Insight into small gaff sail design (~4m²) regarding mold&shape

[noah_no_ark_though]

Hello everyone

this is my first post and I am not a native speaker so forgive me for eventual mistakes and don't mind to correct me =)

The background of my question:
I have an old (50+years) Klepper T8 folding kayak which although is made to be sailed usually doesn't come with any sailing equipment.
So did mine and I decided to construct it myself, I do have some background in various construction/building techniques and a few nice pictures from a folding kayak nerd page (in German, sorry) although I am going a bit bigger with my sails.
folding_side.jpgfolding_sailing.jpg

I have to say, I have never built a sail before nor have I ever sailed before and I am very happy with this prototype, simply because I can actually sail it. The boat has two sideboards and a rudder with foot-steering. Those elements have to make up for the absence of a fin at the bottom.

Now to the details:
I looked at some literature and I researched the youtubez for insight into the curves that a sail should actually have - the one I made so far is flat like a wooden board - and I could use some pointers in regards to
- sail depth at different levels, how much and why
- and foot curve and why(not) rig the foot of the sail all along the foot
- also general help.
I managed to get sail7 and sailcut to run to see how sail shapes generally change and how to actually make them from smaller panels and that curves are like really supportive
I plan to have positive foot- luff and gaff curves and a negative leech curve. Also I plan to have a twist and a sail depth but how would I as the most inexperienced person to do so be able to guess in which direction to lean with those values?
As I am not happy enough with my sails I will rebuild them and I did make some plans and tried to analyze it and sure, it looks like a sail in the virtual world but how do I understand
- which sail depth and twist actually is still useful, and which values are commonly used for sth. like foot curve?
I added two screenshots of the constructed sail to maybe visualize my question a bit (you can ignore the boat shape):
ksnip_20230323-203643.jpgksnip_20230323-204021.png
folding_gaff_plan1.jpg
Additional:
- What is a good gaff angle and why?
- Should I be worried about the whirlwinds at the corners of the sail?
- Is there an idea of jib to gaff surface ratio?
- Do I need to account for mast bend and if so, which are the values that would usually affect that?

If it is still unclear to the reader what I am asking I am also generally happy for any pointers and hints. What is still unclear to me is also: cutting the panels and how to rig the sail to the foot/mast/gaff.
I am also interested in plans for building a small roller for the jib, in case you have anything in mind on that

Also: I did order the "Sailmaker's Apprentice" book so you can refer to that if you know that any of that is well explained there.

[johngsandusky]

Welcome aboard!
Sailmaker's Apprentice is a great book. I also highly recommend Todd Bradshaw's book Canoe Rig, especially as you are sailing a kayak. Todd is a member here, and may well comment. There are also some quick methods of making temporary sails of tarp material, search for "polytarp sails". Tarp sails are a good way to test different sizes and shapes without using expensive sailcloth. You should also watch your local classified ads for used rigs from Kleppers, though older cotton sails may have lost their shape.
Good luck, keep us posted.

[wizbang 13]

Should that wee boat not be Gunter ?

[noah_no_ark_though]

Should that wee boat not be Gunter ?

Well, I have no clue what it should or shouldn't so I went with what I found online.
There is this very detailed sketch that has been tested and approved (in the 70's or so)
ksnip_20230324-170635.jpg
I tried to approach the design from that perspective although I am not going for a besan mast+sail for now.
Also I didn't like the supports in the sail so I went straight for a gaff and just make it bigger instead of that "bat wing sail"/"butterfly sail" (whats the proper name for a gaff like the one in this pic?).

There are also some quick methods of making temporary sails of tarp material, search for "polytarp sails".

