Box? keel sailing hull

Discussion in 'Boat Design' started by lewisboats, Sep 20, 2007.

  1. lewisboats
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    lewisboats Obsessed Member

    A Long time ago I proposed a box keeled hull for a sailing rig but it had too much displacement and was unwieldly. Here is a refinement of said hull (actually a radical redo of a different hull)...rigged as a Gaff Cutter with the mast aft enough to provide a Vee berth forward at anchor (8 Ft+/- for the berth). {Pardon if i don't show the mast as it dimishishes the hull picture enormously.} Quarter berths are provided under a self-draining cockpit floor and a galley and head are provided for amidships (along with another double berth if needed). The design is specifically aimed at the US/Bahama crossing... with an abundance of displacement for stability with less than 4 ft of draft. A Pivoting keel of Lead is provided for...for additional lateral resistance in the crossing (not shown). Length is 24.8 ft with headroom for a 6 footer along the centerline. LOA is 24.8 ft (LWL of 24 ft) with a maximum beam of 7.65' and a beam at the waterline of 6.9 ft. Displacement is 6350 lbs with a fixed lead keel, bottom of the box keel shoe and swing keel of 3350 lbs total ballast. The hull would be cold molded of 3 mm plywood over an initial strip plank to a thickness of 1". The boat would be very trailerable but also capable of some blue water (in decent weather) sailing...So... anything I might have missed?
    Steve
     

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  2. messabout
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    messabout Senior Member

    Steve;
    That looks pretty good. US/Bahama crossing is O.K. in a 24 foot boat, but you must pick your weather carefully. Miami or Fort Lauderdale is a good departure point. The Gulf Stream is beyond vicious in a northeaster. It is like a pond in good weather. With four feet of draft you'll be able to get across the flatland east of Bimini/Cat Cay and on toward the Berry chain. It can get pretty lumpy out on the flats too when the bad weather arrives.

    Plan well, go for it.
     
  3. alan white
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    alan white Senior Member

    Hi Steve,

    Just looking at the cabin top, the ridge at the middle. I don't know the purpose, but for a flush deck the cabin top appears a bit steep for motoring or running (or moving about on the lee side when well keeled).
    Crown works with side decks, but a flush deck is usually relatively flat.
    Sqeezing 6 ft interior height into any light displacement sailboat of that size is a challenge. I appreciate the effort, having worked on a lot of doodles in that size range. The question is how viable the cabin top will be as is.

    Otherwise, seems you've done well with the underbody in terms of capsize survivability especially, and, if construction isn't too complicated with that semi-box keel, good tracking characteristics.
    The boat may pick up some strong weather helm when heeled given the wide afterbody (and this hull will heel easily, being essentially round-bottomed and narrow to boot). The suggestion is a good miansail reefing setup, either in the boom or the mast, to quickly get the center of effort forward. Reefing this boat will definitely start aft and move forward.
    I see a bit of Norwalk Island Sharpie, another boat with a high-crowned deck, come to think of it. I think a solution to that might be as simple as a walking surface a foot wide on each side that is closer to level, not necessarily molded into the deck, but a seperate banana-shaped board set on athwartship wedge-shaped supports, something with some coarse grit on it.
    Nice job, Steve.

    Alan
     
  4. Guillermo
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    Guillermo Ingeniero Naval

    Nice job, Steve.
    What about rock'n'roll? How do you foresee the quality of movements?
    I have estimated the following:
    Motion Comfort Ratio MCR = 26,81
    Heft Ratio HF = 1,37
    Roll Period T = 3,13 Sec
    Roll Acceleration Acc = 0,05 G's
    Stability Index SI = 1,34
    So she seems she'll be quite comfortable, by the numbers.
    What would be the sail area? Could you post a lateral view with rig and sails?
    Cheers.
     
  5. PAR
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    PAR Yacht Designer/Builder

    Steve, what is your logic for the diminutive box keel, as I don't see any advantage it may offer.
     
  6. water addict
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    water addict Naval Architect

    I would echo comments on the cabin/deck shape. Anything sloping or rounded makes for difficult deck work when heeled at sea, and bouncing about. Flat and straight surfaces make much easier footing and gripping.

    I'll also ask about the purpose of the box shape below the waterline- what is the rationale?
     
  7. alan white
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    alan white Senior Member

    I can see that the shallow box makes possible standing headroom maybe three feet wide, and also (slightly?) greater initial stability than if the corners were cut and the bilge was a shallow vee instead. I'm guessing the gain is maybe three inches in height. It appears that the design of both cabin top and hull bottom are what makes the standing headroom possible.
    The structure would eliminate the need for stringers, so no problem with added complication. There's a small wetted surface penalty.
    The cabin top is a bit steep as is--- an advantage when on the windward side, but the opposite if one tries to function on the leeward side.
     
