I'm using a prebent rig, with about an inch and 3/4 spreader rake. I'm finding that my mast is consistently under-rotating, and my old rotator is a tad under-powered to get it just right. For good reasons, I like my old rotator. It's self tending, self tacking, and sets with one pull on a bungie, and can't dismast me if I forget it on a hot jibe or tack, for the twin reasons that I'm supposed to forget it (self tacking), and that the bungie will just pull out of the camcleats. I'd sure hate to lose it.



My thinking is, if I rake the spreaders more, the axis of bend (least resistance) should move; for example clockwise on port tack, (CCwise on starboard). So if the axis of bend moves clockwise on port, the mast should rotate further counter-clockwise so as to line that axis up with the load, right? And on port tack, when you add rotation, you're rotating the mast counter-clockwise. Sounds like a go, right? I'm nervous....



Allright, here's the question for the real experts out there:



Clearly, if it really was that simple, i.e.: "Rake your spreaders more until your mast rotates properly by itself, less if it over-rotates" then people would be doing it. So what'm I missing here? If you rake 'em too far you're not supporting the short cross-section of your mast, and it fails? How far is too far in that case? Or is it something else I'm not seeing?



Thanks, and...

Ed:



Forget trying to use the mast bend to run your rotation. The rig bend characteristics are way more important than how the mast rotates or doesn't. Se the mast up for the proper sail shape and then identify the shortcomings of the rotation control system. If there is not enough boom compression then move the boom bail for the mainsheet aft to increase compression force on the mast. An inch or two is usually all it takes to make a big difference.

Ed,

I'm no expert but I have played with this a little.

If when raking your spreaders you get more mast bend then it will make the mast less inclined to rotate down wind.

But as mark says. You can reduce your prebend so the mast will rotate better but in the same action stuff up the sail/ mast relationship and go like a dog on every other point of sail.

Another point that can limit mast rotation is where the diamond wires attach to the mast.

If this is too high up they will cross the sidestays when the mast rotates. This action effectively has to tighten the rig or stop rotating any further.

Depending on the class of boat there are limits where this attachment point can be.



I'm not advocating you change the attachment point for your dimonds just bringing this to your attention as another possible cause.



A possible sollution to this is rigging the boat a little looser in the first place but this can also have a side affect in as much as it can affect forestay sag on some boats which can have a detrimental affect on your boats ability to point.



I will send you an email of a video I have just taken of a positive mast rotation system that I have just put on my boat.

It may help, it may not.



Regards,

Phill

HI, Mark, Phill,



Thanks for getting back to me.

My post isn't terribly clear. I'm definitely not contemplating using my prebend to affect rotation. Like you, I devoutly believe the curve of the mast must be appropriate (Note, not "identical") to the luff curve of the sail, and I plan to continue to follow my sailmaker's advice on how much the mast actually is pre-bent.

I'm considering if spreader rake, without changing prebend amount, can be used to affect how the mast rotates. I completely agree that the amount the mast is pre-bent must not be changed when considering a different spreader rake.



In looking over your post, I see where we might be disconnecting, here. Are you guys saying that if your mast exhibits a certain amount of prebend, say, an inch, and you rake your spreaders more, then tune your diamonds for that same inch of prebend, you still have a different rig? My point is, it'll rotate differently; what else is different? (Provided you still have an inch of prebend)



I guess the paramount question is: How far can you rake your spreaders without causing your mast to fail?

(One hint on this question: The tangs on my mast which attach to my diamonds, the top ones, are aimed at a point further aft than the tips of my spreaders are at now. Does this imply the mast is engineered to tolerate more radically raked spreaders?)



Thanks again for taking the time to consider this; I hope I've been clearer this time.

This is not a complicated matter.... just like with anything else on these boat, KEEP IT SIMPLE STUPID.... I try to live by it on the water...less chance of a robust simple setup breaking. Set your mast, rudders...etc for the most/best power. THEN deal with your other problems (helm, rotation, etc) w/o messing with you sail tune. Don't use your diamonds to help rotate your mast... Mark has it right.



