Whatever your destination, every Bahamas cruise starts with a forty-mile-or-more passage across the Gulf Stream. One of the true wonders of the world, the Stream is by far the largest and strongest ocean current, carrying a volume 25 times that of all the rivers of the world. When the wind is with the current, passage is smooth and easy. But when the wind opposes the current, it can be the longest forty miles on earth. Whenever boat people play the game of "my water is rougher than your water," even nasties like the English Channel place second to the Gulf Stream.during a Norther.
The most critical element in any successful crossing is an accurate weather forecast. Be particularly alert for cold fronts in the winter and for tropical storms the rest of the year. Never cross when the wind has any northerly component (NW to NE) — even modest northerlies can raise huge seas. Your next concern is the Stream’s current which flows at near—right angles to your course at speeds that vary from near—zip to over four knots but average two and a half knots. Successful crossing requires you to estimate how far that current would carry you to the north and then steer a more southerly course to compensate for the drift. That may sound complicated, but it’s really not difficult. Let’s say you’re on a ten-knot boat heading out of Palm Beach for West nd. That’s a trip of 56 nautical miles on a Rhumb Line course you plot as 095 degrees true. Mark where your intended course intersects the axis of the Gulf Stream and label it Point A. Then draw a line from A in the direction of the Stream’s flow that equals the drift in any given period of time. But keep it simple, use one hour so you can make the line 2.5 miles long to represent the average 2.5 knots of current. Mark the end of the line Point B. From Point B, draw an arc with a radius equal to the distance you will travel in the same time. In this case, since your boat is cruising at 10 knots (your speed through the water), that’s 10 miles. where the arc intersects the Rhumb Line, mark Point C. The angle of the Line BC represents the corrected course to steer to West End, which in this case is 110 degrees true (115 magnetic). The length of Line AC is your average speed over the ground, which you use to estimate progress and time of arrival. If you have GPS, observe (and log!) your position every half hour. Be aware that by applying a constant correction while crossing a current that varies in strength, your course over the ground will resemble a lazy "S" even as your compass reads a (relatively) steady 115. You will absolutely NOT follow the Rhumb Line course; so don’t despair when your GPS shows you slightly off it. You should return to the line as you near your destination. On the other hand, you can use your GPS to make a straight Rhumb Line crossing. In this case, your compass heading will vary, becoming more southerly as you point more "into" the current when its strength increases and then easing back more toward your base course as current diminishes.
Since the Rhumb Line is the shortest distance, there’s a temptation to rely solely on GPS. Especially if you have it guiding your autopilot. However, the aspect of facing more directly into the current during the strongest parts of the Stream can actually make this "shortest distance" the longer way to go — even more so for slower boats. And when your GPS fails, and the odds are one clay it will, it’s best to have the navigational know-how to get there the old fashioned way. Crossing the Gulf Stream by compass and clock is a great way to keep in practice.