shark skin

-- Last Updated: Feb-24-07 7:57 AM EST --

Driling holes in a paddle makes me think of another idea;
Speed swimmers are wearing suites with a shark-skin structure, to reduce resistance in the water.

Is this technique ever used on kayak skins?

What is good for fish and (professionel)swimmers, should be good for us.

Speedo claims 3 - 4% moore (swimming)speed.

You may not be that far off…
I’ve always been a believer in having a non glossy hull for a displacement boat. Started when the top sailboat racers would sand a brand new boat to remove the wax and gloss. Has to do with laminar flow and holding the boundary layer of water.

When I sold my last boat I buffed the hull back to a shine and waxed it to make it look new. I could tell a difference in how it moved though the water.

grayhawk spot on
The sailing club I use to attend held the world Laser championships and to my amazment they sanded their boats before they went out sail.

When i asked why they gave the same answer as you

(wasnt you i spoke to was it)



It Won’t Sell…
Folks want their hulls smooth and shiny. :wink: Do a search and find all the questions about how to smooth back out scratches and like.

The matter is one that pertains more to competitive racers where a faction of a second may make difference from a win to a placement. For the average joe/jane paddler, the speed gain of a minutely gritted hull is akin to the question of what’s faster – an aquanaut or Nordkap. Besides some personal satisfaction in owning a reputedly “faster” boat, it doesn’t translate to much in real world tour paddling.

You get more speed by building up the “engine” than worrying about chasis.


not about top speed anymore…
at my age it’s more about efficiency and not wearing out as fast. Anything to make it easier…

Dentricle skins have been researched…
… for various hulls, swimsuits, etc. Do some of your own and it will save you a lot of wasted time.

There is a lot more to it than the skin friction and boundary layers. Speed ranges are critical, as is actual movement through the water.

Kayak hulls do not move like sharks, nor do they travel mathematically perfect linear paths through flat water. The pitch, roll, and yaw - all while both surging with each stroke and wagging back and forth a bit.

Not big factors at human perception levels - but very important at the scale of small dentricle patterns on the skin.

Keep it simple - keep it smooth (shiny or not I’ll leave to others to debate).

Riblets vs. roughness
I did some experimental work in a water tunnel in the 90s studying the effect of streamwise riblets (i.e. sharkskin) on turbulence production in the boundary layer. Greyak is correct that velocity (speed and direction) of freestream flow with respect to the alignment of the riblets controls everything. Kayaks don’t run smoothly forward through undisturbed water. The unsteady nature of the flowfield messes everything up. My gut feeling is that the overall efficiency would not increase enough to notice - in fact I think it would be downright difficult to measure in a real situation.

On the other hand, roughening a smooth surface will give benefits under the right conditions. Paradoxically the roughness thickens the bounday layer (BL) - this is the layer of fluid that is partially carried along with the hull. While this would appear to be a bad effect, in fact a thickened BL has a more gentle velocity gradient where the water touches the hull. The skin friction is proportional to the velocity gradient, so friction between the water and hull will go down. This is offset to an extent because with a thicker BL, you are having to drag a slightly larger mass of water along with you while you paddle.

For those with long memories (80s, I think it was), this was also one of the mechanisms presumed to give drag reduction by injecting long-chain polymers into the BL, a cold war idea for making submarines more stealthy.

Remember that - and the limitations/ramifications (carrying the stuff, system complexity, detection and environmental issues…

Definitely impractical for a kayak - well, maybe just a small bit for an impressive finish sprint - though the few second gain likely offset by having carried it the whole race.

No free lunch.

SOF = soap on frame
that’s it - once I make my SOF, I’ll soak the skin in biosuds to give it that extra slippery advantage through the water - and it’ll stay clean as a whistle to boot…