Why does a cutter rig point higher & sail faster?

Discussion in 'Sailboats' started by Rich Kinard, Nov 15, 2004.

  1. markdrela
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    markdrela Senior Member

    What type of wing or sail is best for any type of vehicle depends a great deal on what dominates the drag of the vehicle. In general, to maximize glide ratio or pointing angle, one should maximize the overall vehicle L/D (a bit oversimplified for a sailboat, but close):

    L/D = L / (D_wingsail + D_parasite)

    On a sailplane, the D_wingsail (wing profile + induced drags) is nearly everything. D_parasite (fuselage + tail) is tiny by comparison. So

    L/D ~ L / D_wingsail

    which is maximized with a slender cantilever wing like you see on any sailplane.

    But on a sailboat, D_parasite (hull hydro + aero drags) is huge by comparison. So to maximize L/D you want a large lift L to "dilute" this large D_parasite, even if D_wingsail is also made large as a result. Large lift naturally favors multi-element sails, for all the reasons listed by AMO Smith. This is especially the case if the sail area is constrained by rules or whatever.

    An iceboat is probably closer to a sailplane than a sailboat in this regard, so it benefits from a rigid single-element sail more than a sailboat.
     
  2. RHough
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    RHough Retro Dude

    That makes no sense. All you need is a high value of L, not a high value of CL. To get L you can have more area at lower CL (and lower CDi) or you can have less area at higher CL (and higher CDi).

    The value of D includes D(parasite) + Di.

    For a given hull and speed, Dp remains the same so total L/D is governed by the L/Di of the sail plan. The sail plan that gives the L needed at the lowest Di will be the faster boat to weather.

    For any value of L a single foil is superior to multiple foils.

    It is only when other constraints are added that multiple foils become a good choice.

    The only time a multi-element sail plan will point higher if is the vessel cannot carry a single foil of equal area and higher AR.
     
  3. markdrela
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    markdrela Senior Member

    Not really. Di depends only on the span loading and the airspeed. Sail chord and CL don't really matter in this design situation. More specifically, as you increase chord with a given span, L, and airspeed, you find that CL decreases, CDi decreases, but Di ~ (area*CDi) does not change.

    But this may be a somewhat overconstrained design situation. I'll still claim that when the drag is dominated by a hull whose drag is largely independent of what the sail does, then you want the sail to generate as much lift as possible, at least within righting-moment constraints. If the sail area is constrained, then a multi-element sail will easily beat a single-element sail in high-lift capability. This is true for soft sails and rigid sails. Note that Dennis Conner's AC Cat had a 2-element rigid wing.
     
  4. RHough
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    RHough Retro Dude

    If "Sail chord and CL don't really matter ..." then why does it matter that "a multi-element sail will easily beat a single-element sail in high-lift capability".

    If CL and chord don't matter, area is not constrained. Then what does your statement about higher lift with constrained area have to do with anything?

    I'm happy for DC that he had multiple elements in his wing sail, so? What are you trying to say?

    There are only three ways to create more lift, more area, higher CL, or more speed.

    Look at the trim angle needed to produce drive. The higher the L/D of the sail plan, the lower angle the trim angle needs to be to provide the same drive.

    High CL planforms have rotten L/D ratios. Thus high CL (high lift per unit area) sailplans will not drive the boat at as small and angle off the wind.

    If we want to sail close to the wind, and we know we need high L/D to do it, why even consider low L/D planforms?
     
  5. BOATMIK
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    BOATMIK Deeply flawed human being

    Mr Hall can access the articles of Mr Gentry that cover the point with care, logic and consistency - not to mention careful science.

    Gentry covers the cutter rig specifically. Basically the inner headsail operates in such tight constraints from its proximity to the main and the jib that it is very difficult to get consistent lift from it.

    In his general articles about "How Sails Really Work" he talks in more general terms about multi element airfoils.

    For the record ...
    http://www.arvelgentry.com/magaz.htm

    Perhaps you could sneak a peek too.

    :)

    Rhough is right on the money that Righting moment may be more of a determinant in choosing a multi element airfoil. It would have taken me a couple of days to come to the same conclusion!

    Best Regards
    Michael Storer
     
  6. BOATMIK
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    BOATMIK Deeply flawed human being

    Am I flogging a dead horse?

    I probably am being mean to a deceased quadruped but I woke up wanting to post some pics of pretty boats.

    All the boats below have no restrictions on the division of sail area with one exception. You can have any distribution between Main and Jib or Jibs that you like.

    They also measure almost every square inch of actual sail - there is a strict maximum they cannot exceed.

    Funny thing ... no cutters.

