light wind rigging quandary

[techno900]

konajoe said:

[isobars]

[Sailboarder]

I'm with Techno on this. I see the same general changes on a sail.

[Sailboarder]

[techno900]

sailboarder, thanks for the clarification.

Sails without twist reminds me of a great? light wind sail I had about 15+ years ago. A Neil Pryde Light Wind Racer. 10.4. as I recall, there 3-4 cams and a very tight leach. Great low wind power, but if the wind ever got to about 14-15 mph, I was toast. As a result, I only used it when I was sure the wind would stay low. At first, I used it on my Mistral Equipe II XR, & then on my first Starboard Formula, the 175. It was about this time that race sail designs began to include the lose leach and twist.

If you never have gusty winds, a tight leach is great. But those sails are a pain in the ass, arms, hands & back if the wind picks up or gusts. Big sails with a lot of twist are great because of their range and on the fly adjustability.

[konajoe]

Uh-oh, Sailboarder. I think you accidentally disagreed with t900 when you said this:

[konajoe]

[swchandler]

Konajoe, I'm still curious, are you a heavier sailor?

[DanWeiss]

Wow, lots of love flowing in this thread.

Let's go back to think about how sails are made. In the simplest terms, several panels are sewn together along adjoining edges and then sewn to a luff sock or bolt rope (in the case of sailboats). Clew and tack grommets are installed. That's a basic sail.

So why not just cut a triangle and hang from a rigid pole, attach a boom and have at it? If the sail cloth had no stretch at all and the mast didn't bend perceptively the sail would be a flat board. Since masts do bend and sail cloth stretches, sailmakers took up the slack by adjusting the panel seams, install battens and here we are today.

Once rigged and tensioned to the minimum, a sail will respond somewhat differently to downhaul than a sailboat mainsail responds to halyard tension because of how the mast is kept upright and the location of the boom.

On windsurfers, the boom sits in the middle of the deepest part of the draft and mounts to the mast about 1/3 of the way to the top of the mast, whereas the sailboat boom is located at the bottom of all draft and sits about 1/8 of the way up the mast. More importantly, the mast is supported by shrouds about 1/2 way up. The shrouds (plus backstay, forestay and/or jib halyard tension control mast bend. The main halyard does not bend the mast but does change the skin tension of the sail like a windsurfer's downhaul.

The windsurfer's downhaul not only holds the sail on the mast but actually bends the mast tips together because the luff tube is curved more than the luff panel that carries the DH load. With only enough DH to attach the line, we can see the mast slightly bend already, but most of the depth of the sail occurs laterally from the mast to the leach and clew. As we add DH tension, those waves flatten out as the sail is pulled down the mast, leaving a 3D pocket.

Let's now assume that we DH even more, right into the meat of the tuning settings. The boom may be attached but no more than neutral OH tension is applied. I think this is where the conversation took off. As we increase DH we will see location of the maximum draft move forward and down. This is because the mast is softer at the top and the panel seam shaping is less dramatic in the top 1/2 of the sail. We will also see the maximum depth of the draft decrease. This combination of mast bend and seam shaping stunned people in the early 90s who often sailed with far to little DH tension because the correct amount made the leach appear too loose and sloppy and the draft too far forward and flat.

Of course, those sails looking ugly on the beach felt great when powered up on the water. Why? Because the mast now must bend to leeward above the boom, causing the draft to fill out as the skin tension dropped. Each year the sails required increasing levels of DH tension to go faster and maintain stability. These sails were intended to be sailed "overpowered" and finally became truly great by 1995. Yet no amount of seam shaping or battens or cams could give us the feel we now enjoy when planing without 1994/95 mast revolution created by Fiberspar with the Reflex 5000 and improved ever since across manufacturers. The easy-to-control acceleration and terrific stability we now enjoy derives from high carbon content masts that return to column much more quickly and thus spend more time in the intended sweet spot without increased mast stiffness.

Now, back to tuning with OH and DH once in the intended tuning range, but now assume some positive outhaul. Increasing DH will decrease maximum draft and may even move it look like it moved back slightly! Ah, why? Increasing luff tension making the draft shallower right near the mast. Decreasing OH would then move the draft back while making it slightly deeper. Increasing OH would move the draft forward while making the draft slightly shallower.

The point is that most sails -seen very clearly in came sails- will appear to increase draft by increasing DH tension slightly as long as neutral OH tension remains. But what really happens is that total draft is reduced more toward the clew and less at the luff, making the sail look like DH added depth when what it did was round out the entry. Adding OH tension from neutral to positive at this point will increase tension along the seams radiating toward both ends of the mast, but most of the remaining action is around the boom area since that is where the maximum panel shaping exists. This increased tension is designed to decrease draft depth and usually force the draft forward. If the DH tension was now reduced, the relative OH tension would skyrocket, causing the draft to be very flat at the clew then suddenly become very deep in the front third.

To the OP, the takeaway is that modern sails respond as you expect as long as the mast used is exactly the spec on which the sail was designed. Increasing DH tension will reduce depth, as will increasing OH tension. The sail will respond to wind pressure and mast deflection in the same way, so using too little DH tension will eventually collapse the middle of the sail and allow the wind pressure to force the location of maximum draft back as the wind increases and bends the mast even more.

As an aside, and what kanajoe knows, some sails are specifically tuned for racing with far less DH tension than anyone would think is proper. Kona One racers soon discover that you must tune for sailing performance, and a sail driving a subplaning board forward needs a tight leach. The overall rig package is very tunable in ways other performance sails are not, because of the lack of luff curve. This allows the OH to take up the majority of tuning range and makes the Kona rig work great even though it appears a disaster lying flat on the sand. Yet the sail provides huge power at low tension but can be tuned "flat" for planing to perform much like any other recreational RAF slalom sail on a shortboard or just beam reaching with the centerboard up.

[techno900]

Dan, great detail, well explained.