Nautical Navigation Basics (Part 3): What to watch out for during the planning stage

The fact that we’re at part three of this series and just getting in-depth to the “pre-planning” stage is telling in terms of how much there is to navigating safely. Quickly, let’s recap what we learned from Bob Sweet’s The Weekend Navigator so far.

There are three stages of traversing the water: planning, navigating underway and checking. This post is about the first stage—planning—and it’s essential to getting to where you want to go, safely avoiding any charted obstacles. During this stage, you select your course, check it for hazards and adjust it to avoid obstacles. Even if it’s a route you’ve taken hundreds of times, it’s always smart to have everything laid out on a chart ahead of time in case visibility is low or something goes awry with your navigation electronics.

This is when you would employ the waypoint navigation technique discussed in the last part of this series. As you measure out your route, be sure to keep in mind the draft of the boat (the measure of how far its keel extends below the waterline) and the depth of the water. Ask yourself what kind of seabed you’ll be going over. Rocks? Sand? Seaweed? Mud? You can guess which ones are more forgiving than others.

Remember, those depth numbers are a mean value, not the absolute shallowest you may experience. Most charts show MLLW (Mean Lower Low Water) soundings. That’s the long-term average depth for the lower of the daily low tides at that location. During full and new moons, the actual depth could be bit lower at low tide. Well, the tide isn’t always low, so could we safely take a leg over a shallow bottom? Either way, says Sweet, “It is prudent to pre-plan legs of a safe passage with some margin of depth and lateral clearance to obstacles. Only traverse a shallower area when you know for certain that the stage of the tide at that time will give you enough water.”

Finally, the last things of note are “horizontal clearance” (make sure you have plenty of room to steer around obstacles) and “clearance overhead” (don’t make the mistake of going under a short bridge in a tall sailboat). With these guidelines in mind, you should now be ready to hit the water safely. Before you go, though, don’t forget to bring along a NOAA-approved chart from OceanGrafix!

Most of the information for this series comes from Bob Sweet’s The Weekend Navigator. To find the book, follow this link.


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