HOW SAILS WORK

Sails take energy from the movement of the wind and convert some of it into energy of boat motion. Sails are FLOW OBSTRUCTIONS which change the DIRECTION and SPEED of the passing air to generate a LIFTING FORCE.
A sail is not concerned with the true wind- the wind blowing past an anchored boat for example. What matters is the APPARENT WIND which is a combination of the TRUE WIND AND THE FORWARD MOTION OF THE BOAT.

DANIEL BERNOULLI [1738] first noticed how changes in the flow speed of a fluid [air-water] were related to pressure changes.
............Pressure in air drops as its speed increases
............Pressure in air rises as its speed decreases


The venturi tube is a funnel through which air passes. If it is the right size and shape, it improves the air flow by increasing its speed, lowering its pressure and ironing out turbulence. Our sketch shows a normal venturi- note, however, that the gap between jib and mainsail CAN HAVE A VENTURI EFFECT.






[1] Lift is created when a fluid flows around an asymmetrical object because more flow is diverted to one side causing a drop in pressure.




[2] A properly shaped asymmetrical object, a sail for example, diverts so much fluid [air] to the low pressure side that the flow over the other side will be slower than free stream and thus the pressure higher; SO LIFT IS CREATED.






The sum of windward pressure forces and leeward suction forces can be shown by ONE ARROW which shows the strength and direction of the total power by its length and the direction it points.




We resolve total power into heeling force and forward thrust. On windward sailing, heeling force is much greater than forward thrust.






Sail angle has a great effect on forward thrust. Total power is the same but THRUST IS INCREASED 50% by easing sail [boom] by 5degrees. HEELING FORCE IS ALSO DECREASED.

INTERACTION OF TWO SAILS

We noted on our diagram [2] of a mainsail that the convex curve created a narrow channel between the sail and the free air stream. This caused the air mass to speed up. This increase in speed caused a DECREASE IN PRESSURE which is THE MAJOR FORCE IN MOVING A BOAT TO WINDWARD.

Now if we add a genoa that overlaps the lee side of the mainsail, we introduce a stronger lee-side barrier than the free air stream. There is more squeezing of the air between the after part of the genoa and the convex lee surface of the mainsail. The result is a greater decrease in pressure and thus MORE THRUST. The constricted area is known as the slot.



Good slot - just enough squeezing of air.








Slot too closed - too much squeezing of air [main is backwinded-luff].








Slot too open - not enough squeezing of air.





In diagram [d] with a good slot the air flow from the windward side of the genoa adds even more air to the air on the lee side of the main and thus increases the main's total power.

JIB TRIM

The position of the jib sheet lead on deck is critical because it determines the sheeting angle. To approximate the angle, project a line from a point 45% of the luff length above the tack, through the clew to deck.




This point on deck is where your jib fairlead should be.



FOR GOOD UPWIND PERFORMANCE tell tales on the jib luff are invaluable. Get some light knitting wool in a dark red, brown or black and using a needle poke it through your jib in the positions shown. Knot it close to the sail on each side so it can't pull out. Each side should project about 4 1/2 inches.

We can't see the wind on our sail but the tell tales tell us what the wind is doing!

When beating to windward, we want our jib to luff evenly from head to foot at the same time. Go out in a good breeze and bring the jib into the wind. All three windward tell tales should flutter at the same time. If they don't, move your jib sheet lead a little forward or aft until they do. When all flutter at the same time, your sheet lead is at the optimum position. MARK THAT POSITION. Your sheeting angle is now optimal for windward sailing.

When sailing to windward, keep your eyes on the jib tell tales. When both windward and leeward tell tales stream aft, you are achieving close to optimum jib performance. [diagram a]

If the windward tell tale flutters, you are close to luffing and must bear off a little. [diagram b]

If the leeward tell tale flutters, you are stalling the jib and must head up a little. [diagram c]




diagram a- Optimum jib performance -Leeward and windward tell tales stream aft.






diagram b- Windward tell tales flutter [up or down]- Sailing too close to wind- bear off or sheet in.








diagram c - Leeward tell tales flutter [up or down] - Jib is stalled - head up or ease off sheet.


GOOD SAILING!!

DRAFT and TRIM

DRAFT IN THE MAINSAIL

A sailmaker can design the aerodynamic shape of our mainsail for one wind speed only! As the wind force changes, the sail's shape will also change because of sail cloth stretch. As the wind gets stronger the draft gets fuller and the wind pressure moves the draft aft. Conversley, as the wind force decreases, the draft moves forward and the sail flattens. Thus we must use draft controls to keep the right amount of draft in the right location. These controls can increase a sail's effective range to 3 or 4 knots above the sail's designed velocity. So a medium to light sail might be designed for 8 knots but could have a range of between 4 and 12 knots using draft control techniques. The sails we use at Wildwood basically cover our range of sailing wind speed fairly well.

Draft is the source of the driving power of the boat. The more draft you have, the more power in the sail. This is within practical limits- until the sail luffs, or in heavy weather when the boat is overpowered. In light air you want to get as much driving power as possible, so get as much draft as you can while still keeping a good pointing angle.

MAINSAIL DRAFT CONTROL

LUFF TENSION controls the fore and aft location of the draft. Change tension by moving the boom up or down on its sliding gooseneck by altering halyard tension or by using a cunningham. IF the draft in the mainsail has been forced aft by increased wind, it can be moved forward again by increased luff tension. It can, of course, be moved aft by easing luff tension.

FOOT TENSION controls the amount of draft in the lower half of the sail. Create more draft by easing foot tension, less draft by tightening foot tension. Do this by using the clew outhaul.

LEACH TENSION- Most overlooked factor in draft control. A loose leach removes curvature from the after portion of the sail rendering it relatively ineffective. A loose leach causes the deepest part of the remaining draft to move forward. A tight leach curves the leach to windward thus increasing draft in the aft portion of the sail and moving the deepest point of draft aft. We control leach tension with the mainsheet. As we ease the sheet, the leach falls off to windward more and more. This fall off or twist is at its greatest point about 3/4 of the way up the leach. A certain amount of twist is needed. The sail sections aloft should be pointed about 5degrees closer to the wind. If you have the right amount of twist and if the foot and luff tensions are right the ENTIRE LEADING EDGE OF THE SAIL WILL LUFF SIMULTANEOUSLY.

I strongly recommend the use of 2 or 3 tell tales on the mainsail. Fasten them at the leach at each batten. They will indicate when your mainsail is properly trimmed and when it is stalled. If they flow steadily aft, trim is good. If they fall or flutter forward, the after end of the sail is stalled.

GOOD TRIM -Tell tales fly aft







SAIL STALLED
Trimmed in too far,
vang too tight or
sheets too tight.
Loosen sheets.




MAINSAIL FULLNESS WITH BOAT AT REST

Curves show shape of mainsail at rest without full action of wind blowing into them.

[a] LIGHT AIRS
There should be almost no tension on the luff at all. The wind isn't strong enough to alter the sails shape; so start off properly shaped.

[b] MEDIUM WINDS
Hoist mainsail hard enough to draw the flow forward into a fold up the luff. It will blow back as you start sailing.

[c] HEAVY WINDS
Here you need the greatest possible tension on the luff, otherwise, the strong wind will blow the camber aft to an inefficient position.







HOW FULL? MAINSAIL SHAPES
Note how the powerpoint moves forward as the sail is flattened.