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Paul
11-19-2002, 11:48 AM
What is this and how does it happen. From what I understand it may happen when you are running and then turn up? Which brings up a question, if you are running and decide to turn back to head up (pushing the tiller to the boom) should this be done gradually or does it all depend on the wind speed?

Wild Wassa
11-19-2002, 12:11 PM
You are in a position when things can happen. It does depend on wind speed. It depends on how quickly and how well you (and crew) control the boat as well. Be ready for making the turn. The Skipper's words must be clear.

One thing about a wide turn is your not going to abruptly slow down. A centre board stall can happen. Is this what you mean by a death roll ? A complete 360o. Dinghies can turn a cartwheel could this be the death roll?

There is a delayed rolling turn, this can be a death roll, :D .

When the boat goes turtle, that's the real death roll.

Finally, when I'm run 'over the top' of by a faster boat, that's just being rolled.

Warren.

[ 11-19-2002, 01:23 PM: Message edited by: Wild Wassa ]

Tom Lathrop
11-19-2002, 12:20 PM
No Paul, the death roll is oposite of what you describe when turning toward the wind which can result in a broach. The death roll occours when the boat capsizes to windward. Several things can cause this but the most common is that when you are going straight downwind, or nearly so, the sails are well out and they develop thrust to the side of the boat opposite the boom. Can be caused by a twist in the upper mainsail, boom too far out, oscillating spinaker or sailing by the lee.

In the worst case, the boom will wind up straight up in the air, preventing the boat from righting itself if it's a keelboat or causing it to turtle if its unballasted.

Tim B
11-19-2002, 12:24 PM
Alot of the small, planing hull sailors I know use this term to describe what happens when the boat tips over to the windward side. The sail comes down right on top of you, pushing you under the water. It is an un-nerving feeling.

Usually this happens in gusty conditions. Your going along, hiked out pretty far on the windward side to keep the boat from tipping over to leeward. Suddenly the wind dies, or changes direction rapidly and the sails are empty. No heeling pressure to conterbalance your weight hiked out windward and the boat just starts rolling over on top of you.

To get the sails filled again you usually need to head up (turn to windward more). When do this, the turning moment only adds to the boats roll. So a quick turn on the rudder can be that added touch to make sure the boat is going to roll on you.

How to avoid this? Diligence first. Make sure you are aware of what the wind is doing. If its a gusty day, don't sail as close to the wind. Of course if you are racing or in close quarters that may not be an option. As soon as you feel the wind drop get your weight inboard fast. Being ready to move and moving fast is about the best defense. Resist the urge to shove the tiller to leeward. Make a gentle and controlled turn.

If you do happen to death roll - don't panic. Pushing up with your arm will lift the sail enough to get your head above water to take a breath. Swim under water to get out from under the sail. If you are dinghy sailing be prepared to take a swim - always. Its the sail in November when your sure your not going to get wet that will be the one you spill on.

John B
11-19-2002, 01:43 PM
We had a farr 1020 ( 32ft) do one in front of us in the winter racing. His kite was too high and something happened... perhaps his pole beak let go. anyway the kite was high and started to go from side to side of the boat. then the boat stsrted to roll the opposite to the kite.LOL. then it turned to custard. he went about 50 degrees to windward crash jibed and rounded up to leeward(Can you say that) dramatic. that was a kite problem.

Uffa fox always said never let the top of your mainsail go forward of the mast because it induces roll. The kind of boats he was sailing/ talking about, didn't have kickers ( boom vangs)so it was more critical then than it is now( most boats have them) I don't , my boat is old, so it's something I watch for for more than one reason.

John E Hardiman
11-20-2002, 12:18 AM
Here is a look at the physics of a death roll. When running Dead DownWind (DDW) with a spinaker up, the chute sheds vortices off each leach. If the wind is just right, these vortices will begin to oscillate, first one leach will shed a vortex, then the other. The chute then begins to move with the vortex sheet being formed, swinging from side to side as vortex lock on occurs. This side to side motion will cause an apparent change in wind speed and direction on the chute, putting the chute into a reaching condition, while the main is still downwind and effectively contributes nothing to damp any roll. As the chute oscillates, the boat begins to roll, which cause a large apparent windshift on the main also. Depending on the phaseing between the chute and the roll one of two things happens: A)the chute gets over to windward, pulls the boat over to windward, the main backwinds, crash gybes, then with the main and the pole on the same side the boat rounds down or B), the chute gets behind the main with the roll and pulls the boat over into a hellacious round up.

Now many things can contribute to this situation, following seas (which reduce the effectivness of the keel/centerboard), chute too far forward, pole too high or not guyed down properly, no twings or they're not used, tower demons (i.e. the vortex sheet downwind of a bridge tower like the infamous one by the South Tower of the Golden Gate), heavy current lines (causes a wind shift), etc... And the speed at which it can happen is amazeing. In less than 15 seconds it can put a maxi's masthead in the water.

