can anyone tell me what this boat is modeled after

Discussion in 'Sailboats' started by Boston, Nov 10, 2008.

  1. Boston

    Boston Previous Member

    well I got today off
    and it sounds like I need to go study up on modern keel designs
    any suggestions as to good reading
    seems like you are pretty up on the subject

    Bntii and daiquiri
    and I had found the squarekerry site but completely missed the part about the Fara having been found
    thats fantastic
    I cant wait to see her restored
  2. Eric Sponberg
    Joined: Dec 2001
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    Eric Sponberg Senior Member

    There have been innumerable studies, papers, books, articles, etc. on keel design over the decades. Probably the most concise summary on keel design can be found in "Principles of Yacht Design" by Lars Larsson and Rolf Eliasson, third edition.

    For a comprehensive discussion on aero-hydrodynamics, "Aero-Hydrodynamics of Sailing" by C. A. Marchaj (pronounced MAR-ki) is invaluable. This includes the basic development and state-of-the-art science on sailing, including discussions of hulls, appendages and rigs.

    Marchaj also wrote "Sail Performance--Theory and Practice" and "Seaworthiness--The Forgotten Factor". These books are also invaluable for the understandign of sailing yacht design. All of these books should be on everyone's bookshelf for those seriously interested in sailing yacht design.

  3. Boston

    Boston Previous Member

    I think i have Principals of Yacht Design floating around here somewhere
    Ive got High speed Sailing round somewhere to
    I think
    or I used to
    its been a while
    ok a long while since I read either
    and Im not sure I found the time to read Modern Yacht Design
    Ill call a friend of mine on this one
    Sail Performance--Theory and Practice" and "Seaworthiness--The Forgotten Factor
    he's got a used book wholesale house
    so chances are he has it
    thanks for the tips

    oh and hey guys I found a lines drawing
    and framing details
    also a sail plan and static drawing
    no wonder they break ribs
    thing is build light as a feather

    a little bird tells me a few white oak stringers would really help
  4. Boston

    Boston Previous Member

    Hey Eric
    was reading about heavy seas and boat stability
    the more I read
    the more I like your boat
    I still havnt gotten to keel forms yet
    but I had a question
    is your keel some kind of variable ogivoil section or something like that
    or is it a fixed shape
    I notice your mast rotates and was thinkin maybe you got tricky with the keel as well
    also notice some wings on the ballast bulb
    whats that all about Bart
    arent those typically for light wind sailing
    designed to lift the hull a little and so reduce friction
  5. Eric Sponberg
    Joined: Dec 2001
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    Eric Sponberg Senior Member


    The keel on the SC 40 is a constant section modified GA(W) aerofoil of 10% thickness. The rudder is the same section, by the way, just scaled down. The keel has to be a constant section so that it fits properly in the keel casing (close clearances) inside while it slides up and down.

    The bulb is shaped to what I call a Beavertail/Swallowtail bulb (BS bulb, for short--I like the pun). The Beavertail aspect--flat beavertail shape on the aft end--is to form an end plate for the keel blade. Without an endplate, the flow across the keel will tend to slide down the keel and off and under the tip, thereby creating a trailing vortex. The creation of the tip vortex is the very essence or signature of induced drag. The bigger the tip vortex, the more non-uniform is the flow over the keel, the higher is the induced drag. The more you reduce the tip vortex, either by planform shape, foil twist, or by end plate, the lower is the induced drag. Therefore the keel is more efficient. Efficiency is defined as the lift-to-drag ratio, L/D. With lower induced drag, there is less force holding the boat back in speed made good to windward (VMG), therefore the boat is faster in VMG.

    Another way to reduce tip vortex is to change the keel planform shape, either to a more elliptical shape, or by twisting the keel section so that the leading edge of the keel actually points a little to one side. Well, with a lifing keel, you cannot use an elliptical planform shape because in a raised position the keel does not fit snugly into the hull. And you can't twist the keel because it has to work on both tacks. To try to twist the keel while underway would be a mechanical nightmare. So for a lifting keel, you are left with a constant section and constant chord length, and you have to fit it with an endplate. Since the boat's stability favors and requires a low center of gravity, it is natural to make the end plate a ballast bulb. This, you can see, helps me have a range of stability (AVS = angle of vanishing stability) of 180 degrees. This is very rare in yacht design. I achieve the same thing on my design Saint Barbara, which you can also see on my website.

