Query about reducing wetted area

Discussion in 'Boat Design' started by Mr Efficiency, Jun 7, 2013.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    Imagine a hull like a basic sea-sled, with parallel, flat, plumb outside surfaces, running along at speed. All the displacement of water is taking place on the inside hull surfaces, but there is wetting of these outside panels that is retarding forward progress by skin friction. My question is this, what happens if those panels are not parallel, but are widest apart at the bow, and nearest at the stern, if only by a few inches ? At what speeds and negative angles will water no longer contact those slab sides, and thereby free it up from unwanted drag ? Might look odd, but who cares. :p
     
  2. alan white
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    alan white Senior Member

    I think somewhere around 200 mph.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    I'll simplify it, by stipulating "on flat water".
     
  4. groper
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    groper Senior Member

    Makes no sense mate, can you re-phrase?
     
  5. groper
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    groper Senior Member

    Ah i think i know what your on about, it wont work...

    The narrowing of the hull will suck the water inwards and still contact the sides, even at quite large angles...
     
  6. El_Guero

    El_Guero Previous Member

    Try using a foil .... simpler and proven.
     
  7. boradicus
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    boradicus Senior Member

    It seems like that would result in extreme drag. Saltwater flats fishing boats have completely flat bottoms and do very well (gas engine propelled). Then there are those boats that they use in the Florida everglades, but they have flat bottoms too, I believe. I think the saltwater flats boats may use an air cushion on some of the newer models, actually. Correct me if I am wrong about that.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    I'm thinking that if there is deadrise, and an angle of attack, on the "blind" side, water should break away cleanly and not contact the outside, "slab" surface.
     
  9. groper
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    groper Senior Member

    Doesn't work like that. If you could magically not have the boat touching the water, it would sink to the bottom without getting wet.... The boat must be lifted by either dynamic lift or displacement of water. If you design this shape so the water breaks cleanly from it, then it must be lifting ie planing. Whether this works well depends on the aspect ratio, surface area and speed, CoL and CoG. What you described will be a poor lifting surface, so instead it will just push the water out of its way and sink into the hole it created for itself, wave drag will be huge, drag will increase and design is pointless..
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    You are misunderstanding me ( probably my fault !) , but the aim is to get rid of the drag on the outside in the sea-sled example, what is inside that "tunnel" is where the lift occurs, and there is no intent to mitigate the frictional drag in there. I suppose the same idea could be applied to a slab-sided box-tunnel hull by having the tunnel widen gradually from bow to stern, in this case the intent is to stop water from climbing and dragging on those vertical internal surfaces. Wave action obviously complicates matters.
     
  11. tunnels

    tunnels Previous Member

    Get real !! 200 MPH !!
    More like about 24 mph !! it is reliant on the deflection of the water at the front and how far its thrown out ,the speed could be even lower !!:)
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Yeah, anything over 100mph would make me giddy. :D
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Water that is licking at the passing exterior "wall" of a sea-sled is providing zero lift , dynamic or hydrostatic, it just causes frictional drag. The aim is to avoid it, and replace it with air !
     
  14. tunnels

    tunnels Previous Member

    oops got that wrong !!

    having the tunnel widen gradually from bow to stern, in this case the intent is to stop water from climbing and dragging on those vertical internal surfaces. Wave action obviously complicates matters.

    The tunnel need to be the other way round ,slightly wide at the back and narrower at the front but that's defeating the tendency for the tunnel to slightly have an air compressing effect unless you play with the tunnel roof heights , On the inside tunnel walls its possible to angle them slightly and fit a small spray rail to shed and deflect the water and so it wont climb the tunnel wall apart from the bottom 75mm !!
    Most tunnels need to be slightly funnel shaped to collect air !!, the walls need to be wider at the very front and tapering and narrowing for about first 1/4 of the tunnels length, then running parallel to the stern !! also the tunnel roof does a similar thing and for the last 3/4 of its length should gradually drop to the stern with a percentage of the last part having more turndown to deflect spray down wards and to help with the air cushioning effect and semi trap the air inside . at slower speeds they have a tendency to sneeze when the air is trapped by the back end being in the water from going over a wave or a wake or what ever , have seen this happen heaps of times !!;)
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    I understand what you say about fitting a spray rail, what do you mean "angle them slightly" ?
     
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