Hi guys,

I wonder if anyone has any thoughts on why my new home-built spinnaker is not particularly stable when flying. The luff is constantly collapsing - the slightest change in momentum or wind/ boat direction and it folds. The luff, leech and foot lengths are all within a couple of cms or so of my Landy kite although it is much flatter than that spin.

Once it is flying the spin seems to be developing loads of power, it's just so fragile.

I've tried altering the hoist height, both longer and shorter. To a lesser extent I've played around with the sheeting angle. Now the damn thing is really starting to frustrate me as I can't see what the problem is.

I'm wondering whether I need to cut some more round into the luff to make the entry flatter?

Here are a couple of pics:

My guess is luff entry angle and luff shape. Your shape might be to flat in the front, so you have a very narrow 'groove' where the spi flies as you want it.

It's really hard to tell from the pictures. It would be easier if you did some photos from below it and straight upwards. Even better if you had some draft strips on it (I know, not good looking on a spi). It's important that you have it sheeted as you would when sailing when you do this.
If you hoist your old spi and take some pictures from the same angle with the same settings for sheets etc, it should be easy to compare luff shapes. Accumeasure from UK Sailmakers (downloadable from their homepage) is a nice tool for comparing draft and shape trough pictures.

Thanks Rolf,

I'll check out their page.....

http://www.ukhalsey.com/accumeasuredl.asp

John,

My instant reaction (gotta fly to a meeting) is that the top (as you say) needs to be flattened; you do not have a consistant entry angle across the luff:



Bottom is very flat, top is very curved and so you cannot ever get it all to set at the same time.....

could you change that by just changing the location of the sheet block? Would pull more evenly on the leech and pull the top in a little more.

I agree, there is to much "shoulder" in the luff entry in the top third or so. This reduces the angle of attack of the kite to a very small margin, and the bottom of the luff has an entirely different entry giving it a much wider set of angles at which it will work -it should be like trying to set two kites of different cut from the same points at the same time - not easy. Only conclusion that I am aware of is to recut the luff entry.

I'm planning to unstitch the luff tape tonight and then insert a tiny 'dart' of cloth into the seam at the leading edge shoulder. If all goes well, I'll use the kite this weekend and report back.

Good luck John, it sounds like a good workaround if you get the dart outline right (I find it difficult to "guesstimate" this). Perhaps use both stitching and glue to make sure the seams can handle the load?


If anybody would like to have a look at a spi-design I am working on, you can download it from: tornado-spi-27-11-05.saildef

The software needed (Sailcut CAD) can be freely downloaded from:

Sailcut-1.2.0
Qt runtime

Any comments appreciated.. I am not as bold as John, so I will do a 1:10 crosscut model and see how it flies before I order the cloth.

I think you might need to do this as well.

Looking at (what we can see of) the exhaust profile, the leach does look like it's a little open; Putting a dart into the luff will also close the leach slightly, but maybe not enough.

John, I assume you have enough room inthe sail measurement rules to make the mod?

Jalani,

Every one has had their two bob's worth......now its my go.

Its great to see you are having a go at spinnakers as i have made several now & believe I am qualified to offer some comments.

Making mainsails & jibs is not too hard, but spinnakers & particularly assymetrical spin on a cat is very testing. Anybody can make a spin, butvery few can make a "nice" spinnaker.

I agree with Rolf in that the sail is very fine in the front half of the sail & I have too fallen for this trap. But all the comments abou the shape distribution along the luff looks like the tension difference between the luff tape & the spin cloth when you stitched the sail up ( I too have done this.

Before you pull the sail apart try tensioning the luff line a litle to se if you can bring some "feel" back in the kite as tensioning the luff line will put some shape into the front half of the sail.

Becarefull puting some "darts" into the front of the sail as this will reduce any luff round that was there previously & hence you loose some sail area.

Where did you develope the sail shape & panels for your spinnaker?? I use prosail & find it very good, but the "flying" shape is very different to what you see on the boat screaming down hill.

I think your first effort reminds me of mine & your definitely heading down the right path.

Attached is a photo of a grasegal kite off my tornado & my home made kite. I know what are the differences are, can any of the mambers of the group have a go at evaluating the changes needed.??

Regards

marcus

bugger only one attachment in the last post....here is the gransegal kite

jeeze just re read my post ...sorry about the spelling errors & grammar.....its 1am & im off to bed....

I use Sailcut, and it's the same there. The flying shape often differ from what it looked like on the screen. You need to use your knowlegde about sails and wind to get a good result. I made my first jib deeper in the top, and with some nice twist like my litterature suggested. It would probably have set nicely on a monohull, but on a cat the leech was too open.



