Downwind sailing and the evils of rounding up
One of the common pitfalls of sailing a dinghy downwind is that in a sudden gust, the boat will start to head up, leading to a capsize. In this blog post, I'd like to share some of my thoughts on the factors in play and how to best deal with them. This discussion applies to sailing with just main and jib, as well as to those thrill-seekers using a gennaker.
To begin with, why does your dinghy insist on heading up in a gust? There are (at least) two factors involved. To begin with, your dinghy typically has a bit of weather helm: to get technical, the center of effort (COE) on your sails lies slightly behind the center of lateral resistance (CLR) created by your dinghy's foils. In normal conditions, a bit of weather helm makes for a nice responsive tiller that helps you to better "feel" the boat and the wind. However, as the force on the sails increases as happens during a gust, the discrepancy between the COE and CLR is amplified, causing more and more weather helm. This effect will cause your boat to want to head up more strongly during a gust.
However, the natural weather helm caused by the discrepancy between COE and CLR isn't typically what will cause you problems. Instead, it is the additional weather helm caused by excessive heeling. Indeed, as the boat heels more, the hull's interaction with the water creates extra weather helm. Furthermore, your rudder is now only partially submerged, giving you less control over rounding up. And that is just the beginning: assuming that you are sailing below a beam reach, the more the boat rounds up, the more the heeling force on the sails increase. This causes more rounding up, which causes more heeling which causes... you can see that this will probably end with you getting wet.
Now that we know two of the main factors in play, let's consider three different scenarios.
- First scenario: you're sailing on a dead run when a gust hits. The force of the wind on the main will indeed cause the boat to want to round up a bit. However, since you're on a run, it won't cause much heeling, so you only have the first aforementioned factor to deal with.
- Second scenario: you're on a broad reach when a gust hits. Now you're in trouble -- you have both excessive weather helm coming from the force on the main, as well as weather helm caused by heeling. As you round up towards a beam reach, you can't depower the main, and you may end up capsizing.
- Third scenario: you're on a beam reach when the gust hits. Again, both factors will apply, but as the boat rounds up above a beam, the main will start to depower, reducing heeling and weather helm and saving your bacon.
From the above analysis, we see that the danger of rounding up and capsizing is greatest when sailing close to a broad reach. But never fear: we have some powerful weapons at hand in combating the evil demon of weather helm.
First, we would like to be able to depower the main sail while sailing on a broad reach; this will decrease both natural weather helm and excessive heeling. Our first weapon for doing this is the vang or gnav. Easing this control in a gust lets the top portion of the sail twist away from the wind. While crewing for one of our club's most experienced dinghy sailors during the infamous Fast Track squall of May '12, my skipper had me blowing the gnav in every gust, and this is what let us limp back to the club in (more or less) once piece.
Our second weapon in depowering the main is of course the main sheet. If the wind is howling 20 knots and your Bahia is screaming along on a broad reach, chances are that your high boat speed is shifting the apparent wind far enough forwards that your main sheet should not be all the way out. The added bonus is that when the gust comes, you can now ease the sheet to help balance the boat and depower the main. If you instead neglected to sheet your mainsail in at all, you can't depower by easing the main!
Apart from depowering the main sail, the above analysis tells us that we want to do everything possible to keep the boat flat. Anticipating the gusts and aggressively falling off is key. Likewise, hard, aggressive hiking (or even sending crew out on the wire) are also important. Keep in mind, though, that if you are already hiking out hard before the gust hits, you won't be able to up the ante when you need to. Just as with the main sheet, it is highly advantageous to have a bit of room to make adjustment during gusts. Finally, aggressively sheeting in the jib (or gennaker) in gusts so that it stalls a bit will shift the COE forward in the boat, which will also decrease weather helm. Likewise, raking the centerboard aft slightly will shift CLR aft, also decreasing weather helm.
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Comments 3
Great post, Nathan.
One thing that I can't figure out is sheeting in the main on a broad reach. I guess the apparent wind has to be enough forward that the sail has flow on both sides, so that when you let the sheet out you depower it. But if this is the case, you'd be luffing a bit with the sail too far out, no? And if the apparent wind is far enough behind you that you don't have flow on both sides, then power goes as exposed (cross-section) sail area, so letting the main out increases power on the sail, the opposite of what you want.
Am I reading this correctly, or (most likely) missing something?
John
John:
Thanks! First of all, let's suppose that you've sheeted in the main "too far" so that the sail is stalling a bit. You are absolutely correct that easing the sheet will increase power on the sail. However, you will be decreasing the heeling force on the boat, which is the important thing to do to prevent rounding up.
Now, I frequently see sailors at CSC heat up from a run to a broad reach without touching the main sheet, and indeed in the right conditions, the main starts to luff a bit. This seems to be an especially frequent occurrence when the kite is up. So yes, it is in such situations that you certainly want to sheet in.
I might occasionally sheet the main in a little when heating up to a broad reach even if the main isn't luffing. This could partially stall the main a bit and kill boatspeed a smidgen, but by being able to play the sheet I feel I'm much better able to balance the boat. In any case, dumping the main in such a situation will also help avoiding rounding up, as mentioned above. Play around with this and see what you think!
Nathan
To reinforce something you mentioned but didn't highlight.
I was out today with James C (great, wet experience), and he showed me a way to steer downwind, keeping the boat fairly flat. It's bascially what I had learned in the gennaker class last year (but James and I weren't flying the kite at that point) - if you're over-powered, go down, if you're under powered, go up. All with the tiller. It worked nicely (RS-500) without having to move a lot of weight around.