Ever since electronic or sonar/transducer type depth sounders have been around, sailors and captains have had love/hate debates over how a depth sounder should be calibrated and displayed, especially on sailboats. One group believes the readings should provide the depth at the waterline, basically from the surface of the water to the bottom of the sea or lake. Whilst another group believes it should provide the depth from under the keel itself. And yet, another group, albeit thankfully smaller, believe it should be from the sounding unit, wherever it happens to be installed. I’ll leave that latter group out of “this” debate, as that just makes no sense.
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One final step. Anchor over hard sand, close to the depth of your keel. Go for a swim and measure. Like mast height above the water, there is theory and then there is knowing the number at which you will touch. Charted depths are just an estimated historical number, that in may cases dates to the 70s, and tides vary with the wind.
There is also some variability in very soft mud. I’ve been places where I lost the echo.
Thank Drew.
When determining anchor scope, you should not use depth from waterline; you should be using depth from anchor roller, so you still need to add something to the depth reading from waterline. I have a catamaran with the depth reading from the bottom of the keel. With a 4-foot draft and 3 more feet to the anchor roller, I add 7 to the depth reading. For a 5:1 scope i can quickly take one-half of that number then add a zero to determine how much anchor rode to let out. Ten-foot depth reading + 7 = 17. Divide by 2 = 8.5, add zero for 85 feet of rode.
Hi John and thanks for that excellent feed back. You are correct, by adding that extra distance, waterline to the anchor roller, you end up increasing your scope, which is always a good thing! More is better as they say, while still being aware of your swing rate when in a busy anchorage. Thanks for reading and sharing.
Should be with the depth at the waterline. Chart reading needs to be compared to actual water depth. Navigation is easy if I can compare chart work with my position.
Rich
Hi Rich, thanks for your comment. I agree and that’s my setting as well. But, as the previous reader, John Robbins commented, chartsplotters and charts are readings that may be outdated. So keep that in mind. Also, to determine anchor rode ratio, do not neglect the freeboard distance. Read John’s comment for more details.
Setting the depth readout to the water line makes it easier to align the reading to depths on the chart. You still have to adjust for tidal height.
Setting the depth to the keel makes it easier to see when you will go aground. No calculation.
When the boat is on the hard, a laser level makes it easier to take accurate measurements. Set the level to the transducer location and take a measurement to a point on the ground. Then set the level to either the waterline or keel bottom and measure to the same point on the ground. It’s the height difference is entered as the depth sounder offset.
Thanks B.. I appreciate the additional information and technique to set the depth sounder. That’s pretty much how I measure my data as well. I keep that data in my log book specifications page.
Another consideration is asking other boaters for their depth reading. Depth below someone else’s keel is meaningless and yet, that’s what one often gets.
Hi Jack. Thanks for your comment. I do agree with you that asking other boaters what their depth is, will not help as the depth of where you are, is most likely different. I’ve never heard of anyone doing this…
Interestingly I am of both camps and always have two depth displays when on soundings. One (with the 50kHz transducer) is set to total depth and is useful for running a contour like the 100 fathom line. The other, with a 200kHz transducer is set to depth under the keel. They are clearly labeled and both serve a purpose.
Wow, now that covers all the bases! Like wearing a belt AND suspenders at the same time. Lol. Thanks for reading PS and commenting.
My older boat is sailed in a large, shallow body of water that’s productive because of agricultural fertilizer runoff and relatively warm temperatures. The water tends to be opaque, and with shallow depths, a sounder is useful. When I replaced the old neon-bulb blipper with a digital device, I just assumed it was depth under surface. One year on the first sail, my Dad and I looked overboard and saw the bottom. That usually meant “you’re aground” because of the opacity. But this year, the phytoplankton and algae had not bloomed. We decided to check the sounder’s reading with an improvised leadline. Turns out the sounder was reading 3′ low–making the reading the depth under the keel (centerboard up). That seemed just fine so I have left it that way ever since. Adding 3 to the readout is not a difficult math problem.
