Hull shape for simple sailing scow

Discussion in 'Sailboats' started by hospadar, Apr 5, 2011.

  1. BOATMIK
    Joined: Nov 2004
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    BOATMIK Deeply flawed human being

    Howdy,

    I am pondering a few things ...

    The Australian Moths moved from a square hull cross section in the '50s, to a cold moulded wooden hull with round bilges in the late '60s

    Then it went to plywood with a double chine with considerable performance improvement. Chines beat rounded.

    In the latter years of the development of the Moth scows they started to get very brave with chinelines sweeping them in and out relative to the direction of travel quite radically.

    When the very narrow skiff moths started to appear they had rounded chines, but within a couple of years very hard chined, flat bottomed ones seemed to be going well too. They fly now, but that is another story.

    Another series of real boats on real water is documented nicely by the UK Cherub website. Where regular vee bottom and some double chine boats race regularly against quite flat bottomed 90 degree chine machines as well. All seem competitive through the wind range.
    The UK Cherub website is well worth a browse for those interested in design where there are few restrictions in shape.
    http://www.uk-cherub.org/doku.php/history/1990s_designs

    These basic trends seem to have continued.
    http://www.uk-cherub.org/doku.php/boats/boats

    Though most of the photos on the site are of boats going quick - there are plenty of light days too - and if the harder chine boats were at a disadvantage there then they would be dicarded.

    Performance that is "inherent" in the USA inland scows rounded bilge is because nothing else has been seriously attempted. I guess the various class rules restrict the opportunity.

    My feeling is there are too many assumptions inherent in the statements that "rounded is better" and that evidence of real boats suggests that this might be unimportant, but borne out of familiarity.

    Best wishes
    Michael Storer
     
  2. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    What I would say is that low wetted surface high L/B hulls have less drag-and that is epitomized in the inland lake scow form when sailed at an angle of heel. If you took the hard chine shape and superimposed it over the scow shape it would be wider and have more wetted surface..... However, if you can make the immersed hull of a scow with a chine narrower than the rounded form when sailed at an angle of heel the extra wetted surface might not matter. John Ilett once told me that in the Moth class a high L/B,skinny hull was more important than low wetted surface but the Moth is sailed upright. Don't think it would work well on a scow.
    Now, in smaller boats like the Moth scow you can't effectively use the shape though the Butterfly try's.
     
  3. BOATMIK
    Joined: Nov 2004
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    BOATMIK Deeply flawed human being

    Howdy Doug,

    i think the scow moths fit (or fitted - it is the past) very nicely in terms of scow behavior - upwind heeled, downwind flat. The expectation and sailing method was the same.

    You can also see that the hullforms are very similar to the US inland scows, despite the double chine - the rocker is similar, though the restricted class rules allow much more tweaking of hull shape - the boats are fairly similar in terms of inspection.

    The rocker is the giveaway.

    As well as the sailing method - heeled upwind for min wetted surface and waterline (though I more and more find waterline a dubious argument - i think it is more about distribution of volumes - more accurate representation of what is happening), as flat as possible at reaching speeds.

    This is a 1970s boat I documented last year which had wings retrofitted.

    [​IMG]

    More pics
    http://www.flickr.com/photos/boatmik/sets/72157604819711223/

    Best wishes
    MIK
     
  4. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    ===================
    Thanks, Michael-neat info-that scow Moth looks very similar to the Butterfly if you added racks. I love scows from my time many moons ago crewing on a 28' E scow-simply breathtaking!
    I found this bit on the history of the tunnel hulled M20:

    History of the M20
    by James Bland, Past-Commodore, M-20 Sailing Association

    The scow is truly an American invention in one design sailing. The scow has its origins traced to broad beamed, vastly over canvassed racing sloops called "Sandbaggers", which used large crews and moveable bags of sand for ballasting. The scow design was largely an outgrowth of the racing for the Seawanhaka Cup donated by the Seawanhaka Corinthian Yacht Club to promote interest in small yachts. Herreschoff and others had demonstrated the effectiveness of the light displacement broad-beamed "skimming dish" hull type. Glencairn, the 1896 Seawanhaka Cup challenger from the Royal St. Lawrence Yacht Club, was the first true scow and she defeated Clinton Crane's El Hierie easily.

    When heeled, Glencairn's flat section became a canoe like form with greatly reduced waterline beam, and the waterline became elongated both foreward and aft, with the waterline plane becoming symmetrical. In shifting the heeled center of buoyancy farther to leeward, the greater righting moment of hull and crew permitted a larger sail plan on the same beam.

