Mast Rotation

As part of my personal growth as a future Blade owner I have been scouring the archives for this forum. One thing I came across was the recommendation to rotate the mast 90 degrees while the spin is up. My short exposure to spin work didn't include adjusting the rotation. Was this an oversight? Have the spin's changed (flatter) meaning more forward apparent wind, meaning less rotation required? Or, was this just not done that day because it wasn't honking?

It seems to me that the current recommendations are that the bear off requires no significant tuning changes. A slight lowering of the traveller perhaps but most everything stays in the upwind configuration.

Thoughts?

Bruce, if you do not rotate the super wing mast, you are running a chance of snapping it in half.

Twisting the mast sideways allows the main sail to have more attached flow on its lee side.

Also, and more importantly, it allows the unstayed portion of the mast, above the hounds, to be bent backward (sideways bend) by the main's leech. This helps to counteract the forward pull of the spinnaker's halyard.

GARY

Bruce,

I sail the Taipan, same mast as the Blade. Standard dogma is to rotate the mast 90 degrees going downwind. I differ from that.

The aerodynamic purpose of mast rotation is, of course, to keep the flow smooth and attached to the lee side of the mast and main, making a nice, smooth transition from one to the other. I've found that with the Taipan/Blade wingmast, 90 degrees is actually too much rotation for the apparent wind hitting the mast/main when the spinnaker is up. Of course it depends a bit on how hot an angle you're sailing too. So downwind without the spinnaker I rotate 90 degrees; but with the spinnaker my rotation is usually less (unless very light air) than 90 and rarely over 80 degrees. My main has tell tails just behind the mast that I use to set the rotation.

In general, I've found the wing mast likes less rotation on any point of sail compared to a tear drop profile mast.

I don't rotate the mast to avoid some kind of structural mast failure--it's plenty strong and should be fine as long as you keep the main cleated.

Incidently, did you sail one-up? If so, you and I both understand that setting rig just right can be tough with only two hands.

Yup, I guess that was what I was getting at. I felt that 90 degrees would tend to be too much. And I am worried about being a Uni sailor because I find the sheets catch on the braces on my teeth. Yup braces, bifocals and a Blade all in the same year. YeeHa!

Roughly speaking :

advice to fully rotate the mast to 90 degrees is sound advice to everybody new to spinnaker sailing. The chances of snapping or bending a mast are seriously reduced that way. However I'll admit that I play a little with my rotation under spinnaker as well. But then again I'm a sailor off the kind "rather swim, then slack the mainsheet when under spinnaker". You must understand what the rig is doing to safely go look for the edges. But now that I'm quite familiar with the rig I tens to rotate my mast for optimal trim in anything under 10 knots. After that I try to rotate to optimal trim as much as I can, but I keep a constant eye on my mast to see if I'm overdoing it. My experience is that the superwing mast is quite stirdy under spinnaker. I will bend, sometimes alot, but it takes that with a smile. But again, you are doing this under your own responsibility. No builder will warrant any mast when you are not rotating it to 90 degree when setting the spi. Sufficient mainsheet tension is key !




That is the way I run it when it is windy or when I have other things to do like finding my eye through a crowded area.

There are other approaches but I'm finding that "get on with it, focussing on the big things first" is placing me higher. When I have the time or clear water I may well adjust a few minor things as well.

Wouter

Sorry to butt in here, but should I be doing this on my boat? My problem with screwing with the rotation on my boat is that is is yet another thing for us to mess with and then forget when we gybe (before I got the spin, we tried to remember to rotate the mast on the downwind legs but forgot to undo it it before gybing half the time thus, I believe, caused my spreaders to become askew on the diamond wires). I guess my point is that at a point I feel like I need a written check list before gybing or tacking.

On your boat, Patrick, a nacra 6.0 boomless design with a mast rotation inducer instead of a mast rotation limiter. I would try to sail with the rotation cleats fully uncleat under spinnaker. I venture that the hound fitting and sail pressure with rotate the mast far enough under spinnaker. And nothing is more bad then to gybe and forget to release the rotation inducer.

Note that we on F16's have only mast rotation limiters and we only uncleat the limiter lines completely to get up to 90 degrees rotation. So we are not setting any mast rotation under spinnaker. We are just uncleating the line when rounding A and have the mast rotate freely under the loads.

