Sailing Articles
In a sailing boat the tide is king!
I want to explain why we sailors say that the element that has most effect on a sailing boat is not the wind - it's the tide. The reason for this is quite simple - yachts are displacement boats that have a top speed limited by their waterline length no matter how hard the wind blows. Motor boats, speed boats and jet skis are powerful enough to overtake the bow wave and come 'onto the plane'.
Let me explain. As a displacement boat moves through the water it causes a bow wave, which limits its theoretical top speed to approximately 1.5 times the square root of its waterline length. This means a 30 foot yacht has a theoretical top speed of just over 6.5 knots. I have to mention at this point that some dinghies are powerful and light enough to rise over their bow wave and get onto the plane, however, dinghies are rarely used in passage making. So, on passage a yacht usually faces one of three scenarios; wind and tide helping to push the yacht in the same direction, the wind and tide wanting to push the boat in opposite directions, or the tide coming from the beam (on either tack). Looking at all three in turn will explain why the tide is king!
1) You're on a broad reach (the most efficient point of sail for a modern yacht) in a brisk wind and making 6 knots through the water. However, your passage coincides with a spring tide running at 2 knots in your direction - congratulations, the GPS reads 8 knots speed across the ground.
2) Now, suppose you've forgotten the beer, or (more likely) the mother-in-law feels sea sick (sailors never forget the beer) and you have to turn back from whence you came. Now that tide that was helping you along to the tune of 2 knots is now against you, cutting your speed to 4 knots over the ground - exactly half the speed that you were doing before according to Mr. GPS. However long you've been sailing with the tide, it's going to take exactly twice as long to get back to your port of departure.
3) As anyone who has rowed a boat across a river knows, the shortest distance between each bank is not to head straight across, but to 'ferry glide' angling the boat upstream against the current to arrive on the opposite shore neither upstream nor downstream of your departure point. Likewise, a sailing boat skipper must make a 'course to steer' to arrive at the chosen port taking into account the effect of the tide. For example, sailing at 6 knots on a course due east 090 Magnetic and a south going tide means that the helmsman will have to steer 070 to take into account the south going tide. Now all sailing boats make leeway at all points of sail except running dead before the wind - whichever tack they are on they slide leewards slightly - about 5 degrees for a good long keeled or fin keeled design - more for a less performance orientated yacht - even more for a catamaran! What is the relevance of leeway for a yacht sailing in scenario 3? The leeway must be added or taken away from the south going tide depending upon which tack they are on. On port tack (wind coming in over the left side) the 5 degree leeway must be added to the tide, so to go 090 we would have to steer 065. On starboard tack (wind coming in over the right side) the leeway actually reduces the effect of the south going tide - allowing us to steer 075.