    The freak scows won over their strongest opponents when they began to beat the sandbaggers, not only in light air, but in a breeze as well. It was soon evident that the extra weight of the sandbags merely slowed the boats off the wind and the difficulty of moving dozens of sandbags with each tack simply reduced maneuverability. A properly designed scow, with its reduced wetted surface area, longer heeled waterline, and effective planing surfaces downwind, could be effectively sailed by a few crew members.

    In 1898 the Canadian boat, Dominion, carried the scow design to its logical end. She was essentially two semi-circular section hulls with a joining elevated floor above the waterline. When heeled, her waterline increased from 17'6" to 27 feet. She won the 1898 Seawanhaka Cup with ease, unbeatable in any condition. Her superior design was subsequently ruled illegal for the Seawanhaka Cup competition. Dominion would later be part of the inspiration for the M-20 design.

    In the winter of 1898, eighteen clubs from Indiana to Minnesota sent representitives to a Milwaukee meeting which formed the Inland Lake Yacht Association and formulated a set of rules for two classes to race without handicaps in interlake competition. The rules fostered the rapid development of the Inland lake scows.

    The M-20 is one of the newest scows in the Inland family. It was designed by Harry Melges Sr. and Harry (Buddy) Melges Jr. as the scow version of the Olympic Flying Dutchman class. This was the second in the M series, a tunneled hull sloop with spinnaker especially designed for two people. Five different M-20 hulls were built of wood and each was tested against its successor to find the better boat. Finally, in December of 1962, a prototype was produced out of fiberglass. She handled beautifully in all weather conditions and took a sea better than any other scow. The first M-20 competed in Yachting's One-Of--A-Kind Regatta and turned in the fastest elapsed time of all monohulls.

    The M-20 must be considered an advanced scow design and will remain so for years to come. Its high aspect sail plan is similar to the America's Cup Technology currently in use today, and provides a wide range of controls for all wind conditions. The tunneled hull of the M-20 is her most unique feature and provides increased stability and structural stiffness.

    The M-20 is 20' long and 5'8" wide. The tunnel of the hull is refers to the fact that the middle section of the bottom is 3" shallower than the bilges on each side of the boat. This tunnel promotes planing , and aids winward performance. The boat is sailed heeled, with the bilgeboards becoming perpendicular to the waters surface at 25 to 30 degrees of list. Sailing the M-20 heeled reduces wetted surface area and lengthens the waterline. With the bilgeboards down the boat draws 3'6", and 8" with the boards up. The boards are slightly toed in to provide a lifting surface for increased upwind performance.

    The modern sail plan is mounted on a 26 foot mast and 9'6" boom. The M-20 carries 176 square feet of sail and a powerful 300 square foot spinnaker for offwind speed. A full set of sail adjustments keep the skipper and crew on their toes. An adjustable permanent backstay and flexible spar allow competitive crew weights to range from 270 to 450 pounds. as a result the M-20 is popular with families. Male/Female teams, older teenagers, as well as two man teams all compete successfully in this versatile dingy.
     
  5. CutOnce

    CutOnce Previous Member

    Getting back on topic ....

    Other ideas:

    South African Dabchick.

    Kiwi Firebug.

    Both these are small scow(ish) boats used as junior trainers. Fit the original poster's size and type request, but not the capacity or usage issues.

    I'd have to agree with PAR about the notable silence from the original poster ... most of us have to learn the hard lessons the hard way, as good advice tends to sound like negative grumbling to enthusiastic newbies.

    --
    CutOnce
     
  6. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    I would say that for 'lighter' boats, sharp chines make little difference. Especially with sail boats. But as the weight per beam goes up, the hull has to sink deeper to get the required displacement. Then the turbulence around sharp chines could become an issue. And this would be mainly for powerboats, as the original poster specified (rowboat or powerboat). With a sailboat, the extra buoyancy of the leeward chine may more than make up for the turbulence it creates by allowing it to carry more sail area (something that becomes less and less true as the largest section gets deeper and deeper).

    Also, as a sailboat heels, the leeward chine acts more like a dead rise center line, cutting through a chop.

    Phil Bolger once commented with dismay that his cheap 'elegant punt' out sailed a more expensive design he was trying to sell.

    My guess is that with a straight sided scow, any difference would be pennies on the dollar, power or sail. Important in a race, but ignorable otherwise.

    I recommended radius chines mainly for structural reasons, given the material the original poster specified. From what I've seen, fiberglass construction doesn't like chines as much as curves.
     
