How do we reduce skin friction ??

Discussion in 'Powerboats' started by tunnels, Oct 16, 2011.

  1. tunnels

    tunnels Previous Member

    - Introducing "air bubbles" to the pad surfaces? Good grief...why would we do that? A myth about stepped hulls is that the introduction of air into the water that flows under the hull reduces the viscous drag and makes a stepped hull go faster than a non-stepped hull. But in reality, running on air bubbles doesn't reduce the frictional resistance at all. The hull lifts on the water, not the bubbles. So bubbles or "two-phase flow" (water and air) will actually increase the drag.
    The last phrase of that story i dont believe !!!
    The introducion of air must lubricate the boat and the water . Even if the bubbles never touch the hull surface its softer and must allow the hull to slide more easy through the water. .
    So how do we reduce skin friction ????
    Silicone ??.
    teflon ???
    what ????
    what happens as speed increases ?? skin friction is that great ??
    what about slow boats ??
    Slow boats 400 grit wet sand the surface !,would this same thing apply to high speed boats ??
    What works and what dosent or is there some sort of a secret here ??
    So when a hull is airbourne and touchs the waters surface at 150 mph does it feel like the brakes have been applied ?? What ??
    The bubble thing i dont believe because ultra high speed underwater missiles have been developed using bubbles to reduce friction !! Who do you believe ??wheres the evidance !!

    http://www.boattest.com/boats/boat_video.aspx?SubID=2291&ID=2575

    I found this and pooh poohs that idea about the introduction of air as its also done through the step !
     
    Last edited: Oct 20, 2011
  2. michael pierzga
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    michael pierzga Senior Member

    When we were kids we would coat the bottom of the boat with dish washing soap then launch and race. Seemed to work for about 15 min.

    Naturally a smooth bottom is critcal...how smooth. Dont know. I use wet and dry 500 grit on antifouling before regattas...seems to work. Finer than 500 is to difficult to work with.

    On a sailing boat the smooth bottom affect is most noticable during acceleration. Boat slows down as it hit a wave , then accelerates back to target speed faster.
     
  3. masalai
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    masalai masalai

    At a guess, reduce wetted surface, and good design relevant to configuration and purpose... stepped hulls are still used in F1 international boat racing - this runs on LOTS of ethanol does about 150mph around 40ft LOA and 4000kg...

    The next one will do about 6knots and burn 3litres of diesel fuel per hour for that, is also around 40ft LOA and weighs around 4000kg.... If the drift of what I typed is not clear - percentage wise skin friction does little to change results when seeking that last bit in speed within reason... The hulls on the race boat is nice and polished, mine shows a significant deterioration in performance if I have not scrubbed off the slime/weed/green growth from sitting in 'still' waters... The race boat is chasing maximum speed for a given power available... I am seeking best reasonable mileage (current best is 2 miles per litre)... Local conditions have a far greater impact on performance...
     

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  4. tunnels

    tunnels Previous Member

    i HAVE JUST SPENT A COUPLE OF HOURS CRUISING AND LOOKING AT INFORMATION ALREADY POSTED . MORE THAN 90% IS RELATED MORE TO SHAPE RATHER THAN ACTUAL WATER GOING ALONG THE SURFACE OF A SKIN . WATER FLOW THROUGH A PIPE MAKES INTERESTING READING BUT AGAIN SHAPE AND GOING ROUND CORNERS IS THE KILLER LITTLE RELATES TO SKIN SURFACE CLING OR FRICTION !!.
    I EVEN BEEN THROUGHT SOME OLD POSTS FROM BOATDESIGN AND THEY ALSO ARE NOT CONCLUSSIVE IN ANY WAY ABOUT ACTUAL FRICTION . SO THE STEP THING IS LITTLE TO DO WITH ACTUAL SKIN FRICTION ITS JUST MAKING THE BOAT FLY .I REMEMBER MY DAYS OF WATER SKIING !BE IT 20MPH OR 40 MPH THE SKIS WERE STILL SLIPPERY .
    I WANT TO SEE THE RESULTS OF A PANEL OF A X SIZE IN A TEST TANK PULLED AT DIFFERANT SPEEDS AND SEE THE DIFFERANCE IN THE AMOUNT OF PULL REQUIRED AT THE DIFFERANT SPEEDS HENCE DRAG / SKIN FRICTION !! YES ??. :confused:
     
  5. masalai
    Joined: Oct 2007
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    masalai masalai

    Hi Tunnels,
    It is all a matter of what is desired, under what context, how much money will be available to get that last tenth of a percent, and will other variables like wind, sea strength and direction have a more significant impact...

    I was thinking of keeping the hulls "high-&dry" when not being used (the boat is 40ft x 21ft)... That cost is significant and the extra time to clear the berth and put the boat back would be about 2 hours out of the trip, so that makes a spontaneous-day-out almost a non-event for a 10% or so better speed against 12 miles extra distance travelled using present management plan... On days when boating is the choice and the 'outside-conditions-are-not-nice", I scrub the bottom with a stiff broom whilst the boat is standing in shallows less than knee-deep (a 30 to 60minute activity)... This negligible inconvenience and cost keeps things reasonable ... for me and what I do with my boat...

    When going to race at world record levels where money is of secondary consideration then EVERYTHING is done - even if the only benefit is psychological as Dennis Connors did long ago with specially grooved Mylar sheeting to facilitate water flow... Bond returned the complement with the winged keel - - both won in either case... I would venture to declare that both options were psychological only, as far as performance benefit goes...

