Saturday, March 7, 2015

Two Sets of Right of Way Rules

This post should not be necessary. I should instead write about how much fun Nina had sailing the F2 Missile in Corpus Christi, and how she said "it's so easy to schlog" (all 62 liters of it).

But a recent discussion in the Australian Seabreeze showed that there are windsurfers out there who are fully convinced that they understand the right of way rules, but actually do not. I had encountered this problem before, when a windsurfing instructor who had taught hundreds of students explained the wrong right-of-way rules in a certification course. That terrified me a bit.

Why did he explain the wrong rules? He had learned the right-of-way rules for power boats, and thought that the same rules apply to sailboats. That appears to be the case - but only at first glance!

This is the rule for powerboats:
"A vessel approaching from the port side must give way.
 In other words, the starboard boat has the right of way, and should maintain its course. Or simply "the boat on the right has the right of way".

Here's is the rule for sail boats (including windsurfers and kiters):
"The boat on starboard tack has the right of way."
That seems to be the same thing, right? A quick Google image search may even confirm this - here's a picture for power boats:
 And the corresponding image for sail boats:
That's almost the same! The only difference is that the direction that a powerboat change course is defined ("to the right"), while sail boats may change course in either direction.

But hold on - what happens if the wind comes from another direction - say, from below in the diagram above? Nothing changes for power boats, since they don't care about the wind. But for sail boats, this happens:
Now, the sailboat on the left has the right of way, and the sailboat on the right must evade! Here's the little thing that's easy to miss:

  • For powerboats, the right-of-way rules refer to starboard and port as defined by the front and back of the boat 
  • For sailboats, the right-of-way rules refer to starboard tack and port tack, which are defined by where the wind comes from! If the wind comes from starboard (the right), we are on starboard tack; if the wind comes from the left, we are on a port tack.
So for two sailboats on opposite tacks, the powerboat rules (which are defined relative to the boat) will give exactly opposite results from the sailboat rules (which are defined relative to the wind) about half of the time!

I can already hear the argument "but in the image above where the wind comes from below, both windsurfers are going far downwind - that never happens on the water!" True enough - windsurfers mostly sail on  a beam reach. Let's look at diagrams for two boats on a head-on collision course, then. First, here is the powerboat diagram:
Note that both powerboats are supposed adjust their course to the right. For sailboats, the diagram is different:
Here, the boat on the right is on starboard tack and supposed to hold the course, while the boat on the left has to change course. If the wind comes from below instead of above, the roles reverse:
So far, I have talked about "right-of-way rules". That is how these rules are often referred to, but unless you are sailing in a regatta, a better name would be "what-to-do" rules. In the head-on collision case shown above, it's quite obvious that it really helps if both sailors know what they are supposed to do. At least one sailor has to change course to avoid collision; but if both sailors change course, there's a good chance they are still on collision course afterwards! If you often windsurf in crowded conditions, you've probably experienced what this leads to - both of you in the water!

Fortunately, there is a very simple rule for windsurfers to remember: if you are on a collision course with someone on the opposite tack:
  • Right hand is closer to the mast: keep your course
  • Left hand is closer to the mast: change your course to evade collision
Keep in mind, though, that there are plenty of windsurfers (and kiters) out there who never learned the right-of-way rules for sailboats, or who think that powerboat rules also apply to windsurfers. In case of doubt, do whatever it takes to avoid collision. Common courtesy suggests to give beginners and all sailors who are obviously struggling plenty of space, regardless of "right of way".

Just to be complete, I'll end this post with two more right-of-way rules. The first one is that a boat passing another boat on the same course has to stay clear:

Finally, if two sailors are on the same tack, the leeward sailor has the right of way:

There are plenty of sources on the web that explain the right-of-way rules. Some of the powerboating sites fail to even mention that different rules apply to sailboats; others, like boatus.org, have at least a brief explanation. One good explanation that I used as the source of most of my diagrams is http://spinnaker-sailing.com/online-courses/lesson-3.

2 comments:

  1. That is a really good description of why a simple rule is so often messed up.

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  2. I think it explains the behavior of many windsurfers and kiters who used powerboats before windsurfing or kiting. But there are others out there who do not know any right-of-way rules on the water.

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