Calling Snowleopard; Laser death rolls, Aerorigs and that.........

[cameronke]

I have been thinking about Snowleopards solution to Jimi's "Laser death roll" problem all day.

If you continue to take the boom forward of the mast the air flow will improve across the sail to the point at where it will become an effective airofoil again rather than a stalled sail. The healing effect would be to windward would it not? Therefor the sense in sitting on the same side as the boom.

Anyway, then I remembered that Snowleopard has an Aerorig (Doesn't she?). How many degrees can the Aerorig rotate? And have you experienced this effect?

Regards
Cameron

 

[snowleopard]

A sail functions best at an angle of attack of around 25 deg. to the wind. It generates lift at right angles to the wind. With a stayed rig the sail can be eased out to function at the optimum up to about a beam reach; beyond that the sail can go no further so the angle of attack increases and the sail becomes stalled, reducing drive. On a dead run the sail is at right angles to the wind and there is no aerofoil effect.

With a fully-rotating rig (laser, freedom, aero rig or my freewing) the sail can be set at 25 degrees to the wind onto a broad reach. once the wind is aft of the beam the drive pushes the rig to windward and the boat does indeed heel to windward. The effect is not, fortunately, felt on a cat!

On a dead run one could still set the sail at 25 deg. to the wind but the thrust would be at right angles to the course so no progress would be made. The sail must therefore be stalled for a dead run though a few high performance boats can tack downwind faster than running.

The laser death roll results from air flow alternating direction over the sail when it is square to the wind. Allowing the sail to go out beyond 90 deg. ensures that the flow is always in the same direction. The drive is not improved, in fact it is slightly reduced, but you tend to get there quicker if you spend less time swimming.

 

[claymore]

Whislt there is much truth in what you say - I do think you need to factor in critical mass - particularly when its Wee Jamesies hairy and laggardly arse that has probably lingered over lang on the wrang side.

 

[ChrisE]

<<<< If you dont get wet, your not having fun! >>>

Maybe on your puny boat but we pride ourselves on our ability to sail in carpet slippers in all conditions.

 

[pteron]

[ QUOTE ]
A sail functions best at an angle of attack of around 25 deg. to the wind. It generates lift at right angles to the wind.

[/ QUOTE ]

Shurely you mean at right angles to the average chord of the sail?

 

[snowleopard]

No, I meant around 25 degrees. The angle of the resultant force on a sail is very complex. It is the sum of an infinite number of vectors acting on the 3-dimensional surface of the sail. 25 degrees is a fair approximation for an average sail set for close-hauled work. There is a diagrammatic explanation of what I was saying here

 

[pteron]

You are talking about angle of attack, I am talking about the the resultant lift vector which by definition is at right angles to the chord.

The first line on that page says "The wind passing over a sail generates differential pressures, acting at 90 degrees to the surface" which is not "lift at right angles to the wind".

 

[snowleopard]

[ QUOTE ]
... the the resultant lift vector which by definition is at right angles to the chord...

[/ QUOTE ]

Please elaborate. I'd be interested to see the definition you refer to.

 

[pteron]

Arrg, I thought I'd got you!

I am confusing angle of attack with the lift vector, which of course is relative to the apparent wind.

As punishment, I shall dust off my aerodynamics texts and reread three times.

 

[Cobra]

[ QUOTE ]

The laser death roll results from air flow alternating direction over the sail when it is square to the wind. Allowing the sail to go out beyond 90 deg. ensures that the flow is always in the same direction. The drive is not improved, in fact it is slightly reduced, but you tend to get there quicker if you spend less time swimming.

[/ QUOTE ]
Thinking back to the time I was sailing Lasers, running by the Lee was an often used technique when squeezing up to a mark, it being possible to keep the boat moving quite quickly in breezes up to about a force 4 with the sail let out well ahead of the 90 degrees thus not having to waste time gybing in on a mark. Technically you can also maintain the Starboard gybe advantage which again obviously can have its benefits!
Running a Laser by the lee in anything over a force 4 doesn't work too well...in fact the world tends to go green(brown on the East Coast!) and the air tastes wet!