    I seem to recall some discussion relating to maximising performance (movement over the water surface) and the consensus seemed to be that careful design, minimal wetted surface and a fair and 'clean' shape were worthy objectives... Rick Willoughby was at the time designing shapes and propellers to achieve the world record for a boat he designed for human-powered-distance-record...
     
  6. tunnels

    tunnels Previous Member

    Its funny but every post i have come across no one can say a painted shiney surface is x amount of friction at y amount of speed asn % of increase in friction as sped increases
    Every one skirts around the question and there mind is else where and tells nothing !!its all seems to be here say !!guess work .
    Same with the strakes and stepped hulls there seems to be no magic formular written !!!its all just guess work . strakes are a really good example that x amount of flat surface area of strake at y speed will give you z amount of lift + or -a percentage !!!.
    Doesnt anyone actually know ?? you cant tell me in this year 2011 no one hasnt asked the same question a thousand times !!!
    I did :confused
    Found once that 1 foot square of a flat deck on a race boat was capable of giving 12 lbs of lift at 100 mph so that was something to think about !!.:confused:
     
  7. gonzo
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    gonzo Senior Member

    Yes, it is all in "Principle of Yacht Design" by Larsson and Eliasson. The question has been answered to exhaustion.
     
  8. yipster
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    yipster designer

    whats not in the book is boats as mentioned nor foils, wheels, wings and the friction rubbing our flat bottoms in water with high speed
     
  9. daiquiri
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    daiquiri Engineering and Design

    You basically need to study and understand Moody's Diagram: http://en.wikipedia.org/wiki/Moody_chart

    It tells you what you need to know about friction drag:
    1) it strongly depends on the Reynolds number;
    2) it strongly depends on the surface roughness.​

    The Reynolds number depends on the speed, on the size of the hull and on the fluid type. Since the hull size and fluid type are given (water), then Reynolds number depends essentially on the speed.

    Surface roughness is detrimental for the friction drag, so you want to have a surface as smooth as possible.

    Cheers
     
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  10. tunnels

    tunnels Previous Member

    http://www.boattest.com/boats/boat_video.aspx?SubID=2291&ID=2575

    Ok its a sales pitch but i think must have merrits as well !
    The idea is old and the technology is nothing new just the way they have gone about doing its neat .
    I would like to have a ride in each boat and have a look first hand At the whole lot of what they are on about !!
    :p;):p:D
     
  11. MikeJohns
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    MikeJohns Senior Member

    Slavi

    Interestingly and often overlooked, fluid temperature also has a significant affect on kinematic viscocity. Something people tend to forget. Rn can be close to double between the Arctic and tropical waters over a range of 30 degrees C.

    But surely you wouldn't waste too much effort looking for a micron surface finish. The trick would be to concentrate on keeping the attached flow for as long as possible. Probably the 1st meter in reality if you are really lucky. And a planing hull will have the flow continually tripped as will any hull in wave encounter.
     
  12. Stumble
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    Stumble Senior Member

    One of the best ways to reduce surface friction is with the application of nano particle products or specials films. The problem with both of these methods is the upkeep cost.

    There was a 3m film in production for a while that did a nice job of reducing friction significantly, but was quickly banned in most boat racing, and had to be reapplied every few days. Plus it had no anti-fouling capability, so if the oat was in the water for three days you had to sand the boat and reapply it.

    The nano film products also work great, but cost around 1000/ foot/day you want it applied. Even for unlimited budget racers it was too costly to be realistic. There is a thread on here I couldn't find where I detail them with scientific testing results.
     
  13. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Good points on both accounts, Mike.
    I didn't mention the temperature as a possibility of reducing the hull friction because, just like the fluid type, it is a fixed environmental variable not practically modifiable by the ship operator. The water temperature is what it is, in the sea.
    As about keeping the flow attached (and possibly laminar, when organic fouling permits it) as a mean of reducing the overall drag, that's so true. Actually, it's the main theme of hydrodynamic design of many types of transportation vehicles and crafts.

    Cheers!
     
  14. philSweet
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    philSweet Senior Member

    I don't agree, especially when you consider the costs of some of the alternatives. It is not just possible, but quite common to heat the hull or parts of it- think keel coolers and the engine or boiler room of a ship. You then have a temperature gradient at the boundary layer, and that can have a pretty big effect on drag. The mechanism seems to be the temp gradient causes a viscosity gradient. A cooler hull should have less drag than a warmer one. I wouldn't be surprised if an external pipe cooler ended up with less total drag than an internal skin cooler if it was properly optimised. In some respects, its like soaping the hull as Michael mentioned.

    A video of the Kaye effect. The video shows a thixotropic liquid which behaves this way at equilibrium temp. However, you've all experienced this yourself. You poor room temp tea into a glass of ice and it all jumps out and all over your shirt. Not one drop of tea in the glass. same effect, but the viscosity profile was created by the temp change.

     
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  15. brucehallman
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    brucehallman Junior Member

    As to the question: "How to reduce skin friction?"

    There is an implied presumption in this question that skin friction is the most important type of drag. There are different types of drag, and skin friction is not the most important type of drag for many boats.

    I think that skin friction tends to be more dominant/important for slow, low powered boats (like some sailboats and some rowboats). Meaning those where the limiting drag is not the drag induced by the wave making of the boat that is seen with the faster craft.

    So, with slower sailboats, and slower rowboats where skin friction is the dominant form of drag, I believe that the best way to reduce skin friction is to reduce the amount of skin. Hence, for equal displacement, you get the most payoff by having the hull be more compact (meaning less long/narrow). With the ideal shape being short round fat belly, versus long, angular and sleek.
     
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