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Thread: catboat sail design

  1. #1
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    Default catboat sail design

    I'm having a new sail made for my 23' catboat and I've been talking to a number of sailmakers. One of them suggested that the sail needs to be built so that the maximum draft is further forward than on a sloop as the headsails on the latter bend the wind over the main and therefore a different camber is required. Anyone out there with experience who wants to comment/corroborate/explain?

  2. #2
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    Default Re: catboat sail design

    How is the helm now?

  3. #3
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    Default Re: catboat sail design

    The books I have on sail design (ideas might be superseeded) say/ said a catboat should have its point of maximum camber more aft at halfway compared to 0.4 on a sloop. The aim of delaying the position turbulence appears, creates more forward drive at the expense of more heeling force: a catboat has the beam to take it. The extra net power gain forward moment/ vector thrust is 10%. Any further back and forward drive component diminishes.

    (Small Boat Sails by Jeremy Howard Williams 1987- Adlard Coles)
    Last edited by Edward Pearson; 11-20-2018 at 04:49 PM.

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    Default Re: catboat sail design

    I've only had the boat a short while and haven't sailed her much. She came with an old baggy sail and it's also my first catboat so I really can't comment on weather helm.

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    Default Re: catboat sail design

    Todd's your man to answer, but I'll give my 2 c just so that he can correct me and I may learn something.
    The "bending" of the wind is a fact. So with more than one sail relatively on the same horizontal plane the sails further aft needs to be sheeted more tightly than the fore one(s).
    But this is all about the direction of the wind and has nothing to do with the position of maximum draft.
    But, there may be reasons connected to the "channel" between a fore and aft sail that affects things. I don't know if it does or how.

    /Mats
    Yes the avatar depicts me; yes I drew the comic boat pic, it's a joke on the pop song I'm not a robot by Marina and the diamonds

  6. #6
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    Default Re: catboat sail design

    That's interesting Edward. I'm not sure if I really understand. Is there anywhere I could get my hands on the text (preferably digital) you're referring to?

  7. #7
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    Default Re: catboat sail design

    Bill Welch's "The Competitive Cat, Racing Small Gaff-Rigged Catboats Paperback" has the most lucid explanations of the unique sail trim and boat trim issues of these boats. Get it. The pix of how to move the sail's draft about depending on wind strength and point of sail are worth the modest price of this book.

  8. #8
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    Default Re: catboat sail design

    Quote Originally Posted by Ian McColgin View Post
    Bill Welch's "The Competitive Cat, Racing Small Gaff-Rigged Catboats Paperback" has the most lucid explanations of the unique sail trim and boat trim issues of these boats. Get it. The pix of how to move the sail's draft about depending on wind strength and point of sail are worth the modest price of this book.
    Thanks for the comment, Ian. I have the book and I appreciate the ability to change the draft by trimming the sails. However, should the sailmaker shape the sail differently than he/she would for a gaff rigged sloop?

  9. #9
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    Default Re: catboat sail design

    Jeremy Howard Williams was still sticking to his guns as late as 1983 in the fifth edition of his book "Sails". Basically he is saying that once past the point of maximum draft, the flow over the sail's lee side will begin to detach, so by moving the maximum draft aft more, the sail maintains its power phase longer.

    Bear in mind that draft placement is not a particularly exact science. With traditional sailmaking, maximum draft is generally positioned by how far, fore and aft, the luff broadseaming is started. On computer plotted sails, the positioning is pretty much the same, but it is done by shape-cutting panels, not by broadening seams. You are kind of creating symptoms, and where the draft actually ends up is the result of the symptoms you generated. There really isn't any simple thing you do which instantly positions the draft. There are also things like downhauls and Cunninghams, which when tensioned can move draft forward by gathering cloth up along the luff.

    Mats, folks have been arguing for years about what exactly a jib does, but I think most will agree that it provides a lift for the mainsail, allowing the main to be trimmed in a bit farther than it should be without the jib's influence.

    Also.....my radar always goes off when I hear someone mention a baggy mainsail. It can certainly be shot, with the cloth stability worn out and the draft wandering back, but it is fairy often just the result of a shrunken boltrope. The three strand ones do shrink, and then they start gathering cloth along the spars and the result is a baggy sail that no amount of tension (halyard, outhaul, etc.) will flatten out. It can be worth taking a close look at, and can often be fixed by splicing in a bit more rope.