  7. hospadar
    Joined: Apr 2011
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    hospadar Junior Member

    Yo Ho!
    Thanks for the HUGE number of responses, you've given me much to think about!

    It sounds to me from the discussion that any significant rounding of the chines isn't going to make a big difference for how I'll use the boat. I won't be doing anything where sailing performance matters a great deal. I just want a little boat I can practice my sailing on that's easy to haul to my local lakes. I'm not a particulary pro sailer so I suspect many of the subtleties of hull design would go totally unnoticed when I'm actually in the boat. Aside from that, I think the extra stability from a boxier design will work in my favor for all the non-sailing uses of my boat.

    I'm currently thinking I'll stick almost exactly to the PD racer hull design (except made with foam/fiberglass instead of plywood), the only modification being that I might radius the chines a little to help the fiberglass go on and protect the hull from things that might more easily damage a sharp corner. This seems to be a pretty proven and easy to build and sail design. Also, most of the other simple boxy designs I've seen appear to be very similar.

    Another question: It seems that a lot of PD Racers use a single leeboard as opposed to a daggerboard or pair of leeboards. From what I've read, most people who sail the single leeboard arrangement on boats like this seem to be pretty happy with it. It would certainly be easier to build than a daggerboard trunk, and I gather that at the speeds and conditions common to PD Racer style boats, and performance losses are pretty minor. Does anyone have any experience sailing with the single leeboard setup? Any opinions?

    Thanks for all the posts, it's a fascinating discussion
     
  8. CutOnce

    CutOnce Previous Member

    I'm sure everyone here welcomes you. Your response is nice to hear - sometimes people ask for thoughts and never keep us in the loop.

    I'm interested in why you would choose foam/glass as a first boat building effort over plywood. Certainly in larger production runs with above average shop equipment foam/glass can produce consistent, light, quality hulls with less manual labor, but in a one-off situation without tooling, molds and vacuum equipment foam/glass will have a higher effort level than plywood. Finishing a foam/glass boat without molds will be a challenge as well.

    I'm not questioning your strategy -and decisions but it would be interesting to understand why. If you've got experience working with cored/glass fabrication and access to the tools/shop necessary that is a whole different situation. At this size boat I doubt there would be a huge weight/longevity benefit to either method.

    I think your choice of PDR-style is good - but by going with foam/glass you may be opting out of the society of the other folks out there. There is a lot to be said for sailing with a bunch of like-minded characters.

    As far as the daggerboard versus leeboard thing, either one will work, and I'd imagine they'd work about equally as well. In small dinghies, the major difference in performance is in the software, not the hardware (meaning the person in the boat, rather than the boat itself). I know Michael Storer and the OZ crowd split away opting for a little more complex/expensive (and traditional) construction over the simplistic approach of the original minimalist (and low cost) PDR.

    Good luck and let us know how you are doing.

    --
    CutOnce
     
  9. hospadar
    Joined: Apr 2011
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    hospadar Junior Member

    A big part of the reason is that I don't have a lot of experience fiberglassing, and a simple, low-risk design like a PDR boat seems like a good entry point. I considered making a surfboard to learn the basic techniques [of fiberglassing], but michigan (although people do surf on the big lakes) is probably not the best place for that.

    It's possible I may build myself a fiberglass single scull which may sate my hunger for fiberglass, but right now I think the PDR boat is of more broad interest to me (general boat messing about)

    I don't care too much if the fiberglass job is hideously ugly or heavier than it needs to be. I'm going to put a cool paint job on it and get a captain's hat on and throw water balloons at anyone who doesn't like it.

    I did consider this, but that doesn't bother me too much, I don't currently have an overwhelming desire for a duck-shaped trophy =)

    Along these lines, any opinion on the importance of a proper foil (vs just a flat board for the centerboard/leeboard/rudder)? I've seen some people who build a simple foil with a plywood core, foam sides, then fiberglass over the whole thing. That seems like no big deal, but if the performance increase (for a little pd racer in not-too-windy michigan) is minor, I'd rather save myself the trouble.
     
  10. CutOnce

    CutOnce Previous Member

    A proper foil shape makes a WORLD of difference going upwind. If you have to prioritize things - the parts that go in the water and go up in the air have more effect on performance than anything else (within reason).

    It isn't the trophies that people race for, it is the beer, conversation and friendship. Also, having someone around in the same boat can sort out performance issues much faster than trying to so it alone.

    If you do want to get fiberglass experience, you could glass the hull bottom of a PDR (and maybe get away with using a smaller thickness ply). Having a fair surface to start on makes glassing far easier to pick up and "get". Starting from a shaped foam surface (no matter how well shaped) will make ending up with a smooth fair surface much harder. Generally, glass on foam is done in molds.

