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Thread: Using Sailcut for Sail Design

  1. #1
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    Default Using Sailcut for Sail Design

    My winter project: a sail for my canoe, a Hiawatha from Ted Moores' book Canoecraft. I modified the design and built it from plywood.


    The balanced lugsail I'd like to add is based on a design by Tood Bradshaw: http://webpages.charter.net/tbradsha...s/!LUGDWG-.PDF


    I reduced the size to 32sq.ft. and built a scale model for my own education (slideshow). For panel development I used Sailcut (free download).
    Sailcut CAD is a sail design and plotting software. It allows you to design and visualise your own sail and compute the accurate development of all panels in flat sheets.
    I attached the file if you like to try it yourself. todd's fat lug 1.zip


    Last edited by flo-mo; 11-14-2012 at 08:26 AM. Reason: Added an image of Todd Bradshaw's design

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    Default Re: Using Sailcut for Sail Design

    Whatever design you choose make sure that it is simple to operate. In a canoe you will not have the stability to jump up and remedy faults that occure in your rigging or sails. Simpler is better unless you are a veteran canoe sailor. Having a sail that has a lot of artistic merit will not help you if the wind increases and you cannot control your sail.

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    Default Re: Using Sailcut for Sail Design

    Pretty neat! Looks like we probably have more head round than needed, so I might reduce the round some for the real sail. It takes only a tiny bit of round (maybe 3/4" or so on a 32 sq. ft. lug) to give us all the draft we'll ever need up top. The rest of the head round that's added is just to compensate for in-use yard bend, and that will usually be another 1"-1.5" or so on sails that size. More than that and the sail is likely to get awfully drafty in lighter winds when the yard isn't bending all that much. You'll also probably want to switch from spiral lacing to individual robands on the real sail's top. This would give you immunity from group failure and prevention of the slack migrating up and down the yard freely and messing up the sail to spar distance.

    Did it say how far forward it placed maximum draft? If it's in the 30%-35% range, you might move it aft a little bit (40%-45% would be more typical).

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    Default Re: Using Sailcut for Sail Design

    Todd, I was hoping you'd chime in - I did not expect it would happen so fast.

    Too much head round - my mistake. I built it with 3" should be more like 2" - forgot to scale it accordingly.

    Thank you for telling me about the correct placement of maximum draft. This is an important information for me. You are right with your observations and I am glad that the photos of the model provide enough information for judgement of the sail shape. I consciously used more draft than neccessary to make things more obvious. Sailcut's mould dialog is for setting the draft but I was not sure about the numbers.

    The images show the settings I used for the paper model and how they effect the profile. In comparison there are images with the corrected numbers you suggested. I hope this is a step in right direction.


    Paper model


    corrected numbers


    Paper model


    corrected numbers


    Todd, thank you so much for your contribution not only to my thread but to the forum in general.
    Last edited by flo-mo; 10-14-2012 at 08:37 AM.

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    Default Re: Using Sailcut for Sail Design

    The file with the corrected numbers: todd's fat lug 2.zip



    In the dimensions dialog you can also enter a number for twist angle.
    This is what the Sailcut CAD Handbook says about twist angle:

    The twist angle is the angle expressed in degrees by which the top of the sail is rotated with respect to the foot.
    I am wondering is there a typical number for twist angle for a balanced lug sail?

  6. #6
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    Default Re: Using Sailcut for Sail Design

    FLO-MO wondering why the flat bottom section? I will dig out my numbers to compare later on.

    Is that to produce a laced foot? SailCut cannot manage a laced foot with any draft, so I always go for a loose foot design and use a fuller foot shape.

    Brian

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    Default Re: Using Sailcut for Sail Design

    Hi Brian, nice that you chime in too.

    I thought that even with a loose foot it is desirable to have kind of a cupped shape therefore I reduced draft at the bottom section.