    As to the swallowtail feature, flat and square-cornered plates shed their own vortices and create induced drag. Theoretically, one can conceive of a keel design whose endplates have endplates, and then those endplates have more endplates, ad infinitum. You'd actually be overcome by form drag and delicate shapes very quickly. But subsequent to the 1983 historic loss of the America's Cup to the Australians, the Americans did a broad array of studies on keel shapes, winglets, and endplates, and they discovered that if you sweep back the tips of the corners of the endplates into a swallowtail shape, you actually reduce the size of the tip vortex, thereby reducing the endplates own induced drag. So I started putting swallowtail shapes on by beavertail buld designs and came up with the BS bulb. The above is NOT BS, by the way, you can look it up, and there is foundation for the science.

    I might add that you could conceive of a keel with no bulb, but with a scimitar-shaped tip wherein the tip of the keel (or rudder) is swept aft. This, in fact, is becoming more common. One sees it in airplane propellers a lot these days, and you also see it in the tips of windmill turbine blades. Also, you will notice in my rudder designs, including that of the SC 40, and on Saint Barbara and Bagatelle, that I sweep the leading edge of the tip of the rudder aft. This is contrary to older rudder and keel designs of the 1980s and 90s where, instead of the leading edge at the tip sweeping aft, the trailing edge is swept forward. This is an effort to approximate an elliptical planform shape which tends to give the rudder an elliptical lift distribution along the span, which creates the least amount of tip vortex, and therefore has the lowest drag. This is true in concept. But I am of the opinion that such rudders (and keels) with swept-forward trailing edge tips are actually backwards. Rather, the planform shape should be flipped back to front, thereby giving a more scimitar shape to the planform with a swept leading edge tip. This will be a lower drag shape than the reverse.

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  6. Tcubed
    Joined: Sep 2008
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    Tcubed Boat Designer

    The first picture really makes me think 22 1/2 square meter class. (my grandfather had one in Norway)

    These boats sail very nice and point extremely high in sheltered water.

    In big waves they are deathly. Almost impossible to control, pound to windward, and there is at least one report of submarining (never re-surfaced) downwind.

    Good examples of what happens when trying to be as fast as possible within the constraints of a racing rule. Something that hasn't changed, the rules have changed but they produce distorted boats the same.
  7. Boston

    Boston Previous Member

    way to answer the question Eric
    and ya the part about the vorticies causing drag really hit the spot
    I think it might make a nice little alteration in my pond sailor to build it with a steal keel and a bulb
    ( thinnest of possible forward profile and strong as all hell )
    wouldnt the bulb negate the trailing vortex sufficiently to eliminate the need for the platform
    I know some airplanes have fuel cells at the wing tips instead of those little vertical stabilizers
    or does it have something to do with the density difference of the two mediums that one need additional vortex control

    hey T
    drove under is what my grandfather used to call that
    he taught physics at Norwich university but had worked his way through his own schooling as a shipwright
    my family has been shipwrights as far back as anyone can remember
    he and my dad beat the craft into me and I ended up doing the same
    ( working my way through school with it eventually, I just stuck with general contracting )
    drove under is rare
    usually u sail yourself into a breaking wave or a hard chop and just keep going
    but you can also rotate of the top of one and find yourself pointing straight down
    takes a really rough sea though
    you get that kind of water off the east coast were the Labrador current and the gulf stream meet
    I hear it happens in the Bass straight and a few other places, but I never sailed there so what do I know

    thats one reason Ive considered a pilot schooner for my run up to ketchikan
    or the friendship sloop
    either is beyond tried and true in that kind of water
    and pilot schooners being specifically made to sail in the roughest of conditions
    with a capsize screening factor of about 1.4 on the one Im looking at
    and a point of vanishing stability of something like 155deg

    and the friendships being fishermen originally


    I think Ive decided on this one for my pond sailor
    and have found what I think are the lines for it


    now to see about lofting it
    something Ive not done very much of in say thirty years or so
    I dont have room to loft if full size so half will have to do
    and Ill fair it on the forms with the stringers since Im keeping em in anyway to stiffen it up some
  8. Eric Sponberg
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    Eric Sponberg Senior Member

    Not sure what you mean here by "platform". There are bulbs and then there are bulbs. Some bulb shapes will work better than others in reducing the tip vortex, and then the bulb itself has its own vortex. To be really accurate one would have to go into the wind tunnel or towing tank to view flow around the keel and bulb. The differences in mediums are taken into account by proper scaling to Reynolds number. This is why keel and bulb shapes can be tested in a wind tunnel where smoke can be seen more readily than dyes water in a towing tank.