  10. #10
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    Default Re: catboat sail design

    Both Jeremy Howard Williams books Small Boat Sails (192p) and Sails (at 461p the bigger book) are very well written, easy to read and comprehensive, the latter especially so.

    You can now buy them on Amazon UK for 1 pence each, plus delivery. Totally bargainacious Christmas reading.



    https://www.amazon.co.uk/Sails-Sixth...2791861&sr=1-2




    https://www.amazon.co.uk/Small-Boat-...2791861&sr=1-7

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    Default Re: catboat sail design

    Quote Originally Posted by Todd Bradshaw View Post
    Jeremy Howard Williams was still sticking to his guns as late as 1983 in the fifth edition of his book "Sails". Basically he is saying that once past the point of maximum draft, the flow over the sail's lee side will begin to detach, so by moving the maximum draft aft more, the sail maintains its power phase longer.

    Bear in mind that draft placement is not a particularly exact science. With traditional sailmaking, maximum draft is generally positioned by how far, fore and aft, the luff broadseaming is started. On computer plotted sails, the positioning is pretty much the same, but it is done by shape-cutting panels, not by broadening seams. You are kind of creating symptoms, and where the draft actually ends up is the result of the symptoms you generated. There really isn't any simple thing you do which instantly positions the draft. There are also things like downhauls and Cunninghams, which when tensioned can move draft forward by gathering cloth up along the luff.

    Mats, folks have been arguing for years about what exactly a jib does, but I think most will agree that it provides a lift for the mainsail, allowing the main to be trimmed in a bit farther than it should be without the jib's influence.

    Also.....my radar always goes off when I hear someone mention a baggy mainsail. It can certainly be shot, with the cloth stability worn out and the draft wandering back, but it is fairy often just the result of a shrunken boltrope. The three strand ones do shrink, and then they start gathering cloth along the spars and the result is a baggy sail that no amount of tension (halyard, outhaul, etc.) will flatten out. It can be worth taking a close look at, and can often be fixed by splicing in a bit more rope.
    -----
    Great to get your point of view, Tod. In summary, would you build a single gaff for a catboat the same as you would if it had a foresail?
    Also a good point about baggy sails and how to remedy the problem to give them a longer life. In my case the sail is 40 years old and really needs replacing.

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    Default Re: catboat sail design

    You might also ask your question over on the Catboat Association Facebook page. If I was looking to have a catboat sail made I would be looking for a sailmaker with a lot of experience with the type.

    Jim

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    Default Re: catboat sail design

    Thanks for the pointer. I'll check it out.

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    Default Re: catboat sail design

    Look around for someone with catboat gaff sail experience. We're lucky here to have Mark and Julie at Squeteague (https://www.squeteaguesailmakers.com) with the right skills. Look at the sails of other gaff cats in your area, assuming there are a few.

    By the way, what boat do you have?

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    Default Re: catboat sail design

    Quote Originally Posted by Ian McColgin View Post
    Look around for someone with catboat gaff sail experience. We're lucky here to have Mark and Julie at Squeteague (https://www.squeteaguesailmakers.com) with the right skills. Look at the sails of other gaff cats in your area, assuming there are a few.

    By the way, what boat do you have?
    I've got a William Garden 'Family Cat' 23' LOA. I'm on the West Coast of Canada and catboats are few and far in between.

  16. #16
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    Default Re: catboat sail design

    Garden really shows his versatility, his heart of New England, in this design. Very nice.

    These long gaff highly peaked cat sails are a bit different from the flatter gaff angle more common on gaff sloops. You'll want someone who gets it.

    If you can't find a sailmaker locally who has plenty of experience, you might consider an Atlantic coast loft. Import taxes might be horrendous to deal with the likes of our various excellent locals who make many sails of this type, but even though cats rather thin out north of Cape Ann, I'll bet that some of our Canadians, a naval architect in Nova Scotia springs to mind, could direct you.