The position of spreader and tension not only set the initial state of the mast, but also dictate how the mast will react under load (sheeting/dh..etc) So, by screwing with your sweep, you may not change how much INITIAL prebend you have, but you WILL change the bend characteristics of the mast. It may then depower too quickly or not quickly enough or just not bend properly to suit the sail (inversion/poor shape).



Another interisting point... on a Nacra 6.0 or other boomless rigs, battens make a HUGE difference in how your mast rotates...



like Mark said, on a boomed boat, move the bail aft. Personally I think there is a problem in your rotation system if the mast is not rotating properly. I hate those auto bunge systems. They don't rotate far enough and move too much...



Unless you have a "personal" sail maker (Randy/Saber/other) cut your sails for YOU, you have to be careful with the numbers you might get. I got a brand new set of sails for my 6.0 ~2-3 yrs back (sold it)... called EP sails and they gave me mast/batten settings...well... they were WAY off. For doing the initial setting, measureing your sail and mast is best, but once you start to know your rig, you can adjust based on feel.



I don't remember who said it, but one of the masters said, "I just try to get my boat in the ballpark....the rest is sailing..."



my 2 c's



Will_R

Thanks, Will.

My sails were made by Randy Smyth, personally for me. I started out using his batten numbers, prebend and rake settings.



Most of the top half of my sail wouldn't flatten, no matter how hard I DHauled (18:1) and sheeted (6:1). So I increased the amount of prebend slightly, and the sloppy luff got better, and the deep pocket got somewhat flatter, but still deeper than I like. So I'm already experimenting with getting this beauty dialed in... so I'm in no way convinced the spreader rake is necessarily dialed in right already. So in experimenting for the right spreader rake, what explicitly should I be looking for when I sail test it... what specific effects does too much/little rake have? (Other than the change in rotation)



And I totally agree with you on the K.I.S.S. principle - that's why I have a one piece bungie rotator in the first place! I can solo my sloop rigged 5.0 and keep up in a tacking duel handily, partly because I never have to touch the rotator. Sorry you don't like them



I guess as I predicted in my first post, this is not the way to go... I'll just beef up my bungie and let the spreaders find their proper setting independently.



Thanks again,

Randy tends to cut his sails pretty full....but if you want to talk to someone about how to get it right, he's the one.



I know for the most part, I don't mess with sweep much... just tension. increase the prebend for heavy air. You can get the sail just as flat w/o as much dh tension (less sail stress). Hence if you increase the diamond tension and pull just as hard on the dh, you should get the sail to flatten at the top.



So, i assume that you have a pinhead on your 5.0? The way sails (square vs pin) react to dh is different and has to be considered. Just on general principle, I would figure between 1.5-2" of sweep. That seems to be a good range for a lot of different boats. I know on the 6.0, we ran less tension than on most other boats because the mast was stiff enought to support itself (and we wanted a LOT of power), but we could also dh it (8:1) and flatten it well (I'm a fairly large guy). I think it was like 1.75 sweep ,~725lbs (outer), and ~250 (inner) on the 6.0. Been ~2yrs since I sailed one.

Ed,



The rotation of ypour sails has to do with having the right shape, and timing of your controls. i.e. items like closing off the slot will keep your mast counter rotated.



The spreader rake is a method of providing prebend in the mast to match your sail. Diamond tension is what you use to keep the mast stiff for a fuller pocket. Combining the 2 allows you to have a stiffer or looser mast with the same prebend by adjusting the speader rake.





Matt

Hi, Will,

Thanks for all the advice. Randy built me a square top, since I'm more interested in sailing as hot a boat as I can than in one-design on a 5.0! I'm gonna be in a portsmouth fleet whenever I race anyway, eh?