I'll take this as a learning opportunity and have a shot:

The luff on your spi looks looser than the Gran spi. Longer with more round?
The head section on the Gran sets better, perhaps due to more panels. But the Gran spi also look like it have a horizontal cut (dart) made in the back section to tighten the aft section and the leech.
It's very hard to say from these pictures, but your spi looks like it has the same draft, but more shape to the luff section.
With a looser luff and more shape, I guess you had to go very deep to make it fly properly.

Why dont you take pictures from beneath the spi and towards the halyard block, like we do on jibs and mains? I find it easier to compare shapes that way.


I just bought an old Pfaff 1222 with triple stitch z-z and 'walking foot' for US$ 150,- Should be good for both ripstop and dacron. Issues like different tension in luff tapes and ripstop should be easier to handle with this machine. Looking forward to being able to sew zippers and luff tapes without basting them first.


The F-16 forum certainly is the most interesting and technical..

Just to update those who took an interest in my sail building attempts:

Thanks for your input Marcus, but I hadn't put a luff line in when I built the kite. Perhaps next time....

Anyhow, as I said I would, I unstitched the luff tape down to about half height (I figured that the bottom half of the spin didn't look too bad). Then I opened up the upper horizontal seam through two panels. I made a small dart of ripstop about an inch deep and just over a panel wide and then re-stuck the panels. I then resewed the luff tape, taking extra care that the tensions in spin cloth and tape were the same.

I tried it out last weekend (had to scrape the frost and break the ice off the boat ) - winds were 'challenging' at 12 - 35 knots! - and although it IS better - it's still not quite right. I now suspect that I may have built too much twist into it, as although it's basically the same dimensions as my Landy kite, it looks totally different and the sheeting angle doesn't look right. The foot comes tight waaay before the leech, so I'm going to rig up a sort of adjustable sheeting arrangement and have a play with that.

BTW the sail was designed on Sailcut 6 and the shape is the result of discussion with Dave Tugwell at Datchet who also posts on this forum from time to time. David has already built quite a few spinnakers for his Stealth (and for other people too!) so he's been a great help. However, he is still experimenting as am I.

The panels were cut professionally and I simply stuck and sewed the whole thing together. Next one though is going to be wholly sewn as I didn't like the end result of the sticking process - it's too easy to leave an edge of tape showing and then you've got to de-sticky it with talcum powder, also alterations and adjustments are less easily done when you've got sticky remnants to deal with.

I'll keep you posted on progress - or lack of - (if you're interested)

John,

Sailcut 6 is not well suited for asymetric spis. You should try Sailcut CAD instead (the open source version). Just use the "jib" choice.
I talked with Robert Laine about Sailcut 6 and asymetrics, and he agreed that Sailcut CAD would do better as you will have problems specifying the shape you want in Sailcut 6. The design envelope and buildt-in controls are more suited for slower boats with less apparant wind in Sailcut 6. Sailcut CAD is not ideal either, the design envelope is a bit to tight, but it's better than Sailcut 6 (it's ecpecially tight in the leech area, for a spi. Hard to get the shape I want).

What file format did your sailmaker accept for cutting?

Could I ask how much twist you designed into your sail? Twist is (as you no doubt know) also a function of sheeting angle/tension, and I dont think you need to design much twist into a kite.


glue/stitching: I have made a few kites (not spis) without basting the seams with basting tape. I am much happier sewing when the panels are glued, as I can sew much faster. Unless your sewing machine is sunk into a large table or the floor, I think you will have lots of wrinkles and alignment issues. I would also recomend a 'walking foot' machine or a Pfaff with IDS if you dont want to use any kind of glue. Otherwise, you will find that the bottom panel is fed trough the sewing machine faster than the top panel. The same goes for luff tapes.
Basting/gluing takes time, and you need to be accurate as you say. But it goes faster after a while when you find your technique.

Please do keep us informed, I find this discussion very interesting (as I hope others do as well).



Rolf

Rolfe,

I agree with your comments ont the two T kites. The main change I was considering on the next T kite was to increase the number of panels in the luff and put a little more bull nose. Since both of my T's are now gone I will start to think of an efficient kite for the blade sixteen we are building. I think I would like to see my first try a bit fuller than "Jalani's" pictures more shaping in the luff. Wait and see I suppose.

With regard to seam tape, I use twelve mm tape on fourteen mm seams and you dont have the problem with the seam tape exposing. I also only stitch the cross and vertical seams only.

Marcus