Wouter

Wouter,
Thanks for the info... much help and one less thing to worry about. You da man,
pp

Bobcat ,

Reduce rotation reaching w/ the kite ? - NEVER !

Think in terms of apparent wind - the kite (and it's big luff curve), proceed the mainsail INTO the wind when reaching ! Mainsail profile gains (reduced drag), are small because the fullness of the main's luff is behind the kite as the boat reaches downhill . That is until ...

... you hit a big wave or shoot a fast gybe while reaching w/ the kite. At the instant the apparent wind swings astern you'll feel the rig of the boat "load up" substantially as all that force (has anyone calculated the N's of force at this moment ? - gotta be huge!!), from the true wind slams into the sails from astern. If you've derotated by 10 degrees you've reduced the masts ability to elastically absorb the sudden load (by 11% ?) because the mast won't bend off as well in this profile. So, where does the sudden load go ? Into the sails (bad) and bows ( bad/slow/wet)... Also , when masts bend they get shorter base to tip. Derotating reduces the diamonds effectiveness to keep the mast below the shrouds from shrinking too much ( bad/ dangerous/expensive ).

Interesting. At a recent regatta we found that the boat was massively over powered downwind and that in big breeze all it want to do was got to china. In desperation I decided to use our standard big breeze upwind settings, downwind. Thats max downhaul, zero rotation . This had a huge effect, it was faster (reduced drag?) and could sail deeper and made the boat semi-controlable. I didnt get alot of time to look at the mast, but it didnt break and if it was ever going to it would have been then! Only problem was the gooseneck got pretty much destroyed.

Paul57man ,

My sense is that most if the speed reaching w/ the kite is due
in large proportion to the kite alone. Trim the main as you would to "A" mark - this is, I think we all agree, fast in the F16. I just wonder if you guy's who derotate the mast are doing so to reduce mainsail profile (drag) and if so
how much reduction in drag could this gain?

I think the kites are still being cut too full(don't want to sound deluded but I wonder if most sail manufacturers are underestimating the speed potential of these boats?) ; betcha flatter kites will prove faster.

Hey, by the way, the fastest i've ever gone w/ the kite was when my mainsail was inverted (battens flipped to windward)
if I could figure out how to do this every time I gybed in windy conditions I would. Sounds nuts, but try it...


PK

I'm still very careful with derotation under spinnaker. I mean there is a bottom line there somewhere. What I'm looking for is some nice entry in lighter winds and some flexing of the top to lee in the stronger breezes. The last has the same effect as derotation when sailing upwind. You depower the top and seem to be able to cary more grunt down low. However my mos effective use is in making the final stretch from the bottom mark to the finishline, which at my (club) races is nearly always a reach. By derotating the mast I can calm the boat down on the reach and thus can point slightly higher on this reach then most others while carrying a spinnaker. Letting out some traveller in this situation will really bend of the top and depower the tig enormously. I use this to handle the gusts. However, I do feel that I'm looking for the threshold with respect to breaking the mast, so be very careful everybody. I don't want anybody to just copy these tricks without fully understanding what they are doing and blaming me when it breaks.

Personally I found the superwing mast to be very abuse resilliant but then I also know when to hold back and go for safe.

I don't think I ever derotated the mast while under spinnaker then the rotator arm pointing halveway between sidestay and sterns. And the mast was really bending off then. Ohhh, by the way I do feel that derotation is less dangerous when sailing solo. The mast was designed and build to handle 2-up loads and the 1-up loads are just seriously lower, as a direct result you have noticeable more margin to "play" with. This maybe very interesting to you.





I'll venture to say that that is because you only sail the boat solo. For 2-up sailing you'll want the kite to be slightly fuller. Next time when you order a kite, order it as a specialized singlehander spinnaker, then you indeed get what you want. Just don't expect to win much 2-up races with it.





I had that happen to mu top 3 or 4 battens as well and indeed it felt really fast. I seem to recall that this happens when your top of the mast is allowed to bend away to leeward. This can be caused by derotation.

By once again, to everybody, be careful out there. By derotating your are cutting into your safety margins and once you've used that up, it is SNAP ! Everybody will be doing this at his or her own risk.

Wouter

Just a small comment on the rotation discussion.