AM I correct that by "sweep" you mean the amount of rake in the spreaders? I've been gradually tweaking up and have hit 2.0" My prebend is up to about 2" as well, measured standing up, unloaded. As I had hoped and you just predicted, this weekend's sail showed a cleaner luff, with less DH, so the top leach stayed firm with less mainsheet, meaning the whole sail did more work. Amazing how the top leach on a square top is so responsive to DH!! Before this latest increase to prebend, I needed to DH so much to clean up the luff that the top wasn't willing to stay on the job without heavy sheeting, which flattened the bottom of the luff but left the top luff very deep indeed. A teensy bit more prebend later, things are much better.



I think I've finally entered the useful prebend range, just on the "Light air" end... I'll just dial up the prebend slightly now for medium and heavy air; it's still a little full to really go fast.



Oh, yeah! It rotates slightly easier since I added the last 2and1/2 turns on the spreader adjusters, bringing my spreader rake up to 2". Thanks for telling me that 2" wasn't extreme. I'm less nervous to be back that far, now.

Thanks, Matt!



That's exactly what I needed to hear, and it agrees with my experiences this weekend. I needed to have a slightly softer mast with slightly more prebend. I could have just tightened the diamonds to get more bend, but then the harder mast wouldn't have flattened in response to sheeting. So when I raked more, while thinking about rotation, what also happened, was it slightly exposed my short mast-axis to bending load, making a softer mast, with slightly more prebend! That's why it worked. This weekend's sail went well, see post below.



So Mark as usual is right of course, you can't just lightly screw around with spreaders while thinking only about rotation, it just happened that I needed more rake, for other reasons, which I didn't initially go into in my original post, not wanting to dog on a hugely excellent sail, withoug good reason to drag it into the discussion.



Eureka!!



So, the simplified rule-of-thumb (on a prebent rig) is:



More rake gives you a softer mast by allowing you to get the same prebend with less diamond tension; incidentally it lets the mast rotate more too. Less rake does the opposite.



(Naturally, you have to understand your sail and what it needs before you change your rake.)



Anybody notice any other effects of changing spreader rake?

Ed,

I'm no expert but I noticed one thing about too much spreader sweep coupled with tension. I found that 2" on my Inter was messing up my leach and causing the sail to s-curve when sheeting hard--more so when I used any DH. More than likely I need a new sail--but the one I have has to last til next year. I am now somwhere around 1.5 to 1.75 and my leach now stands up right, sail flattens out nicely with DH with little S-Curve except when I really wail on the main sheet. Now I am experimenting to find the right tension settings for the diamonds to see what works best.



Whatever the right settings, I think other posts nailed it on the head---match the prebend (mast) to get the best sail shape. Use the settings others are using as a ballpark as each sail and mast is different, even if its one design.



As for mast rotation, I have found no difference between more prebend or less, I think rig tension makes the biggest difference.



Regards,

Don S.

I20

Hey Don,

Stupid question: What precisely is S-curve? Is it when the leach makes an 's' with the very edge, as seen from behind the sail, or do your battens actually bend off near the leach in an 's' shape, as viewed from the top down?



Maybe I need to look more carefully, but I've not noticed either condition. (Except for the squaretop opening up the top leach under heavy heavy DH; but that's a design goal, right?)

Its when the top 3rd or more of your sail inverts . Once I started sheeting (regardless of DH), my sail would invert and never really flatten out--very slow!.--This happened regardless of whether I used stiff or flexible battens. Taking out some of the Pre-bend--seemed to make much of the problem go away--except when ALOT of DH and Sheeting is applied



I seem to have more of a range new between a full sail and a flat sail when needed.

Sorry to be such a dope, Don.



You mean the top third actually pops inside out, with belly to windward, etc? I do get that sometimes, when tacking, the sail needs to be 'popped' over to the other side and the top third is the last to go over. You mean that can happen other times?



Or are you saying proper mast tuning will reduce the difficulty popping the sail when tacking?



Thanks