Main Sail Runner (or not)

[savoir]

I took a look at the Z Spar website and can't see any obvious adjustment point. From reading their installation instructions it seems like the only solution to a saggy boom is to buy a bigger spring.

[bmz]

"I want my top batten to be parallel to the boom at the leech, which means that the sail has a moderate twist to it."

That is hard for me to envision; as I see it, the only way your top batten would be parallel to the boom, is when you have no twist in your sail.

[savoir]

You are putting a different interpretation on the word " twist ". With mainsails it means the line between the clew and head as sighted up the leech. No twist means a straight line and a pretty tight mainsheet. In such a situation the top batten would be pointing up to windward and that might not be very fast.

[cwyckham]

Originally Posted by savoir

You are putting a different interpretation on the word " twist ". With mainsails it means the line between the clew and head as sighted up the leech. No twist means a straight line and a pretty tight mainsheet. In such a situation the top batten would be pointing up to windward and that might not be very fast.

Exactly. The sail is curved. At the boom, the leech actually points to windward. As the leech twists off higher up the sail, you get to the point where the sail is twisted enough that the leech is pointing straight back.

It helps to draw out the curved cross sections of the sail to be able to see it

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[bmz]

if you have not set your traveler to windward, and your outhaul is tight (to flatten your sail for close hauled sailing), the leech should not to windward.

[bmz]

BTW, a windward leech always acts as a brake (the resultant force vector is aft).

[savoir]

Not if the mainsheet or vang is set too tight.

[bmz]

I just gave you the Newtonian explanation; the Bernoullian explanation is that the lift at the leech is aft.

[cwyckham]

Look at the upper surface of an aircraft wing. The trailing edge points towards the ground. The net lift is positive. The Newtonian description of lift is that by deflecting the air downward, the resultant is lift upward. The air leaves the wing in the direction the trailing edge is pointing.

A flat sail isn't a very good wing. Curvature is important. We only flatten them to depower them.

The boom will need to be dropped significantly to leeward to have the leech at the boom facing aft. There's no good reason to do that. Boom on center with curvature in the sail is the correct place to be in light to moderate wind. The leech will point towards the upwind side of the boat. That's a good thing.

[bmz]

When the trailing edge of an airplane wing points down, the wing is actually slowing the plane, but providing maximum lift (when the airplane needs acceleration more than lift, the trailing edge is horizontal). The same thing happens when the leech is to windward(less forward vector, more healing vector). The forward force vectors on a sail are maximize when the luff is tangent to the direction of the wind and the leech is tangent to the direction of the boat. This means a very flat sail when you are close hauled, and a very full sail off the wind (increasing to a semi circle when you are running).

[bmz]

Another point--although the same rules of physics apply to airplane wings and sails, there is a very significant difference: airplane wings do not provide forward propulsion; the lift provided by an airplane wing is perpendicular to the direction of travel--the same thing as heel on a sailboat. Hence, beyond the basic physics, they are not analogous.

[cwyckham]

bmz, you obviously are very interested in the physics of how a sailboat works. Might I suggest that you find yourself a copy of the excellent book, "the symmetry of sailing?" You will love it.

Airplane wings and sails (when used upwind) both produce lift by the passage of air over a curved surface held at an angle of attack. Of course they're analogous.

The trailing edge of a wing always points down. Otherwise there would be no lift. It's called the angle of attack. It doesn't matter whether you want a low or high lift coefficient, you always need a positive angle of attack.

You absolutely do not want to have the luff parallel to the wind and the leech parallel to the keel. Can you imagine what the sail would look like on a beam reach? There would be zero chance of the air staying attached. You'd have huge drag and no lift. The boat would go sideways.

Also, remember that when you are beating, the boat absolutely will go slower than when you are cracked off and can drop the boom a bit. However, the name of the game isn't boat speed, it's VMG. You need to be able to point.

In light to moderate winds, you do that with the boom on centerline and a moderate curvature in the sail. The only way it is physically possible to get your sail set right is if you are controlling the boom position from somewhere upwind of the centerline of the boat. You can do this with traveler up or a double sheeting system (not double ended).

[bmz]

"You absolutely do not want to have the luff parallel to the wind and the leech parallel to the keel. Can you imagine what the sail would look like on a beam reach? There would be zero chance of the air staying attached. You'd have huge drag and no lift. The boat would go sideways."
Next time you are out and on a beam reach, look at your genoa; if you are a good sailor, your luff will be parallel to the direction of the wind and your leech parallel to the keel, AND the air will be fully attached. I have rigged my boom to have huge outhaul travel in order to achieve most of this with my main as well. Moreover, as I said, although the physics are same, an airplane wing gets ALL of its lift perpendicular to its direction of travel, while the objective of sail trimming is to minimize the lift (force vector) perpendicular to the direction of travel and maximize it parallel to the direction of travel. Hence, if you trim your sail to look like a vertical airplane wing, your boat would truly go sideways.

[bmz]

I guess I always have an afterthought. I am sure you have seen racing genoas with telltales running from the luff to the leech on both sides of the sail--but you have never seen a main with telltales like that; the reason is that the way just about everyone trims their mains, most of the windward telltales would be stalled.

[cwyckham]

Lift, by definition, is perpendicular to the direction of the air flow. Direction of travel doesn't come into it. I'm sure you can find some videos of the last America's cup that give a convincing case study for boats doing quite well with wings for sails.

You shouldn't try to make your main look like your genoa in a beam reach. You should try to make your genoa look like your main. This is why people Barber haul or use reaching struts.

You have convinced yourself that everyone else is doing it wrong and even changed the rigging on your boat. Maybe all those professionals actually know what they're doing.

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