    But what do I know!
    My entire sailing experience is fooling around with a laser for half an hour and though I actually did not know what I was doing it was so much fun.
    I am eager to learn more about sails and sailing so I am thankful for any contribution by more knowledgeable sailors (that means by everybody) but especially by such an experienced one like you.
    Last edited by flo-mo; 10-14-2012 at 09:22 AM.

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    Default Re: Using Sailcut for Sail Design

    Nice project. I'm building a 12-foot sailing canoe (50-50 sail/paddle) and also plan on a small balanced lug for a sail. I'm playing with Sailcut also, so this is good information. Before I make the sail for the canoe I'm getting a kit from Sailrite to re-rig my old Cape Dory 14 (yes I know...faux-wood hull!) as a lug yawl. The kit is inexpensive and I figure (as Todd and others suggested) that it'll be a good way to learn more about sewing lugsails. I've made a number of other sails previously (marconi, gaff, and jibs), designing them myself manually, and they all turned out pretty well.

    Good luck with your sail rig, and let us know how it turns out!

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    Default Re: Using Sailcut for Sail Design

    It's typical with either computer design or whatever you call the "by hand and by eye" sort of traditional cutting that some of us old guys do, to bring the sail's draft back in as you approach the boom on both laced and loose feet. This is done with the broadseams along the foot on traditional sails or with panel-shaping on computer sails. It keeps the foot round from flapping and gives us a bit of the old "end-plate effect" (helps keep the wind pressure from sneaking around the bottom of the sail).

    Since we are dealing with a light, easily accelerated hull with a fairly high speed potential here, one could make a case for keeping the draft pretty far forward. In general though, since a lot of canoe sailing is done in fairly moderate conditions in a rather vulnerable open boat, I think easing it aft to a somewhat more "normal" position (40%-45% aft of the luff) usually gives better all-round performance for folks who are just sailing for fun.

    Twist has always existed, though being able to measure it and plan for it is new with the advent of computer design. An example of a place where this capability really comes in handy would be a tall, skinny catamaran mainsail with eight or nine compression battens spread along its height. The wind direction itself is twisted to leeward up high and the compression battens often may twist the leech to weather a bit (the wrong way) and cause a hooked leech (over-trimmed up high when trimmed properly down low). The tall sail and batten tension can create a "perfect storm" for this and greatly reduce the effectiveness of the top half of the sail. Being able to make specific, fine-tuning adjustments to the sail's twist can give us a much better sail for that boat, and a computer that can mentally juggle a whole bunch of figures and angles accurately and at once sure beats an old sailmaker with a pencil and a pad of paper.

    On lower-aspect, more traditional sail types, the twist is much less of a pressing issue and has largely (and successfully) gone pretty much ignored for generations of traditional sailmaking. It's still going to happen, but the amount present by basically doing nothing to the design to encourage or discourage twist will tend to fall within reasonable parameters. Even if you design a traditional sail type with a computer, the program can probably tell you what the twist will be, but it would be unusual for it to actually modify the cut much at all away from the same basic shape that a traditional builder would do.

    For example, These are the 39 sq. ft. and 15 sq. ft. balanced lugs for Geoff Chick's self-designed sailing canoe/yak. No adjustment was made for twist in the design phase, yet it is visible and seems to be falling within a reasonable range.



    Here's another one on a Skerry. We're probably usually looking at twist in the 7-10 degree range when sheeted in for upwind sailing as being typical, and that seems to happen without needing to do much about it to the design.


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    Default Re: Using Sailcut for Sail Design

    Model canoe + model sail:




    After a lot of consideration I think will use a sail design that is similar to the Solway Dory Expedition Rig or Michael Storer's Drop in Lug Sail





    Both sail designs have in common that mast yard, and boom have the same length which should make storage on bord easier.

