Hydrophobic properties of boat hulls

[copria]

By increasing the hydrophobicity of canoe hull material, will drag be reduced? Have hydrophobic materials been used on boats?

My thoughts:
Yes, there is less surface contact with the water. Thus less viscous (friction) forces.

 

[Borek]

Surface contact is almost the same, as it mostly depends on how deep the hull is submerged, which is in turn function of its geometry and required buoyancy. Hydrophobicity has nothing to do with this part of the problem.

 

[copria]

Say we are dealing with a superhydrophobic material. Then the water is only in contact with the top of the asperities.

See: http://en.wikipedia.org/wiki/Hydrophobe

 

[Borek]

It doesn't change the contact surface.

 

[fzero]

The silicone-based hull paint on the Emma Maersk is claimed to result in lower drag. From http://www.emma-maersk.com/specification/

"Instead of biocides, used by much of the industry to keep barnacles off of the hull, a special silicone-based paint is used. This increases the ship's efficiency by reducing drag while also protecting the ocean from biocides that may leak. The silicone paint covering the part of the hull below the waterline is credited for lowering the water drag enough to economize 1200 tonnes of fuel per year. "

However, I'm not sure that the frictional drag force, which depends on surface area of the entire hull, scales exactly the same way as the displacement drag force, which depends on the effective cross-section. I don't think the benefit to a canoe would be as much as for a container ship in relative scales.

 

[copria]

I am confused on how Borek came to the conclusion that the contact surface does not change. I could understand if that viscous drag does not change because when the water is on top of the asperities, there is the same downward force on a smaller area (thus viscous friction is not reduced), but I do not understand why the contact surface does not change.

 

[copria]

I am thrilled to that you have replied to my thread!
Thank you!

Would a boat hull made with a hydrophobic material travel faster in water than an identically shaped boat hull made of aluminum? (assuming that the displacement drag of both boats is the same)

 

[fzero]

I am thrilled to that you have replied to my thread!
Thank you!

Would a boat hull made with a hydrophobic material travel faster in water than an identically shaped boat hull made of aluminum? (assuming that the displacement drag of both boats is the same)

Probably (also assuming weight and propulsion were the same), but I couldn't point you to any useful references that would help settle the issue. You might look into information about performance racing boats, since they would be using cutting edge paint or gelcoat finishes to maximize performance. Again, I don't know any specific studies to point you to, this is way out of my area.

 

[JaredJames]

Surely, given no other factors change (hull geometry, mass etc), wouldn't the deciding factor here be smoothness?

The smoother the boats hull the more efficiently it would travel through the water.

Which is why a silicone based hull paint can help so much.

The key would be developing a paint / material with the smoothest surface possible. The lowest coefficient of friction you could achieve between the hull and the water.

Strictly speaking I'd like to think all boat hulls are hydrophobic.

 

[copria]

At a microscopic level, some superhydrophobic materials are very rough, yet water will slide off of the surface at a lesser angle than that of a very smooth, non-hydrophobic material.

In trying to understand this phenomena, I imagine a boat hull (made with a superhydrophobic material) resting just above the water (less than a millimeter). Just as the positive poles of magnets repel each other. Is this a correct visualization of the properties of a hydrophobic material?

 

[Borek]

I am confused on how Borek came to the conclusion that the contact surface does not change. I could understand if that viscous drag does not change because when the water is on top of the asperities, there is the same downward force on a smaller area (thus viscous friction is not reduced), but I do not understand why the contact surface does not change.

Think how buoyancy works. It is all about pressure and contact surface, very basic HS physics.

 

[JaredJames]

In trying to understand this phenomena, I imagine a boat hull (made with a superhydrophobic material) resting just above the water (less than a millimeter). Just as the positive poles of magnets repel each other. Is this a correct visualization of the properties of a hydrophobic material?

Definitely a misunderstanding of the principle there. This isn't magnetic repulsion. The water still contacts the material. To build a boat, it certainly wouldn't 'float' a mm above the surface. It's weight would hold it in the water identically to a boat made of SS.

 

[Quinzio]

If small quantities of air can be trapped into the asperities of the hydrophobic material, then water air friction can reduce overall friction.

 

[copria]

I understand that magnetic repulsion has nothing to do with hydrophobicity. I was confused about the definition of the hydrophobic effect. Some defined it as a repulsion of water. Upon further reading, I found that, "The less surface area is exposed the fewer water molecules have to suffer the loss of conformational entropy, the lower the energetic penalty. So the hydrophobic effect is an entropy-driven process that seeks to minimize the free energy of a system by minimizing the interface surface between hydrophobic molecules and water." (from http://www.bio.brandeis.edu/classes/biochem104/hydrophobic_effect.pdf") There will always be direct contact with water. The tendency is not to repel water but to reduce the interface area between the hydrophobic material and water. I get it now.

 

[copria]

If small quantities of air can be trapped into the asperities of the hydrophobic material, then water air friction can reduce overall friction.

This is truly what I am getting at. This prompts me for another question. Because of the air trapped, is it appropriate to say that half of the boat hull is in contact with water (for purposes of understanding assume that the top of the asperities comprise of half of the water contact) and half is in contact with the air between the hull and the water?

 

[JaredJames]

http://en.wikipedia.org/wiki/Hovercraft

 

[NobodySpecial]

You can get very high low friction and high speed boats by creating a layer of bubbles between the hull and the water - there is a Russian torpedo that can do several 100km/h by doing this.

 

[Quinzio]

This is truly what I am getting at. This prompts me for another question. Because of the air trapped, is it appropriate to say that half of the boat hull is in contact with water (for purposes of understanding assume that the top of the asperities comprise of half of the water contact) and half is in contact with the air between the hull and the water?

I don't really know what percentage is in contact with air and then the rest is in contact with the material.

Say half/half is just a guess which is supported by no evidence.

 

[JaredJames]

You can get very high low friction and high speed boats by creating a layer of bubbles between the hull and the water - there is a Russian torpedo that can do several 100km/h by doing this.

Do you have a reference for this torpedo? Rather interested to see it.

 

[Borek]

http://en.wikipedia.org/wiki/VA-111_Shkval

 

[JaredJames]

Wow, I'm impressed. But how would you apply that to a ship? I could understand adapting it for a sub, but a boat?

 

[copria]

The Russian torpedo relies upon supercavitation to produce the bubble of water vapor around it.

There are relationships between cavitation and hydrophobicity.
http://doc.utwente.nl/59080/1/controled_multibubble.pdf"

I question whether the microcavities, which comprise of the space between asperities on a hydrophobic material, would create an area of low pressure.

 

[NobodySpecial]

I suspect it would be rather expensive in fuel if you intended a voyage of more than a couple of km - and didn't have a really-really good reason to go that fast.

There are ice breakers that use bubble cavitation to reduce the hull-ice friction