    A stray thought: how does the sail's throat attach? Head tension is adjusted mostly by the peak halyard and that means the throat connection matters a good deal. Any sailmaker you use should have access to the exact attachment there, at the tack, at the clew, and at the peak. If your sailmaker is remote, provide detailed photos with a ruler in frame for scale and perhaps for the throat and tack a mechanical drawing with exact dimensions. The sailmaker may suggest modifications.

    G'luck

  17. #17
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    Default Re: catboat sail design

    Quote Originally Posted by Ian McColgin View Post
    Garden really shows his versatility, his heart of New England, in this design. Very nice.

    These long gaff highly peaked cat sails are a bit different from the flatter gaff angle more common on gaff sloops. You'll want someone who gets it.

    If you can't find a sailmaker locally who has plenty of experience, you might consider an Atlantic coast loft. Import taxes might be horrendous to deal with the likes of our various excellent locals who make many sails of this type, but even though cats rather thin out north of Cape Ann, I'll bet that some of our Canadians, a naval architect in Nova Scotia springs to mind, could direct you.

    A stray thought: how does the sail's throat attach? Head tension is adjusted mostly by the peak halyard and that means the throat connection matters a good deal. Any sailmaker you use should have access to the exact attachment there, at the tack, at the clew, and at the peak. If your sailmaker is remote, provide detailed photos with a ruler in frame for scale and perhaps for the throat and tack a mechanical drawing with exact dimensions. The sailmaker may suggest modifications.

    G'luck
    Thanks for the pointers, Ian. Any suggestions for Canadian naval architects or sailmakers who might have experience in this regard would be helpful.

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    Default Re: catboat sail design

    Quote Originally Posted by Jim Ledger View Post
    You might also ask your question over on the Catboat Association Facebook page. If I was looking to have a catboat sail made I would be looking for a sailmaker with a lot of experience with the type.

    Jim
    I just checked and that page doesn't seem to be active anymore.

  19. #19
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    Default Re: catboat sail design

    I appreciate everyone's input on this subject. I'm a bit surprised that there doesn't seem to be a real clear consensus on where the max. draft should be built into a single gaff. I would have thought it would be an important design consideration.

  20. #20
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    Default Re: catboat sail design

    Forget FaceBook. Try https://www.catboats.org

    Marmalade's huge roach full length batten sail is interesting. It's 90 square feet bigger than the design. Something dynamic about it actually reduces heeling compared to the old sail. The organization of the three radiating upper battens is essential to keep the weight of the roach from thrusting the lower battens forward and around the mast. The sail in incredibly stabile but "playing the peak" on and off the wind matters much, as does the general issue of outhaul and halyard tensions.

    The general apex of the sail's curve is in a line from the peak drifting a little aft on it's way to the boom. This puts that apex about 1/3 or a bit more abaft the mast.


  21. #21
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    Default Re: catboat sail design

    I think Leitch and McBride in Sidney made some ( at least one that I knew of) of those sails originally. There was also an old guy Barker, I think his name was, again in Sidney. I heard of him again just a couple of years ago, so possibly he is still around.
    I'd seriously consider a sail maker on the east coast with relevant experience, such as Ian suggested....Mark and Julie at Squeteague.
    I note that some of the Garden Family cats had a Marconi/Bermudan sail, so I am thinking you may want the draft a little more forward of the middle...
    Wow great photo Ian. Really shows the draft is forward....

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    Default Re: catboat sail design

    Quote Originally Posted by daggo View Post
    I appreciate everyone's input on this subject. I'm a bit surprised that there doesn't seem to be a real clear consensus on where the max. draft should be built into a single gaff. I would have thought it would be an important design consideration.
    I think the reason for no clear consensus is that an awful lot of sail design hinges on a bunch of "it depends" moments. Marconi or gaff? How much bend is there in the mast/boom/gaff? How heavy is the sailcloth? Typical local conditions? Lots of variables. Also, with a couple of notable exceptions most of us around here simply order up a set of sails from a loft or get a kit from Sailrite and hope for the best. There is a certain amount of sail-designer voodoo involved, most of us just don't have the experience to make more than qualified observations on the subject.
    Steve

    Boats, like whiskey, are all good.
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    Default Re: catboat sail design

    [QUOTE=Ian McColgin;5734696]Forget FaceBook. Try https://www.catboats.org

    Marmalade's huge roach full length batten sail is interesting. It's 90 square feet bigger than the design. Something dynamic about it actually reduces heeling compared to the old sail. The organization of the three radiating upper battens is essential to keep the weight of the roach from thrusting the lower battens forward and around the mast. The sail in incredibly stabile but "playing the peak" on and off the wind matters much, as does the general issue of outhaul and halyard tensions.

