Is this an innovative concept?

Discussion in 'Boat Design' started by kengrome, Jan 3, 2010.

  1. kengrome
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    kengrome Senior Member

    I've always appreciated the look of Sallie Hyde, an Atkin boat, but I want a smaller one that uses panels no longer than 16 feet to minimize scarph joints. I also want it to plane comfortably in a chop so "getting there and back" can be a fast and pleasant experience most of the time.

    I came up with this concept:


    The topsides are basically a 14.5 foot Sallie Hyde look-alike. Below it has a flat bottom and a free flooding wedge shaped box keel. It is a short but substantial load carrier and should be seaworthy in most conditions.

    Here's the concept for its use:

    Powered by an outboard mounted on a hydraulic jack plate, the "up" position will align the prop with the center of the aft end of the box keel. This is the position for running at low speeds. The box keel protects the prop from grounding. Water is drawn in through the oval holes in the front of the box keel and exits out the back.

    To accelerate to planing speeds the prop is lowered to the "down" position so that the top of the prop is below the bottom of the box keel. As the boat begins to plane its wide flat bottom rises up and away from the water's surface, and the water in the box keel drains out completely. Now the boat is planing on the box keel's triangular shaped bottom which provides a smooth high speed ride with very low vertical accelerations -- in other words a soft and comfortable ride at planing speeds.

    The disadvantage to this type of hull is that it draws 12 inches below the boat's bottom -- not a big deal considering that the outboard needs this much clearance to run anyways. Advantages are more numerous:

    - A low speed displacement boat that planes comfortably at high speeds
    - Great directional stability because of the wedge shaped box keel
    - Substantial roll resistance (like a water ballast chamber) when at rest
    - Excellent propeller protection at low speeds and in shallow water
    - Substantial grounding and puncture protection for the hull bottom

    I've never seen a free flooding box keel designed or used like this before, so I'm wondering if I'm the first to come up with the concept?

    Attached Files:

  2. tom28571
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    tom28571 Senior Member

    Wow Ken,

    If this works, you will be teaching something to a lot of us.

    I like the general concept of box keels. I like the general concept of a flooded stability chamber. Never seen anything like your version. How this one is expected to plane is beyond me though. There seems to be a great surplus of drag.
  3. alan white
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    alan white Senior Member

    The flooding idea is one thing but the passing of water through the keel is frictional disaster. Both inner and outer keel surfaces count for wetted surface, and from what I see, at displacement speeds it's going to eat power like crazy and at anything more (semi-planing speeds) it seems the power required would also be huge.
  4. eponodyne
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    eponodyne Senior Member

    One unlooked-for benefit of this monstrous drag, however, would be a really hefty wash off the prop at manuevering speeds, which might make docking a relative breeze.
  5. kengrome
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    kengrome Senior Member

    There will be lots of drag at displacement speeds because of the substantial wetted surface area both inside and outside the box keel. I don't think it's going to prevent planing with a reasonably sized engine but the only way to know may be to build it. Assuming 50 pounds per HP to plane and 1500 pounds displacement it should plane on 30 HP so a 40 HP outboard is probably the best choice for this boat.

    I have more than one design for the box keel. My first was double-ended, typical of Seabright hulls:


    This image is very close to the original Sallie Hyde hull shape designed by Mr. Atkin. I think he tilted the bottom of the box keel to help the boat lift up as speed increases. It seems to me that this would work much like a water ski.

    This boat is designed for an inboard engine which I did not want so I replaced this box keel with the free flooding type shown at the top of this thread. The free flooding box keel has a triangular bottom like a Seaknife hull. Seaknife hulls are known to create soft rides at planing speeds. When they come down on a wave their narrow profiles sink into the water gradually rather than stopping abruptly and pounding.

    A nice ride is one of my priorities.

    Attached Files:

  6. Loafer
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    Loafer New Member

    I like your ideas Ken
    would you consider keeping the box keel and where it tapers down have the keel bottom flare out .
    this would give you more planing surface at speed or could be extended out to act as ahydrofoil
  7. kengrome
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    kengrome Senior Member

    If I'm understanding you correctly I actually did something like this a year or two ago in one of my old designs. Basically I extended and widened only the bottom of the box keel without changing the sides. It was simple with this design because the whole boat was only about 2.5 feet wide (without the amas) so all I had to do is use a longer wider piece of plywood for the box keel bottom.

    The idea was to provide a flat surface so the hull could semi-plane at speed without ruining low speed efficiency too much. I never got a chance to build it but I'm pretty sure it would have worked well. I developed the concept for an inboard powered banca in the Philippines. Here's a freeship image with 4 different views which I think will illustrate the concept reasonably well:

    Attached Files:

  8. PAR
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    PAR Yacht Designer/Builder

    I've had some success with a similar concept, though I just provide additional planning area and don't attempt to carry so much displacement in an appendage of dubious value to a high speed craft.

