Preventing Deathrolls

R

rockingaswake

New Member
I Just bought a laser used and took it out sailing last Sunday. There was between 15 and 20 knots of wind and a few times while running, the boat becan to rock uncontrollably and deathroll. Any specific cause for this, maybe too much vang. I've been sailing since i was 8 (I'm now 17) and i have never experianced this rocking with any other boat.
 
probably too mutch vang... try to keep your boom to a maximum 90 degre angle with your mast... else your CE CLR is gonna be bad & it will be rocking... also try healing you boat a little to windward to center CE over CLR... I don't have time to explain very well... cheack out drlaser.org there are probably some hints on that..
 
no no! NO VANG or very little at least! watch your leach and dont let it flutter about and start to roll in! jus put your centreboard down a little more and keep your balance!
 
Next time your running downwind, with the boom out at about 90deg, have a look up at the top of the sail. All sails have some degree of twist, so that you'll probably see the angle of the sail at the top is more than 90deg and could well even be beyond 90deg ie pointing forwards rather than back.
Now imagine what happens when you get a strong gust. The wind hits the top of the sail and deflects off to leeward. It obviously also hits the bottom of the sail, which is square and pushes you forward. But the strong leeward push from the top of the sail, which is also at the end of a long lever arm (your mast) creates a strong torque which pushes the boat over to windward.
If the vang is loose there will be more twist and so this effect is exaggerated.
So to prevent this, sheet the main in so the boom is not near 90deg, pull it in until you can see the top of the sail is also below 90deg. This is obviously slower but safer.
Also you can keep the centre board down. Again this is slower but creates a bit more counter torque to stop the roll. In a real death roll situation though it just gives you a bit more time to react.
The alternative is to leave the boom/sail out (at near 90deg) and sail by the lee, ie bear away so the wind is now flowing in reverse over the sail. Then you can't get that deathroll torque. There are heaps of other posts and drlaser/rooster info' on BTL sailing.
 
From my experiance and tips from my coach, the best ways to prevent deathrolling are:
-Don't let your sail past 90 degrees this includes the bubble in your sail caused by letting the outhaul off.
-Tighten your BoomVang, not enough vang causes the wind to "spill" out the leach making the boat rock violently back and forth.
-Drop your dagger board more, the extra board in the water stabalizes the boat preventing rocking.
-If you are still having problems controlling your boat i would suggest depowering your sail (Tighten your outhaul and cunningham). If the previous tips did not help it sounds like you are really overpowered, and should depower your sail until control can be maintained over the boat. Hope this helps.
 
The fundamental answer is "don't sail dead downwind".

One of the reasons that the boat starts rolling back and forth when you're sailing dead downwind is that there is no established flow over the sail. But, as the boat rolls in one direction, the sail sweeps through the air and flow is established. This generates a lift vector (force) which points forward and in the direction the sail is moving. The second component of the force (pointing in the direction of the roll) actually drives the mast further in the direction it's already moving. Heroically, you get the roll stopped before the boat capsizes (with the mast pointing dramatically somewhere off to the side). What happens next? The boat starts to roll back upright - whew you saved it... Guess what, the same thing occurs, only this time in the other direction. The new lift drives the mast in the other direction, only this time the roll starts from further over and has more distance to build up. Sooner or later your heroics aren't enough anymore and you crash.

How to get better?

First, study Steve Cockerill's article "The 4th Dimension - Downwind Sailing":
http://www.roostersailing.com/articles/4TH DIM WEB ARTICLE.HTML
He presents a very different way of dealing with the problem:
What do you do? [...] Do you push the rudder away from you (head up) or pull it toward you (bear away)? For most of you it is probably inconceivable to pull it towards you, but that is actually the right answer!​

Next, make sure your vang isn't too loose. If you're an ILCA member, Mark Jacobi's article, "Ideal Vang Tension Downwind" is a good place to start:
http://www.drlaser.org/ILCA/VangMJ.html

Another good downwind article is Ed Adams' "Zig Zag Your Way from Top to Bottom":
http://old.cruisingworld.com/zigzag.htm

Finally, go out and practice. A lot of it is balance (if the boat doesn't start rolling the problem never develops, even if you're sailing dead downwind).

