Air Subduction in Kayak Paddles and cavitation in lee boards

I dont know if subduction is the right word to describe the phenomenon of air getting sucked down along the back side of a kayak paddle, but there I am using it.  Dictionaries indicate that common usage of subduction only applies to the geologic phenomenon of one piece of the earths crust getting shoved under another but here I am with no one to stop me and so I am expanding the usage of subduction.
I mention subduction because it is a phenomenon that impacts paddle efficiency adversely but looks so normal when it happens that its easy to miss entirely.  Subduction happens because pulling the blade of a paddle through the water creates a low pressure region on the back side of the paddle.  If the pressure is low enough, air will get sucked down along the blade of the paddle in response to the lower than atmospheric pressure there.  The result is reduced efficiency of the paddle.  Why? Because the amount of thrust you get from the paddle depends on the pressure difference between the front and the back of the blade.
Furthermore, this phenomenon seems to happen only with paddles that have long narrow blades.  At the start of the stroke with a long bladed paddle, only part of the blade is in the water and part of it is still out of the water, as you pull on the paddle, low pressure is created on the back side of the blade and the relatively flat surface of the blade that is sticking out of the water directs air down the back of the blade.  I suspect that water coming off both edges of the blade creates stable vorteces on both sides of the blade with a low pressure area in the middle that becomes the pathway for air subduction.
Commercial paddles with short, wide blades dont generate this phenomenon because the blade is fully immersed in the water before power is applied to it and the only thing sticking out of the water is the circular loom which does not produce any pathway for air to travel down to the low pressure area behind the blade.
But the phenomenon of air subduction with a long skinny blade is not unavoidable.  Depending on how the paddle is held and moved through the water the phenomenon can be avoided.  If the paddle blade is moved sideways as well as straight back, water moving across the face of the blade will not form the vortices that allow air to travel down the face of the paddle.
Since air subduction is dependent on low pressure on the back of the paddle blade, the lower the pressure, the higher the likelihood of subduction. Pressure is force divided by area so the same amount of force on a smaller area will create a greater pressure differential. And since on paddles with long narrow blades, only part of the full blade area is submersed at the start of the stroke, much more of a pressure differential is created across the face of the blade than in a paddle with a short wide blade where the whole blade is already fully submersed at the start of the stroke.

Lee board on a shallow draft Dutch craft.

The lee board deployed.  The angle at which the board is deployed controls the amount of lateral resistance that the board supplies.  The backward rake also allows the board to kick back and out of the way harmlessly should it hit bottom.

Not surprisingly the phenomenon of air subduction can appear anywhere that you move a  fin shaped object through the water like for instance a lee board on a sail boat.  The purpose of the lee board is to provide lateral resistance to a sail boat when it is sailing at an angle to the wind.  Some sail boats use keels to achieve the same results but lee boards are more handy in places where the water is shallow.  And they dont take up space in the cargo area of the boat like a centerboard does.  Lee boards are generally used on relatively slow moving boats which cargo boats generally are. And the triangular shape of the lee boards on Dutch craft puts most of the surface area of the board toward the bottom of the board, keeping the top of the board relatively narrow to minimize air subduction.
A slightly related phenomenon to air subduction is cavitation.  Unlike air subduction which happens when a foil pierces the surface of the water, cavitation generally happens when a foil is moving rapidly while fully submerged. If the speed of the foil is sufficient, it can create pressures low enough to cause water to vaporize. If you remember your high school physics, the temperature at which water turns from a liquid to a gas gets lower and lower as pressure drops.  The net effect of water turning to vapor is the same as air getting sucked into the water, it reduces the efficiency of the moving foil.  Generally, cavitation occurs in high speed propellers, but it can also happen in vertical fins on fast moving water craft like the one I posted about a few days ago.  Read more about it on the Vestas sail rocket site.

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lhydroptere in Alameda

The French boat lhydroptere is currently in Alameda being overhauled at Nelson Marine before heading to LA where the team will try to beat the world record time for the LA to Honolulu trip.  Apparently, the boat did over 45 knots in San Francisco Bay. If this sort of thing interests you, you can find full details at the lhydroptere website.
If you go to the website, note that everyone is wearing wetsuits.  Because these boats are fast they make for lots of spray.  Sailing these boats is apparently more of an athletic event than a pleasure ride.

Here it is. The boat has a main hull and two outriggers.  The foils extend down from the outriggers.

A closeup of one of the foils, folded up from the outrigger.

And another view of one of the outriggers detached from the wing.  You can also see the steering wheel of which there are two, one on each wing.

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Ventilation Air Subduction other reasons why paddles dont work as well as they should

One of my readers recently informed me that what I called air subduction already has a name in the field of boating, namely, ventilation.  OK, there we go, the internet once again comes to the rescue and dissipates darkness by spreading light, at least in the linguistic domain.
The original post, the one just before this was all about ventilation diminishing the efficiency of paddles.  Ventilation, the sucking of air down the back side of a paddle blade is a problem but can be fixed by appropriate modification of ones paddling technique.
But the problem, fundamentally of getting the best bang for ones paddling buck is that there are too many variables to the deployment of a paddle by a human and inadequate means for measuring input energy vs. forward propulsion, the two key numbers one needs to measure efficiency.
I thought the problem was more or less insoluble.  But perhaps it isnt.  Quite coincidentally while launching my kayak to do some paddle testing, I fell into conversation with a painter who has been commissioned to do paintings of some of the Americas Cup boats. He told me that the boats have a person on board whose title is tactician and he advises the helmsman, the person who actually controls the boat.  The tactician sits at a computer which gets constant inputs from a variety of sensors and then suggests ways to wring additional performance out of the boat.
Thats all I know, but I suppose that as software and sensors get more sophisticated, some of that learning paid for by sailing syndicates might also be used to increase the efficiency of lowly kayak paddlers.  I havent done any serious research on the topic, but who knows, paddle design might be advanced from art to science, assuming of course that someone cares enough to spend the money to do the research.

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