Hydrophobic = Afraid of the?

Discussion in 'Boat Design' started by rambat, May 11, 2013.

  1. rambat
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    rambat Member at large

  2. rwatson
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    rwatson Senior Member

    So how do you repel water when your boat is in it ?

    Water doesnt 'stick' to a hull, it sticks to objects, in the atmosphere, due to surface tension.

    The youtube video shows objects repelling water applied to them, when they are not immersed.

    To do this with a boat hull in the water, you may need to invent antigravity paint.

    You can look into low friction hull coatings, and turbulence reducing hull design.

    Think about it.
  3. tom28571
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    tom28571 Senior Member

    Those who have looked into coatings for reducing skin friction have always reached the conclusion that there was no coating that could do this. One point often given is that wax could actually increase skin friction and was worse than no coating or any other coating. Since wax is also hydrophobic and has similar properties to the material being proposed, it is likely that it would have the same results. The reasons given are that what is being called skin friction is actually the force required to overcome shearing of the water in the boundary layer.

    Not being an expert in hydrodynamics, I don't know the answer but I have made some simple experiments and have not been able to show any difference in drag due to whatever coating was on the test piece. As long as the surface was maintained in the same level of smoothness, I was not able to detect a difference. For some reason, I did not try wax.
  4. rambat
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    rambat Member at large


    I guess I need to clarify, hulls on Earth have skin drag in water due to its viscosity "stickiness". I am not sure what planet it does not. This new product seems to retain/hold a fine layer of micro-bubbles onto its outer surface. I think wax and Teflon have a effect on surface tension to prevent sticking. However, this seems different. If we calculate the wetted surface area drag out of a performance calculation it should be better? Still have form and wave making drag.......
  5. messabout
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    messabout Senior Member

    That magic stuff may have some value as a deterrent to algae formation on the hull. Not likely to have much if any value for reduction of skin friction.

    Sailors are wildly creative with this sort of thing. Graphited paint, soap inclusion, turbulators, air inclusion, teflon coatings, and on and on. So far no meaningful improvement has been discovered. Maybe this is the long sought solution but I have serious doubts. That is not to say that we shouldn't continue to try. Unfortunately, the gods of physics continuously plot against we mere mortals.
  6. michael pierzga
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    michael pierzga Senior Member

    Nano particles of Silicium SiI4 work well on the bottom. I dont think it increases speed , but it certainly makes cleaning slime off easy. nothing seems to stick.


    The particles seem to be viable for about two weeks. I use it on the bottom of yacht ribs
  7. rwatson
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    rwatson Senior Member

    Well, no. Hull drag is from friction, not stickiness.


    "In a viscous fluid, a boundary layer is formed. This causes a net drag due to friction. The boundary layer under goes shear at different rates extending from the hull surface until it reaches the field flow of the water."

    "frictional drag that is given by the shear due to the viscosity. This can result in 50% of the total resistance in fast ship designs and 80% of the total resistance in slower ship designs."

    so yes, the friction component is a major part of the problem.

    Having bubbles on the hull surface has been done mechanically as previously mentioned, with with all sorts of devices, but you need a lot more than a few transient bubbles from a chemical reaction to get any useful results.

    Bear in mind, any bubbles on the hull are under enormous pressure, and may contribute to friction rather than resolve it.
  8. Village_Idiot
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    Village_Idiot Senior Member

    They were on their way to figuring it out back in the mid-1990's:


    I don't know what has become of their work, but if it has been implemented into industry (airliners, etc.), it may take several decades to show up on the consumer end.

    Basically it boils down to creating random protrusions on the surface. As water rolls along a surface, friction causes (boundary) layers of water to roll up into vortices along the surface, much like rolling up a rug or carpet along a floor. When those vortices get large enough, they break away from the surface and disrupt the laminar flow that is occurring beyond the boundary layer. The trick with the random protrusions is to break up the vortices before they become too large, thus reducing disturbance to the laminar flow. This was shown in the lab to reduce boundary layer friction by 13%.

    However, if the protrusions are not random, but uniform, the exact opposite phenomenon occurs. Drag is INCREASED by up to 20%. Both applications have merit in industrial uses.

    The 13% reduction in the lab would probably result in something like 3-4% reduction in field conditions; however, that is still an order of magnitude greater than most other treatments have shown.

    Btw, the protrusions were quite small, maybe less than a millimeter.
  9. daiquiri
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    daiquiri Engineering and Design

    Gents, if this discussion is about marine applications (is it?), then please do not forget the name of this bad guy: Marine Growth.
    He will always manage to successfully spoil all your millimetric efforts.


  10. Mike Graham
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    Mike Graham Junior Member

    Fighting marine growth (not reducing drag) is actually where these sorts of coatings have some usefulness.
  11. tom28571
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    tom28571 Senior Member

    OK, that is something different to consider. However making calculations based on the assumption that it will work to reduce drag is jumping the gun a bit. Do us all a favor and make some experiments to see if it works before making calculations. We have all been made to believe that a large air bubble surrounding a submerged hullform greatly reduces drag, as in the hyper fast torpedo.

    Up to now, Mr Watson's comments have held sway BECAUSE of stickiness of water to the hull surface, whether waxed or not. That is, a water film on the hull surface does not shear and moves at the speed of the hull and must, therefore experience drag producing shearing through the boundary layer from the surface until it matches the speed of the ambient water. Injecting some materials (long chain molecules) into the boundary layer can significantly reduce drag but this is ruled illegal by all racing bodies.
  12. Southern Cross
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    Southern Cross Senior Member

    What do you think about using this on the deck in addition to or in lieu of a Kiwigrip as water between the deck and shoe cause one to fly head over heals on occasion.

  13. keith66
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    keith66 Senior Member

    Years ago back in the late 80's there was a product called sea slide that appeared to reduce friction on surfaces coaed with it. Apparently the then IYRU banned it due to its friction reducing qualities. It did make wet things very slippery thats for sure!
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