tides, currents, and leeway 113
How to Measure the Eff ect of Current in Th ree Easy Steps
In navigation, the word current includes all eff ects, both mechanical and natural,
that combine to push a boat from ahead, astern, or one side. Tidal currents,
ocean and wind-generated currents, helm error, engine or engines out of sync,
even leeway—all these go into the term “current” as it applies to navigation. Tidal
currents are those currents directly aff ected by the vertical rise and fall of water
(tide) in a geographic area. Ocean currents such as the Gulf Stream are not infl u-
enced by tide.
Your GPS receiver has a cross-track error function—abbreviated XTE
or XTK—showing how far right or left you have wandered from your plotted
trackline. It also tells you the direction to the next waypoint, but it won’t tell you
whether steering that course places you in danger. For that reason, it is far prefer-
able to stick to your original trackline (which you presumably have plotted on
the chart and know to be hazard-free) than to wander off that trackline and then
steer a new course to bring you back to it.
Here too GPS provides a ready answer: You can simply adjust your course until
your GPS receiver tells you that your course over ground (COG: see below) and
the bearing to the next waypoint (i.e., your trackline) are the same. But how can
you stick to your trackline without GPS? Doing so will be easier once you know a
quick and easy method that tells you the direction and strength of the current.
FINDING THE DIRECTION AND SPEED
OF ANY CURRENT
As mentioned above, the set of a current is noted in degrees true. If the current sets
045 degrees, then it is fl owing toward 045 degrees true. Also as noted, the velocity
of a current is called its drift. Like boatspeed, drift is expressed in knots.
For example, let’s say you’ve used your tidal current tables or atlas to calculate
the current. You determine it has a set of 045 degrees true and a drift of 3 knots.
If you stopped your boat next to a buoy, what would happen? Th e current would
push (set) the boat in a direction of 045 degrees true, and in 1 hour you would
end up 3 miles northeast of the buoy. Once you fi gure out the set and drift of any
current in the world, you can compensate for it. Below is an example.
At 1200 we obtain a fix from crossed bearings and plot a trackline to the next
waypoint. The TR is 90 degrees magnetic, and our speed is 4 knots. At 1230 we
obtain and plot a second fix. How is the current affecting our boat?- Plot three separate positions. All set and drift calculations require you
to plot three basic positions. Start with a known position. Plot a second
position based on information from a reliable navigation source (visual or