What is the general concensus regarding upwind mast rotation for those using a pre-bent mast (i.e. tensioned diamond wires). My understanding of the theory suggests that I only need to rotate my mast enough to present the leading edge cleanly to the apparent wind (over-rotated on the downwind leg if not using spi), yet a fair number of the photos I've looked at recently show cats with the mast really cranked around, even while beating. What are your thoughts?

I don't know what pictures you saw this in, but have you considered that the boats in those pictures may have been running their rigs with loose diamonds?



My feeling is that if they had pre-bent masts and were over-rotated, they were probably trying to increase power in light wind or serious chop.



The foil's effective draft is deeper when the mast is rotated further and this provides more lift.



The top portion of the mast above the hound will be in a position that will induce bend on its minor axis; this adds power as well.



Thirdly, the same top portion will be prevented from bending off or away on its minor axis, taking the leech to lee with it, and spilling off a lot of pressure. This prevention of sideways bend up there prevents the associated loss of power from spill-off.

> I don't know what pictures you saw this in, but have you

> considered that the boats in those pictures may have been

> running their rigs with loose diamonds?



While I don't know about all the boats out there, I'm familiar with most of the popular designs. The photos I was looking at were mostly N6.0's and I20's. The reason I was asking is that I tend to leave the mast to its own on the windward leg, but I wanted to get some feedback on what others have found to be effective.

Kevin-

Can't comment on the use of prebend in the Inters and N6.0's directly but there is a nice article on the Boyer/Taipan/AHPC website about wingmasts (which are all prebent to my knowledge in cats) that discusses rotation. I realize the N6.0 mast is not a "wing" mast but it might be helpful. Basically, the way I understand it- with the prebent rig the mast is actually "bent" fore and aft at rest and the lower section, supported by quite tight diamond tension, is "free" to bend more fore-aft but quite limited in it's sideways motion- The top portion of the mast (above the diamonds) however is still "free" to bend as it desires (and due to it's shape this is more side-to-side than fore and aft).

Rotation affects the sail differently above and below the hounds in that above the hounds it also effectively controls mast bend, while below it primarily controls depth of the sail/mast (of course it affects the wind entry and airflow over the mast/sail leading edge always) and therefore chord of the total "rig". Unlike a "soft" diamond rig, where derotating the mast serves to "power up" the rig, derotating is generally used to "depower" the rig since the chord is reduced and the top of the sail/mast is allowed to "spill" during gusts.

Which pictures are you alluding to? Perhaps I and others could look at them and comment directly?



Kirt

Here's one of Jim Zellmer at Spring Fever on his N6.0. (I know that Jim is successful , winning his class at the event.)

Kevin-

Hopefully some of the 6.0 boys will "jump in" here- But I was at SF and presuming this is Friday (only racing day we had sun and wind!)- it was fairly light air and if this is the first race it was a reach all the way- They are trapped out but hull is barely (not?) out of the water so I would assume they were going for power for here and perhaps traveler is off center (ie close reach?) so sail is really set up more for reaching than upwind. I also notice the long spreader arm is just in front of the jib, indicating to me you couldn't really run much less rotation and not have it affect this jib adversely. On the Taipans and "A" cats we have tell tales about 2-4" aft of the mast to help indicate whether the sail/mast "joint" is fair.

Too bad you can't get Jim to comment!



Kirt

Kevin



I general, on boomless rigs, you let the mast rotation take of itself.

Kirt,



I think you're right. It does look like a close reach in the photo. Thanks for your suggestions as to what might be going on there. My reason for beginning this thread is to try and sort out some of the subtleties of what's happening with mast rotation on a pre-bent mast. Given that the N6.0 has so many strings to pull, my learning has focused on the more important ones first, with the idea of sorting out the nuances of the others next in the order. Mast rotation for points of sail other than downwind is something I'd considered of lesser value when things are happening quickly on the race course, but I may be wrong and I'm hoping that other sailors out there can give me feedback.



It was only when I was pasting in the picture in my last post that I crossreferenced the sail number to the Spring Fever results to find out who the sailors were. Jim is a fellow Vermont sailor. Jim, are you out there, or does anyone have any contact info?



Cheers,

Got an email with this advice that I thought I'd share:



"Randy Smyth gave a good basic sail control talk at SF; thought someone would have mentioned that. He suggested that in light air you rotate the mast until the leeward side sees a smooth curve; this gives you more power. I suspect that's about where that 6.0 is in the picture (although the reach theory is good, most people don't change rotation just for a reach, especially a short congested race like that one was). Once you don't need more power (around single trap conditions), you rotate the mast back until its aligned with the apparent wind (equal sail "indentation" on both sides of the mast). This gives you less drag. Randy didn't seem too keen on under-rotating to depower; maybe too much drag? Anyway, on my Hobie 20, the common setting is "pointing at the shrouds", and this seems to give the low drag setting. The mast pretty much goes to that position. In lighter air, I have to force it out to the smooth curve setting. I tried this the next day at SF in the lighter air and it did seem faster, but then again I also changed some other stuff. The Word according to RS..."

Gang,



I sail a I-17

I just bought a new main and had this converation with Charlie Oglietree of Ullman Sails ( he is an A-Cat sailor too)

The Mainsail only gang is a different animal,...so beware of bota w jibs,..the science to the entire rig is different.

For Example, the Spinnaker boats mast set up is diferent because the mast must support the spinnaker in heavy conditions and not break,....a good thing,...yes!

Kike Figuroa, the Olympis Tornado sailor from P.R., just finished the CORT series here in the Caribbean,...get this,...his diamond wires are 800 pounds,...I repeat,..this is not a typo,...800 pounds. It is like solid steel at that point.



In closing,...the unis,...the spin,....the main and jib,....each are special with there own science.



Bruce

St. Croix



ps,...since I sail in open classes,....the new main will be a rocket compared to the stock sail!!

WOuter

Wouter,



As I have just finished the $ with Ullman, and the preliminary design needs,.....my weight,..the wind and waves in this area,...and I told Charlie that I DO NOT sail in class,...I gave him free hand to build me what I needed.



Which led to his formula for Uni-rig diamond tension.



As follows;

Light air - diamond never to go slack

Medium - diamond same as above

HEavy - almost, but not slack,..on the verge.



Tension



Light - set diamond wire to 'almost slack' tension.

Medium - 1 to 2 full rotation of turnbuckle from light position

Heavy - Maximum of 4 complete turns of the turnbuckle from light setting



Pre-bend



Heavy - 2 to 2 1/2 inch rake on spreader

Medium - 1 1/2 to 2 inch rake on spreader

Light - 1 inch or so rake on spreader

( ..this is my interpretation, Charlie O. said never more than 2 1/2,..less as it gets lighter.)



If you go the the North American A Cat site,..look for results,..you will see 5 or 6 out of 10 boats had Ullman sails ( all different platforms),...except for Skip Elliot, he had a Elliot-Pat sail, of course..!



The price was very good, by the way,...$1130.00 USD without battens ....20% less than the stock sail from Performance!



Bruce

St. Croix