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