In the Great Lakes we don’t have tides bit we do have long-term quasi-cyclical water level fluctuations, as well as an annual seasonal cycle. I’ve lived through several periods of 6′ changes. Charted depths on the Lakes are to Low Water Datum, so we always have to do some calculation to determine what the chart numbers mean right now, depending where we are in the season and in the long-term fluctuation.
Chris Campbell
Lakes Michigan and Huron in Michigan
Hi Chris and thanks for reading PC taking the time to share your « situation ». I too sail in relatively shallow waters and this year, in the St-Lawrence, with record low waters, many boaters hit and grounded themselves it what we’ve always assume we’re safe areas. Another thing to watch for in our conditions, is false readings to thick underwater plant growth.
Having a swing keel is also another thing to consider with a depth sounder. With one of my former boats that had a 2 to 4.4 feet swing keel, I had set the alarm at 5 feet, will setting the reading to actual water depth. (Water surface to bottom of the pond!)
My older boat, a Seafarer Polaris (1961, 26′ sloop) has a centerboard and not a swing keel, so it’s more forgiving. The board can just swing up. I went without a sounder for a couple years. There was a known shoal area of small rocks off a river mouth. When water levels are low, the board can bounce a bit on them. It’s always alarming. The original board was steel so I hated to bounce it and knock paint off. Copper antifouling plus steel make a battery, so I always tried to keep my primer intact. A number of years ago I replaced the original board with bronze, so corrosion is no longer an issue.
Once my Dad and I were sailing toward a shallow area and decided it was time to come about. We threw the helm over and nothing happened. That moment we had grounded on sand, silently. We cranked the board up and the bow fell off on the new tack. It’s nice having a functioning sounder to avoid surprises like that.
Chris Campbell
I love these stories. Especially when dads are involved. My is no longer with us, but I have terrific memories with him and sailing.
My Dad is gone too, but sailing is when I heard most of his stories. My eulogy at the funeral was a retelling of several of them, which illustrated his principles and character. One, the sailing one, was just interesting. At the end of WW II he and a buddy were in a little restaurant in the Netherlands. A local yacht club was having a meeting there. Yanks were their heroes so they asked the two Americans if they would be interested in sailing the next day. On the boat, Dad observed a line of bullet holes on the bright-finished hatch cover. He inquired. The skipper replied that he had been strafed by a German pilot one day, who did that out of pure malice. He vowed never to cover the holes. Later, way offshore, Dad asked about the depth. He expected some vast number. The skipper grabbed a wooden pole, poked it overboard, and said “About 6 feet.” We have Dad’s B&W photos from the day.
Great story!
Ironically, I was measuring for depth sounder offset just today. One refinement to the article. Measure WL, Transducer, and bottom of Keel to concrete when on the hardstands. Then subtract for the blocking (keel above concrete) and calculate draft and depthsounder offset.
Measure WL both port and starboard, then take the mean. My boat lists to starboard by one inch. Negilible generally, but I was taught to round off only at the end of measures and calculations.
I am of the Waterline camp. That corresponds to the chart, accounting for height of tide, and interpolating for our location. I set the depth alarm to account for draft, plus room for error and time to tack away. Tides vary in water I sail. “Will I be aground at low tide 0 dark thirty, when I rather be asleep” is common question.
Hi Randall, thanks for being a PS reader and taking to time to share your technique. You are correct that knowing you’re personal reality is important, if not critical. Sailing in sea water, where tides play an important role versus fresh water is a consideration one must be on guard for. Not only tides, but how the boat actually floats in salt water versus fresh water must be noted. I, like you, am a water line depth sailor. I give myself a 2 feet extra clearing under the keel on my alarm. Maybe more than necessary, but that’s where my comfort level is. This past season, with record low water levels, thick and dense seaweed growth was a big factor is setting off false alarms. But also help me steer away from these areas before getting caught. There’s just so much weed cutters can do! Thanks again Randall.