    I am planing to use these Bamboo poles for my mast, yard and boom (all three 7'):


    This is my new design (36 sq.ft.):




    and here is the Sailcut-file: expedition lug sail 3.zip

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    Default Re: Using Sailcut for Sail Design

    For comparison I add the images of the Sailcut data of the new sail design as well:








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    Default Re: Using Sailcut for Sail Design

    What does the top edge of the bottom panel look like? Straight edge or a curve? It would be unusual for the bottom not to curve inward toward the boom a bit, cupping the bottom of the sail. As shown, it creates a straight line of tension between the tack and clew corners, and anything outside that line (the foot round) will be prone to flapping, especially as the sail ages. The tack seam is usually heavily broadseamed (or panel-shaped for computer designs) so you have quite a bit of draft along that lowest seam. To bring the draft back in along the boom and give you some end plate effect on a cross-cut sail, they usually use one of two methods. #1 would be to split the bottom panel vertically (perpendicular to the cross-cut panel seam) and about 40%-45% of the way aft and reassemble it with a broadseam as you approach the foot edge. This would be standard procedure for most loose-footed cross-cut sails to give the sail's bottom a bit of a cupped shape. #2 is sometimes used on sails attached along the boom. The bottom panel would be lofted out with a straight upper edge, the foot round along the lower edge and a bit of excess fabric at the leech end. Then the panel is actually flipped over when it's attached to the rest of the sail. The curved part with the foot round is sewn to the straight lower edge of the panel above it and then the excess fabric at the aft end is trimmed to complete the leech shape. This makes a bottom panel that creates draft along its upper side and then comes in to a straight line along the boom for fitting it with a bolt rope, slides, slugs, etc. that have to live in a straight line along the top of the boom. I remember way back when I was first learning sailmaking reading this and thinking "If I cut my panel to match the lofting, then why the hell would I then want to sew it to the sail upside-down?" but, it does work and makes for a shelf-style foot.

    In any case, I can't tell a lot from the drawing, but it would be nice to have a little shape along the foot and to have a means of preventing the round from flapping.

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    Default Re: Using Sailcut for Sail Design

    I found a couple of photos that show foot shaping on a cross-cut. This jib has the vertical broadseam cut into the foot panel. The upper end of the vertical seam would be the standard seam width (.5-.6") and as it nears the bottom of the sail, it gradually widens and flares slightly more near the bottom, increasing the overlap to help round the bottom of the sail and solidify the foot round against flapping. Computer-cut sails would do the same, but they usually do it by cutting the fabric in curves and keeping the seam overlap constant everywhere.



    Here is another example on the mainsail of this pair. In this case, there happens to be a panel seam that intersects the foot, below the tack seam. Since it intersects the foot at about the right spot, a little bit of broadseaming along the lower end of the panel seam will shape the loose foot without needing to make any other cuts or seams. Whether or not this possibility exists on any sail is pretty much a matter of the sail's tack angle and the panel width you choose, but if the opportunity exists, you might as well take advantage of it. It doesn't take a lot of shaping or a particularly wide broadseam, and too much will curl the foot, but when done right, it gives the sail a nicely firm foot and also looks better.


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    Default Re: Using Sailcut for Sail Design



    Here are the panels:

    The top edge of all panels is always staight and the bottom edge is curved.

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    Default Re: Using Sailcut for Sail Design

    OK. As I suspected, they haven't built any shape into the bottom along the boom. The line of tension between the tack ring and the clew ring is a straight line and anything below that straight line (the foot round) then becomes just a flap, and will do so. It would be a very good idea to split that bottom panel as seen on the tanbark jib above, and reassemble it adding enough to allow for the seam overlap and maybe 1/4" or so of broadseam (gradual seam overlap increase) as you approach the bottom end of the split. The easiest way to do this is probably to plot the panel on paper or loft it out on the floor with masking tape, rough-cut the panel with some excess at the ends and bottom, make the broadseam and then lay it over the lofting and cut it down to the final lofted shape. The end result will be a foot area that both works and looks better.