  8. PAR
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    PAR Yacht Designer/Builder

    The box keel concept I understand well and enjoy the advantages it can provide. With this insignificant box, offering little toward displacement, increasing eddy making, chine steer, etc. I fail to understand (not trying to be coy Steve) what advantage it offers. A couple of inches in headroom could be handled easily, several ways without the underwater areas suffering as much.
     
  9. lewisboats
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    lewisboats Obsessed Member

    Actually it runs to about 4.5" additional headroom...but I have also decided that the trade-offs are not worth it. There would be less problems with the additional windage of raising the superstructure than the hydrodynamic dirtyness that the box keel adds. I was just working through a thought to see if it was workable. It is...but its kind of like reinventing the wheel and giving it 6 sides...it doesn't work as well as the one we alread have.

    Steve
     
  10. PAR
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    PAR Yacht Designer/Builder

    Yacht design = concessions in convoluted compromise . . .
     
  11. alan white
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    alan white Senior Member

    Height is never a problem if the standing headroom doesn't extend too far fore and aft. I myself have been working on a St Pierre dory yawl, which could use some headroom. There is a space between the two sails on masts from the forward end of cabin hatch to the mizzen mast 3 ft abaft of the cabin, about six feet total.
    The main boom swings within a foot of the seven foot long cabin top (the dog house is fully 38" from deck to cabin-top at the centerline).
    Forward of the cabin (which is entirely aft of the centerboard), the deck, from main cabin to toilet/storage cuddy forward of the mainmast is a big 8 x 8 area below the main boom.
    By limiting the cabin to seven feet long (galley aft and settee/berths forward), the boat does not have excess windage, nor does it look very high.
    The whole of the aft main cabin is, in other words, fully 6 ft 2 in or so high inside. The forward cabin, the cuddy, is for the toilet and storage, and two more berths on occasion.
    I learned a long time ago that what's really needed is standing headroom in the galley and settee area. This allows traffic to and from settee (berths) and galley without ducking.
    If the cabin is limited to just that, the boat doesn't look or behave top-heavy.
    I think, unless an under-28 ft boat is pretty heavy (beyond trailerable), the only way to get headroom is to keep it short fore and aft unless you want to battle winds going to windward and to sail on your mooring.

    Alan
     
  12. lewisboats
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    lewisboats Obsessed Member

    Well...I ditched the box keel and the headroom is down to about 5.1 ft. I added side decks or rather narrowed the cabin to provide for the decks but still kept the raised shear profile (Adjusted slightly ;) ). Draft has been reduced to 29" before a pivoting keel drops down. The actual hull shearline has been straightened to give a bit more interior room at seated height. Disp is down to 3885 lbs loaded, with the (projected) dry unloaded hull running about 2500 lbs...of which 1375-1500 lbs is lead in the keel(s). I am still debating on a bilge keeled configuration so the boat can ground level. Unfortunately coming up with a bilge keeled anything in Freeship runs quite a few hours of work...of which I don't have right at the present moment. I'll work on it. Attached also is the hydros for the shape...some of it doesn't exactly fit with the established norm...ie the LCB is a bit forward of the midships location and the prismatic cooefficient is a bit high due to the wider after section. Once heeled tho...things shift a bit forward due to the flair of the sides forward of the max beam area. I would anticipate this easing the increase of weather helm most boats with a wide beam aft tend to generate.

    Steve
     

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  13. alan white
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    alan white Senior Member

    I liked the idea of box keel = standing headroom. I think it can be done, but with the box keel as lowest board-up draft. A dagger to one side of a 2 ft wide box would give lateral plane. A thick iron shoe bolted to the bottom of the box would be plenty ballast if the rig was low'n'long and the bottom to each side of the box was flat (put the box where feet stand at 18" below water level, and the highest point of the cabin top can be a mere 4 ft 10in above the water). This might involve a 24" dog house, but seated headroom could fit under the side decks.
    You had a solution. It was pretty workable. Unless you add some lateral plane to the last design, you won't go well to windward. An in-keel foil board with 5 ft draft would work.

    Headroom is very wonderful, you know.
     
  14. tom28571
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    tom28571 Senior Member

    Before you become too enamored with a box keel sailboat, look at some of those that have already been built. Lancer 25, S2 26 and Rinell 23 are some that come to mind. They all sailed like pigs. Headroom may be nice in a small sailboat but what you give up is even nicer.
     

  15. alan white
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    alan white Senior Member

    Yet I feel the concept could sail well, because we are talking about shapes here. The box needn't be vertical sided, the outer corners needn't be hard.
    Nor does the width need to exceed 16-18" (and now were approaching the shape of a shoal keel).
    We know that many shoal keels sail very well, and their keel chords are enough to allow foot traffic inside. I would say that if well designed, at least in the belly of the hull, the arrangements could be done cleverly enough to mimic at least the "plank-on-edge" types.
    Yacht design slowly moves out of the dark past when all hulls needed a bilge to keep the occupants dry. Wood hulls leaked. Nowadays, hermetically sealed hulls without frames (and without floors) but instead monocoque or skin-based structure can do away with the bilge entirely.

    Alan
     
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