Sailing Navigation - Depth Chart Symbols Every Skipper Needs to Know
Did you know that you can use depth curves for safer sailing navigation anywhere in the world? Or that a tried and proven method to find your way home in pea-soup fog has been used for well over 100 years?
Look over any nautical chart or chart plotter software and you will note curved lines or enclosed circles. These depth contours show areas of equal depth. Check along the line or circle for a break that shows a number (see illustrations 1 and 2).
Contour Line Secrets
Depths are written in numbers divisible by 6. For example, the line closest to shore might be 6 feet, the next line 12 feet, the next 18 feet, and so forth (illustration 1). But never assume that contour lines follow a consecutive pattern of 6 foot increments.
For example, in illustration 2, the contour lines jump from 18 feet to 30 feet to 36 feet. Study your navigational chart to decipher the depth information.
Contour Circle Secrets
Some charted contour circles show depths in breaks along the circle (illustration 3). Like contour lines, depths may not be consecutive between adjacent contour circles. Notice that the contour circles show 18 feet, then 30 feet and then 60 feet!
Sometimes you will see contour circles without depths shown in a break along the circle (illustration 4). Use this easy method to find the depth of any contour circle:
* Look for the lowest depth just outside the circle (highlighted in illustration).
* Find the highest depth just inside the circle (highlighted in illustration).
* Determine what number--divisible by 6--would lie between the two.
In the illustration, you see 19 feet of water just outside and 16 feet just inside the circle. This indicates an 18 foot contour circle.
Five Ways to Use Contour Lines or Contour Circles
Use these five tips to backup your sailing navigation, increase your skills, and boost the safety of your small cruising boat in any marine weather.
1. Set waypoints at contour circles
Use depth contour circles to back up your nautical gps or chart plotter. Draw your course line across the circle. Set a waypoint to alarm at the entry and exit points. Correct your depth sounder to low tide.
When the alarm sounds at the entry point, check the depth sounder. It should show that you have entered shallower water. When the alarm sounds again at the exit point, you have cleared the underwater bank and returned to deeper water.
2. Follow contour "boundary lines
You could also use a piloting technique used for more than 100 years called "longshore piloting". Find a depth contour line that you can follow along the coast to a prominent object (like a sea buoy) near your destination.
Plot your course a few miles to the left or right of the prominent object, on the upwind or up current side. Set your depth-sounder alarm to sound as soon as you reach the depth contour line. When you hear the alarm, turn downwind or down current, watch the depth sounder, and follow the contour line to your destination.
3. Transit races with caution
In some ocean areas, underwater mountains rise on each side of a narrow passage. One famous area in New England has been named "The Race".
Transit a race with caution. Underwater mountains plunge from high peaks (yellow highlight) to deep canyons (green highlight).
There, the current squeezes between the two underwater peaks with enormous velocity. Note how depths change from 300 feet in the middle to less than 50 feet on each side.
If possible, time your passage through a race at slack current. The passage will be easier and you will maintain better control over your small cruising boat.
4. Beware of staggered or missing soundings
Chart plotters fall far short of nautical charts when it comes to detailed soundings. Their small screen real estate forces the manufacturers to sacrifice detail in order to keep the screen uncluttered. This reason alone should be enough to convince any prudent skipper to carry navigational charts.
Scan your chart for signs of inconsistent or scattered soundings. Large gaps between soundings warn that this area hasn't been surveyed well enough for safe sailing navigation (illustration 5). Better go around and find a better anchorage with soundings closer together!
In part III, we discussed the extreme hazard of going near a spoil area (also called a fish haven or spoil bank). Illustration 6 shows two spoil areas just outside of Key West. As you can see, no soundings are shown. Stay clear to stay safe!
5. Use nautical charts for detail
Before you approach landfall, round a point of land, or attempt to enter a cove, harbor, or inlet--pull out the largest scale nautical chart available. This gives you the best orientation and detail for the safest sailing navigation anywhere in the world.
Follow these chart navigation depth secrets to keep your small cruising boat safe and sound. With these tried and true techniques, you will gain the confidence to sail over the horizon--wherever in the world you choose to cruise.
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