Let's Get to the Bottom of the Lee-Bow Effect Once and For All

[Dockhead]

About a year ago, we had a great discussion on here which went right to the bottom of the question of how to cross moving bodies of water -- the principle of constant heading crossings and calculating course-to-steer. Hats off to major brains Seaworthy Lass and Andrew Troup who did amazing work, getting all the way into the math, creating mathematical proofs, and Seaworthy even came up with an original technique for how to do it without a computer!

Why don't we turn the same kind of attention to the Lee Bow Effect, which has puzzled me since I started sailing in the English Channel? There is no really clear consensus on whether it is ridiculous nonsense, or whether it really works.

There is a fair amount of public information arguing that it is a myth. See for example:

Destination One Design - Preparation

https://groups.google.com/forum/#!to...ng/TFWS6xe55KQ

June | 2008 | Campbell Sailing

Now, we need to define exactly what we're talking about, since there are several layers to this onion called the lee bow effect, and several possibly different phenomena.

1. Sailors tacking upwind across the Channel sometimes tell you that you should keep the lee bow into the tide. They say that like that you shorten your passage and arrive faster, because the tide takes you less far off the rhumb line so your passage is shorter.

I submit that this is obvious nonsense, for the same reasons we got into when discussing Course To Steer across moving water. Your boat sails through water, not on the seabed, which it doesn't know, so the distance over ground sailed is irrelevant. Your boat only feels the distance sailed through water, and if that distance is identical, then it makes no difference whether you tack with or against the tide.

2. Some racers say you can get a lift from the tide pushing against your bow from the lee side, holding you against the wind and allowing you to sail closer to the wind.

I think this is also nonsense, and I agree with the arguments in the last article linked above.

3. If your boat could profitably use a bit more wind, then you can get a lift by tacking so that the tide increases apparent wind during that part of the passage you really need it.

I think this one is true. Of course the apparent (actually, true) wind is increased when the current and wind are in opposition, and is reduced when they are together.


There are probably some other variants of the Lee Bow Effect.

Discuss!

Paging Seaworthy and Andrew!