    --
    CutOnce
     
  11. hospadar
    Joined: Apr 2011
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    Location: Michigan

    hospadar Junior Member

    Well, I do love beer...

    I plan on sticking to the PD Racer hull shape regulations (which permit a 1" radius on the chine, and I plan on using 1" foam, so that's all I'd probably do anyways), so I'm hopeful that when ballasted to the appropriate weight (perhaps with a cooler of the aforementioned beer) or just with my big self in it, it will perform very similar to a PD racer.

    I also just remembered a question I had about rigging, on a lot of the small PD racer and similar style boats I've seen, the mainsheet appears to be secured to the tiller, usually (as far as I can tell) with a fairlead right above where the tiller pivots and a clam-style cleat. I assume this is just a convenient way to secure the sheet and doesn't have anything to do with sheet-to-tiller "autopilot". Am I correct in my assumption?

    I'm currently waiting on one of phil bolger's small boat rigging books, but I've been wondering a lot about this as all the other elements of rigging one of these boats seems pretty straightforward.

    Thanks again!
     
  12. CutOnce

    CutOnce Previous Member

    There are as many ways to rig mainsheets as there are people, but typically they fall within a few general groups. Generally there is NO sheet to tiller autopilot setup - keeping sheet to hand is the safest and fastest way to get from A to B. Cleating mains generally is a cruising concept, not a dinghy thing. Don't let that stop you from freeing a beer hand though - cleats can be useful.

    Boom end sheeting off the transom usually is handled with some type of bridle that splits the attachment of the sheet so the tiller and rudder can act normally without interference.

    Bridles can be done many ways - with a becket block attached to the transom bridle and the sheet then 2:1 from boom (bitter end) to becket and back to boom, then running either to hand or a turning block forward on the boom.

    A "Y" splice in the mainsheet is another way, where the two tails attach to the sides of the transom and the sheet runs 1:1 to a boom end block and then forward to a turning block. In a boat as small as a PDR, 1:1 sheeting isn't tough and it certainly makes your connection to the sail fast and sensitive. The more mechanical advantage you have, the more line is needed to run it and move.

    Storer-type lug sails are different again. It really depends on your choices as a builder.

    --
    CutOnce
     
  13. hospadar
    Joined: Apr 2011
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    Location: Michigan

    hospadar Junior Member

    I just last night got a hold of an instant boats book from my library, and the "Pointy Skiff" looks appealing. It looks like a shorter, less elegant goat island skiff (the GIS is too long for me). It's has a very hard-chined profile, so I imagine it would fill my stability requirements well, but the plans I've seen are only for rowing or motor. Could a boat like this be expected to sail well with a simple rig? I suspect the answer is yes, it seems like if a PDR can sail, this would sail just fine since it's not much more than a pointy PDR

    Here's the boat: http://www.upwoodenboat.com/pointyskiff.htm
     
  14. CT 249
    Joined: Dec 2004
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    Location: Sydney Australia

    CT 249 Senior Member

    If you want a boat like that, why not got for a proven sailer like the SigneT?

    It's a flat-bottomed ply cruiser/racer designed by a great dinghy designer. LOA is 12'4", hull weight 72kg. Claimed to be very easy to build because of the flat bottom.

    [​IMG]

    [​IMG]
     

  15. ancient kayaker
    Joined: Aug 2006
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    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    8' is a bit short for adult use. Is there a reason for that - like you want it to fit standing on end in your garage over the winter? If so the PD Racer is a good choice and very easy to build. If you can stretch it to 10' or 12' there are plenty of designs that will look and act nicer than the PDR under sail, oars or a small motor.

    A flat-bottomed boat is very easy to build and stable in the water. All those nice square corners are great for storing stuff, stuff like oars and rods will stay there and not roll around and get under foot.

    Rounded chines adds effort and all you get back is maybe a nicer looking shape. This mostly shows up when the boat is upside-down so it’s not much use in the water. You lose on stability which may compromise its utility as a swimming and fishing platform.

    Professionals build elegant rounded hulls mostly because they sell; boats in this size are bought by yacht owners as auxiliaries and a round-bottomed boat looks nicer than a flattie when it’s upside-down on a yacht, and probably tows better too. A small round-bottomed boat is going to be a royal pain to climb into from the water.

    Let the experts here know where you plan to use it (river/small lake/big lake etc) before you finalize the design choice so they can advise you better.
     
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