    I'm a little surprised that they have reduced the draft so much at the tack seam, since that part of the sail has the widest chord and the potential to produce a lot of power, and that their entry angle is so flat all the way up and down the sail when they have plenty of opportunities to round it a bit with all those panel seams. A flat entry may yield a slightly higher top speed potential (on the right hull) but tends to be rather intolerant of minor steering/trimming errors. Their entry angles are nearly as flat as their exit angles off of the leech, which would be pretty unusual on just about anything but an iceboat. Some of that may just be perspective on the drawings, but in general, any time you have so many seams along the luff and the shape-refining ability that those seams present, it's a shame not to take advantage of it.

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    Default Re: Using Sailcut for Sail Design

    Next iteration:

    I came to the conclusion that I will not use Sailcut for the final design of my sail.

    Though it is great for quickly making a 3D model of a 2D sail plan and for developing sail panels there are restrictions which make it impossible (at least for me) to design the sail the way I’d like to.
    There is no way to have a lot of draft in the lower section of the sail and having the foot laced to the boom at the same time. Although I intend to use a loose foot I understand the foot still should not have any draft.

    With Todd’s generous help I think now I know what the shape of the sail should look like.

    To achieve what I desire I proceeded with ProSurf a program I often use for hull design. I took the 3D model of the sail I created with Sailcut and loaded it in ProSurf to modify it. As ProSurf is not intended to be used for sail design the process is more complex but then again I have better control of shaping.

    Here are two results (I favour the second) of my new attempts:







    The first one has constant panel width and a “hard chine” at the seam next to the foot.
    (Camber ratio = 9%, draft position = 40%, entry angle = 28, exit angle = 13)

    The second one has a split bottom panel for a smoother transition from maximum draft to no draft at all and broad panels at the sections where there is almost no change of shape.
    (Camber ratio = 8%, draft position = 40%, entry angle = 25, exit angle = 11)

    I know it may seem strange how much effort I spend on the design of this simple sail but it is a great way to learn to understand the principles of sail shaping and chances are I finally end up using broadseaming.





    Last edited by flo-mo; 11-14-2012 at 06:44 PM.

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    Default Re: Using Sailcut for Sail Design

    Here is the process of making a scale model of the sail:












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    Default Re: Using Sailcut for Sail Design












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    Default Re: Using Sailcut for Sail Design

    As ProSurf is a program for hull design it is easy to generate a topographical contour map of the two sails (shortly discussed in this thread: http://forum.woodenboat.com/showthre...hape-a-Sail-Is). It is just the equivalent of the buttocks of a hull.


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    Default Re: Using Sailcut for Sail Design

    That's pretty neat. Now the question is will it plot the panel shapes for a radial?


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    Default Re: Using Sailcut for Sail Design

    I did a 4 sqm canoe gennaker using Sailcut https://docs.google.com/document/d/1...thkey=CIHIs_MH

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    Default Re: Using Sailcut for Sail Design

    Hi,

    Thanks for an interesting thread. I am going to design and make a lug sail, based on the Goat Island skiff sail measurements using sailcut. I am wondering about the need to have gaff or luff round in the sail. With light to moderate winds, the mast and gaff will remain relatively straight, and in stronger winds the bend will flatten the sail, which is good. So why have gaff or luff round in the sail? One other thing: is it possible to print 1/1 drawings from from the panels in sailcut? I have access to large format printing, and would use printed paper panels for cutting the cloth.