    The general apex of the sail's curve is in a line from the peak drifting a little aft on it's way to the boom. This puts that apex about 1/3 or a bit more abaft the mast.

    _________


    That's really interesting. It looks almost identical to Stuart Hopkin's (sailmaker in Chesapeake Bay) conversion of a Marshal catboat, except he added a foresail and a mizzen. He also maintains that it's not necessary to have a track and cars on the mast as the lacing allows the battens to slide beside the mast when the tension is released.

  24. #24
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    Default Re: catboat sail design

    Okay, I'm really intrigued by the rig on Marmalade. Is this your boat, Ian? I'd like to find out more.

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    Default Re: catboat sail design

    First
    Quote Originally Posted by mohsart View Post
    ...
    But, there may be reasons connected to the "channel" between a fore and aft sail that affects things. I don't know if it does or how.
    Then, in a reaction
    Quote Originally Posted by Todd Bradshaw View Post
    ...
    Mats, folks have been arguing for years about what exactly a jib does, but I think most will agree that it provides a lift for the mainsail, allowing the main to be trimmed in a bit farther than it should be without the jib's influence.
    There may be a hint in a post by Mark Drela on the boatdesign.net forum and the discussion that followed.
    ... The reality is that the overall lift of a rig is almost entirely determined by its planform and by the local incidence of the rearmost portions of the rearmost sail. In contrast, the lift is almost unaffected by:
    * the incidence of the front parts of the sail (i.e. the jib or jibs)
    * the size, number, or orientation of the slots, if any
    * any overlap between the sheets
    https://www.boatdesign.net/threads/s...36#post-744698

    In one of the following posts, Mark Drela then wrote
    It's not very useful to consider the loadings on the "leech" and "luff" regions separately, or on the mainsail and jib separately, because they always interact very strongly. The mainsail generates a big "drag", but in doing so it causes the jib to have a big "thrust".
    [...]
    Similarly, to determine the drive of any sail rig all we need to worry about is the overall circulation (which is determined mainly by the "leech return angle"), since this is what determines the overall lift. Twiddling with the jib shapes, slots, overlaps, etc., merely spreads this given lift among the various sheets, but it cannot change the overall lift.

    The jib, slots, etc., are important only in that they affect the overall viscous drag of the rig. Redistributing the loads will also affect the yawing moment and the helm trim, so that's another but probably secondary consideration.
    https://www.boatdesign.net/threads/s...37#post-744976

    My 2 cents.

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    Default Re: catboat sail design

    So considering all that, back to the original question: should the maximum camber on a single sail be further forward or further aft compared to a main sail/jib setup?

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    Default Re: catboat sail design

    "The Aero- and Hydromechanics of Keel Yachts" by J.W. Slooff states: "... for single sails, a position at 30–40 % of the chord gives the best lift/drag ratio", but also "Information on the optimum position of maximum camber for interacting fore- and mainsails is, unfortunately, not available" and "Results of computational fluid dynamic optimisation studies (Chapin et al. 2008) suggest that with the position of maximum camber at x/c = 0.30 the minimum drag angle attains its lowest value for fc /c 8 ≅ 0 0. . For the performance of a sailing yacht this means (see Chap. 4) that a moderate amount of camber (about 8–10 %), with the position of maximum camber at about 30–40 % of the chord, is favourable for close-hauled conditions. This in particular at high wind speeds, when the sails have to be set for moderate lift to avoid excessive heel.
    Figure 7.4.12 implies that a more rearward position of maximum camber is favourable for reaching conditions when the sails have to be set for maximum lift.
    ".

    So your guess is as good as anyone's I think. Or, in other words, it depends on what you want to optimise for.

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