Adjustable Outhaul On Mainsail

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Bill Roberts
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Adjustable Outhaul On Mainsail

Post by Bill Roberts »

Why do SC and ARC products not have an adjustable mainsail ourhaul?
Years ago, back in the 70s and early 80s I experimented with adjustable outhauls on the foot of mainsails.
I tried many different rigs. One was a simple adjustable manual rig, lever on the boom actuated system. After several hundred tests sailing against standard boats, no outhaul adjustment, no benefit was found.
Another system was an outhaul system that varied the outhaul position, clew corner, with mainsheet tension. At max or much mainsheet load, the outhaul was in the full outward position. As mainsheet tension was eased for off wind sailing, this eased the outhaul pulling force was overcome by a strong shockcord that pulled the clew corner of the mainsail forward. Again no boatspeed benefit sailing against a standard SC20.
Why these results? Some other brands of beach cat came from the factory with adjustable outhauls.
Well, here we go: As the outhaul on a mainsail is eased, the sail becomes fuller, more camber. This can, according to Aero 101 text book, lead to a higher cift coefficient for the airfoil. Sail thrust is a function of lift coefficient X sail area X wind speed**2. So what's wrong. Well first of all as you ease the outhaul and the leech moves toward the mast, the projected sail are goes down, becomes a smaller number. With no change in boat speed at test, it means the lift coefficient went up about the same percentage as the sail area went down. So it is just an even trade off.
Another possibility is that the sail camber for sailing to windward was also the optimum sail camber for reaching. (I'm talking prespinnaker days.) because at test the boat with the more cambered mainsail went slightly slower that the standard, non adjustable outhaul, boat.

I even went further than this. I made a set of soft fiberglass battens, easily bent. Then I rigged a wire leech line, 1/32" flex cable, that went out and around the end of each batten which extended out beyond the leech of the mainsail at each pocket a couple of inches. I sewed a fishing rod eyelets to the exit of each batten pocket at the top and bottom of each pocket. The wire ran up the leech through the eyelets and out and around the end of each batten. The wire terminated at the head of the sail and on to a little 6 to 1 Holt Allen winch fastened to the boom. By tightening the wire the sail battens were put in more compression against the mast and they bent, bowed, more. This was another way of controlling sail camber with a much greater sail camber variation range. Guess what??? The boat with this fancy sail camber control system at test went slower than the standard non adjustable mainsail boat.
At this point I decided adjustable outhauls on mainsails was not a benefit. Today with spinnakers which keep the mainsail sailing to windward when the boat is sailing off the wind or even downwind, the nonadjustable mainsail outhaul conclusion is even more substantiated.
Kevin Keller
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Re: Adjustable Outhaul On Mainsail

Post by Kevin Keller »

Thanks Bill for that info. I had once asked Tom why he did not have an adjustable outhaul.
J Drew
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Re: Adjustable Outhaul On Mainsail

Post by J Drew »

Bill,
After studying this, I understand your explanation.
My question is about this part
Today with spinnakers which keep the mainsail sailing to windward when the boat is sailing off the wind or even downwind, .[/quote]
I can think of a couple different possibilities, but I'd like to know the correct reason rather than be confused by an incorrect assumption
Bill Roberts
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Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: Adjustable Outhaul On Mainsail

Post by Bill Roberts »

Hi John,
The reason the mainsails on high performance boats, catamarans, are trimmed in like the boat is sailing to windward even though the boat is actually sailing well off the true wind or even downwind is that the relative wind vector is so far forward. The relative wind vector is bent forward severely as boat speed approaches 1.5 to 2.0 times or even more times true wind speed. Also the spinnaker itself is a contributor to bending the relative wind vector toward boat center line. The big windbender is boat speed.
Bill Roberts
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Joined: November 17th, 2003, 9:13 pm
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Re: Adjustable Outhaul On Mainsail

Post by Bill Roberts »

More considerations to my first points: When the mainsail outhaul is eased, the sail becomes fuller at and near the bottom of the sail relative to the fullness change at the top of the sail. Actually the top region of the sail may not become any fuller at all. The sail cloth at the front of the sail, the luff area, and in the lower region of the sail moves to a more abeam direction. The luff area sail angle near the top of the sail changes very little or none. These relative motion changes in the luff area of the sail are in a direction to "reduce sail twist". Now you need a certain amount of sail twist to match the twist in the relative wind direction entering the sail. Therefore when an outhaul is eased, sail twist should be readjusted to keep the airflow attached to the sail and obtain max sail thrust. The relative wind has twist in it because the true wind has a velocity gradient in it for about the first 100ft above the water. The greatest wind velocity changes occur in the first 50ft above the water.
One other point: Just because an outhaul is eased and a sail becomes fuller, the airflow may not stay attached to the sail with more camber in the windspeed you are sailing in. Contrary to popular opinion, flatter sails are faster in light winds and medium camber sails are faster in medium winds and full camber sails would be faster in strong winds if we could hold the boat down in strong winds but we can't. So that makes all kinds of things change as to the optimum sail camber and size for the best sail, the fastest sail, in strong winds.
J Drew
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Joined: September 9th, 2013, 12:39 am
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Re: Adjustable Outhaul On Mainsail

Post by J Drew »

Bill,
Understanding what you have explained, brings me to the next question.
Is there a minimum wind speed needed to obtain relative wind?
I know that in order to create relative wind, you have to adjust the angle of attack and gain boat speed, but what I'm wondering is, can relative wind be obtained in very light wind? Let's say 5 or less
Additional sail area? Taller reaching the wind gradient better?
Bill Roberts
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Joined: November 17th, 2003, 9:13 pm
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Re: Adjustable Outhaul On Mainsail

Post by Bill Roberts »

You have relative wind as long as the boat is moving, even very slow. All sailboats sail on relative wind, even monohulls. Relative wind is the vector sum of boat speed and true wind speed. Example: If you sailing at 10 mph square to the true wind blowing at 10 mph, the vector sum is a 14 mph breeze, relative wind speed. The direction of that relative wind vector is 45 degrees across the boat. Tell tales show relative wind direction. If you hold up a wind speed measuring device on your boat while sailing, it is measuring relative wind speed. If you come to a complete stop, the wind gauge is measuring true wind speed.
J Drew
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Joined: September 9th, 2013, 12:39 am
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Location: n. florida

Re: Adjustable Outhaul On Mainsail

Post by J Drew »

That makes sense, now.
Hence the word "relative"

Thanks Bill, once again you've taught me something
John
Kevin Keller
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Re: Adjustable Outhaul On Mainsail

Post by Kevin Keller »

I never really thought about that either when I sailed monohulls for fun. It took me bombing around the bay on my Supercat wondering why the wind direction was always changing no matter which direction I was going to figure that out. I can't believe I had never thought of that prior.
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