Box Keel

Discussion in 'Boat Design' started by Brands01, Feb 22, 2007.

  1. Brands01
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    Brands01 Senior Member

    I'm interested in designing a seabright skiff (as a theoretical design project), similar to outlined by Dave Gerr in Nature of Boats, and Aitkin. The idea of a hull that operates very efficiently at displacement as well as semi-displacement speeds, and is also very seaworhty due to its low COG is of great appeal.

    I assume that it is all the flat sections in the underbody that produces the lift required to reach higher-than-displacement speeds.

    What factors should I be focussing on to ensure it is capable of operating at the speed I nominate? Is prismatic coefficient most important? (I am refering purely to hull shape as opposed to power and weight).

    I know this is a broad question, but any suggestions will be greatly appreciated.
     
  2. FAST FRED
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    FAST FRED Senior Member

    AM attempting something similar , and believe low weight , DL well under 100 is needed for a good performing boat.

    Look in power boat section for plans from Atkin I posted with article.

    FF
     
  3. duluthboats
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    duluthboats Senior Dreamer

    I have for a long time been fascinated with this hull form. It seems to be a frequent topic on this and other forums especially with the current interest in efficiency. There is a huge variety of box keel boats, from the long-established Sea Bright, the many Aitkin versions including his tunnel hulls, to the contemporary displacement glider. Things to remember are increased efficiency dose not mean increased speed. The DWL is very important, and I think the length to beam ratio is critical. Small changes in the box can mean big changes in displacement so try to get a handle on the required displacement early in the design. The advantages of this hull may be individually small like better use of interior space, the ability to sit upright on a mud flat, the allowance for the drive shaft to be near parallel to the water line, and the prop being protected by the box but they all add up to make this hull very appealing.

    Gary :D
     
  4. FAST FRED
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    FAST FRED Senior Member

    "to the contemporary displacement glider."

    The folks in Austria still have a web site , but I think there gone.

    The interesting part is these boats were designed by Atkin for simple construction in wood.

    Only the "power glider" version was done with modern materials that might allow a good improvement.

    Don't know to look at submarine shapes or bows of a modern tanker for a direction to modernize Atkins work.

    FF
     
  5. Brands01
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    Brands01 Senior Member

    I've been having a good look at the Aitkens website for some time now, and there's no doubt that his boats all work. Its the science of how this particular hull form works that fascinates me, along with the benefits as mentioned by Gary that make it so practical.

    FF, I had a read of of the Pocket Cruiser thread, and there is definate interest in the hull form. It seems that the DL ratio of under 100 is required for a slender hull that can reach high speeds without planing. I don't think this is practical for me, and I'm sure not many of Aitken's designs would have had a DL of 100 or less.

    Rather, I would be envisaging a higher DL and a hull form that develops a small amount of dynamic lift to allow a speed length ratio of higher than 1.34.

    Reading through the pocket cruiser thread highlighted to me that I know nothing of the science of planing. So I plan to research this area some more, then I will return to the drawing board with more knowledge in hand to attack this very interesting project!

    How are you going with your plans?
     
  6. PAR
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    PAR Yacht Designer/Builder

    I've been working occasionally on a 39' Atkins (John), box keel motor yacht, in the last couple of years. It certainly doesn't have the ability to plane, nor an especially low D/L though can be pushed past it's theoretical speed limit with surprisingly small amounts of power. At 11 tons (net and dry) she's no small craft (christened Namaka and one of the designer's favorite boats). With her fresh 50 HP diesel she touches 11 knots, but generally does 8 - 9 in all but dead flat water. The efficiency of the type is reasonably well documented, but I don't expect particularly high speeds from the hull form. They have a point of rapidly diminishing returns and get unstable if pushed beyond this limit, which I've done in over powered skiffs. Another point to ponder is backing maneuverability, which generally sucks in fixed shaft versions of the type.
     
  7. FAST FRED
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    FAST FRED Senior Member

    "With her fresh 50 HP diesel she touches 11 knots, but generally does 8 - 9 in all but dead flat water. The efficiency of the type is reasonably well documented, but I don't expect particularly high speeds from the hull form."

