Relation between skin friction and speed

Discussion in 'Boat Design' started by hprasmus, Sep 12, 2010.

  1. bearflag
    Joined: May 2010
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    bearflag Inventor/Fabricator

  2. sorenfdk
    Joined: Feb 2002
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    Location: Denmark

    sorenfdk Yacht Designer

    Guys;
    Henrik hasn't delivered/been able to deliver any proof that this new product works. That does not necessarily mean that he's selling snake oil!
    We have all been presented with products that promised the world to us, but didn't deliver. That does not necessarily mean that Henriks product doesn't work!

    Henrik;
    There is no general mathematical relation between skin friction and speed. Too bad, but that's the way it is.
    Please come back if/when you can publish tests that prove the effectiveness of your product. Or better: Call me (or my company) - we'd like to get a head start!

    Couldn't we all just leave it at that and get on with our lives and business? This isn't funny any more!
     
  3. tunnels

    tunnels Previous Member

    Yes slippery coats are just a part of the answer and would need to be used in combination with design of the actual surface and design of the surface areas . Like aircraft shape can and does enhance the flow along and around different parts of the air craft and even to this day theres still a lot of learning going on.:D :p
     
  4. terhohalme
    Joined: Jun 2003
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    terhohalme BEng Boat Technology

    Friction formula

    Friction formula of William Froude:

    Rf = 161*(0.00871+0.053/(8.8+L)*Sw*V^1.825 (N)

    where
    Sw is wetted surface (m2)
    V is boat speed (m/s)
    L is waterline length (m)

    The only problem here is, that the friction is not between boat surface and water but between water molecyle layers near surface. And you just can't change friction between water molecyles easyly...
     
  5. Vulkyn
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    Vulkyn Senior Member

    So the problem is not in the 1st water layer in direct contact with the hulls surface, but the 2nd in contact with the 1st layer? (figuratively speaking)
     
  6. tom28571
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    tom28571 Senior Member

    And the third and the fourth and so on.
     
  7. ancient kayaker
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    ancient kayaker aka Terry Haines

  8. MikeJohns
    Joined: Aug 2004
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    MikeJohns Senior Member


    Laminar flow is the desired condition, with laminar flow there is a very small shear zone and small level of surface vorticity. Laminar flow trips due to a couple of factors, one is due to flow and shape creating what's termed an 'adverse pressure gradient' the other is due to surface roughness, length of body and relative velocity.

    A smooth surface delays the transition due to surface roughness.

    If you take the pressure gradient factor out then it's well described, for example in pipes the surface roughness factor is a significant player in flow rates. The same principles apply.
     
  9. wardd
    Joined: Apr 2009
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    wardd Senior Member

    how about a layer of air injected?
     
  10. Vulkyn
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    Vulkyn Senior Member


    I am sorry but i have to get a dictionary and boating terms just to figure out what you just said ..

    Forgive my lack if knowledge :S
     
  11. HardWare
    Joined: Sep 2010
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    HardWare New Member

    I coated my boat's hull with a hydrophobic silicone mix(not as good as rough teflon, but cheap) and it does make a measurable difference.

    Making a hull slippy works great, but I've always wondered if you could make a design more efficient with turbulence, a hydrodynamic golf-ball effect.

    I remember seeing a pretty odd design years ago which appeared to used a bubble film on the hull.
     
  12. ancient kayaker
    Joined: Aug 2006
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    ancient kayaker aka Terry Haines

    From all I've read about silicone it is going to be difficult to paint over it.
     
  13. MikeJohns
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    Location: Australia

    MikeJohns Senior Member

    It works and the more air you can introduce the better the result :)
     

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  14. jesdreamer
    Joined: Jan 2013
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    jesdreamer Junior Member

    I found lots of animosity and hard feelings in this thread but little info. Ancient Kayaker in comment #73 did post some numbers which do make a point. He said that 4Kt was around 80% of his canoe design's hull speed. (I find most folks talking of hull speeds around 5Kt or so). And at this speed friction was around 50% of total resistance. Then that a small reduction in friction would produce a really small reduction in total resistance, and that it would be much less a factor at higher speeds. Now Hprasmus in his original post said results on his coating formula showed 50% reduction in friction at 15Kt and 30% at 10Kt -- Both of which suggest a ratio of 0.03 -- so the relation looks linear. Thus at a typical hull speed of 4 or 5KT we might expect around 15 or 16% reduction in friction which itself is a fraction of total resistance. Most paddlers can get up to this kind of speed w/o too much effort so should really only show interest in a coating to get above hull speed. While the coating's performance improves at higher speeds, unfortunately above hull speed friction rapidly becomes a very minor factor in overall resistance -- so I see little motivation for interest in such a product -- Am I missing something??
     
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