I don't know. Plumb line is an interesting idea! That would for sure settle the question.

My notes say 4.5 feet from the bottom of the keel to the transducer (6 foot keel).

I use 2-foot offset on the instrument to give me a tiny margin. I know I draw 6 feet so I just want to know the real depth and I try to avoid anything under 10 around here.

This is helpful, I'll just go with that. You have a 30+ I take it @bigd14

 

[robcally41]

Same situation, I installed a new transducer (airmar 810) and dropped my 30+ in the water over the weekend before taking measurements. I'm horrible at math and am confused....

If the depth from the transducer to the bottom of the keel is 4.5 feet and the draft is 6.0 feet, why would you use a 2.0 foot offset? Wouldn't you use a 1.5 foot offset so that your reading depth is from the waterline?

Again, apologies for being dense. Thanks everyone, this forum has been incredibly helpful!!

Rob

 

[Dave G.]

Wouldn't you use a 1.5 foot offset so that your reading depth is from the waterline?

Yes if you want to know the depth at the waterline it would be (+)1.5'. If you wanted to know the depth under your keel you would use (-) 4.5' offset.

 

[bigd14]

why would you use a 2.0 foot offset? Wouldn't you use a 1.5 foot offset so that your reading depth is from the waterline?

I wanted that extra little bit of margin just in case. Probably not necessary but that’s what I was thinking when I set it up years ago and I’m too lazy to change it!

BTW the shallowest depth I have seen on the sounder is 4.2 feet at my slip during the lowest river flows and very low tide in the fall. I think the boat is docked right on the edge of a steep underwater slope or maybe the keel has excavated itself a little channel. I haven’t been stuck yet…

 

[markvone]

Now my point is, and actually my question, wouldn’t it be useful to have ”0” offset on the transducer with (in my case) a 6 foot alarm that would indicate a distance from the hull (where the transducer is mounted) and 1’-6” under the keel? Offset or not it pays to be cautious in thin water. Maybe this is all just a personal preference? Is setting an offset to the bottom of your keel a rule written in stone? I’d like to see what others write about this question.

 

[Dave G.]

I think it is definitely a personal preference based on what you're most comfortable with. No matter how you do it the key is remembering what you actually did or didn't do. I now check it occasionally to "remind " my old brain of the settings especially when I'm venturing into new waters. Now if I can just remember how to get to that screen, hmmm...

 

[markvone]

Sorry for the blank post above, ran out of time getting this reply together.

I'm a big fan of simple. Plus, I started navigating before GPS and without LORAN in my little E23. We had speed and depth, compass and paper charts. Often in the fog in New England, we would navigate using the bottom contours and compass bearings to sound-producing nav aids.

So, my depth display is set up to display the actual water depth (like Loren and bigd14*). This requires a POSITIVE (+) offset** for the distance from the transducer to the boats waterline. (+1.5 feet for the E30+). The number you have to remember is the boat's draft. Easy for me.

Maybe not so easy for kids and non-sailor guests. So, another approach (like bgary) is to add a NEGATIVE (-) offset*** for the distance from the transducer to the bottom of the keel (-4.5) for the E30+. Everyone is told "NEVER GO BELOW ZERO FEET!". Anchoring only requires you remember the boat's draft for proper scope.

IT IS VERY IMPORTANT THAT YOU GET THE +/- CORRECT FOR THE OFFSET YOU ENTER.

NOW, If you add a fudge factor for either of these two methods, you have to remember that and you add the possibility of the following issues:

** You don't want a positve fudge factor LARGER than the actual distance from the transducer to the boats waterline. Any value LARGER will measure MORE water depth than you have!
* bigd14 - if you have +2.0 feet offset, you are 6 inches closer to the bottom than you display
*** You don't want a negative fudge factor SMALLER than the actual distance from the transducer to the boats waterline. Any value SMALLER will measure MORE water depth than you have!

Mark

 

[bigd14]

* bigd14 - if you have +2.0 feet offset, you are 6 inches closer to the bottom than you display

Yikes! I’ll double check that immediately. Thank you. It’s also possible that I am misremembering the actual setting which I set back in 2018. I calibrated everything according to the manual but tried to add 6 inches. Maybe I subtracted it. I’m thinking I’ll just set it to actual depth from waterline this time. It’s interesting to see how others handle this. Good reminder to double check how the depth finder is set when chartering or using other boats.

I’m not sure why this offset business hurts my brain so much….

 

[markvone]

EDIT TO *** IN POST #13: should say: You don't want a negative fudge factor SMALLER than the actual distance from the transducer to the boats KEEL BOTTOM. Any value SMALLER will measure MORE water depth than you have BELOW THE KEEL!

This issue is hard to visualize in your mind's eye because we typically picture ourselves ON our boats. It's much easier if you put yourself off the boat in space looking back at it. Here is a diagram which is worth a 1000 words.

Mark

 

[robcally41]

EDIT TO *** IN POST #13: should say: You don't want a negative fudge factor SMALLER than the actual distance from the transducer to the boats KEEL BOTTOM. Any value SMALLER will measure MORE water depth than you have BELOW THE KEEL!

This issue is hard to visualize in your mind's eye because we typically picture ourselves ON our boats. It's much easier if you put yourself off the boat in space looking back at it. Here is a diagram which is worth a 1000 words.

Mark

View attachment 46943

Very helpful diagram, thank you!

 

[Bolo]

Sorry for the blank post above, ran out of time getting this reply together.

I'm a big fan of simple. Plus, I started navigating before GPS and without LORAN in my little E23. We had speed and depth, compass and paper charts. Often in the fog in New England, we would navigate using the bottom contours and compass bearings to sound-producing nav aids.

So, my depth display is set up to display the actual water depth (like Loren and bigd14*). This requires a POSITIVE (+) offset** for the distance from the transducer to the boats waterline. (+1.5 feet for the E30+). The number you have to remember is the boat's draft. Easy for me.

Maybe not so easy for kids and non-sailor guests. So, another approach (like bgary) is to add a NEGATIVE (-) offset*** for the distance from the transducer to the bottom of the keel (-4.5) for the E30+. Everyone is told "NEVER GO BELOW ZERO FEET!". Anchoring only requires you remember the boat's draft for proper scope.

IT IS VERY IMPORTANT THAT YOU GET THE +/- CORRECT FOR THE OFFSET YOU ENTER.

NOW, If you add a fudge factor for either of these two methods, you have to remember that and you add the possibility of the following issues:

** You don't want a positve fudge factor LARGER than the actual distance from the transducer to the boats waterline. Any value LARGER will measure MORE water depth than you have!
* bigd14 - if you have +2.0 feet offset, you are 6 inches closer to the bottom than you display
*** You don't want a negative fudge factor SMALLER than the actual distance from the transducer to the boats waterline. Any value SMALLER will measure MORE water depth than you have!

Mark

I like the idea of a "positive" offset since I know my shoal keel depth is 4'-3" and with a depth alarm set to go off at 5 or better yet 6 feet it would be less likely for me to run aground. Notice I wrote "less likely". Sailing, like the rest of the world, is full of chaos and stuff happens. Next haul out (this winter) I'm going to measure the distance from the transducer to the water line on my E32-3 unless someone here knows that measurement.