Checking my sonar logic (before low tide!)

SJI Sailor

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Nov 28, 2018
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804
Non-Fluid Motion Model
American Tug (formerly had C30)
I wanted to run my sonar logic by folks for a second opinion, before the tide goes out this evening.

We are docked with sonar reading 11.8 ft. Tide will go out another 8 or 9 feet. Assume 9 ft. That gives 2.8 ft clearance at low tide from the sonar location.

Figure that sonar is about 2ft below water line (2019 C30 sedan, installed next to the head). There is 0 offset on the Garmin setup. So that will net 4.8 feet from water line at low tide. Then take -3 ft draft (in reality probably a little bit less), for a net clearance of 1.8 feet from the bottom. So we're fine.

Sound right? (Another data point: we're on the winter dock at Sucia Fossil Bay, east side of the dock, and the neighbor says he's never seen anyone with a problem even in larger boats.)

Thanks in advance for a second opinion!
 
Sounds right to me. If it were me, I would stay on the dock. I usually also look for Active Captain comments to see if other people have noted anything unusual.

John
 
Your logic sounds good to me. Might be useful to test your sonar accuracy by measuring the water depth with your boat hook when you get to lower tide values. I've stayed on the Sucia dock in all tide conditions and never had a problem with my R27 OB.
 
I would determine the accuracy of your offset setting. What I would do is put a sinker on a fishing line and drop it overboard and let it down until you hit bottom. Then mark the line at the water. Pull the line up and measure the length. Then see what your depth sounder is reading. If it is different than the depth you determined on the line adjust the offset accordingly so they both match, if you want actual water depth. You can further adjust it if you want depth under hull, which will be the difference between the actual depth and your determined draft.
 
Thank you everyone for checking my math and the advice!

As it happened we did mark a boat pole and measured near low tide. At that point the sonar read 2.5 ft, and the pole read 4.5 ft from the water line to the mud bottom. So that implies that our sonar transducer is about 2 ft below the water line, as I had expected (and we had 54 inches water - 30 inches draft = ~2 feet clearance from maximum draft).

We'll see again this evening, when low tide will be 0.5 feet lower. We should see sonar read 2.0 feet, which will clear our draft by about 1.5 feet. Not too nervous because there is a mud bottom. And there is good company here, as our C30 is docked between two RT 31s!

I suppose if I set an offset of 2 feet then I could read clearance directly on the sonar. OTOH I would like to have a bit more clearance and not have to worry. And having an offset would just confuse me more 🙂
 
SJI Sailor":3cczwm9r said:
I suppose if I set an offset of 2 feet then I could read clearance directly on the sonar. OTOH I would like to have a bit more clearance and not have to worry. And having an offset would just confuse me more 🙂
I agree. I originally entered an offset and then took it back out again though I still mentally treat the sonar reading as the “water depth.” If things get that close, I want to do the math like you did, think about what kind of bottom is under me, and be sure of myself. For me, it’s intuitive to visualize water under the transducer head. It adds another step to add or subtract an offset. If I were to enter an offset at some point, it would be to give me the reading of the true depth under my keel/rudder. Enjoy the rest of your time at Sucia!

John
 
My 2 cents worth. Your transducer is not located at the maximum draft point meaning that some part of your boat sits deeper than the transducer face. So what I would do is take the sounding depth and add it to your maximum draft which should give you the bare minimum water depth to consider safe. And, IMHO, is cutting it close. It's one thing to churn up milkshake, but another to hear prop chatter on something hard. I get nervous when the 4 foot alarm goes off on my boat, meaning I may have a clearance of only 2.5 feet.
 
I agree and definitely would not want to be in motion in such shallow water! This was sitting on a state park dock in shallow water and with a negative low tide. We might have paid more attention to that in advance (but had the option to switch to a mooring buoy, so it wasn't a mak or break issue).

As a minor update for the record, we measured again on the second night and concluded that the sonar transducer (C30) was closer to 1.5 feet below the water line (not 2.0). So you are quite right and any reading under about 1.5 on the sonar would be near contact. That assumes a uniform bottom, of course.

Anyway, agree with all the advice, and also John's note to keep it simple and do the math! Also to get out and measure when you have such a chance 🙂
 
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