In order to get the most out of this section you will need to know how to plot courses and find distance on a nautical chart. If you'd like a refresher go to Plotting Positions & Courses.
Currents have two components; their direction and their speed. Tidal currents vary in speed and reverse direction. Other currents, like the Gulf Stream, do not reverse and are quite steady over long periods of time. We call the direction of a current its set and is usually noted in degrees true. True is used because it's not affected by changes in magnetic variation. So, many of us will need to convert true to magnetic; for a guide on converting go to True & Magnetic.
Interestingly, the direction or set of a current is defined as the direction in which it is traveling unlike wind which is the direction from which it comes; a rather important distinction.
We call the speed of a current its drift and is usually noted in knots which should be quite familiar by now. If not (no pun intended), then see Speed, Time, & Distance.
A simple current (one that's steady) will be noted on the appropriate chart. There will be a current symbol with the speed written on it. For examples see Unlocking The Chart. If it's a tidal current, which is always changing, then you will want to consult the appropriate tide and current tables to find out details.
Below is a fascinating video from NASA showing ocean surface currents.
(If you are unable to view this video you may see it at NASA: http://www.nasa.gov/topics/earth/features/perpetual-ocean.html
Navigating in current. Currents can be tricky to navigate; just ask anyone who has sailed the coast of southern New England. There are better and worse times for sailing Vineyard Sound. You don't want to be caught going against its strong tidal flows.
A GPS will be very handy for figuring out what the current is doing to your boat at any point in time. Caution must be exercised here because of the nature of an electronic GPS receiver; it's a gadget that depends on a source of power and is prone to break downs in the harsh marine environment. If you know how to compensate for current on a chart you will be a more confident and better sailor.
Example. From Ft. Lauderdale, FL we would like to go to Bimini in The Bahamas which is some 50 nautical miles distant. We'll be out of sight of land for the most part so will need to sail by compass heading. The direct heading to Bimini is just south of east or about 120°. However, there is the mighty Gulf Stream flowing north that races through the Straits of Florida. How do we find our way? What heading should we use? First, we'll get out the appropriate chart. We'll need a chart that shows both our departure point and arrival point. Let's see what we can come up with.
There are several things to note in the above chart. We'll start our journey from the Sea Buoy at Ft. Lauderdale in the upper left corner. Then proceed to Bimini. The course heading above is the magnetic heading of our desired track or course made good. Our compass heading will be reading something different as we compensate for the Gulf Stream. We will be 'crabbing' into the current and will need to figure out how much 'crabbing' is the right amount. You can see the Gulf Stream is active all the way across the Straits of Florida.
The current in the chart reads from 1.8 knots up to 3.3 knots. We usually figure the current averages 2.5 knots for the duration of the trip. That should be good enough to get us there. To find the direction of the current we look at the current arrows and average them together to give us a direction of true north.
Now, our work is to figure out the compass heading that will get us to our destination. To do this we could use the chart above but, for the purposes of illustration, we'll be using a larger scale chart. Remember, larger scale means smaller area. We'll zoom into our departure at Ft. Lauderdale to work out the details of our course. Once we've got it figured out for a small section of our trip it will hold true for the whole trip.
We have several components to deal with at the same time. We have our speed and direction through the water and we have the speed and direction of the water itself. An easy way to work with this is to break it down into two sections, work each one out, then put them together to solve our problem.
As in many navigation problems it's a matter of first knowing what you are trying to find out and then, what the first step is. We are trying to find what compass heading will produce our desired track. We know the speed (drift) and direction (set) of the Gulf Stream. We also know our boat's speed, or do we? We'll need to come up with our best estimate of what our speed, through the water, will be. We know our boat and in the prevailing weather conditions we'll be a bit conservative and say our speed will average 6 knots on the crossing. Now, all we have to find is our compass heading!
We'll be using the chart below which shows the Sea Buoy at the entrance to Port Everglades (the port of Ft. Lauderdale). I've already placed our desired course on the chart. Now, let's get to work!
We've got everything, in the above chart, to work out what our heading should be. We've got our course line drawn and labeled, we have a compass rose handy to provide magnetic headings, and we've got our longitude scale along the right edge to give us distances.
There are more ways than one to skin the proverbial or, in this case, find the heading to use to stay on course. The easiest way is to figure out the set and drift of the current for a period of time, say one hour, then figure out our boat's speed and direction for the same period of time, and add them together to come up with the answer. This may sound complicated but once you see it on the chart I'll bet you'll notice the ease and clarity.
First, we separate out what the current's set and drift will do to our boat for one hour. We've stated above the Gulf Stream's set is due north and the drift, in that direction, is 2.5 knots. So, if we sat at the Sea Buoy for one hour and did not run our engine or raise sail, the Gulf Stream would carry us due north for a distance of 2.5 nautical miles. Ok so far? Using our dividers we go over to the side of the chart and measure out 2.5 minutes of latitude which is 2.5 nautical miles. Then, we take this distance and draw a line due north that is 2.5 nautical miles long starting at the buoy. Here's what we've got now.
Now we have taken care of the full effect of the Gulf Stream current on our boat. Since we've compensated for the current now it's time to get sailing! This is a graphic representation of the forces that affect the boat and is not time based as the real world is. So, what we can do now is to ignore the current, because we've taken care of it already, and sail down to our desired course line.
We've said that our boat's speed through the water is 6 knots and, just like we did with the Gulf Stream current, we'll draw a line representing the boat's progress over a one hour time period. 6 knots for one hour makes for 6 nautical miles run. Again, we measure it off the latitude scale with our dividers. Between 05' and 10' we have 5 nautical miles so we'll stretch just a bit further for a total of 6 nautical miles. Once we have our dividers set we place one end on the point where we would have drifted to and then swing the other point to intercept our desired course line. We'll stop at the point where the 6 nautical mile long line just touches the course line and draw as shown below.
Now all that's left to do is to find the heading of the last line and that will give us our compass heading to steer by. We find the angle of the line with the parallel rules and the compass rose and come up with 143° magnetic. We would round this to the nearest 5° increment to steer by so we'll put 145° on the chart. The completed chart work below:
This shows us that to maintain a course of 120° we will actually need to steer 145° on the ship's compass because of the northward flowing Gulf Stream. How long will it take us to get there and what is our actual speed along the way? We know our speed through the water is 6 knots, which is what our knotmeter will read but what of our real progress made? Our speed over ground, as its called, will be less because we are pushing against the Gulf Stream. Our resultant, the track of the boat, is the lower part of the triangle formed above. We know its angle and if we measure the length this will give us our speed in knots because the lower line is a measure of our real world progress over a period of one hour.
We use the dividers, with one point at the Sea Buoy and the other point where we intersected our desired course line, and measure 4.2 nautical miles. We will be using up about a third of our boat's speed to push against the flow of the Gulf Stream but we'll get there! The trip should take; 48 NM/4.2knots = 11 hours 26 minutes.
This will get us all the way to Bimini where we'll celebrate with a rum drink and a dinner of cracked Conch. Does life get any better? Oh, and do remember to pronounce Conch as 'konk' and Cay as 'key' (small island). Now that you know how to get there, I'll meet you in Bimini!
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