Optimum sail angle

A

abenn

New Member
I'd be interested to hear if anyone has any theoretical backing for what the optimum sail angle is on a laser upwind. The general rule is that in any reasonable wind the mainsheet is pulled block to block with the traveller tight, so that the traveller block and sail clew sit as close to the deck and the gunnel as possible. Its said that the traveller should be kept tight both to keep the boom low as well as stop the traveller block from coming inboard. How come this is the best position for the sail ? Was Kirby such a brilliant designer that he worked it all out this way ? ie the width of the hull at that point.

From a simple point of view, the forward drive of the sail comes from the relative angle of the sail to the wind and I appreciate how there's an optimum angle of the sail to the wind, to get best VMG, no stalling etc. But given the sail is at this angle to the wind and on the gunnel, what would be the problem with rotating the hull under the sail to point higher, ie pulling the sail closer to the centre line of the boat. The sail should be "driving" just the same, but now the boat is pointing higher ??

Hmm maybe not, - the sail is obviously carried by the boat, so if the boat starts pointing higher, the sail goes with it, so even if it stays at the same angle to the (real) wind, the apparent wind will move forward i.e. luffing.

I haven't sailed on other boats, but I do see pictures of bigger boats with the main virtually down the centre line, which seems to be the best way to point high. Anyone got any references on why this isn't so for the laser ?
 
abenn said:
I haven't sailed on other boats, but I do see pictures of bigger boats with the main virtually down the centre line, which seems to be the best way to point high. Anyone got any references on why this isn't so for the laser ?
Click to expand...

Sorry, no references, but that raises an interesting question. What happens if you put a real traveler on a Laser. I have one on my Europe and I can pull the mainsail to the center line or above. It points like a dream.
 
Don't forget the action of the centreboard (in terms of direction of movement and opposing the leeward forces from the sail). If you rotate the hull to windward (i.e. moving the boom further inboard to point the hull higher into the wind) you are also rotating the centreboard.

Ian
 
Moving the end of the boom closer to the center and eventually putting it on the center line would stall the small dagger board. It's very similar to the situation in the Finn, the board isn't big enough to maintain lift when the sail is at that close an angle to the wind.

I sail E scows mainly, as most know they have bilge boards/lee boards. We routinely have the main traveler on the center line of the boat and in some case in lighter air above the center line (only to ease the sheet and induce a ton of twist for good acceleration). However the big difference is both in placement of the boards and sheer size. our boards draw nearly 4 feet of water and are over 2 wide from front to back, it makes a huge difference.
 
I think it's also a question of leach... as usually is on a Laser.
Let me try to explane:
Starting from a block-to-block sailing condition, let's image (or try) to move the blocks towards the center of the boat, what will happen is that the aft end of the boom will be lifted some inches up in reason of the forced inclination of traveller line with a loss of mast bending and so with an impact on leach setting (don't forget that a laser sail is normally manufactured with a "default" hooked leach).
Thinking to cover that loss of bending force using the vang will move the boom out again as well as reduce leach tension.

Ciao
 
You are correct that on may boats they have the boom on the centreline and this is fast - but if you try this on a boat such as a laser this will not work- you will stall the boat completely.

The main reason is that the boats that have the boom on the centreline will have a foresail - this has the effect of changing the airflow over the main which allows the main to be brought onto the centreline. Such a rig will point higher than a single sailed boat such as a laser.( If you look at the top of the main on such a rig you will see that it is actually in the same position as a laser ( it has a lot of twist) as it does not have the effect of the jib).


So keep the boom on the corner of the transom and you are about right- try to bring it in closer and you will kill it!!
 
oatsandbeans said:
...The main reason is that the boats that have the boom on the centreline will have a foresail - this has the effect of changing the airflow over the main which allows the main to be brought onto the centreline. Such a rig will point higher than a single sailed boat such as a laser...
Click to expand...
Isn't the forward drive in this situation coming primarily from the jib? The angle of the jib relative to the hull is giving it the forward component of that sail's lift.

In a cat-rigged boat such as a Laser, the lift from the sail is more or less perpendicular to the sail (unless there's a ton of twist). If the boom is at centerline, the lift is perpendicular to the hull - straight sideways - so no forward drive on the hull, and everything stalls.
 
The sail on the Laser is much flatter than on those bigger boats where you bring the boom to the center line. I wonder if a flatter sail needs to be eased to develop power. A Sunfish (the old style sail is completely flat) has the same issues as the Laser -- bring the sail in too far and you lose all power.
 
Master Mike you are correct that when a laser sail is pinned in close to the centreline, the resultant lift has very little forward component to it that is why the boat "parks". Re the two sailed boat, the two sails affect each other- the mainsail leach makes the wind in front of the jib be lifted (I know that this bit sounds wierd!),- and the main delays separation on the leeside of the main, which allows it to be sheeted closer. The upshot of this is that the two sails enhance each other and the comnination foil that is produced is more efficient than would be expected from looking at the individual roperties of the two sails. I hope that this makes sense.!!
 
When boats with jib/genny sail in after a race it is noted that the expereienced sailors let the main out over the corner of the transom when the fore sail is dropped. This was the case with everything from J/24 to Snipe. Those who kept their main in the middle were soon passed in the important race to the hoist. When Dennis Connor's jib ripped in that windy AC race off Perth, he first had the traveller dropped down. The jib bends the wind. Without it, like on a Laser, the one air foil must be out more.
 
Any Sail without a jib in front of it acts like a jib and therefore cant be pulled to the centreline. Boats with jibs that dont go to the top of the mast have loads of twist in the sail above the point where the jib attatches to the mast. This is because the Jib directs the airflow around the sail.
 
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