Equipment Outfitters

Plotting

Having marked the courses between buoys, the “legs” of your intended trip, you will want to measure them. And, having measured them and added them up for total mileage, you will want to figure out how long it will take to make the trip. Since distance divided by speed equals time, you will first have to learn how to tell boat speed. If you know your speed and distance on a leg, you can compute time in hours.

Course

Examine the chart to determine the best way to get from your departure point to your destination. You may be able to go straight between the two. More likely, however, the trip will require several doglegs to avoid shallow area and hazards.

Using a pencil and parallel rules, draw straight lines between the navigational aids you plan to use along the way.

Distance

At the top of the chart is a scale in nautical miles. Using the dividers, determine the distance of each leg. Write the distance for each leg beneath the course marked on the line, followed by the letters “nm” (short for nautical miles).

Speed

A tachometer can be used to determine a vessel’s speed through the water. Thus, at any given engine rpm, the skipper knows what speed the boat is making.

To come up with a speed table, choose a mile-long course between two known navigational aids. Run the course each way at a given throttle setting, then do it again and again at increasingly higher settings. Average the time it takes to cover the distance at each setting, convert the average into seconds and divide the seconds into 3,600 (seconds in an hour) to get speed. If the measured mile is a statute (land) mile, speed will be in mph. If the measured mile is a nautical mile, speed will be in knots.

There are various ways to determine boat speed. Some vessels have speedometers. Larger ones are equipped with navigational devices, such as loran, which compute speed electronically. Smaller boats can calculate velocity by timing their running time on a measured mile at different engine rpm. To minimize the effects of wind and tide, timed runs should be made in each direction at a given rpm.

Dead Reckoning

On long legs between known navigational aids it is difficult to know your exact location at all times. However, by keeping track of your compass course, time and speed on each leg, you can “reckon” where you are at any given moment. Constantly updating your estimated position isn’t as necessary in familiar waters as in unfamiliar ones. But knowing where you should be is critical in any emergency.

Waypoint

Dead reckoning involves recordkeeping. Each time you pass a waypoint, for example, you should pencil in the time next to it on the chart. If you know your compass course and speed from that point, you can determine your location at any given time along the leg that you are traveling by multiplying your speed by the time that has elapsed since you passed the last waypoint divided by 60 when dealing in minutes. You should also be able to predict your arrival time at the next waypoint.

Checkpoints

Dead reckoning information should be updated as frequently as possible.

To check your progress on a two-and-a-half-mile leg (the long one on the chart at right), you should note the time you came abreast of Long Neck Point.

On a long leg of a journey, tides and currents, even wind, may push you off course, slow you down or speed you on your way. The longer the leg, the greater the possibility of deflection. One way to correct and update your dead reckoning is to use waypoints along the way to check your progress on each leg. Look for conspicuous landmarks along the shoreline, nearby headlands and well-defined land contours.

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