You set out one fine morning for a day’s sail. After arriving at the end of the Potrero Reach, you get your sails up and turn to a course of 214 magnetic (214M) because you know that is the direction of Raccoon Strait. On that course you know from past trips there are a couple of buoys which you will pass about a quarter mile off to starboard, and one off to port about a half mile. All is well in the world, and you commence to sailing and socializing.
What you didn’t take into consideration is the 1.4 knot ebb crossing your intended course at nearly right angles. Even though your bow is pointed at 214M, the course made good is 200M. You are pointed at clear water and Raccoon Straight, however, the boat is headed directly towards R “8” a very hard and unforgiving buoy!
Here’s a quick easy visual check to determine if you are on a collision course with “something”. If that “something” … in this case the buoy … appears to be fixed in relationship to the background, you are headed straight for it. If the background seems to be moving to one side of the buoy, you are going to pass on that same side.
With a bit of advance planning you would know steering a course of 228M (call it 230 because it’s a nice round number) would put you right down the center of Raccoon Strait. Now all you have to do is hope the wind co-operates and you don’t have to do a couple of dozen tacks to get there.