Relation between skin friction and speed

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

  1. hprasmus
    Joined: Sep 2010
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    hprasmus Junior Member

    Hi, I am new to this forum. I am also marketing a skin friction reduction coating product that has been tested in pro tank tests.

    It was found that a body of untreated Gelcoat compared with the treated surface would reduce skin friction by approx. 30% at 10 knots and approx. 50% at 15 knots (the highest speed tested).

    Here is the issue: what will this translate to in terms of increased speed for a boat/surfboard?

    I hope someone can help me here!

    The results can be viewed here:!/photo.php?pid=31097162&id=1166912263&ref=fbx_album

    Thanks - Henrik
    1 person likes this.
  2. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    If what you say is true, you have done something that has eluded everyone else who has tried to do the same thing. Other experts have proven that no coating applied to a hull can have any effect on skin friction. The theory says that the fluid flow at the surface of the hull is always zero, so no coating can have any effect. All efforts that have been successful in reducing skin friction have involved working with the boundary layer or laminar flow in one way or another.

    There was a rush in using artificial slime to sailboat hulls in the late 1960's. After a brief period of unrest, the sail racing authorities concluded that there needed to be no prohibition against the coatings since they did not reduce skin friction. These coatings were supposed to emulate fish slime.

    One experimenter, perhaps Brittan Chance, drilled tiny holes in the hull bottom and injected the material into the water flow. This did have the desired effect by reducing friction caused by sheer in the boundary layer. Of course, this was promptly outlawed and rightly so. The same material is used in fluid pipes in power plants and other similar applications to reduce drag from turbulence and also in pulling wires and cables through conduits. It is exceedingly slick stuff and I tried some on my racing sailboat. It proved to be way too much trouble for unknown results.
  3. yipster
    Joined: Oct 2002
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    yipster designer

    the laminar flow on a dimpled golfbal and possible uses for boats has in detail been discussed here before
    sharkskin for racing swimsuits do exist and win but i read from the producer only a max of 4% reduction in drag
    there is a sort of sharkskin sticky foil for on cars too but only good in hotspots or reverse effects may occur
  4. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    If you had such a simple solution to reduce skin friction by 30-50%, you would probably soon become one of the richest men in the world. Shipping and airline companies would cover you with gold to get their hands on something like that. You will hardly manage to sell those claims to a person with some knowledge in this field.
    What does terms "treated" and "untreated" mean in the above citation? If "treated" means "sanded. washed and coated with your product untill the smooth appearance", while "untreated" means "rusty and covered with marine growth", then the above marketing claims may be true.
  5. hprasmus
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    hprasmus Junior Member

    X friction reduction = Y speed increase

    This is what I am after.

    Somewhere I found results an Americas Cup team conducted (can't seem to find it again unfortunately). Their results showed that a 6% skin friction reduction resultet in a 2% speed increase at max. hull speed.

    So, if someone could come up with a graph showing friction reduction and speed increase from 0 - 50 knots, that would be great.

  6. hprasmus
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    hprasmus Junior Member

    2% speed increase

    Further to the above. Say that the yacht is sailing 1 hour at 8 knots she will be 300 meters ahead by the finish line ! That's a lot.

  7. messabout
    Joined: Jan 2006
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    messabout Senior Member


    I wish that such a miracle could be wrought.
  8. hprasmus
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    hprasmus Junior Member

    Well it can be bought in 4 -5 weeks time (a kit incl. cleaner and primer will be about $ 25 to cover 4 m2/40 sqf). To you who suggest I will be a millionaere on this product, I am sorry to inform that this is not intended for super tankers. It is targetted towards boats/boards/kayaks without anti-fouling. And the effect is at its peak for 1 week for racing but longer for leasure.

  9. gonzo
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    gonzo Senior Member

    The skin friction as a percentage of hull drag varies with design and speed. There is no magical formula. I have a hard time accepting your claims. You say "it was found". By whom and how?
  10. hprasmus
    Joined: Sep 2010
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    hprasmus Junior Member

    It has been tested by the Technical University of Munich:

    For competitive reasons I can not release more data than what you can see in the graph I have posted.

    I realise that friction varies with form. So how would it work on a Laser dinghy and a windsurfer. Just approx. ?

  11. Eralnd44
    Joined: Oct 2006
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    Eralnd44 Wanderer

    i am wanting to discover more on this. please help with more info.
  12. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Are you asking these questions in hope that someone with do the math for you, or to supplement your lack of understanding on the hydrodynamic issues you seem to desire ignoring, as you attempt to tease with your non-disclosure announcement?
  13. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    There is no simple formula like that. It will depend on the type of boat, the speed range, the proportion of the drag due to skin friction, etc. You can use a program like Michlet to experiment yourself. You'll be able to put in a hull shape and vary the skin friction to see what a difference it makes in drag. But from there to a difference in performance requires knowledge of how the propulsion (be it sail or power) varies with speed, too, in order to see what speed results from a change in the skin friction drag.
  14. fg1inc

    fg1inc Guest

    Hmmmm....2% speed increase, for 1 week, for $25.00? On something that's going 5 knots?
    Seems like a lot of money for a very small result.
    And that graph looks a little fishy to me. Perhaps it was done by a student?

  15. ancient kayaker
    Joined: Aug 2006
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    ancient kayaker aka Terry Haines

    Henrik, the forum gets a lot of proposals that look like recycled old ideas or impractical new ones, so forgive us if we can seem a little suspicious at times. I won’t pass judgement on the technical merit of the concept, I will just try to address your question as best I can, although you probably know, and certainly should already know, all of the following.

    “X friction reduction = Y speed increase”

    The total resistance of a displacement type hull passing through the water is made up of skin friction and wave making resistance; the latter generally incorporates form drag in most common analytical tools. Neither are linear, both are influenced by heeling and wave/form drag does not a have a simple relationship to hull shape or speed.

    Therefore the problem does not lend itself to straightforward analysis. For every individual combination of a hull and speed a separate calculation must be made, and the relationship between speed increase and a slight reduction of skin friction will be different for each case. A family of graphs showing friction reduction and speed increase from 0 - 50 knots can be generated for various amounts of skin resistance reduction but it would only apply to a specified hull.

    In short, we don’t know, cannot know, and nobody can know the ratio X/Y, except for a single case such as the one you seem to recall for the AC. That example sounds about right for a canoe, by the way, but I doubt it applies to an AC boat at speed. Also, the analytical tools generally ignore the effect of water surface conditions.

    I’ll mention a marketing caveat if I may: skin friction is a relatively minor part of drag in a small boat moving fast: a recreational boater is unlikely to notice any improvement. A tanker will need the improvement to last considerably longer than a week and can hardly go intro dry dock every voyage. Therefore I predict that this new product will not sell in commercially viable quantities. Of course, racers will pay almost anything to get the slightest edge but that’s going to be a small market, so if it really works you should probably rework your cost projection so you can make some kind of profit from the exercise.
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