Boundary layer in water


by ken
Tags: boundary, layer, water
ken
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#1
Sep19-06, 07:08 PM
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Hi

Sailing 30 yrs ago we used to wax and polish hulls to maintain a protective and as gloss a surface as possible. However some say that anything that beads water such as wax causes more drag than a smooth surface that water will sheet over.

Note Racing rule 53 " SKIN FRICTION
A boat shall not ejest or release a substance, such as a polymer, or have special textured surfaces that could improve the character of the flow of water inside the boundary layer."

I believe this is not intended to prevent the use of protective treatments such as wax but does not allow the use of chemicals that can leach into the water to give advantage.

Is there anything in this idea that protective treatments might affect the boundary layer differently to a high gloss natural surface?

Discussion on effects on polar molecule alignment, fresh vs sea water etc please?


Regards,
Ken
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Clausius2
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Sep19-06, 07:54 PM
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Texturized surfaces cause the enhancement of the turbulent boundary layer. The separation point is displaced downstream, and even though the viscous stress is increased the pressure drag decreases such that the final skin friction is globally decreased, assuming Reynolds numbers large enough for being far from the transition point.
ken
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Sep19-06, 11:06 PM
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Yes but given equal surface finish, is there a difference between a surface that beads and one that sheets? This is to do with molecular polarity isn't it? So how does this effect boundary flow?

Ken

Clausius2
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Sep20-06, 12:33 PM
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Boundary layer in water


I don't understand what you mean with equal surface finish. This has nothing to do with polarity, but with fluid dynamics.
ken
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#5
Sep20-06, 04:34 PM
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OK

Beading is not just a function of surface quality though it is one factor. Dull wax will still cause beading.

If you spray water on a waxed surface, the water will bead due the polarity of water and non polarity of the wax molecules. The polar water molecules of align strongly to form surface tension against the wax (beading). However if the surface is not waxed but composed of polar molecules such as might be the case when OH radicals are bonded, the water molecules arange more randomly and will sheet across the surface as there is little surface tension against the hull.

OK so this is not the same situation as a boat floating in water but there are some who say that a waxed surface or anythig that causes water to bead in the above example will cause more skin drag due to surface tension against the hull. However I thought the the 1st layer of molecules within the boundary layer stops regardless but on the other hand, if the surface molecules are aligned, this must also align the next row and so on to a decreasing degree so I'm not sure how significant alignment may be within the boundary layer.

In both cases we are talking about a highly polished surface and only considering the effect of molecular alignment within the boundary layer.

So my question is there anything in this assertion or is it totally irrelevent?

And also is this effected by salt content ie fresh vs sea water?


Ken
Clausius2
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Sep20-06, 08:33 PM
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Quote Quote by ken
OK

Beading is not just a function of surface quality though it is one factor. Dull wax will still cause beading.

If you spray water on a waxed surface, the water will bead due the polarity of water and non polarity of the wax molecules. The polar water molecules of align strongly to form surface tension against the wax (beading). However if the surface is not waxed but composed of polar molecules such as might be the case when OH radicals are bonded, the water molecules arange more randomly and will sheet across the surface as there is little surface tension against the hull.

OK so this is not the same situation as a boat floating in water but there are some who say that a waxed surface or anythig that causes water to bead in the above example will cause more skin drag due to surface tension against the hull. However I thought the the 1st layer of molecules within the boundary layer stops regardless but on the other hand, if the surface molecules are aligned, this must also align the next row and so on to a decreasing degree so I'm not sure how significant alignment may be within the boundary layer.

In both cases we are talking about a highly polished surface and only considering the effect of molecular alignment within the boundary layer.

So my question is there anything in this assertion or is it totally irrelevent?

And also is this effected by salt content ie fresh vs sea water?


Ken
Do you mean that polarity (i.e. intermolecular forces) play any role here?. The order of magnitude of those forces are much smaller than the macroscopic drag caused by the bulk fluid motion. Now I think about it, the wax is causing the surface to be smoother, and I do know it, because I apply wax to my car. So wax should enhance a smoother flow. I don't get your point at all.
ken
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Sep20-06, 09:04 PM
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Quote Quote by Clausius2
Do you mean that polarity (i.e. intermolecular forces) play any role here?. The order of magnitude of those forces are much smaller than the macroscopic drag caused by the bulk fluid motion. Now I think about it, the wax is causing the surface to be smoother, and I do know it, because I apply wax to my car. So wax should enhance a smoother flow. I don't get your point at all.
Yes, Inter molecular forces relating to water being a polar molecule. This is the reason for surface tension. Well it's not my point in fact it is an assertion form others that wax reinforces surface tension in contact with the hull. It is also well documented that adding substances to the water that break surface tension significantly reduce drag, hence the rule.

So they say wax increases drag despite any improved surface.
I'm not convinced they are right or wrong.

You need to bear in mind that very small differences in performance have a significant effect on the outcome of sail boat racing. A difference of 0.1 kts is nearly 400m over a 2hr race. That is the difference between 1st place and last. If the average speed around the course is 10kts then 0.1% on performance is still around 40m and therefore very significant.


Ken
Chronos
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Sep21-06, 01:12 AM
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Surface tension jumps to mind. Certain chemicals reduce surface tension. You can try this at home. Place a wooden toothpick in a pan of water. Sprinkle some camphor [moth balls] powder into the pan - the toothpick will be drawn toward it.
Clausius2
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Sep21-06, 11:18 AM
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I think I am gonna leave this thread. It goes nowhere.
ken
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Sep21-06, 08:52 PM
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Quote Quote by Clausius2
I think I am gonna leave this thread. It goes nowhere.
That's disappointing.
I've been trying to clarify my original question and then you raised the question of magnitude of effects so I tried to clarify the order of magnitude that would be considered significant.

I hope you don't have a problem with somethig I wrote?
Though you say intermolecular forces are small, there remains the question of how small so I feel we haven't got to a definate answer quite yet.

Regards,
Ken
NoTime
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Sep23-06, 08:46 PM
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I think your question is perfectly clear.
You want to know what effect wax has on sailboat performance.

Unfortunately, I don't have an exact answer for you.

Some quick searching implys that it is unlikly that there is any effect.
Unless it helps keep the hull clean.
Having crud growing on your hull is definitly bad.
TGarzarella
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#12
Sep25-06, 10:26 PM
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It's a good question. I used to do flow studies and simulations with water passing along Si wafers whose surface could be hydrophobic (water repelling, like your waxed, beading surface) or hydrophillic (like your unwaxed surface).

I don't recall encountering this effect (water flow affected by surface conditions). The models usually say that the flow is zero at the surface and increases quadratically as you start to move away. But that model may have been over simplified.

I would tend to believe that its true. If the surface is hydrophillic, there will always be a boundary layer of water on the surface. The friction that is felt is between water laminae; essentially the viscosity of the water itself.

On the other hand, if the surface of the boat surface is hydrophobic, the water is repelled by the surface. The contact angle is large and there is no sheeting action. The flow is probably turbulent near the surface, which will slow down the boat, compared to the laminar case above.

...Sorry, I'm just thinking out loud as I write this. I hope it's correct
Chronos
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#13
Sep25-06, 10:44 PM
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This is not difficult to test. Make identical model boats out of hydrophobic and hydrophillic materials. Give each one an identical push in a bathtub, or similar calm body of water, and measure their respective velocity over the same distance.


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