A column by Nigel Barron
Ask Nigel
How Low Can You Go? Every summer without fail, boatyards learn about new supposedly uncharted rocks in Puget Sound. That’s because inevitably someone, somewhere, will run aground. These rocks, it turns out, have names… names like Bainbridge or Vancouver Island. This phenomenon, however, brings up an interesting point about depth transducers on boats and how they work. A friend recently asked me how accurate his depth gauge is. A depth transducer will read how much water is below the transducer. But how far below the water is the transducer? A newly installed depth transducer comes without any offsets. It’s up to you to figure out what the offset should be or to communicate to the people installing the instruments what you want the offset to be. Some people prefer to set their instruments up to read accurately how much total water the boat is in. This would mean adding an offset to account for the transducer being usually installed a few feet below the surface. The logic here is the numbers displayed on the depth are going to more closely resemble the numbers on the charts. For myself, I just want to know how much water is below the keel, because I know without doing the math that I am going to float. The first step is figuring out what your depth transducer is reading, and then what number you want it to read. The easiest way to do this is to get a piece of line, tie something heavy to it, and drop it in the water next to the boat in the slip. Pull it up, measure what you have, and compare it to the instruments. If your line tells you that there is 20 feet of water, and you want to have your instruments to tell you how much is below the keel, you have your starting point. If you draw 6 feet, then you would want to set the offset so the instruments read 13-14 feet (I like to leave a little buffer). If you want to have it read 20 feet, add accordingly.
With your depth transducer now accurate, it’s also important to know where you’re going. Some places you go all the time will be familiar, and you’re not likely to have any problems. For others, it’s a good idea to review your charts or other publicly available information. If it’s somewhere you think is questionable, do the math on if it’s a good idea to go there or perhaps wait for the high tide. It’s important to also understand the relationship between the charts and reality. Because of our strong Northwest tidal exchanges, bathymetry (underwater topography) is always moving and silting over. For example, West Point and Meadow Point don’t necessarily reflect what’s shown on charts anymore. If you’re not sure, respect the buoys or channel marks. This image is taken as a screenshot from Nobeltec TimeZero and shows the traditional Raster chart along with a 3D view with color depth shading. The red line is at 7.5 feet. Most chart plotters can set the shallow depth shading for the shallow and safety contour. It’s another way of interpreting the data to give people information they can use. Lastly, pay close attention to the tides. NOAA-charted depth is expressed as MLLW (mean lower low of the water). Most Canadian coastal charts show this as LLWLT, meaning the surface of lower low water, large tide. It is important to know the difference with the tide book you are using. Nigel Barron was born in England and developed his sea legs at an early age. He is currently the Project Manager at CSR Marine in Seattle where he has worked for 15 years as a rigger, then an installer, and now in his current role. He is also the captain of Crossfire, a Reichel-Pugh designed and McConaghy built custom race boat from Seattle.
59
