David - One more question(s) to the list of questions.

Your yard and pocket extends in front of the luff.  Is that because you are working with the sail cloth at a right angle to the yard to align the stresses or does it help the yard stay in front of the mast when hoisting/dousing or does it do both or does it do something else?  :)

As always, thanks so much for sharing your knowledge!

Erik

Erik and Evi Menzel Ivey wrote:

Hinges - per the posted drawing, your hinges are essentially hollow.  Are you using a line inside of the batten sections and hinges to hold it all together as a unit?

Yes, I put a line through the central part of the batten and the two hinges, to make it all easier to assemble into the sail, but not through the forward and after batten sections. This was possible because I had some tension in the pocket. I need to change this, slackening the fit of the sail to the battens so as to get full value out of the small amount of barrel cut camber that I have, so I will need to have a line linking all components.

Upper battens not flipping as easily as lower ones - If I understand it correctly that would be because they are not sheeted and depend on the luff tension and sail pressure to flip? 

It's only the wind pressure in the sail  that causes the hinged battens to flip. The upper leech and luff are under tension because of the weight of the lower part of the sail and battens, and the upper part of the leech is convex. This puts the upper battens under compression. When a batten flips, its ends first move further apart, until the batten is straight, and then move back towards each other. It's the moving apart, against the compression, that inhibits flipping in the upper battens. 

Boom -  Not that I would have thought of it, but it makes sense that the hinged boom will act differently from the battens since it is only supported by the sail panel above it.  Since the battens above have forces pulling downward at the luff and leach, and from the weight of the sail and sheetlets, how do the the topping lifts (lazy jacks?) and mast lift lift pulling upward help?

What I meant was that the topping lifts and mast lift, if they are trying to support a double-hinged boom, must support each end of each of the forward and after sections. A fixed span on the mast lift is the main change from normal practice.

Batten pockets Ahhhhh.... It is always these nuggets of information that scare me because they make me feel like I know so little.  I was wondering if the small amount of wrinkles that seem to radiate from the upper luff corners of the the lower panels are due to the small distortion from the pocket design or is there another reason?

I think this is due to having hauled the sail too tight along the batten. I will try a slacker fit next season.

Erik, I've inserted some answers below.

Erik and Evi Menzel Ivey wrote:

David - thank you for the detailed description of the Weaverbird planform.   

Aspect Ratio:  Your comments about the max AR for the planform are encouraging.  I think I also understand the concern regarding the straight line panel diagonal distance and how positive stagger is affected by camber.  Both you and Arne have written about it at times.  More camber means more length of cloth, and the method of panel design suggested in PJR doesn't make allowances for it.  If I understand correctly, a larger batten angle decreases the critical diagonal, but is that related to "This will mean that the pitch of the panels will be reduced,"  I don't quite understand "pitch" in this context.  Also, am I correct in my understanding that  your current sail design is in part relying on restrictive BP to induce positive stagger. 

By batten pitch, I meant the vertical height between battens, "P" in PJR. In a high AR sail, it can be difficult to stop the luff from moving back and fore; even when there is good control over the top of the sail, using a THP or LHP, and the bottom part is constrained by a tack parrel, the middle part has some freedom to move, introducing a curve in the luff, concave or convex, or alternating. Short batten parrels stop this happening, so much so that the sail sets automatically in the right place, with zero stagger. I don't have to do anything after hoisting sail except take the slack out of the YHP. This is very convenient, but comes at a price - more friction. More friction when hoisting, and downhauls being more desirable when lowering. This is acceptable in a small sail, but may be less acceptable in a maximum-size sail. I would be inclined to think in terms of what Arne has called "semi-short batten parrels" - not so long as to allow the sail to move fore and aft completely freely, not so short as to bind on the mast when hoisting or lowering. With these, I would use an upper luff HP and a mid luff HP, to control the fore and aft position and also to act as downhauls.

Sail balance, yard angle, yard length:  These sound like an iterative approach is necessary.  On Arne's design, with a 10% sail balance and the yard length equal to P, a 70 deg yard angle seems to result in a sling point sufficiently aft of the mast but not too far.  Makes sense from the geometry that if the yard length doesn't change, the sling point moves aft and shortening the yard moves it forward again.  Other design elements such as sail balance limits to still allow hinges to work and cloth orientation coming off the yard are noted. For our boat, a 10% sail balance seems "fit" depending on whether equal sized sails or a schooner rig.

The balance of my sail is 15%. It can't really be any less with a low angled yard, so if you/Raven can only accept 10%, then you will need to go with the higher yard angle of Arne's sailplan.

Hinged Battens:  Understood!

Hinges/Battens:  I have not sense of the loads on the junk sail so your comments are much appreciated.   As a first guess I was thinking the hinged batten should be stronger or at least as strong as the straight battens on a purely cambered sail.  Are the stresses and loading on the battens significantly different (lower) when they are allowed to hinge?

I think that there is little difference between straight and hinged battens in the amount of bending load developed in the tubes, and the bending load peaks in the centre of the batten. However, there are local "bursting" effects near the hinges.

Some added questions:  

Are the hinges held within the batten by the batten pockets or are the hinges held in the battens by some other means.  

As Arne says, the hinges need to be held together. The pockets can tend to slacken in some circumstances (eg flat sail, bulging pocket), and in the early days of using hinges, there were cases where they separated. Some kind of line, strap, or lashing should be devised, depending on the detail design of the hinge.