I already have the temporary sail, made from very cheap GradeB Tyvek - very amazing for this size of sail I have to say. Now it is time to make a not so temporary one - also from Tyvek ^^

Anyway, if the wind picks up to more then 4 or 5 bft I am in big trouble as most of my boat is over the water and my hull becomes as much of a sail as the sails So I don't have to worry too much about the forces on the sail canvas which is why I am going for Tyvek now. I I actually manage to produce a good sail I might replicate it from another material

[Todd Bradshaw]

You would be much better off building the batwing mainsail than trying to build a gaff sail. To start with, batwings are cut flat, saving you from trying to address all the shape issues that the sail design programs are throwing at you. Secondly, a good gaff mainsail is one of the most difficult to design and cut properly. Chances of a beginner being able to make one that actually sets and sails well are pretty much zero. Thirdly, your mast is most likely not sturdy enough or well enough supported to handle a gaff mainsail properly. It is also questionable whether or not you will actually gain much by having a jib when it has no shrouds to keep the luff tight. You are often going to experience a lot of luff sag, at which point the extra unwanted draft is likely to contribute more unwanted heeling force than sail power.

Batwing canoe sails, one Egyptian Cotton, one Dacron, both flat cut, tolerant of a fairly light mast and either type would move your boat just fine. Tyvek will work for a while, though it is no substitute for real sailcloth.

bats.jpg

Another good choice, which would work better and be at least somewhat easier to build and shape would be a balanced lugsail. It would also tolerate your light mast better.

34sf lug2.jpg

34sf lug plan.jpg

[noah_no_ark_though]

Thank you Todd, for your insight.

You would be much better off building the batwing mainsail than trying to build a gaff sail. To start with, batwings are cut flat, saving you from trying to address all the shape issues that the sail design programs are throwing at you. Secondly, a good gaff mainsail is one of the most difficult to design and cut properly. Chances of a beginner being able to make one that actually sets and sails well are pretty much zero.

That is most probably true. Still, I will try as I enjoy failing as much as succeeding and anyway it's more about the joy of building then the actual sailing afterwards.

Thirdly, your mast is most likely not sturdy enough or well enough supported to handle a gaff mainsail properly.

Do you have any info to explain that more clearly? Is the mast bend different between the sail types? Because in my opinion the force of the wind needs to go through the mast in the end, so as long as I catch the same wind the mast also needs to be able to withstand that force, no matter the sail. What am I missing here?

I went for this shape in case this becomes interesting for anyone:
ksnip_20230412-113930.pngksnip_20230412-113945.png

Which in total produced the following cut:
ksnip_20230402-124733.png

Now I am waiting for the arrival of the sailmaking book to understand the mistakes I might make when cutting/sewing before actually wasting the material =)

[Todd Bradshaw]

That is most probably true. Still, I will try as I enjoy failing as much as succeeding and anyway it's more about the joy of building then the actual sailing afterwards.

Well, building to that "plan" will certainly achieve what you enjoy. You apparently wish to build something which looks like a gaff sail, but don't care if doesn't work like one should. In my opinion, that is truly a silly waste of rather expensive materials, but whatever floats your boat (regardless of whether or not it will actually move it).

A gaff sail needs a pretty solid and non-flexible mast, more so than something like a lugsail. The lug will pretty much maintain its shape, regardless of mast flex. They are kind of like flying a kite from a pole. The pole can flex a fair amount without totally ruining the sail shape that you are trying to achieve. A gaff sail, on the other hand, gets its draft by attaching the sail's luff round securely to a pretty stiff and straight mast. That extra cloth is then forced aft, forming the sail's draft, positioned by broadseaming (or panel-shaping in the case of computer cut sails). A mast which is too flexible will be changing the sail's draft and its position (chord shape) constantly, which is a terrible thing to do from a performance standpoint. In addition, the gaff jaws themselves will exert a good amount forward pressure and bending force on the mast at the spot where they intersect the mast - which is not a place where you want your mast's shape to bend and be distorted.