Breaking masts was common in the Tornado class when the spi was introduced, before the sailors learnt to handle it. Keeping the downhaul on, to depower, was a really bad idea. Letting the mainsail out was also a quick way to go mast-shopping.
After a while, sailors became confident and began experimenting with mast rotation and sheeting in various windstrengths. Now, we rotate the mast to get a nice profile and sheet the mainsail actively. That is, until we start trapeezing, then we rotate the mast fully and the skipper begin using the traveller instead of the mainsheet. Top teams would probably still use the mainsheet instead of the traveller, but we can not afford a new mast so we keep it safe. This is with the relatively slender Marstrøm mast.
It's faster with less rotation and an active mainsheet, but you need to weight it up against possibly breaking your mast, as Wouter says.

PK, when talking about a deep spi, how deep is your spi and how flat do you want to go? The current Tornado spi's are about 20% in the deepest part, and they build a lot of virtual wind. With a flatter spi, you can go higher on the run and build more virtual wind, but you will probably hurt in low-wind conditions when you can't build enough virtual wind to justify the extra distance. Rick White sails with a "hooter" (I would call it a reacher), a spi flat enough to use roller furling, and swears by it. He also makes it go well in weak winds, but others have mixed experiences with it. It's up to you, but it's questions and experimentation that makes the boats go faster. Then again, out of 10 ideas you try out, only 1 will make you go faster, and that is also expensive


I am currently look at spi shapes, and are pondering the 'vertical' component of the spi's resultant force (force from the sail is generated perpendiculary off the surface, so you get a small vertical component). As we are a heavy team, we dont need much help keeping the bow up, except in the heaviest weather. If I could re-shape the spi so we could get a larger forward driving component, we could go double trapeezing downwind in a blow instead. Need to weight this against possibly backwinding the mainsail badly in the top, so it's all about compromises as usual. And of course, double trapeezing with a heavy team downwind might break the mast, which brings us back to the current topic..

Rolf (hello),

- I want a kite to be cut this way : Foot to one half luff height flat as a code zero ; from this point to the head - full to maximize the vertical component of the force (this is what I kept trying to achieve by moving my block to kite clew into the tramp). I don't want a kite that goes limp when I foot because it's too full to keep the flow across lee and windward surfaces.The boat and I are about 370lbs. - i'm not worrieed about a kite w/ grunt - gimme flat so I can bear away without the kite bagging backward.

- Wouter (hello) - I think I probably still have more prebend in my mast than anyone sailing an F16 solo. Maybe this is why i'm afraid of my mast
bending w/ the kite (less diamonds support w/ more spreader rake).

PK

I dont think it's possible to fly what you want. If you build it flat in the lower part, the upper part will probably not fly if buildt full. You can open the upper part up a bit relatively to the lower part, but only to a certain limit.

If you want an experimental kite, why not get to work on it?

I'm just starting to really look at the essential "why's" of fast vs. not so fast. The answers are really tough to discern and i'm amazed at the volume of misinformation readily passed along by veterans to neophytes (not here though...). At this point i'm hoping someone smarter than I will tinker a kite that suits the potential of the F16 - then, i'll buy one !

PK

Interesting thread. I don't have a superwing mast or a spi yet but want to understand these issues so I did some elementary diagramming of wind direction, boat heading, and mast rotation and looked at the forces on the mast by the spi and the masts stiffness/flexibility in these scenarios (admittedly a very crude analysis). At issue, as discussed above (and I've read before) is the need to fully rotate the mast under spi to protect the mast from breaking. The problem is when I diagram out the geometry (superwing mast profile on a cat on different downwind headings with different amounts of rotation) it doesn't seem to confirm this. For example, what rotation will stiffen the mast vs allow it to bend off looks like it depends just as much on the particular heading downwind as it does on the amount of rotation. For example:

1) When heading deep downwind, less rotation puts the major chord of the mast more in-line with the wind direction so rotating completely on this course would seem to make it more bendy and breakage prone

2) When heading downwind as high as possilbe (close to a beam reach as you can under spi) max rotation puts the major chord of the mast close in-line with the wind so rotation completeley on this course looks like it would indeed prevent breakage

3) When heading on a typical broad reach (ie, 135 deg off the wind), it looks to me like you can never get the major chord of the mast in line with the wind and the best you can do is to get it diaganolly across it (still better than across the minor chord); BUT it looks like you get that scenario both totally unrotated and fully rotated - only when paratially rotated is the minor chord parallel with the wind. Since both fully rotated and non-rotated seem to be mast protective and fully rotated presents a better luff entry angle, it seems obvious why the latter is preferred.