To recover when the "rolli-pollies" start, sheet in on the main, choke down the foot of the chute, and reach up to get the main to start damping out the roll. Besides, most boats go faster reaching a little than DDW anyway.

[ 11-20-2002, 01:23 AM: Message edited by: John E Hardiman ]

John R Smith
11-20-2002, 03:56 AM
Big Moto-Guzzi motorcycles also would indulge in a "death roll" from time to time - always downhill, on a trailing throttle. The oscillations would build up, and the only way to cure it was to accelerate . . .

But I digress. Sorry ;)

John

rodcross
11-20-2002, 05:09 PM
"...then the boat stsrted to roll the opposite to the kite.LOL. then it turned to custard. he went about 50 degrees to windward crash jibed and rounded up to leeward(Can you say that) dramatic. that was a kite problem."

Could you run this by me again, slowly? Turning 50 deg. to windward, then gybing doesn't compute for me. I thought I was prepared. Now I'm feeling unprepared... Its one hell of a rudder that can do that.

You're making me nervous.

edited part: I have dumped a boat to windward on a run and ended up with the sail between me and the air...well, to be more precise...with the sail between me, plus a couple of people precious to me, and the air.

[ 11-20-2002, 06:24 PM: Message edited by: rodcross ]

David Tabor (sailordave)
11-22-2002, 06:09 PM
Well those are some pretty good explanations. As I frostbite in a LASER it was of interest to me; I "knew" what happened, just not the physics of it...till I got edjamacated.
Basically in a dingy when you are sailing downwind and ease the main, if the vang isn't cranked down tight the top of the mainsail really twists off to leeward. When the boom is almost perpendicular to the wind the top of the mainsail is actually forward of the mast. Think about this for a moment...
This means when the wind blows from astern or almost astern, the top of the sail is generating LIFT to windward (windward being defined as the side opposite the side the boom is on) Since only the top third or so is doing this it accentuates the problem by virtue of the fact that this lift is applied to the top of the mast.. Therefore the boat heels to windward,then loses lift, heels back to "normal" leeward, and the cycle repeats itself, each oscillation getting bigger. Very quickly you lose control and capsize to windward. Not fun, esp. cuz you're usually planing along pretty quickly 8,9,10 knots as verified by my GPS.
So what do you do to prevent it? Well, first you honk down on the vang to minimize twist. Then you can sheet the main in so the twist isn't fwd of the mast. And head up some, 15,20 degrees so the lift generated at the top is converted to fwd motion and not to the side of the boat. Of course when you're racing that's not going to get you where you want to go!
You also want to raise the board ALL the way up so as not to "catch" in the water when you jibe. You want to allow the boat to sideslip a bit as you come off the broad reach/run. Heavy weather sailing in a centerboard boat advises this. Although it seems counterintuitive (put the board down, lower the center of gravity) you want to diminish the area of lateral plane so the boat doesn't trip in the water. And remember: "the harder it blows, the tighter you set your lines" vang, halyard, outhaul, cunningham, mainsheet.

Quiz time. Anybody know why a "cunningham" is called that? ;) Answer tomorrow! Assuming knowledge of what a cunningham is...

rodcross
11-22-2002, 07:04 PM
Briggs Cunningham is a guy. Notable Long Island sailor and car racer. He knew that if you yanked down on the luff of a sail, you could take the draft out of the sail, spill some wind and point higher in heavy air. The trouble was, class rules limited how far you could drag the boom down with the downhaul. The masts had bands painted on them, at the top and at the bottom. If the head or the tack were outside of the bands, you went home.

He put a hole in the luff, about 10% above the boom, ran a line through the hole and pulled. What he got was more than he expected. The luff smoothed and reduced draft. What he didn't expect was the dramatic reduction along the boom.

The cunningham is an amazing device.

David Tabor (sailordave)
11-23-2002, 02:11 PM
And the kewpie doll goes to RODCROSS!! Very nicely done lad. (1958 BTW) Or so I've read; I wasn't even a twinkle in pop's eye yet!

Now if I can keep from death rolling tomorrow; winds have been real strong today. Gusts to 30, hopefully they won't all blow out by tomorrow....

Chadd Hamilton
11-25-2002, 08:59 AM
Funny this topic should be brought up, because I actually had this happen to me friday afternoon. I was sailing my 8' Acron Tender in a tidal creek and was going windward in gusty winds with the daggerboard up as not to drag bottom. Well, that was a mistake!! With the main all the way out, a nice puff hit, the boom went up and the boat headed up to weather and I nearly teetered my a** into the water. A textbook example if I ever saw one. Lesson learned.

Well, it had to look hillarious to the motorists driving by...