    The speed is directly related to the fineness of the boat and the weight.

    I don't think the slim lightweight hulls available today will find a S/L of 2.5 or 3 to be out of range for semidisplacement .
    Although a D/L of under 100 was probably beyond Atkin for EZ construction in wood in the 60's.

    AS the multi-hullers have found after a LB ratio of at least 5 or 6 , the losses to wave making are far less.

    What IS the best shape for the box keel remains a question , that I don't think many tanks have investigated.

    I would certainly accept unremarkable backing , for the ability to run aground , or take the ground on a family cruiser..

    FF
     
  8. moTthediesel
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    moTthediesel Junior Member

    I have been thinking along the same lines for some time. What I'm looking for is a "retirement" boat that has enought room for a couple to spend several months aboard, yet is narrow and light enough to be moved and stored on a trailer. It would need to operate at moderate speeds (10 to 12k cruise) and show execellent fuel efficency (say 10nm/gal +).

    The box keel hull has much to recommend it for a boat like this. To show this kind of performance with a very small diesel (say around 30 hp) the boat must have a long dwl (I'm toying with around 37'), narrow beam (say 7.5'), and very light weight (5000# or less dry). The design I've been noodling around with has a long raised deck forward with a wheelhouse aft, rather like a Lake Union "Dreamboat" style. One of the advantages of the box keel for this is that it allows a deep, narrow "aisleway" that gives standing headroom through the fore cabin without resorting to outrageous freeboard or a "crazy-crown" deck. It also, of course, puts the engine weight (considerable by proportion in a boat this light) low and allows for a near horizontal prop shaft.

    Have you all read the artical in WoodenBoat #189 by the late Robb White? His description of his version of Atkin's "Rescue Minor" was a real eye opener for me. He refers to the tunnel Seabright hull form as two boats in one, and as I think about it, it makes sense. The long narrow box keel provides most of the bouyancy, directional stability, and as it quickly reaches hull speed it's wake fills the tunnel void under the stern of the "upper" hull. This upper hull gives the accomidation room, reserve bouyancy, and it's extra beam adds stablity and maybe a little dynamic lift aft.

    His belief that the captured stern wake allowed the boat to "surf on it's own wake" while tantilizing, is perhaps debatable. There can be no debate about the pictures in that artical though, the image of Robb and RM zipping along trimmed flat in just inches of water while making virtualy no wake needs to be seen to be believed.

    moT
     
  9. FAST FRED
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    FAST FRED Senior Member

    Your dimensions are right on, as the proposed boat could fit inside a sealand box , allowing cheap shipping to a new cruising area every season.

    At slow speeds 10nmpg is easy , most sail boats do it.

    With such a vessel the ability would exist , for folks with thicker wallets to cruise at 5nmpg at a substantially improved speed.

    The hassle is most cruising engines in the 120hp and up class are 1200lbs , for a reliable industrial conversion , rather than a diesel auto conversion.

    That will bring the trailer weight close to 8000 lbs with empty tanks but batteries and cruising gear aboard.

    How does one obtain this article , can you post it?



    FF
     
  10. moTthediesel
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    moTthediesel Junior Member

    Fred,
    I'd never even thought about that container thing until I saw your post about it somewhere else (Passagemaker board maybe?). I'm not sure I would ever do it, but it sure does open up possibilities --

    Here's a link to the Robb White site:

    http://www.robbwhite.com/index.html

    We was a great writer and a genuine free thinker, his death was a great loss to boat noodlers everywhere.

    Here's the picture I was talking about:

    [​IMG]

    If you want that WoodenBoat story, PM me, I'll scan it to you.

    moT
     
  11. fcfc
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    fcfc Senior Member

    Have you ran some numbers for this ?

    5000# is around 78 ft^3 for volume. A DWL of 37' and a Cp of .64 typical for a semi displacement hull will give you a midshipsection of 3.3 sqft.
    With a Bwl of 7.5', If the section is triangular (Cm = .5) , with chine at waterline level, you would had a canoe draft of 0.88' = 10 " .