But the above anaysis doesn't take into account the clocking of apparent wind as the boat accellerates. If at speed the wind always clocks forward to say 30 degrees off the bow then only a partially rotated mast setting would put the major axis parallel with the apparent wind (and at apparent wind speeds the boat's heading in relation to true wind diminishes in importance).

Also, in reality, when sailing downwind under spi, it is my understanding you are really never sailing a straight line. To get going initially you head up, then fall off as you gain speed and the apparent wind builds. You also sail "S" curves to maintain optimal speed and mananage gusts/waves.

It seems like it's much more complex than "rotate fully to be safe downwind" and "derotate a little in lighter air or when singlehanded to go faster but risk breaking the mast". Your course is constantly changing, you're accellerating and decelerating and as a result the apparent wind is clocking forward as you bear off initially and ocsillating foreward and aft to a lesser extent as you go through "S" curves and gusts. Plus the true wind and apparent wind are shifting with gusts. So if you just set one mast rotation for all downwind sailing it seems like at times the the fully rotated mast position is not the most protective position and in fact no one position could be. The issue as to what is the fastest mast rotation position for these different conditions is an even more complex anaysis but not part of this question...

So regarding which single mast rotation position is most mast protective under spi, what am I missing?

Thanks in advance for educating me on these interesting and complex isues!

Jerry

This is simple to answer; you are forgetting the leech-and-mainsheet combo that acts as a backstay. (when the mainsheet is not slack)

What you do by 90 degrees rotation, mainsail traveller near to centre of the rearbeam and with the mainsheet on, is to support the mast in its weakest plane by a tensioned leech.

The stronger plane of the mast (fore-aft) is then presented to the side of the boat. Meaning that the mast top, which is unsupported in a sideways direction, can count on the stonger plane to keep it from bending to much and collapsing.

Naturally the spi is mostly pulling foreward and here the leech tension keeps the mast top from bending forward and this is alot more important then the strength of the weaker plane of the mast. Of course if you dump the mainsheet in a situation like that ....

What happens when you derotate the mast. First you rotate the stronger plane more to the fore-aft direction of the boat where its strength is pretty useless as the leech tension is more then enough to stabilize the mast top. At the same time you are presenting more of the weaker plane to the side-to-side forces. These side forces are smaller then the fore-aft forces BUT the weaker plane of the mast is disproportionally weaker in that way so in fact you are increasing the risk of damaged.

That is the reason for the following rule of thumb when sailing under spinnaker :

-1- 90 degrees rotation on your mast
-2- pull the mainsheet on tight
-3- NEVER EVER uncleat or dump the main during a gust or a fully powered up spinnaker.


These three rules are simple enough for any bozo to remember and that saves alot of money to both the owner (bozo) and the builder (warranty)

As you have discovered yourself know. The REAL workings of the boat under spinnaker are alot less easy to understand and misconceptions can quickly end up in damages.

More experiences sailors are introduced to the more nuanced techniques of trim under spinnaker when it is clear that they understand what is going on and can be trusted to understand what can and can't be done safely.

Wouter

Does it make a difference whether the spinnaker is attached at the top of the mast or lower on the mast?

I can't answer that accurrately if it is not specified in which difference you are interested in. There are things that are hugely different between the two situations and things that are no different.

Wouter

Sorry, I thought the discussion was about mast rotation.

Thanks, Wouter, for your input, I always appreciate your taking the time to share your perspective!

Yes, I was already very well aware of the need to use the mainsheet leech tension as a "backstay" - I didn't mention it in my post but rather assumed it would be the case (ie, it would be a constant). As you point out, even with adequate leech tension and the traveller centered, there is a risk of mast breakage if the the mast is not sufficiently rotated.

You said that the leech tension is sufficient to support the mast in the fore-aft direction of the boat (regardless of mast rotation) opposing the mostly forward pull of the spi, and when unrotated, the reduced support of the minor chord of the mast will allow it to bend and possibly break abeam. I understand what you are saying and the logic you are using but I wonder if there's more to it than that - an if there aren't some lessons that might be learned from those nuances? I'd like to try to take it a bit further...

