Main Sail Power

[Ken Prichard]

We had a long day sailing in light air from Catalina Island to Marina del Rey and the conversation got to a lot of different topics. One of these was where, on a main sail, is the most power generated? Near the luff or the leach? We all put forth several different theories but no one was really sure. Anyone know?

 

[Don Guillette]

The power starts from the top down and here's why. The wind power you feel at deck level is not what is going on at the top of a 45' mast. The wind power at deck level is slowed down by friction generated by the surface of the water. The wind power at the top of the mast is blowing 60% harder then at deck level. To compensate for this wind speed differential, the sail maker builds twist into the sail. Without the built in twist you could not control the sail or boat. Twist is one of the 4 element of sail trim and that is why a sail boat has sail trim controls to reduce and increase twist on the main and jib plus all the other controls that are used to adjust the other 3 elements.

With all that in mind, it takes the entire sail to make the boat move so no portion of the sail is more important than the other. It is like a airplane wing - no part of the wing is more important than all its parts.

On the sail back from Catalina (which I've done more than a few times) how was your main and jib set up to get the most out of the light conditions?

 

[Ken Prichard]

Here's some specifics:
The boat was a Beneteau First 38 with a 135 genoa and a main with 4 full battens. We were making about 4 knots in 7 knots of wind and were getting a small push from the last of an ebb. We were close hauled at about 70° and the boat was standing up, with less than 10° of heel.
All telltales on the foresail were perfect. The cars were right where they should be. The main was traveled up so the boom was just leeward of center. The vang was on so the top batten was parallel to the boom and the draft was about 10% at 45% so the outhaul and halyard were good. All the telltales were flying except the top. If we eased the main sheet to fly the top telltale a small bubble began to show at the luff. It didn't seem to effect speed and didn't happen if the wind picked up to around 9 knots or better. We didn't have barber haulers so we didn't try to change the slot. This led to our discussion of the location of power in the main.
One position was that if you thought of both sails as being one sail then because of the overlap of the genoa the first half of the main did not contribute as much as the back half and that 60% of the mains power came from the back 40% of the sail. C&C's are notorious for having a bubble near the luff and most owners will tell you to ignor it. Some of us agreed, some of us didn't.
That's the kind of conversations you have when you have too much time on your hands in light air.

 

[Bill Roosa]

the vectors

Imagine the sail broken down into lots and lots of little squares. Each one has two forces, (imaginary as they are the net representation of all the tinny tiny forces of the air molecules bumping or pulling on the sail fabric) a friction force that is opposite to the direction of the wind and a pressure force which is normal to the surface of the little sail (sticks straight out from the center). The bigger the pressure difference between the two sides of the square the bigger the force vector that we commonly call "lift" (airplane term it is really thrust) it is always pointing toward lower pressure. The faster the wind blows across the little square or the rougher the surface the larger the force vector that we all call drag.
The lift vector can be oriented in any direction but consider the standard sail shape. All the lift vectors point generally to leeward, some point more forward (luft area) and some can even point backward (leach area). The drag forces all point toward the stern of the boat. I'm going to ignore the drag forces to simplify the discussion.
The forces that point forward to any extent pull the boat forward, the ones that point to leeward make the boat heel and cause the boat to slip sideways in the water. The ones that point aft slow the boat. The ones (almost all of them BTW) that point forward and leeward or aft and leeward contribute to both.
The little squares with the biggest pressure differential are located just forward of the max draft. Course this area is smaller (fewer number of little squares) than the area behind the max draft line so determining which actually contribute more to pushing the boat forward is complicated.
Sufice it to say get as many of the little squares vectors pointing forward as possable and as few pointing aft as possable

 

[RichH]

We had a long day sailing in light air from Catalina Island to Marina del Rey and the conversation got to a lot of different topics. One of these was where, on a main sail, is the most power generated? Near the luff or the leach? We all put forth several different theories but no one was really sure. Anyone know?

Although not intuitive, the max. 'force' is generated 'just behind the luff'.

....and most 'theories' that one finds in 'sailing books' are STILL wrong and and have been wrong since the time of the Wright Brothers, who proved they were wrong.
For a 'technical explanation' of how Sails yield 'lift' go to www.arvelgentry.com ---> origins of lift ---- how sail 'really' work, etc. - for deeply 'technical' (aerodynamic) articles written for the 'layman sailor'.