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Mast Rotation #65174
01/21/06 12:47 PM
01/21/06 12:47 PM
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Edmonton, Alberta, Canada
bobcat Offline OP
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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?



F16 Blade 716
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Re: Mast Rotation [Re: bobcat] #65175
01/21/06 02:10 PM
01/21/06 02:10 PM
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St Petersburg FL
Robi Offline
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Bruce, if you do not rotate the super wing mast, you are running a chance of snapping it in half.

Re: Mast Rotation [Re: bobcat] #65176
01/21/06 02:47 PM
01/21/06 02:47 PM
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US Western Continental Shelf
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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


Santa Monica Bay
Mystere 6.0 "Whisk" <--- R.I.P.
Re: Mast Rotation [Re: bobcat] #65177
01/23/06 01:40 AM
01/23/06 01:40 AM
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Central California
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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.


Eric Poulsen
A-class USA 203
Ultimate 20
Central California
Re: Mast Rotation [Re: ejpoulsen] #65178
01/23/06 11:29 AM
01/23/06 11:29 AM
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Edmonton, Alberta, Canada
bobcat Offline OP
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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!



F16 Blade 716
Re: Mast Rotation [Re: bobcat] #65179
01/23/06 06:57 PM
01/23/06 06:57 PM
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Wouter Offline
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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 !


Quote

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.


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



Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: Mast Rotation [Re: Wouter] #65180
01/23/06 08:15 PM
01/23/06 08:15 PM
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Michigan
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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.

Re: Mast Rotation [Re: PTP] #65181
01/23/06 09:10 PM
01/23/06 09:10 PM
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Wouter Offline
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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 Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: Mast Rotation [Re: Wouter] #65182
01/23/06 09:22 PM
01/23/06 09:22 PM
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Michigan
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Wouter,
Thanks for the info... much help and one less thing to worry about. You da man,
pp

Re: Mast Rotation [Re: bobcat] #65183
01/24/06 04:27 PM
01/24/06 04:27 PM
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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 ).

Re: Mast Rotation [Re: pkilkenny] #65184
01/26/06 10:45 PM
01/26/06 10:45 PM
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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.

Re: Mast Rotation [Re: paul57man] #65185
01/27/06 01:49 AM
01/27/06 01:49 AM
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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

Re: Mast Rotation [Re: pkilkenny] #65186
01/27/06 06:39 AM
01/27/06 06:39 AM
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Wouter Offline
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Quote

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'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.


Quote

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.



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.


Quote

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...



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


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: Mast Rotation [Re: Wouter] #65187
01/27/06 07:17 AM
01/27/06 07:17 AM
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West coast of Norway
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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 MarstrÝm 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..

Re: Mast Rotation [Re: Rolf_Nilsen] #65188
01/27/06 10:36 AM
01/27/06 10:36 AM
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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

Re: Mast Rotation [Re: pkilkenny] #65189
01/27/06 11:04 AM
01/27/06 11:04 AM
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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?


Re: Mast Rotation [Re: Rolf_Nilsen] #65190
01/27/06 11:50 AM
01/27/06 11:50 AM
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Quote
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

Re: Mast Rotation [Re: pkilkenny] #65191
01/27/06 04:21 PM
01/27/06 04:21 PM
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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

Re: Mast Rotation [Re: rbj] #65192
01/27/06 05:12 PM
01/27/06 05:12 PM
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Wouter Offline
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Quote

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



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



Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: Mast Rotation [Re: Wouter] #65193
01/27/06 05:20 PM
01/27/06 05:20 PM
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Key Largo, FL & Put-in-Bay, OH...
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Does it make a difference whether the spinnaker is attached at the top of the mast or lower on the mast?

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