The issue comes down to better understanding the sideways forces on the mast that will cuase it break when unrotated (assuming that the leech tension and spi pull counteract each other in the fore-aft direction of the boat).

I would think that the spi, like other foils, has a center of effort perpendicular to the foil, so the pull would have a foreward, leeward, and upward component. Looking at it from above I would guess it might be pulling approximately 20 or 30 degrees to leeward off the bow (fore-aft direction). If the traveller is centered, the leech tension acting on the top of the mast would be almost directly aft. If the traveller is slightly off center, as if often the case, the force would be to leeward of aft, maybe 10 or 15 degrees.

So if you map all these forces on a vector diagram, if my crude analysis is correct, an approx 90 degree mast rotation is needed to provide the counterbalancing (static resistive) force to the (dynamic) leeward forces generated by the spi and slightly off-center traveller. In that scenario, significantly less mast rotation does look like the mast would be insufficiently supported against those forces, especially in a gust, (although slightly less than 90 degrees looks like it would be fine as well). Interestingly, with a slightly off-center traveller, the leech tension is actually part of the problem - instead of contributing to mast support a portioin of it's pull is actually pulling in the same direction as part of the spi's pull (to leeward) instead of opposing it. This suggests that the more you ease the traveller, the more critical it is that the mast remains fully rotated and vice versa.

If the traveller is centered fully, according to the vector diagram, the optimal mast rotation to protect the mast looks to be less slightly than 90 degree rotation (by 10 or 15 degrees). But if you wanted to have the option to periodically partially dump the traveller in gusts, it would seem that 90 degrees rotation would be a good and safe overall compromise.

I'd appreciate your insights on these thoughts as I hope that a better understanding will be useful to me when learning to sail with the spi.

Jerry

I think you have gotten it.

So indeed 90 degrees is safe. And the issue for more adventurous sailors is to learn how much safety can be traded away for improved flow over the mainsail. Nobody knows yet where exactly the threshold is. That is with respect to the rotation/windstrength combo. Some of us, however, have learned that there is some leeway in trading off safety to performance, the superwing mast seems to be able to take some abuse. But we need still need to be very clear about that this experimentation is risky business.

Wouter

Thanks, Wouter.

I guess that's where we are -- adventurous -- because the people in my family who sail the Taipan 4.9 with spinnaker leave the mast rotation set basically the same for downwind as it is for upwind, and no positive mast rotation is used. We just basically let the mast rotate to where it wants to be, which seems to be about 45-50 degrees.

Are we living on the edge? Well, at least we have been warned by people with more experience and engineering know-how than we have.

I would guess that the most common causes of broken masts when sailing with spinnaker are mainsheets being inadvertently released or positive mast rotation devices not being released for a jibe.

P.S. I still would like to know what the difference is, as far as mast rotation recommendations, for a masthead spinnaker versus a fractional spinnaker - but maybe that should be a thread on the main forum rather than here.

Yep, I wouldn't recomend anyone to do anything that I do on a boat. Another interesting point is that maybe if you are using long spreaders with a lot of rake when you use no rotation the rig starts to act more like a 18 foot rig where both diamonds can counteract the foward pull of the spin. I think there is a lot of foward pull if your running deep. Now days in big breeze we never release much main or traveller because we are already heading so low that if you do this your just exposing more sail area especially the twisted head which just pushes your noses in. Its a matter of steering and holding on. I think you want as powerful kite as you can get as long as it can handle the apparent wind, its just a matter tuning the rest of the rig to suit. Also I think that drag reduction has to be a big factor when you have more than 20 knots of apparent wind.

Ps Inverted mains are cool we sail upwind with one all the time super fast.

Wouter, just a clarification in light of Mary's comment:
on the Taipan/Blade superwing rig, I was under the impression the boat is usually rigged with a mast rotation limiter but no rotation inducers. So, whe going upwind you would pull in the limiter the desired amount and just release it for downwind. Mary indicates that downwind the mast doesn't want to rotate as far as 90 degrees. I'm curious if this is what you and others have found? And if that's the case do you just let it rotate as far as it wants or do something to make it rotate more?

Thanks, Jerry

Correct.





Correct.





