Skinny Class 40

Discussion in 'Boat Design' started by Paul Scott, Jan 16, 2023.

  1. Paul Scott
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    Paul Scott Senior Member

    5FD1EF7F-67B0-4330-B73E-28EFC32CD41F.jpeg 4A30253A-9D08-4888-842A-CA7C40F3F49A.jpeg "fcfc, post: 941216, member: 7793"]Are you speaking of this :
    www.balta.fr/Classe 40.html

    PS : in 2023 class 40 french rules, daggerboards and lifting devices are not allowed.
    In french :
    "Nota : les dérives sont interdites. Les plans porteurs, c’est-à-dire tout appendice visant à générer une force verticale, sont interdits."[/QUOTE]

    looks close to Amati…. See above
     
  2. Paul Scott
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    Paul Scott Senior Member

    Some quick & dirty skinny 40er s. They are based on flat bottomed IC shapes I’ve messed with as initial hull forms, but I hope there’s some like against like continuity in a box boat kind of aesthetic. You’ll see statics first & then wave and friction drag. When these shapes heel, according to this modeling, wetted surface goes down, but wave drag goes up quickly, pretty much proportionally to the upright wave drag. (I’ll get some max beam shapes up if anyone is interested). Round hulls have slightly lower wetted surface drag at lower Froud #s, but wave drag starts out lots worse, at least in this software. The main thing I’ve taken away from messing with these shapes is that class 40 boats are heavy. (Lindsay Llord had some interesting things to say about heavy planing shapes.) what I’ve done here is kind of Bolgeresque, & I hope it might amuse. 74EBB494-79C9-4B2F-9666-FC39D12F14BA.jpeg 3D534899-0457-481E-A70B-44FABAE7CDAA.jpeg 451EAFB7-C93E-4F14-A985-66B088AAB3A2.jpeg A19E9E1B-C6BD-4E73-AFC8-3B92C29EAC29.jpeg 8402F6BB-B7F4-4DCF-867E-596250FE6C07.jpeg A910B495-D9E2-49B4-9A9F-BE095B76E118.jpeg
     
    Last edited: Jan 20, 2023
  3. skaraborgcraft
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    skaraborgcraft Senior Member

    I often wondered about this Bolger boat, as a means to get myself, my essential belongings and tools for a new life in New Zealand.

    upload_2023-1-20_10-35-23.jpeg

    I would have ditched the leeboard for a vertical ballasted daggerboard. I think even a small V in the bottom would have been an improvement unless shoal draft was an absolute must.
     
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  4. Paul Scott
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    Paul Scott Senior Member

    I wonder if the anti squat devices would be legal- they work on the windsurfer level
    F3576EFC-8195-4921-AF7E-49970DE80165.jpeg
    Phantom Race Windsurf Board https://windsurf.star-board.com/windsurfing-boards/windsurfing-longboards-raceboards/phantom-race-windsurf-board/
     
  5. skaraborgcraft
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    skaraborgcraft Senior Member

    Would that be the "wings" aft, like they retro-fitted on a lot of ferries after the MS Estonia sinking?

    Cant recall where, but i think Bolger may have said the area aft was possibly may not have worked as well on the bigger boat, as it was basically an enlarged 11ft EEK ( IIRC). ,or just far too much displacement aft.

    I came across his Blackgaunlet 11 some time after, the round bilge version with a foot more beam, and considered that a better boat for someone not looking for a disposable one-way frieghter. Still a skinny with almost 5-1 beam ratio.
     
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  6. Paul Scott
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    Paul Scott Senior Member

    The skeg ( What to call it?) is a way to add displacement aft and increase the prismatic coefficient, but it is alarmingly draggy, at least with my software… maybe he was looking for a way to move the mast a bit aft?
     
  7. skaraborgcraft
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    skaraborgcraft Senior Member

    If that comment refers to the Economy cruiser. then "skeg" seems like an understatement. I think the drag thing is what he was probably referring to. Having said that, its still a long waterline, so even at displacement mode, probably easily driven. He drew a lot of stuff that was never built, so i would not be surprised by excessive drag. I can imagine seeing a LOT of bubbles at night in the wake of that boat. Sorry for the thread drift.
     
  8. skaraborgcraft
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    skaraborgcraft Senior Member

    How does a froude number relate to S/L. I understand round bilge is better at low froude, but if one never wanted to excedd hull speed @1.3, how much draggy is a chine hull compared to the round bilge, if already skinny?
     
