Understanding Water Depth
While water can be dangerous, so can the stuff that lies below it. Thus it is important to know, for sure, how close it is to fragile stuff on the bottoms of out boat such as the propeller and rudder if fitted.
Depth sounders are reliable electronic instruments that determine the distance from the sounding transducer to the sea floor by using ultrasound pulses. These devices can be set to alert sailors when sailing close to shore, near atolls, or over other objects where you are uncertain about the water’s depth. Fishfinders can be incorporated with depth sounders, allowing sailors to also check for fish activity, should they wish to catch the “big one.”
Keep in mind that when cruising over areas that may have highly irregular bottom surfaces, such as coral, large rocks, and sunken objects, you may not have ample time to react to your depth sounder’s warnings. You must know where you are on the chart and be able to read the chart.
Harbors are notorious for having fluctuating depths due to currents and poorly scheduled dredging. Be wary of water depth any time you cruise into a new and unfamiliar harbor.
Depth Animation
In the olden days, they used marked lines attached to lead weights to determine depth. Nowadays, we use sonar signals traveling at the speed of sound to measure the depth (and to determine if there are fish around for dinner). Still, a prudent boater will have on board a backup lead-weighted line. Recreationally speaking, it’s not practical to have a bowman calling out the depth of water every minute. Thus, almost every vessel these days has an electronic depth finder. Every experienced boater will tell you that the cost of a depth finder is worth the investment. If your vessel does not have one, get one.
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Anecdote: Lake Travis in Austin, Texas is an amazing lake for boating. My friend and I used to go out a lot at night. The lake can be quite challenging in that the level goes up and down with the seasons and el nino and el nina. After a few times running aground at night, my friend decided to install a depth sounder. The next time we went out we ran aground. My friend yelled “how can we possibly run aground when the depth sounder says 40 feet?” On closer inspection, the digital LED numbers read 4.0 with a very small decimal point. A not-so-complimentary shout-out to manufacturers reading this – why would you do that? |
Offset
Depth sounders have a feature that allows the depth reading to account for the depth of the keel of a sailboat or the depth a powerboat lies in the water including its propulsion gear. For example, if your boat’s draft (how deep it lies in the water) is 3 ft (1 m) deep and the real water depth is 10 ft (3m), you will have only 7 feet (2.5m) of clearance. Thus, you want the depth reading to show 7 ft (2.5m) not 10 ft (3m). Caution however, when you set up your device—make certain that (1) your offset number is accurate (with an extra few feet for safety), and (2) that you are entering it properly with a negative or positive offset using a plus “+” or minus “-” symbol according to the owner’s manual or manufacturers instruction. Thus, you’ll need to read the manual of the individual device (some manufacturers do it differently from others). Don’t assume a negative or positive offset when you set up the device. In the image below for setting up a sailboat with a much deeper draft we assume a negative offset device.”
Depth Offset
Tides
Tides are the regular rise and fall of the ocean due to gravitational forces from the moon and the sun. Tides vary in height all over the planet from zero to 50 feet (15 m), but for each specific location, they rise and fall a similar amount each cycle.
Be especially wary of tides when anchoring. Anchor at high tide, and you may find your boat resting on the bottom in a few hours. Anchor at low tide and you might find your boat drifting away in a few hours because the amount of anchor rode (anchor line) you put out did not account for the extra depth.
Fortunately, the depths listed on navigational charts are marked with the low tide depth. But this is not all you need to know about tides. Boating is endlessly technical in its own nature. Further studies are always required.