I don't have that problem. My mast goes pretty much to 70-85 degrees when the limiter is released. But then I don't slacken the outhaul from its upwind setting. Under spinnaker this is not needed at all; and loosening it will indeed only hamper mastrotation. I don't have any devices that induce mast rotation on my boat. I planned to have one but decided to just see how it went during the first season. In that time I didn't felt a need to have an inducer and so I just forgot about it.

Pretty much I will sail with whatever mast rotation results from having uncleated the limiter. On my boat this is also very low friction, I'm not to sure about how low friction the limiter setup on the standard Taipan is. I also believe the foot on my F16 mainsail is shorter then on the standard Taipan 4.9's like the one Mary is commenting about. I think the big difference is wether the outhaul is released when going downwind or not. An experienced spi sailor can be recognized by totally ignoring the outhaul once it has been set to suit the (upwind) conditions of the day. Nearly all racers use this single setting from start to finish if the boat is balanced upwind.

Wouter

Decided I'd jump back in to explain what I do in light of Wouters comment and add a subtlety about the Taipan rig:

When I round the top mark, I release the mast derotator (ie rotation limiter). After hoisting the kite, building apparent wind resetting the main tight and traveller where I want them, I then may or may not pull in the mast rotation as I mentioned earlier in order to get good flow over the mast. Like Paul (Taipan F16 #300), I know I'm going fast when I see just a hint of backwinding on the main, although I keep it to a minimum. I've never needed to pull the rotation in closer than 70 degrees, and I feel the superwing is plenty stout to take this load, having tested it heartily. But I firmly believe the superwing requires a lot less rotation on every point of sail to be effective compared to pear-shaped section masts.

Wouter mentioned that his rig rotation naturally going to 75-80 when the rotator is uncleated. I have a rotation maintainer (not a true positive rotator, like a nacra, because it can't get you past 90 degrees), but it is lame and I rarely use it (note that it is usely when sailing the Taipan without the spinnaker).

Anyway, I tinkered with the rig a bit to find out how I could control how much the mast will rotate naturally, ie with the derotator and rotation limiters uncleated. I've found that the key control for this on the Taipan is the side stay tension. So when I want to make the mast more free to rotate, I go with less side stay tension; if I want less natural rotation, I go with more tension. So in light air--when I really need the rig to rotate to 90--I set up the rig loose; in heavier air--when I don't want it rotation as much because the apparent wind is further forward--I set up the rig tight.

The result of this is the in heavier wind, I round the top mark and let off the rotator and the rig is limited to about 70-80 by the tight shrouds. Then it often ends up being just about right in terms of air flow without me needing to fiddle with any control lines.

So I like to do what Wouter says he does--sail downwind with the derotator uncleated. I just wanted to point out that on the Taipan you can set up the side stay tension to control the amount of "natural" rotation.

Wouter and Eric, thanks for the clarifications and tips, very helpful indeed.

Eric, your comment on rig tightness effecting rotation makes perfect sense; obviously you can feel that when you manually rotate the rig in no wind after chaning rig tension. Also, sometimes the rig rotates freely to a point and rotates a little further only when significant force is applied so it must be wind strength dependent as well when the rig is stiffer.

Wouter, does a tighter OH induce more rotation because the mast pushes harder on the back of the mast?

A few more rotation issues:

Do you guys think that more bent masts (whether prebent or via DH/mainsheet) rotate more easily than unbent masts (all other factors being equal)?

In your experience, does increased mainsheet tension increase or decrease rotation, all other factors equal?

Finally, from looking at the forces of the spi on the mast top (admittedly very crudely) it actually looks like the spi's pull on the mast acts to de-rotate it, as opposed to the main's pull which acts to induce rotation. The overall net rotation you get looks to depend on that balance although the pull of the spi would be entirely wind strength dependent whereas the main forces would be a combination of mainsheet tension and wind stength dependent. Does this make any sense to you? Maybe these factors can be used when sailing to tweak mast rotation under varying wind conditions?

Jerry

Increasing mainsheet tension decreases rotation and twist, but not much.

With regard to the spinnaker affecting the mast rotation, it should not have much affect with the proper set up. Keep in mind that the top of the spinnaker should be rigged with a metal bail or rope system that allows it to rotate freely around the front of the mast.