    Your first boat will have to drag a huge "hole" behind that open triangle, box thingie. It would never get up on plane with reasonable amounts of power per pound, let alone get a prop to enjoy living behind it.

    The second hull might see some semi plane speeds, but I'd be surprised if you'd get more then 2.5 S/L out if it, again with a reasonable power to weight ratio.

    The last hull has as much drag as the second, but a second, submerged planning surface and what appears to be a hook in the stern quarters. Yep, you'll probably need a hook or tabs, but how much. I think you'll just create a huge amount of suction aft and drag the stern down further.

    Consider just the need for additional surface to plane on. Maybe something adjustable that can feather at displacement speeds, but canted at 2 to 4 degrees can offer lift to a stern that is trying to squat under increasing loads and speed. Forget about the box keel, it's not doing anything for your design.

    With the exception of a couple of Atkins very small box keel boats (I have one of John's last box keel designs), they never really got over 2.5 S/L. They only did so with higher power to weight ratios than are practical in larger craft (my boat is 40'), but this is fairly easy in a boat two guys can lift into a pickup.

    Again, your box keel does nothing except disrupt water flow to the prop. The box keel offers shoal draft and efficiency without resorting to great beam. The engine can live very low in the hull, the shaft angle is much straighter, freeboard can be kept low and the bulk of the craft's displacement is carried in the box.

    A couple of models could solve this debate pretty easily. One of a typical warped bottom planning hull and another of the last hull you have. Tow them at S/L of 3 and up and see where you stand. This isn't fast, but is just up on plane at around 11 knots (depending on actual LWL), hell at S/L 4 you're still only doing 15 knots. Tow them on a balanced yoke and see which has more drag, as it will be the one lagging behind. The wave train shapes will get you close enough to your target S/L ratios.
  9. Fanie
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    Fanie Fanie

    Since that keel is wedge shaped it is going to act like a displacement hull amd will only pierce the water and will not want to plane. It is only going to displace the water. It's shape does not promote lifting the hull up on the water.

    I agree with Par on the aft part of the keel - it is going to make a lot of drag. While it is possible to make a dustbin pane with enough power, I doubt that would be the idea.

    A boat on the plane does so on the aft third or less of the hull, usually around a meter. IF that boat for some reason does get on the plane on that keel I suspect it is going to be very difficult to keep it steady. The slightest sidewave is going to grip either side and throw the boat.

    Also, again IF you get it on the plane, a planing boat rides up and down to find balance. The first time that nose moves down, nothing is going to keep it up because there is no support surface, and it is going to make a spectacular nose dive.

    Sorry Kengrome. But don't give up. Some good may come of it, if only to learn something.
    Have you thought about hydrofoils ?
  10. wet feet
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    wet feet Senior Member

    Rather than add to the previous comments,which are entirely accurate and should be noted before amending the design,I will add a word about a flooding keel.My sailing club had a four metre Avon Searider RIB as a rescue boat-those of you familiar with the type will know that the bottom chamber of the rigid hull floods.The aperture through which the water entered and left was a circular hole of about four inches diameter in the transom.It took a few seconds to empty or to fill and the motor was a thirty horsepower Mariner.Top speed was adequate for the intended use.I would suggest using a similar concept as the flow patterns with the proposal would almost certainly create a huge amount of drag.
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    FAST FRED Senior Member

    would you consider keeping the box keel and where it tapers down have the keel bottom flare out .
    this would give you more planing surface at speed or could be extended out to act as ahydrofoil

    The usual ATKIN solution is to use reverse deadrise to be the surface the boat runs on at high speeds.

    R-D stops the stern from squatting the usual problem with semi displacement boats.

  12. DGreenwood
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    DGreenwood Senior Member

  13. PAR
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    PAR Yacht Designer/Builder

    There are no box keel design other then the very small ones, that do much better then S/L 1.8, which isn't even semi plane mode. The hook Atkins used was to keep the bow down when the hull was pushed past 1.4 S/L ratio.
  14. TollyWally
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    TollyWally Senior Member

    I've always been interested in the Atkin hulls. I'm certainly not arguing with you but after reexamining my copy of "Motor Boats" by Atkin I find several claims for his box keel boats doing better than that S/L 1.8

    One example being Islamorada, a boat designed for your waters, 34' 8" l.w.l., 10' beam powered by a 115 hp Lycoming turning a 20 x 16 wheel with a 2:1 reduction. The owner claimed 18 mph a few months after taking possession and fooling with the prop etc.

    Several other box keel boats were mentioned with similar claims of speed but as I recall this is the only one that had actual post build trials results noted.

    If I remember correctly you either have or know about one of the last box keel hulls to come from the younger Atkin's drawing board. I am very interested in your opinions and real world experiences.

  15. Submarine Tom

    Submarine Tom Previous Member

    I drove a 12 passenger Burtram that had flooding holes in the transom

    similar to what "wet feet" discribed. It worked pretty well and planed

    after a slight delay. To me this makes more sense than having any holes

    forward. Less wetted area, and much less drag.

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