It seems like any tension to stretch the sail along the batten pockets would make it harder for the hinges to articulate.  So shaping and the smoothness of the sail has to rely on the initial cut.

See below - if the pocket goes into tension on one tack, there is some camber inhibiting/limiting effect. There is a stronger effect in the top two battens, whereby they are reluctant to flip from one tack to the other, if there is a lot of compression on them (this argues in favour of a lower yard angle and an even division of the change of angle in the leech from top panel to lower leech); and if the hinges have too much of an angle of articulation. I only use one hinge in the top batten, with an angle not exceeding 10 degrees. The next batten can have two hinges, with a total angle not exceeding 15 degrees, and the lower battens can have two hinges, with a total angle not exceeding 20 degrees. The bottom batten (boom) can have a total angle not exceeding 20 degrees also, but it has to be remembered that this angle also obtains in the vertical plane, so must be controlled by the topping lifts and mast lift. I have found these angles by trial and error, and if I stay within them, I get good articulation in all but the lightest of airs.

Your batten pocket design is such that the batten symmetrically "bulges" the sail and the pocket.  This shows nicely in the photos and would encourage the sail to set the same on each tack.  I noted that at the mast, where there is not pocket to allow for the BP to attach the battens to rest directly on the mast, the sail remains flat.  Could this possibly cause a distortion of the sail shape if the diameter of the battens - on a larger sail - is larger?  

Actually, I aim to make the pockets such that the batten "bulges" the sail only one quarter of its diameter. You have correctly identified that if there are gaps in the pockets, and if the sail is not flat, there must be some distortion. It would be easier to sew the pockets on flat, so that the batten's centreline is in line with the sail, and this would fix another thing: if the sail is flat, and the pocket does all the "bulging", then the hinges tend to come apart on one tack, go into more compression on the other. This is undesirable. But if the pockets are sewn on flat, there can be no gaps in them. So, I compromise with just a little bulging of the sail. 

Erik

David - thank you for the detailed description of the Weaverbird planform.   

Aspect Ratio:  Your comments about the max AR for the planform are encouraging.  I think I also understand the concern regarding the straight line panel diagonal distance and how positive stagger is affected by camber.  Both you and Arne have written about it at times.  More camber means more length of cloth, and the method of panel design suggested in PJR doesn't make allowances for it.  If I understand correctly, a larger batten angle decreases the critical diagonal, but is that related to "This will mean that the pitch of the panels will be reduced,"  I don't quite understand "pitch" in this context.  Also, am I correct in my understanding that  your current sail design is in part relying on restrictive BP to induce positive stagger. 

Sail balance, yard angle, yard length:  These sound like an iterative approach is necessary.  On Arne's design, with a 10% sail balance and the yard length equal to P, a 70 deg yard angle seems to result in a sling point sufficiently aft of the mast but not too far.  Makes sense from the geometry that if the yard length doesn't change, the sling point moves aft and shortening the yard moves it forward again.  Other design elements such as sail balance limits to still allow hinges to work and cloth orientation coming off the yard are noted. For our boat, a 10% sail balance seems "fit" depending on whether equal sized sails or a schooner rig.

Hinged Battens:  Understood!

Hinges/Battens:  I have not sense of the loads on the junk sail so your comments are much appreciated.   As a first guess I was thinking the hinged batten should be stronger or at least as strong as the straight battens on a purely cambered sail.  Are the stresses and loading on the battens significantly different (lower) when they are allowed to hinge?

Some added questions:  

Are the hinges held within the batten by the batten pockets or are the hinges held in the battens by some other means.  

It seems like any tension to stretch the sail along the batten pockets would make it harder for the hinges to articulate.  So shaping and the smoothness of the sail has to rely on the initial cut.

Your batten pocket design is such that the batten symmetrically "bulges" the sail and the pocket.  This shows nicely in the photos and would encourage the sail to set the same on each tack.  I noted that at the mast, where there is not pocket to allow for the BP to attach the battens to rest directly on the mast, the sail remains flat.  Could this possibly cause a distortion of the sail shape if the diameter of the battens - on a larger sail - is larger?  

Erik

Erik,

Thanks for your interest in the weaverbird planform, in connection with re-rigging your Freedom 40 Raven. I'll start this general topic now, and leave it to you to start another topic specifically about converting Raven.

As it happens, I've just finished making a sail for the YM Senior 16ft that Martin Norris is building in Cockermouth, quite near to me. This sail has an area of 13 sq m, and is directly scaled down from Weaverbird's sail. I've made it flat, because Alan and Gloria Parsons have kindly made available the spare cloth left over from making Badger's sails, some 10 -15 years ago. It's 9 oz/sq yd, and I judged that it would be very difficult to build any shape into a small sail in heavy, stiff cloth. All the camber will come from hinges, with their conical angle slightly increased from that which I used on Weaverbird.

If the planform is to be scaled up, the first thing to do is to add another panel on the bottom. I only needed 5 sheeted points, but a bigger rig would be better with 6 sheeted points. This would increase the aspect ratio. It would also make the planform more suitable for a ketch or schooner. I'm confident that the planform can be used on a two-masted rig, as it's a development of the ketch rig that Tystie now has.

More later.