    If you use a sqare keelbox, 1.5' wide, 1.5' draft (1.5 wide external mean internal width barely above 1' keelside thickness and some framing : one feet atwartship for floor, or a very narrow engine).
    Section of the keelbox would be 2.25 sqft. remaining surface for the bottom is 3.3-2.25 = 1.05 sqft, span on 3' each side. Again with chine at waterline level and triangular sections, the intersection between the bottom and the keelbox would be 4" below waterline.

    If the keelbox is 2' wide, and 1.5' deep, your hull bottom will be barely touching the surfave of the water ...

    And what layout do you plan for living months aboard ?
    5000# dry will give you at max 2000# payload for crew and tanks.
    Where do you plan to put engine, tanks water and fuels, batteries ?
    If you put them in the keelbox, you will no longer be able to use its volume for increasing headroom.
    Another point a small engine will be 2' net high, If you add bottom thickness, bottom clearance, engine heigth, top clearance, sound insulation thickness, floor/hatch thickness, you will have an overall heigth or nearly 3'. With a 1.5 ' draft, the top of your engine hatch will be 1.5' above waterline. How do you handle it in your layout ?

    I have tried things in the O1 forum, just to conclude that it is very hard to make numbers match ... I have not found any usable/practical layout. You either go houseboat/bolger style with high supertructures (and shallow draft) that will limit you to very protected / inland waters, or you have more sea kindly supertructures, but liveability (ie headroom) suffers a lot.
     
  12. moTthediesel
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    moTthediesel Junior Member

    Ok, here's how I figure it --

    First, you need to bear in mind that the box section keel we're talking about is a good bit shorter than the rest of the hull. Say for an LOA of 38' the box keel would be about 30' long. Say a maximum beam of this box is 3', that's 90 sqft. If we multiply by a fineness factor of say, .65, we get 59 sqft. Figuring 14" depth we get a volume of 69 cuft, or 4279# fresh. With 12" depth = 59 = 3658# fresh. So yes, the box keel provides most of the bouyancy for the hull, with the wider upper hull only immersed a few inches. I'm no expert, do you see a problem with that?

    Remember, the keelbox is very long, the engine is very small. Engine would be mounted in the wheelhouse area under a motorbox. I don't mind motorboxes at all, had one in my CC SeaSkiff for years, made a good place to put the plate of crackers and cheese! Plenty of room under the wheelhouse sole for tankage. Fuel tanks don't need to be huge for a coastal cruiser that gets this kind of fuel economy anyway.

    That still leaves a full 20' length forward under the raised deck for accomidations. Nothing fancy there, but a good full double berth forward, a simple galley, settee/dining, composting head, and a shower. No air conditioning, no mechanical refrigeration, no bells, no whistles.

    I'm still just doodeling here, but I don't see anything that can't be done. When I get something polished up, I'll post it here for a complete thrashing ;)

    moT
     
  13. sal's Dad
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    sal's Dad Atkin/Bolger fan

  14. moTthediesel
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    moTthediesel Junior Member

    I've seen the Gerr boats but I hadn't seen that Alligator, -- both interesting, but the Atkin boat is closer to the kind of minimalist cruiser I've got in mind.
    moT
     

  15. FAST FRED
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    FAST FRED Senior Member

    From the folks on this board , and lots of other reading I think I may have figured out WHY the Atkins boats perform so well.

    I think the "box keel" works like the submerged pods on a SWATH boat , floating most of the hull with no big surface wave.

    The small under hull reaches "hull speed" and feeds water to the barely submerged prop , so the description "she surfs on her own stern wave" is mostly true .

    Whatever the displacement all the Atkins Sea Bright Skiff hulls have the transom just kiss the surface at rest , as a displacement boat would.

    AS the bow rises from the under hull starting to plane the accelerated prop wash in the simple tunnel aft lifts the stern , so she rides mostly flat.

    When one boat was overpowered the stern had too much lift , so steering became wierd until lifting strakes were added , to lift the bow.

    So far what I have come across has all been good , Atkin , Tollman Skiff, Power Glider all seem to use the concept.

    If this is how it works, it sounds like a good cruiser could be made.

    Does anyone have BAD things to report about this style hull?

    FF
     
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