It’s that almost uncontrollable rolling of the boat from side to side that happens when running dead downwind. Often resulting in a windward crash gybe (a.k.a. A Chinese Gybe) and you in the drink.
The vortex that peels off behind the sails sucks your sails to one side, this motion heels the boat over, the flow over the sails changes and it then goes back the other way. This oscillating motion get bigger and bigger until you either tip over or you take action to restore the flow over the sails to there normal flying state.
Von Karman Vortex Street
This peeling off of eddies, or shedding of vortices is known as the Von Karman Vortex Street.
This same thing happens behind a truck at speed, which you may have observed as the trailer unit of a truck sways from side to side, or you may have even felt the buffeting from side to side when driving close behind the truck.
The oscillation of your boat from side to side builds up more and more as each vortex peels off and becomes larger, having a great pulling force. Your keel acts like a pendulum underneath and helps to maintain the rolling. The movement amplifies and before you know it you’re in the drink (possibly even swimming). Typically your boat will have turned to leeward and capsized to windward. The mast will have come to windward which places the sail force vertically to windward of the centreline, this makes the boat want to turn the boat to leeward, also as the boat is heeled to windward the shape of the boat also promotes the turning to leeward.
Sometimes the simple action of removing your spinnaker pole can release a vortex which initiates the Death Roll.
The oscillation of the Death Roll can also be initiated by a movement or action. This could be how your boat is moved on a wave or how the boat is being maneuvered.
It’s just as important to know how to avoid allowing thedeath roll as it is to know how to get out of it.
The root cause to the Death Roll starting is to do with air flow. When you have your sail right out when running dead downwind and they just act like a parachute. The wind pushes into them and air spills around the sites, it’s this spilling of air that can incubate the vortex street.
Don’t let the Death Roll take your Soul
The alternative is to have you sails flying with air flowing across them. Typically from the front edge (luff) of the sail to the back (leech), but it could also be from the leech to the luff (sailing by the Lee). The important thing is to have the air steadily flowing in one of these directions. With an air flow across your sails there is no way the vortex street can develop, and thus no way the death-roll can start.
On a keel boat – Avoid sailing dead downwind
Just don’t do it. If you look at most boat polars you’ll see that going dead down wind is slow. Head up a few degrees, induce a flow across your sails. You’ll avoid the Death Roll and your sails will produce more force enabling you to go faster.
Everyone likes going fast
In a Dinghy – Sail by the lee
To maintain air flow across your sail in a dinghy learn how to sail by the lee and how to transition into and out of using it.
Sailing by the Lee
To do this when running downwind set your main sail up so that the air flow is from the leech to the luff (back to front) of the sail. To control in gusts you’ll need to sheet on to pull the leech closer to the wind to reduce the sails angle of attack. Instead of letting the kicker (Vang) off – pull it on, this will prevent the leech from blowing out and increasing the angle of attack in a puff.
If you feel the roll is developing or you are in conditions where it’s more likely than normal for the Death Roll to develop there are some things you can do, and there are some myths which you should not do.
Pull on your main
Sheet that main in more than you think. By pull it in you make it easier for the air to flow across it (from Leech to Luff), and it shuts down the size of any vortices that can be created. Also as mentioned in sailing by the Lee pull on your kicker (Vang).
Spread your weight out
Really this is a bit like being on a merry-go-round. Remember how you go faster when you are closer to the centre, and slow down as you move your weight out. You might also remember that it was harder to push when everyone was at the outside, but easier when they were in the middle. Your boat is the same, if you are near the middle it’ll easily roll, if you spread yourselves out to either site you’ll have a greater moment of inertia and it will be harder for the rolling to develop.
There is no point trying to counter the roll by trying to move quickly to counter it (especially on a keel boat), you’ll need to use all of the boats dynamics to get it under control.
Anchor the Spinnaker Down
If the shoulder of the spinnaker can freely move it makes it an easy target for the vortex street to get to work on rolling the boat. Pull the spinnaker in down and in close by; sheeting in, pull on your barber haulers, and lowering the pole. Also set kite a little fuller than normal (pool forward) to ensure that the flow across it is kept constant and that the luff doesn’t collapse.
You can also set up your main so the air flows from the leech to luff and then into the spinnaker creating a steady air flow.
What to do when you’re in the drink
Smoking the Halyard
On a keel boat, or any other boat with a spinnaker, then one of the best ways to remove the force from the spinnaker trying to pull the boat over, or keep it tipped over is to release the spinnaker halyard, letting it out. Keep hold of the sheets so you can easily retrieve it.
The San Fransisco/Californian Roll
No it’s not a type of sushi, and you can’t eat it, although doing one will feel like you are. As your dinghy has tipped over to windward (in that the boom is up in the air), and to leeward in that it has tipped downwind – you’ll probably find that your mast and sail have sunken in quickly and you are close to upside down almost immediately. Now you can try to pull your boat back up the right way by pulling it up normally with the sail on the downwind side, but you’ll probably find this slow, very slow as the boom is not trailing the mast in this motion, instead it’s probably folding out to one side and thus the main sail is holding up your recovery. instead you can do a San Fransisco Roll where you pull the boat up with the sail to windward, and then expect it to be blown over straight away again, so you keep a hold of the centerboard as this happens, getting pulled under the water and then pop up on the windward side of the boat with the boat now tipped over like normal (mast and sail to leeward), which you can now right like normal.
Cavitation happens to foils at high-speed. It has limited boat speed for years, including the catamaran’s of the 34th Americans cup. But now the game is about to change as Paul Larsen’s SailRocket has smashed the sound barrier of sailing.