How to Use the “Snow Fence Effect” to Gain Downwind

Overtaking downwind can be tricky, and passing even one boat can take a long time and a fair amount of effort. But if you are down the fleet there is a way that you can make quite a few places with minimum fuss.

I was racing quite recently and rounded the windward mark towards the back of the fleet.

(Aside: It is starting to feel like I am painting a picture of myself as a particularly bad starter. Someone who can creatively innovate many new ways to be down the fleet after the first beat. I mean, there's this incident that I'd rather forget; or this thoroughly ill-considered plan; or this episode of righteous indignation that turned out to be not that righteous after all; I even have video evidence of my bad starting.

I reckon the reason I write so much about my bad starts is that they throw up more useful lessons than the races where I start well, sail well, and finish well. But maybe I should do a bit of work on my starting too.)

As I rounded the top mark and bore off onto the run (we were sailing a windward/leeward course) I could see the boats ahead of me on the leg. As is quite common, they were spread in an arch heading down to the leeward mark to the right of the direct course to the leeward mark.

A Chance to Gain Downwind

The fleet have all gone right on the first run.

This normally happens as the boats behind try to sail on the wind of the boats in front, while the boats in front try to protect their lead and sail in clear air. Luffing matches, or at least the beginnings of a possible luffing match, all lead to boats sailing above the rhumb line.

If you are down the fleet this is an opportunity to gain places.

In this instance the opportunity was there for me. I bore off and gybed onto port and went out to the other side of the course, well away from the boats ahead, and went hunting for the best breeze and boatspeed I could find on the left of the run. There are a couple of reasons for this.

The first reason is that it means I won't get caught up trying to overtake one boat. If I concentrate on attacking the boat ahead's wind to try and overtake them the chances are that, even if I do this well, I will gain one or possibly two places on the run. On a really good day I might even get lucky and gain three or four places by the time we round the leeward mark. But realistically I'm looking at a possible one or two places gained as I get tangled up in trying to sail past to windward, or slipping by just to leeward of the boat in front.

Whereas, if I avoid direct confrontation, I have the opportunity to gain more positions by having better downwind speed and allowing the others to slow each other up as they attack and defend against each other.

Which leads me to the other reason for heading to the other, uncongested side of the run: the snow fence effect.

Wherever there are a few boats close together there will always be less wind, even on the windward side of these boats. It is most noticeable at the start, where you get a lot of boats lined up together. Of course there is less wind behind these boats, hence the need to avoid being in the second row at the start. But there is also less wind to windward of the start line than there is elsewhere on the course. and this is well demonstrated by a snow fence:

The Snow Fence Effect can help you gain downwind

A good example of how wind acts when dealing with an obstruction

As you can see from the picture, the wind approaches the obstruction and then tries to find a way through or past it. this leads to it flowing up over the obstruction, and there is an eddie or backdraft on the windward side. If you look at a snow fence on a windy, snowy day you'll notice that the snow is lower immediately to windward of the fence, as the backdraft blows some of the snow back away from the fence.

What this means for us as sailors is that, if we can avoid being caught in this effect then we will be sailing in better breeze than those boats that are caught in a group - even if their wind isn't directly being blocked by the boats behind them.

On the day in question I got myself well clear of the fleet and concentrated on sailing fast. By the leeward mark I was up to second, slipping inside third position as I got water at the leeward mark (another benefit of going left on the run).

I had gained eleven positions on that one downwind leg.

There is no way I would have achieved that if I had engaged with the boats ahead of me like the rest of the fleet had.

There are, of course, some other considerations to take into account when deciding which way to go on the run. In this instance there was no major difference in breeze across the course; the current was negligible; the fleet had all, to a man, gone right. But one of the benefits of the run (as opposed to the beat) is that you can change sides of the run fairly quickly if you have badly misjudged one of these factors.

So next time you round the top mark down the fleet, give some thought to the snow fence effect and how you can use it to gain downwind.

One thought on “How to Use the “Snow Fence Effect” to Gain Downwind

  1. Great post – I have never heard of the snow fence effect, but it maks a lot of sense.

    Another reason to go left may also be to avoid the sheep mentality of doing what everyone else is doing (“everyone is doing it, so it must be the right thing to do”). I generally try to focus on what the best sailors are doing and, then if I understand (or think I do), I will follow suit. But I admit, it sometimes takes a strong will to ignore the thundering herd, and sometimes the thundering herd is right.

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