The plan you show has some serious problems. First of all, the deepest draft (shown in the three profile drawings) is way too far forward. A gaff main's deepest draft is generally around 40%-45% aft of the mast. Secondly, there is no shape along the head (gaff). The panel seams don't appear to be adding any shape to the sail, other than that curious little bit at the throat seam (a place where you generally don't want to be adding much shape). The leech would require at least three battens, otherwise all that roach is going to just flap like crazy, and the same thing could easily happen with the foot round, because there is nothing present to give it some shape. As straight as the panel seams are up front (especially the lower one) there is nothing there which will generate the steep entry angles being shown in the lower two profile drawings. Essentially, your bottom drawing can't possibly produce what is shown in your top drawing - which itself would not be the shape you want or need on a gaff sail anyway. So if you "enjoy failing", you are set up perfectly to do so, and much of it is being caused by the computer problem itself.

[noah_no_ark_though]

Thank you Todd for your very detailed answer.
I was surprised myself that a shape as in the dimension table and mold shaping would produce that little amount of shape and when I built a model of it in 1/10 proportions it also seemed to have less shape then intended. I will do some testing in the 1/10 version of the sail to see how to change those parameters to my liking. Maybe I confused some value for the printing of the panel drawing, thanks again for pointing it out..

The battens at the leech are already planned although not in the drawings - those are more a sketch to get an understanding of the process of making the panel shapes.
I didn't understand why I would add any shape along the head (gaff) but of course you are right it makes only sense to have it there.

I do also understand the point of how mast bend influences the gaff shape. That is something to address definitely and I have already found some ideas on mast shape/thickness/stiffness here in the forum and hope to be able to do good here. At the moment my mast is just an aluminum tube that I got at the home depot but of course I could inspect some other diameter/thickness to support a gaff more then at present state. If anyone reading this has an idea about determining wall thickness of the aluminum pipe to use for a mast of that scale: I am super curious!

As for the wasting of good material: I agree, there is waste, but also there is learning and as such in combination I don't see it as a total loss. Sure, at some point I want to be sailing a nice boat that is a lot of fun and challenge, but what I meant with "I don't mind failing" was to acknowledge that that way is long and I don't plan to jump to that end with my first go =)
I am encouraged by your patience and I am very curious where this road will lead me.

[Todd Bradshaw]

From this side, it looks to me like you are jumping into the PhD program without ever bothering with the undergrad studies to learn the basics. I am not aware of anyone, myself included, doing that successfully when it comes to making decent sails which work properly. There are vast and obvious gaps in your knowledge on the subject and a mediocre at best computer program is not going to make up for them. You aren't learning anything by using it and the best thing you could possibly do is ditch the program, get a couple of good books on traditional sailmaking and start over - on your hands and knees on your floor with strings, push pins, masking tape and a couple of battens. Sailmaking isn't all that difficult, but there are a lot of tedious little details and factors which have to be accounted for. It's not really one of those things where you learn by making mistakes. Instead, you learn by studying.

The round missing along the head of your sail is a good example of a couple of those things that need to be accounted for. The curve along the head will be the sum of two different factors. On a small sail like that you would probably have an inch or so of round added to create a small amount of draft up high, and another maybe 1.5" to 2" of additional round added to account for the bend that a small gaff will probably have (which would eat up and cancel the round that was put in to create draft). Neither you nor your computer program knew enough to include either of those factors.

I think I have this Dropbox photo spread set up so that anybody can open and view it. It has 32 photos showing the design, lofting and production of a couple of small lugsails - traditional-style with no computer work other than drawing the original plan. A gaff main of similar size would be a pretty similar process, though the lugsails have straight luffs without round and a gaff sail would have luff round.

https://www.dropbox.com/sh/5ztjj8bmt...clSgdPoKa?dl=0

(for some reason, the series starts at the bottom of the list and proceeds upward)

I still think that building a gaff main that size is a very dumb idea when other sail types would be a much better choice. As for your tube mast, consider that even a simple lateen rigged Sunfish uses a 2" diameter aluminum tube in order to generate sufficient stiffness and strength. Just wall thickness on a small tube isn't going to do it.