See this URL for further pictures, diagrams and explanations:

http://www.geocities.com/f16hpclass/Tech_genaker_mast_setups.html

actually most current spinnaker halyard setups have a fixed point in the middle of the mast profile at the mast top. from here a line is connected to the halyard block which than is also running on a rope [in earlier times this was a metal half arc ring] to allow the block to move to both sides.

so basically as the main force attach point at the mast top is fixed and not rotating there is still a tendency that every spinnaker tries to de-rotate the mast. how much this force is noticeable all depends on the other forces like those from the mainsail trying to rotate the mast and mainly depending on the distance between the spi halyards block running on the rope or ring and that top attachment point. so more distance you have between them, so less the tendency of de-rotating becomes visible.

In lack of a small spi yet I am using the large F18 spis on a 4.9 super wing mast. to hoist those long luff spis my spiboom is very low and still the spi halyard block is very close to the end of the mast [I am aware size of the spi and position of blocks do not fulfil current F16 class rules]. with such a close distance between the ring rope and the top block (I think it's less than half a metre) the tendency for de-rotating is extremely noticeable and I constantly have to use the mast-out-rotator to keep the mast rotated. while this surely helps to protect your mast that even a strong gust cannot easily de-rotate your mast, its tricky when jibe as you shouldn't forget to detach and reattach this rotation device!

I noticed a similar tendency on the A-cat already where the masts (and specially the marstroems I was using) are much more soft sideways than the stiff super wing mast.

Regarding the safety of the mast, feel very safe if you run a 17 m² spi single-handed on the super wing mast. you must do something terrible wrong (like opening the mainsail when burying bows etc...) to kill this mast. sailing double handed the forces on the mast (we talk about dynamic loads) generated by the spi become much higher and that is where in strong wind I start worrying about the durability of high aspects 21m² F18 spi attached close to the mast top... but as long as my mast can deal with those forces those following the F16 specifications should feel really safe with their super wing section!

Actually there is one methode that doesn't have this tendency at all. Go to

http://www.geocities.com/f16hpclass/Tech_genaker_mast_setups.html

And look at drawing 4 with the bottom 2 photographs.

I've tried this system myself but I'm now using the setup shown in drawing number 1 where The top line has a loop that is just thrown over the top of the mast and hanging of the mainsail hook fitting. This seems to work out well when the spi gate is at 7.5 mtr and thus a mtr below the mainsail hook fitting. I don't notice too much derotation under spinnaker and all derotation that I get I seem to want anyway. The effect in this setup is pretty small if it is noticeable at all, in my opinion.





WOW ! a F18 21 sq. mtr. spi on a spi gate that is 0.5 mtr higher on the mast then the F16 gates ! This is great info Dirk. You are easily putting the mast under twice the bending load that a normal singlehanded F16 sailor would. If the mast is holding up well under that then the solo F16 sailors have a considerable margin with respect to keeping the mast tops on board. I would expect this to easily translate into possible a significant derotation of the mast while singlehanding. Note how this excludes doublehanding, everybody, here the sail forces are alot higher due to much greater righting moment of the crew.

Thanks Dirk I think I will try to derotate my mast further when singlehanding under spi; just like Eric and Paul. I feel this should make the singlehander setup noticeably faster. Flat is fast !

Wouter

hi wouter, can you post the pic here, you mentioned. geocity as well as the whole f16 site is not accessible from china (like some other american websites).

regarding margin, yes I also believe solosailors should really feel safe with this very stiff superwingmast. but the best margin is still to have a look on it. looking from the mastbase to the top significantly shows the deformation the mast has to deal with. I never [knocking on wood] in 12 years spi-sailing broke a mast but damaged once a mystere 6.0XL mast when the spihalyard camcleat failed and let the 25m² spi suddenly completly slip in a gust. thats why I empahsize on the understanding what dynamic forces can causes...

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Not true.

1, Mainsheet tension will decrease twist.

2, Mainsheet tension will effect mast rotation.

but this depends on the sheeting position relative to the boom (if present).

If when sheeting in, the sheeting point is aft of vertical(behind the back beam), the boom will be pushed forward, now assuming the boom is attached to the trailing edge of the mast (most are) then the mast will be pushed forward and so rotated more (in the absence of a limiter). Reverse is true if the sheeting point is forward of vertical.

If there is no boom, then it's the position of the sheet mount on the sail; Generally I would think that the sheeting point would be forward of vertical so get control of the sail with no boom present.

Thanks Wouter, Eric, Dirk, and Scooby for the excellent discussion.

Jerry