Go easy on me, Newbie here...Concave Hull design question...

Discussion in 'Boat Design' started by j. h. weil, Dec 13, 2006.

  1. j. h. weil
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    j. h. weil New Member

    Thank you all for dealing with a newbie, this is a great site, Im sure someone here can shed some light on this question... Hope this isnt sailing only-this is a powerboat hull question...
    So here goes...
    Ok, in terms of efficiency of a planing hull, wouldnt a concave, narrow hull (for example a 16-20 ft john boat) be the most efficient. That is to say, able to stay planed off with the least amt of horsepower possible. More than a flat bottom?
    Granted, comfort is potentially lost. Is it more efficent than a flat bottom? doesnt the concave "channel" energy and provide a captured lift by retaining the compression generated inwards...Its hard to describe...
    In the world of surfboards, concave or double concave bottomed boards are effectively turbocharged by this, making them much faster ( ask Kelly Slater)and I wondered if it would translate into boats. For a waterway cruiser or lake boat, this may work.
    Is there a term for the "highest level of hull efficiency" + the "most economic rpms of a particular engine"? Where can I learn about these things?
    Many thanks to whomever can shed some light on this for me...
    Thanks so much, JHW
     
  2. marshmat
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    marshmat Senior Member

    Hi JHW,
    As to the concave-bottomed jonboat, if I read you correctly, you're talking about starting with a flat bottom design, and making the flat part concave- thus the chines would be the lowest points, and in the centre it would be somewhat higher?
    I'm not sure if this would make it more efficient. It would certainly make the ride harsh, if it isn't already. It's hard to beat a flatbottom for planing efficiency in calm water.
    Some V-bottom hulls, notably the little fibreglass jetboats that are somewhere between PWCs and real boats, have concave bottoms. In this case you take a V hull, keep the chines and keel where they are, but make the normally straight bottom panels concave. I have heard that this can work well, but it is tricky to get right.
    There's always tradeoffs. A hull that's very easy to propel on plane in calm water might be impossible to handle in chop. Something that holds plane well in 6-footers, might be a dog in the calm. And then the engine.... well, ideally, the rpm-power curve of the engine will match nicely with the speed-power curve of the boat..... but it rarely happens that it works out so nicely. It's all about tradeoffs- what characteristics are most important to you?
     
  3. j. h. weil
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    j. h. weil New Member

    Hey Marsh, thx for replying! Yeah, I was reading a magazine and that idea hit me...And youre right, I dont think this would work well in any thing but glassy rivers, light chop at best, but Im pretty sure-in theory- that this is solid. Of course youre right about other conditions, it may not work worth a damn...
    There an experimental airplane called the xb-70 valkyrie, that had wingtips that folded down at speed, thus trapping the shockwave generated by a moveable duct-youd have to see it to understand, unless you have already, coining a term called compression lift...I see the concave hull as this-greatly simplified...anyway, I hope an actual engineer will chime in a get me straight, and nice talking to you!
    'H
     
  4. stonebreaker
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    stonebreaker Senior Member

    Many very high performance planing hulls use a pad - a long, usually flat section of hull to get the most performance at high speed. Hydrostream boats are well known for this design. They have a flat pad running forward from the stern, usually around 18 inches wide and about 6 feet long. As Matt mentioned, there are always tradeoffs, but this is a pretty good one - the pad is too small to support the hull except at relatively high speeds, so you get the smoother ride of a vee hull at lower speeds. The trade-off comes in a lack of stability - several of the fastest hydrostreams are known for chine-walking and take a skilled hand to drive fast. I'm talking speeds of around 80-90 mph with a 150 hp outboard. Some of their later models have outboard sponsons molded into their hulls, effectively creating a hull with two tunnels, but they trade some speed for better stability.
     
  5. marshmat
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    marshmat Senior Member

    Agreed on the Hydrostream hulls, stonebreaker.... never driven one, but from what I've seen they're not exactly easy to drive. Incredible speed from very little power though, in the hands of a good pilot.
    JHW, I'm familiar with the Valkyrie, and I'm pretty sure that unless your jonboat is going to be supersonic, the principles will not carry over. The shocks formed by the wings of a supersonic aircraft are incredibly powerful pressure waves, similar to what we think of as sound waves but about a hundred orders of magnitude more powerful. Shaping and controlling these shocks is a huge factor in supersonic aircraft.... but a totally different principle from what a boat encounters.
    A successful example of the principle you appear to be trying to articulate is the planing tunnel hull, especially if the tunnel narrows as you move aft. (Think v-hull, but cut it down the centreline and move the two halves apart, leaving a small space.... real ones are of course a lot more sophisticated but that's the basic idea.) In short- air enters the tunnel at the bow. As you move aft, the water between the hulls is forced up as the tunnel narrows, and the air is compressed. Thus the hull is lifted higher while on plane, reducing drag. There's a bit of a black art to designing these things so that they work, I'm not very well versed in it; but I think this is the closest successful example of the principles you seem to be thinking about.
     
  6. Willallison
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    Willallison Senior Member

    Contrary to what you'd think, concave sections, like you are talking about can not only be efficient, but also smoth riding in rough water.
    The Hickman Sea Sled is generally accepted as the pioneer of the type.
    There's a thread discussing them here:
    http://www.boatdesign.net/forums/showthread.php?t=4654&highlight=sea sled

    There are others too - just do a search for Hickman Sea Sled....
     
  7. yokebutt
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    yokebutt Boatbuilder

    Hundred orders of magnitude?
     
  8. PAR
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    PAR Yacht Designer/Builder

    Will, you beat me to it, I was going to bring up the Sled. Great boats, I've been in a few and surprisingly nice rides, from a square boat. Some of he Boston Whalers use a similar concept.
     

  9. marshmat
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    marshmat Senior Member

    Ahh right, the sea sled.... amazing design, to this day. I thought of it and the Whaler as being more of a multiple tunnel configuration, though? But then we get into an argument over definitions.
    yoke- OK, maybe not a hundred orders of magnitude... exaggerating a bit.... but a helluva difference nonetheless, the pressure ratio across a Mach 3 shock is 10.33:1, compared to 1.00014:1 for the most powerful sound waves of a really, really powerful rock concert at ~115 dB.
     
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