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Taking Bearings On a Small Boat

This tip is provided by Bill McNiel, one of our ever growing number of "experts." Bill has been boating for 37 years, including 34 years as an owner/operator of an offshore fishing service in the Gulf of Mexico and has taken numerous cruises in the ICW and offshore.

Bill is a member of the Fort Worth Boat Club and has been a member of the United States Power Squadron for 34 years. His official capacities have been as Squadron Commander, Squadron Cooperative Charting Officer, District 21 Education Officer and 10 years on the Piloting Course Committee.

Taking Bearings On a Small Boat

By: Bill McNiel

Accurate bearings are critical to small boat positioning. Small boats' freeboards and shallow drafts greatly increase the ratio of drift to advance when underway and the possibility of dragging anchor.

Following are methods for taking bow-on, compass, hand bearing compass and relative bearings. I will also discuss use of the three-arm protractor for plotting lines of position (LOPS) obtained from bearings.

The simplest bearing to take is the bow-on bearing. The operator aims the vessel’s bow at a landmark or an object on the water and reads the compass direction. The resulting reading is a compass bearing. The accuracy of this method can be greatly improved by providing sights on the vessel that are parallel to the keel and in the normal line of sight of the helmsman. Simple items to use are a vertical line of tape or heavy thread attached to the windshield and a pop rivet or golf tee placed vertically near the steering station.

Where the steering compass is mounted on a small boat may limit the skipper’s ability to take bearings over the ship's compass. When the compass is used to obtain compass bearings, be sure to use the deviation for the boat's heading at the time of the bearing to convert the bearing to true before plotting.

My experience with hand bearing compasses is mixed. Soon after we acquired a 24-foot outdrive sport fisherman, my wife gave me a beautiful hand bearing compass. Three years later, after many attempts to compensate it, several lost contests because of bad positioning and plenty of reverified chart corrections, it became a conversation piece. On a cruise across Corpus Christi Bay, a sailing member of another squadron demonstrated his latest high-tech "hockey puck" hand bearing compass as he sat on the motor box. He was taking bearings on a charted tower. None of his bearings crossed within a mile of our obvious location, which we verified by another technique. Otherwise, I have had excellent results with my hand bearing compass on sailboats and my current 21 foot outboard sport fisher. I recommend compensating and trying before you buy. They’re great devices when they work!

Relative bearings are angles relative to a boat’s heading measured clockwise from dead ahead. This measurement is usually made with a pelorus, which can be purchased or easily made at home. Because the 0- to 180-degree line on the pelorus must be parallel to the keel, you may have difficulty setting the pelorus in enough positions to give 360-degree coverage. If your boat has a hardtop, you might consider a periscope pelorus. I have seen government surplus periscopes mounted on hardtops but don't know about their origin. Remember, using the pelorus requires two people; one to stabilize the course and read the compass and one to take the bearing and read the angle. Knowing the boat’s heading is important when taking a relative bearing, as you must use the deviation for the boat’s heading when converting the bearing to true before plotting.

In my opinion, the second most useful navigation instrument after a compensated compass is the basic plastic sextant. This instrument provides adequate accuracy and allows one person to position accurately. From anywhere on the boat, a relative bearing can be taken by sighting on the compass or a line parallel to the keel in one glass and a charted landmark in the other glass. You must use the boats heading to obtain the deviation before plotting the true bearing as an LOP. I find that inverting the sextant handle up for portside bearings and handle down for starboard-side bearings works best.

The most accurate method of locating your position is using the sextant to measure the two angles between three charted objects. The center object should be closest to you. Plot the angles on transparent paper laid over your chart or set them on a three-arm protractor. Match the LOPs through the objects on the chart. Your position is at the apex of the angles.

This method can be accurate within feet. I once plotted angles measured from a docked vessel during a cooperative charting seminar, and the plot showed my exact position in the water alongside the dock. A position obtained by this method always matches my lateral position in the channel. Operation of the three-arm protractor is elementary; I use small pieces of masking tape to maintain the settings on the protractor while I am plotting.

Another type of bearing commonly used by fishermen in the Gulf of Mexico is the boat's course offshore. It is usually referenced to the end of the jetties or bar and combined with the water depth to give a position offshore. The slope of the bottom in the Gulf of Mexico is fairly constant, so the accuracy depends on the steadiness of your course, sea state and the calibration of your depth finder. I have heard stories that fishermen broadcast false or coded locations of fish schools to avoid unwanted company.

I hope you’ve gained some ideas for broadening the use of your small boat. In addition to compensating your compass, calibrate your gas tank gauge, depth finder, pelorus and speed curves for load and motor/outdrive angles. In closing, I’ll paraphrase the old joke about getting to Carnegie Hall. If someone asks, "How do I get to a safe harbor?" the reply is "Practice, man, practice."

Related Articles:
Nautical Chart Reading 101
All About Nautical Charts
How to use Dividers and Parallel Rulers
Identifying Aids to Navigation
Regulatory Markers
Nautical Miles and Statute Miles

 

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