what is the best way to determine the impact of current to your speed over ground? for example if i am going 5 knots and i want to go 50 miles and i am going with a current of 3 knots - does that increase my speed by three knots? also what is the impact of sea height/winds on this situation - such as 5' seas with 15 mph winds- going against or with?

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The effects of current and wind (known as Set and Drift) on your vessel will always depend on actual boat speed and direction. In the example you mentioned, the answer is Yes, your boat's forward speed over the ground would be increased by 3 knots with a 3 knot current in the direction you are traveling.

It is not too difficult to determine the effects of wind and current when they are on the bow or stern. The trick comes when the wind and current are from an angle to the direction you are traveling or when they oppose each other in some fashion.

The easiest way to learn this type of navigation is to take a class that makes you leave your GPS at home. However, here are some basics you can take with you:

Let's say you are traveling at 5 knots water speed, due north to a destination 50 miles away, on a compass course of 000. You would plot a DR (Dead Reckon) course on your chart from where you started to where you are going with estimated positions every 5 miles along the line.

Every hour, you take a position fix and plot that on your chart. A position fix is an accurate notation on the chart of your current position that can be achieved with GPS, Celestial (for long passages), or with visual land marks.

When you plot your first hourly fix on the chart and it is to the right of the DR line you drew on the chart and a little further along that your original estimate, you know that there is something affecting your direction and speed. It could be current and/or leeway due to wind and wave.

If you measure the distance you were set off course, and it is one mile to the right of your DR and 1/2 mile further on, you can use this information to correct your course (and speed if you are under power). From experience, I can tell that the True course is 007 at 5.5 knots. To correct this, I would adjust to a compass course to 350 degrees to arrive at my destination now 45 miles away. Because conditions can change over the estimated 8-9 hour trip, you would continue to take position fixes every hour and make corrections until you arrive.

GPS has mostly eliminated the old school way of navigation. However, it is prudent to know how to do it the old way. It is also prudent to ALWAYS plot a DR on your chart, and use the GPS to plot fixes. On most chart plotting GPSs, the planned "route" that you program into the GPS is the DR that the GPS tries to keep you on. Just remember that a GPS can suddenly malfunction and having your paper charts with the DR and position fixes could save you one day.

thank you shane. that helps a lot.


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