    Another interesting thing is that some have a restriction on mast height. These tend to be sloops.

    I suggest that if you want the cutter to be faster than a sloop you simply need to restrict the mast height further than in these examples - like maybe half :) without reducing the allowable sail area.

    However, even in this case the solution may be to go for two masts of the same max ht and a single jib on the forward one - but that is just intuition speaking - I have no hard facts to support my position at all!!

    Note too the wide variation of hullforms and stability.

    Anyway - enjoy the pics of all the lovely boats and remember why we all love this sailing thing!

    Michael Storer

    [​IMG]
    Australian NS14s - max hoist of main above deck 18ft/5.5m

    [​IMG]
    Bembridge Redwings classic long keel yacht designed a long, long time ago.

    [​IMG]
    A Class Cats

    [​IMG]
    Trimaran by Dubois to Formula 40 Rule. The F40 rule demands that there be a headsail - so it ends up being the size of the minimum storm jib allowable. If this restriction was not there the rig would look very much like the A-class.
     
  7. SuperPiper
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    SuperPiper Men With Little Boats . .

    Deeply Flawed Human Being:

    So without design limitations, the jib would be small, as small as possible, or non-existent.

    It makes sense to me and your anecdotal evidence seems logical. Is there a NA onboard that may comment to this? Is Eric Sponberg following this thread?
     
  8. Skippy
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    Skippy Senior Member

    Look in the Aerodynamics thread. Tom Speer discusses how the jib reduces drag from flow around the mast.
     
  9. brian eiland
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    brian eiland Senior Member

    MacLear cutter rigs

    Frank MacLear of MacLear & Harris did some big, stout, cutter rigged designs; both with a boomed mainsail and boomless.

    He also stepped the mast quite far aft to give the double headsails room to work. I was significantly influenced by these designs.

    I just recently gathered some info together and posted it here:

    Boomless Mainsails, Frank MacLear
    http://boatdesign.net/forums/showpost.php?p=160630&postcount=63

    Angantyr, cutter rig
    http://boatdesign.net/forums/showpost.php?p=160631&postcount=64

    Boomless Cutter
    http://boatdesign.net/forums/showpost.php?p=160633&postcount=65

    Falcon II
    http://boatdesign.net/forums/showpost.php?p=160536&postcount=221
     
  10. yacht371
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    yacht371 Yacht Designer

    The Cutter rig is less effective than a sloop of equal sail area BUT can often set more sail area. Racing rules measure sail area, so racing boats need to get the maximum performance out of a fixed area. Cruisers on the other hand can set as much sail as they like, and a cutter rig is one way to get more sail area without raising the mast. From personal experience the cutter is very fast on a close reach where the double slot seems to work best. Upwind, I haven't seen one yet that gives as good a VMG as a proper sloop rig.

    A single sail (cat rig) is even more efficient if the mast is streamlined and rotating. The disadvantages are such that you will only see this rig on racing catamarans that can remove the mast when not sailing.
     
  11. brian eiland
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    brian eiland Senior Member

    I think it depends on how you define the terms "effective' and "efficient" you use above.

    The "single sail is even more efficient"? I would refer you to posting #106 above, and could provide quite a bit more:
    http://www.boatdesign.net/forums/showpost.php?p=40229&postcount=106
     
  12. lewisboats
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    lewisboats Obsessed Member


    There's enough roach on those to make a Rastafarian Proud!
     
  13. brian eiland
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    brian eiland Senior Member

    Attached Files:

  14. Paul B

    Paul B Previous Member

    There doesn't seem to be any more overlap than the Jib, both sails are sheeted near the front crossbeam.

    I've also heard they've been furling the jib while going true upwind with the Code Zero.

    The use of this sail basically changes the boat from a fractional sloop to a masthead sloop with a much larger "J" dimension.

    This setup is only good for very light air, where the extra power is going to allow hull flying much sooner than the conventional setup.

    I'm sure you would love to use this as a way to argue for the aft mast cutter configuration you are fixated on, but you are barking up the wrong tree.
     

    Attached Files:


  15. brian eiland
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    brian eiland Senior Member

    I'm having a real hard time telling exactly where this sail is sheeted, but I seriously doubt it is sheeted near the front beam...just wouldn't work with the clew that high

    I'll have some more on this subject later....probably over here under Sail Aerodynamics

    Have you read AMO Smith's paper recently posted there??
    http://boatdesign.net/forums/showpost.php?p=215039&postcount=313
    You might have a look at page #518, Multi-element Airfoils, and his references to the 'dumping effect' or 'dumping velocity' that Tom Speer has addressed on several occassions.
     
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