  9. Boston

    Boston Previous Member

    well on your site
    ( nice site by the way )
    on the first rendering you show a keel bulb with a flattened area towards the back
    rather than a cylindrical section
    I called that a platform
    I guess I should have called it a wing
    didnt have em when I was a kid so Im not so familiar with the terminology for em today

    I was looking through keel shapes
    which kinda answered the question for itself
    as the most modern ones are torpedo shapes
    with no wings
  10. JesperW
    Joined: May 2006
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    Location: Stockholm Sweden


    Apparently not too many Swedish people in this forum.

    The yacht in the original question is a M-30.

    Belongs to a family of M-15 / M-22 / M-25 / M-30 yachts named after Lake Mälaren, the lake on which the capital of Sweden (Stockholm) is situated.

    The number is the sail area in sqare meters.

    Both early all-mahogany and later fibreglass versions exist.

    The owners club webbsite is at (All in Swedish I am afraid)
    There is a gallery of various M-boats racing at

    Designed in the 1930's by Lage Eklund.
    LOA 11.5m
    WOA 2.06m
    Draft 1.4m
    Displ 2.4t

    The original lines drawing:


  11. Boston

    Boston Previous Member

    way to go
    do you have the rest of those lines drawings in there somewhere

    most folks have been really helpful about what these boats do well
    Ild love some additional input on what they dont do well
    obviously they are going to have trouble in rough water

    Im likely to try this one


    but all I have is these miniature lines for it
    Ill try the sweedish site
    Ive got a translation program in here somewhere
    but any aditional input it greatly appreciated

  12. JesperW
    Joined: May 2006
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    Location: Stockholm Sweden


    What they do well ?

    Let's see....

    Look extraordinarily beautiful, go to windward better than downwind, being very small to live 4 people in, and get the crew and interior wet as soon as there are waves.

    That's about it :)

    My advice if you're interested would be to contact the M-båt association.
    I think there is one wooden new being built. And the forms for GRP building are available. is the official contact. I'm sure she speaks english and can help you further.

    ...that's gotta be worth some rep ;-) !

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  13. Eric Sponberg
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    Eric Sponberg Senior Member

    Well, I do have the benefit of being of 100% Swedish descent, even though I am third generation American.

    This lines plan that you present was also offered to me by my client for the SC 40 as a representative example of the kinds of lines that we should use as a model, so that particular plan did provide me with some inspiration.

  14. Boston

    Boston Previous Member

    thanks guys
    I just lost my job this morning
    so Im kinda "dead in the water" till I think of something
    maybe I should start building boats for a living
    its always been what I wanted to do anyway
    that or marine biology
    ( neither likely here in Colorado )

    well done working with it Eric
    drawing is all of about three inches by eight
    would drive me nuts to have to extrapolate the fuzzy bits
    its dam near starting from scratch

    hey Jesper
    good info
    does kinda look like it would be easy to broach
    and the right wave would help that out just fine
    little push and round you go
    pretty sure they dreamed up fin and skeg for just that reason
    and the lack of free board means your going to be bailing
    course I'ld just be goofing around in the thing and not likely to set up a pole and a spinnaker anyway
    hell Im kinda lazy about my sailing and sorta get things set and just tack my way about on a course that lets me not even touch the sheets
    its a lake
    not like Im actually going any were anyway
    kinda like my job scene

    dam the hole job thing is killing me
    I was really looking forward to building something

    there is a star class races round here all the time
    pathetically easy to build ( chine form = plywood, cheep cheep cheep )
    and its rigging is locally available
    also I could get a wrecked one and salvage the bits and pieces

    I gotta quit thinking about building anything till I get this job thing resolved
    what a pain

    on the bright side
    I think Ive got enough stuff round here to whip up a nice Bloody Marry

  15. TeddyDiver
    Joined: Dec 2007
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    TeddyDiver Gollywobbler

    Just don't end up hanging in the same thread with SH :p
    Sorry Sean couldn't resist:D
    pouring Capt. Morgan, stirring in my mouth, no shaking.. Got promoted last week.. no more norwegian moonshine..
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