  9. myszek
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    myszek Junior Member

    I found my old results about Class 40 boats.
    "wide" is a hull of a maximum allowed width, 4.5m
    "narrow" is 3m wide, with a high ballast coefficient - quite feasible for anarrow hull, even without hi-tech materials.
    Unfortunately, Class 40 rules restrict maximum (as well as minimum) righting moment at 90deg heel. It is well below the feasibility limit for the narrow hull.
    Hence:
    "narrow restricted" with a ballast coefficient limited by class rules.
    And finally,
    "DSS" is the latter, with a DSS foil added.
    All polars obtained by my crude VPP program, basing on drag computed by Michlet. The wind speed was 8m/s
    40_compare.jpg
    As you can see, the present class 30 rules strongly prefer wide hulls, and they ban clearly better solutions.

    regards

    krzys
     
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  10. Paul Scott
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    Paul Scott Senior Member

    Now that I’ve getting my Vacanti up and running again, I’ll post some friction/wave drag plots soon, but in general, IIRR, a box section and a round section are really close at low Froud #’s in friction, but as long as the boats are upright, the boxy hull, if double ended, and light enough (which the open 40’s aren’t), is better upright than a round hull, especially at higher Froud. Once the boat starts heeling, things change.
     
  11. Paul Scott
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    Paul Scott Senior Member

    That’s what I’ve been finding. Gutelle (probably spelled that wrong…) points out in his book on yacht design, that shallower hulls are faster, especially in the heavier boat range. Llord covers heavier planing hulls, and wider is better- but you need more power. At the class 40 weight range, the main thing I can think of is if weight was down deep enough in a skinny hull, drag reduction might be effective, as well as windward wide hull weight in foot pounds in waves, you might be able to do without a bulb? Reducing windage might help, as well as wetted surface? Finding a better gear sailing farther off the wind like PRB (IMOCA) has might help, as well as better Wetted Surface/Sail Area in general, as well as better upwind performance, less drag from the massive sail area required by present class 40’s (although sail drag downwind can be considered helpful), as well as easier sail handling, less crew load, like lugging sails to windward, and a less punishing ride, but all these things were available according to Tom Wylie’s design approach to Ocean Planet, but Bruce wound up doing what he could towards a Finot type sailplan. Different approaches to VMG might be culturally bound? The Red Cigar’s sailing angles didn’t look much different from the fatties in her class, recently. As I’ve mentioned before, Amati (skinnier and lighter than Class 40’s) in anything above 20 TW goes better downwind with main only, but our mast is farther forward, but our main is smaller, and I can soak lower with full main up so there might be a tweak there for the Class 40’s? Jib oriented vs main oriented….

    On the 90 degree weight test, when the boat is at 90 degree heel I’m trying to understand why wide vs narrow would change anything except the weight of the deck & cabin?- but getting to 90 degrees would be vastly different….
     
    Last edited: Feb 15, 2023
  12. skaraborgcraft
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    skaraborgcraft Senior Member

    To this non-racer, in light winds, it was hard to tell without a gps that a ssquare chined boat was more draggy than a round hull.......what i did notice was the chined hull did have better roll damping. Racers have no comfort consideration, i prefer to keep my bacon and eggs in my belly and not sprayed over the saloon.....
     
  13. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Several quick thoughts:
    • The handicap winner of the Sydney-Hobart just sailed was a relatively narrow Reichel/Pugh design. Had the Reichel/Pugh maxis raced this year, one of them probably would have been first-to-finish.
    • There is the Fast 40+ class.
    • What about a ballasted catamaran, basically a scow with a bulb keel (or two, one on each side), but with the middle of the hull cut away? The hulls could be round in section with canting keels, the windward one able to lift the bulb out of the water like Q.
     
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  14. CarlosK2
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    CarlosK2 Senior Member

    Froude number / 0.3 = SLR
     
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  15. myszek
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    myszek Junior Member

    Still thinking of a skinny 40, I recently obtained an interesting result.
    Never before analysed a skinny scow. So, here it is, hard chined, only 3m wide.

    slimscow40_plan.jpg
    Designed to heel up to 30deg, with the axis of the immersed body turning up to 3deg in the "good" direction (i.e. the keel has a 3deg angle of attack without leeway).
    The comparison with a "fat" scow (i.e. 4.5m wide) scow is interesting. To satisfy the maximum 90deg stability rule, the wide hull has its CG about 40cm below the waterline, while the narrow one 60cm below the waterline. Therefore, the maximum righting moment is only 1.3x bigger for the wide hull.
    On the other hand, the drag of the skinny hull is 1.2-1.5-fold considerably smaller:
    drag_compare.jpeg The aerodynamic drag is also obviously smaller for a skinny hull. With a lower mast, you can fly the same sail area with a similar aerodynamic efficiency and similar thrust. The result: slightly better performance of the skinny hull.
    polar_compare.jpeg
    regards

    krzys
     
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