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    Default Re: Using Sailcut for Sail Design

    Quote Originally Posted by flo-mo View Post
    Model canoe + model sail:

    I am planing to use these Bamboo poles for my mast, yard and boom (all three 7'):



    A lot of Victorian yachts used bamboo for spars, with spruce ends glued in.
    There is a risk of the bamboo splitting in between the nodes so the Victorian spar makers put a tight seizing on the spar between each node, think barrel hoops keeping the walls in compression.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Using Sailcut for Sail Design

    The principle of luff round doesn't work very well on small balanced lugs, so we generally have to get our shape through other means (broadseaming and head round). Part of the problem on small lugs is that the lightest cloth we can use is still proportionally rather stiff and isn't likely to stretch to shape. Secondly, the luff should be pretty heavy-duty and built up with extra layers to withstand extended periods of fairly high downhaul tension without getting stretched out. I'll typically double the luff tapes binding the luff edge, so that edge is at least five layers thick everywhere, and may be eight or nine layers thick in areas where there are corner or reef reinforcement patches. Luff round on most sails helps create draft by being pulled straight and forcing extra cloth toward the middle of the sail, but with that many layers on a small sail and no spar present to lace the luff to on lugsails, it's usually impossible to get it to straighten out. You end up sailing with a convex-curved luff out front that won't sit still and tends to wobble back and forth, trying to find a stable angle to meet the wind with.

    You still need to create draft, but in this case, luff round like you might use in a 3-sided sail just isn't a good way to do it. As a result, luffs are cut either dead straight, or even slightly hollow if they're long ones to help maintain goof luff tension. I'm still not experienced with Sailcut, and plotting traditional 4-sided sails is a fairly new thing for computer programs, so I don't know how up to speed they are these days. In the past, the results were less than impressive.

    Head round along the yard (it's a yard, not a gaff on a lug) is there for two reasons. A small amount (like maybe 1" of round on a sail that size) is designed-in to add extra cloth for creating draft in the upper part of the sail. When forced toward the middle of the sail by being lashed to a straight yard, it furnishes the extra cloth needed to make the "belly" in the upper sail, and the belly is then positioned by broadseaming for straight-sided panels (traditional) or by cutting curved panel edges and joining them (computer-plot-style - yields the same shape). We don't need an awful lot of draft up there due to the short chord length, so the amount of head round added for draft creation up there is pretty small.

    Picture this - Your sailing along and your yard bends downward 1" at its ends due to the sheet and downhaul tension pulling against the halyard tie-on spot. If all the round you built into the sail's head was the 1" for draft creation, then the bending yard has just cancelled it out. There is no longer extra cloth to force downward for draft creation and what you end up with is a sail with a very flat top section - too flat to work well. For this reason, we also need to add more round to compensate for this inevitable yard bend. That way, the yard can still bend some (which it will) and we will still maintain a bit of head round to make draft with. It wouldn't be unusual on a sail that size to add an additional couple inches of head round as a bend allowance (1" of round added for draft, 2" for bend allowance, 3" total round added) all in order to just get the sail to set properly while actually out sailing.

    In some conditions (light winds) the yard may not bend as much. In heavy winds, it may bend more than we allowed for, so our bend allowance has to be a rough estimate, aimed at the "average" conditions we plan to sail in. Depending on the conditions of the day, sometimes it will work better than others and there just isn't any way around that. Making your yard pretty stiff seems to be the best way to minimize the problem, and really whippy yards are just going to magnify it, constantly changing shape, and the shape of the sail attached to them. In any case, you will still get the desired result of flattening the sail somewhat in heavy air when it is under high sheet tension and making more draft in moderate winds when the yard is less bent. We just don't want it to be too erratic and unpredictable due to an overly flexible yard. Unfortunately, there is no formula for bend allowance. It's based on the gut feelings and experience of the sailmaker. Some of the early versions of 4-sided sail computer software didn't seem to take spar bend into account at all (which is nuts) but I don't know what the current status is. The best way I've found to stay in control of the bend issue for my customers has been to be pretty specific about the construction and dimensions of the wooden yards they build. That way, I'm at least aimed at a familiar target when I cut the bend allowance into the sail. Bamboo can certainly work for a yard, but each piece is different and the amount of guesswork involved in making the sail set properly goes up somewhat any time you build the yard from a different material.

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