Good luck. Sailing downwind is one of the most challenging points of sail. And, unlike upwind, body-weight isn't all-important.

Cheers,

Geoff S.
 
while i was in Youthfest 04' (strongggggggg winds)... i found something. if you do not empty your cockpit with water. than its stables you... no dethrols, then, i capsized and the water emptied... then deathroll, deathroll, deathroll!
 
iain_CAN165061 said:
while i was in Youthfest 04' (strongggggggg winds)... i found something. if you do not empty your cockpit with water. than its stables you... no dethrols, then, i capsized and the water emptied... then deathroll, deathroll, deathroll!
Click to expand...


I strongly disagree with the above.

The extra weight slows you down. A lot - while others are surfing down waves or planing on reaches, you will be dragging 100lbs or more additional weight. This makes the hull unstable, not more stable as the force trying to drive the hull forward is absorbed by the rig. That energy has to go somewhere and it's usally in the form of increased loads in the mast (ie perm bends) or increased force slightly off center

The cockpit full of water moves around, in reaction to the hull heeling. This extra movement will hardly ever be in concert with movements you may be making to balance the rolls.
 
while i was in Youthfest 04' (strongggggggg winds)... i found something. if you do not empty your cockpit with water. than its stables you... no dethrols, then, i capsized and the water emptied


Note the last point after "then" I think that it says it all!
 
w.e man, im talking 3-4" of water... not the whole bloddy cockpit. and hav u tryed it? once ur surfing... there isnt much of a difference. and believe me, if i herd it from some retard "iain_CAN165061"... id b thinking.. 'what a hack'. but reli, there is a huge difference in stability! believe it or not.
 
well, i only weigh 120, so i guess that makes up for it. and it cant b 30 lbs... u gotta b kidding me. double check your measurments.
 
I'm satisfied with my original measurements, why dont you get off your arse and attempt to prove me wrong?
 
couse i dont hav the time right now, bogged down with school, how bout bysaturday night.... start counting down the minutes.... : )
 
There is a lot of argument on this, but you should try bagging your sail a little more and raising your centerboard about 6-8 inches. If you watch Robert Sheidt this is what he does(along with perfectly sliding through the valleys of the waves in a Godly way)

hope that helps
 
Darryn is about spot on with his weights
1 cu. ft. of water = 62.425 lb.
1 cu. ft of water = 7.48 gallons

I don't have exact measurements on the Laser cockpit but my guess is roughly 36"X22"
 
well i didnt read all these replys but the best thing i can say is look back. if you look back in heavy air you can clearly see the puffs and just tighten up your vang a little so it tightens up the top of your sail and when you see a puff tighten up your main a little. most people say thats not a good idea and i agreed tell my coach gordy bowers told me that. and it works!! and also if you start to death role dont head up head down a little NOT MUCH!! good luck dont go bending masts!
 
The faster you go, the more force is exerted on the hull the more initial stablity your boat has, of course, things happen faster and wipeouts can be more brutal this way, but you may as well go for it, if you try to slow down the rig gets loaded up and the boat gets unstable. Another thing, you definately never want to sail dead downwind in a laser. For one thing, its just slow, in any breeze. When the wind picks up, it makes the boat very unstable, with a rocking motion that you can't really determine which way its going to go. Crack off a bit by the lee or come up to a broad reach. Personally, I think being by the lee is a lot faster. Keep the boom in at 90, when you are by the lee, the leech is the leading edge of a foil, so you don't want the boom perpendicular to the wind direction. The same thing goes for gybing, don't slow down, this loads up the rig, you want to gybe at max speed, when there isn't much pressure on the rig.
 
-The more vang, the less twist there is in the sail, so there is fewer chance to deathroll

-When sailing with strong winds, don't pay out the rope too much

-When rolling, tighten your rope, or steer to luff.

-Centreboard up!!!!! It's the same effect when you take a knife and try to move the knife through the water, with high speed. A little bend is enough to 'capsize' the knife.

sorry for bad english.... ;)
 
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