A little Hydrodynamics

I gotta know.

That’s my burden, probably comes from the ADD (Attention Deficit Disorder) that so clearly determined my career path (Motorcycle mechanic, nuclear reactor operator, PR flack, software entrepreneur, technology marketer, direct and interactive agency founder, surf bum). When I start looking at something critically, I have to have some kind of authoritative or at least comprehensible answer for the question “why is it like that?”. And I really can’t let the issue rest until I know.  Questions pop back into my head at any time, distracting me from whatever I’m doing. It might take years, I might still be looking for some particular answers when I kick the bucket. But I gotta know.

When I started looking at SUP race/distance boards they looked “wrong” to me. The majority had long, tapering pintails. How could that work? Most of the folks I talked to said the idea was to disturb the water as little as possible, to join the flow back at the tail of the hull. That sounded a bit too zen for me. It looked to me that the flow would start separating from the rails as soon as they started to converge. That you’d have turbulence on both sides, causing not only parasitic drag, but also uneven buffeting force against the rails and the stabilizing fin, which would add more drag.

The Ku Nalu board is known to be a relatively fast standard board with a short nose and squash tail that permit straight rails

Most race boards, even 12′ ones like this Naish Glide, tend to have long pintails

Joe Bark’s racing boards, like this 16 footer, are even more extreme

While the S.I.C. F16′s and F18 tend to have blunter tails

I talked with designers like Mark Raaphorst whose F16′s figure large in the top ten finishers at almost any race, almost any place. Most of Mark’s open class race boards have a somewhat flattened tail. I asked him why his boards didn’t have pintails.  I didn’t take notes, so these aren’t really quotes–just the sense of what I recall  Mark saying:

Some of them do. Depends on what people are using the board for and where they paddle. If people are going to paddle in flatwater then a pintail is great, but in swells, especially following ones or in high wind, the squarer tail separates flow cleaner and the straighter rails let you catch the swells and plane quicker.

I suggested that the pintail might balance buoyancy better and he said: Nah, look how little difference we’re talking about. He grabbed a pintail template off the wall and dropped it onto a F16 tail. I could immediately see that it wasn’t much. This is all about flow and making a board catch a swell, or not.

Mark’s explanation made sense–sort of, but I still didn’t really know why a pinail would work in flat water.  One of the guys on the Stand Up Zone came to my rescue with an email address for a guy named Al Bowers who he said was an aerodynamicist for Nasa and a Surfski paddler. I sent him this email:

A fellow Stand Up Zone member gave me your email address and said you might be able to blow some smoke away from the issue of watercraft tail design.  I’ve seen that most Surf Canoes, OC’s and racing SUP boards have a pintail, and this doesn’t seem particularly logical to me. Most of the designers I talked to say something like “All the successful designs have Pintails” or “The idea is to disturb the water as little as possible” but neither of these are satisfactory answers.

Viewing the issue strictly from a viscous drag and friction side, it seems the pintail would have more drag for a similar waterline length since the flow would (I think) be separating along the length of the converging sides and the unequal turbulence would create side thrusts as it does in race cars. Seems to me that straight rails and a relatively abrupt square or squash tail would offer less drag.

I would appreciate your views on the subject. I’ve done quite a bit of searching on the web, but haven’t found many technical papers that offered enlightenment. I’m not planning to design any craft, I’m just looking to satisfy my curiosity and perhaps stimulate some new thinking on these issues. People tend to design things because they have “always been done that way”. Absent budgets for tank testing and sophisticated analysis I don’t think attempts to make a better hull will be very creative without some discussion to spark experimentation.

Thanks for your time in considering this request.

In  remarkably short time I got this reply:

Thanks for the note. I dunno if you’ve received any info on my background. I spent 20 years as an aerodynamicist in the Fluid Mechanics branch here at NASA dryden (located at Edwards Air force Base). I love aircraft, especially low speed and motorless flight (both a hang glider pilot and a sailplane pilot). I did my graduate work on laminar flow and sailplane airfoil computer code validation against wind tunnel and flight research data. I’m also an avid sailor (since I was a kid), paddler, and rower. Used to swim a bit and freedive. The one thing I never did do was surf (long story here, but I didn’t have much time for it, you see I also backpack, hike, do photography at a very serious level, and a bit of climbing). I’m also a bit of a musician (guitar mostly), did some studio work a zillion years ago. After I did my 20 years as a working aerodynamicist, I was promoted to the position of Chief of Aerodynamics, and then did a stint as the Deputy Director of Research at NASA Dryden, and I did an invited residency as a Special Assistant to the Associate Administrator of Aeronautics at NASA Headquarters in Washington DC. I am currently the Director of Aeronautics Projects at NASA Dryden. One other bit, I have sat on the board of Cal Poly University (San Luis Obispo) for 16 years for Mechanical Engineering, and two years ago I paid for four students to do their research project building a custom ICF Sprint K1 based on an optimal genetic algorithm (we were trying to rebalance the wave drag to skin friction drag, the theory said we should see a 3-4% decrease in drag at sprint speeds, we didn’t succeed, but the kayak worked as predicted, it failed due to other problems; I still have the boat and the molds). BTW, my wife and I paid for that project out of our own pockets.

ENOUGH! It turns out your friends are right. A pintail IS the optimum for most of the paddling we do. The flow REALLY does stay attached and flow back along the sides, FOR MOST OF THE PADDLING WE DO. Note the caveat at the end. Below hull speed (that taking the LWL in feet, take the square root of that, then multiply by 1.34, and that is the approximate “hull speed” in displacement mode) we are operating in pure displacement mode. Above this speed, we start to experience some hydrodynamic lift. By the time we get to double the hull speed, there is enough hydrodynamic lift that we start to leave the displacement mode behind. This regime is the transition range. Above this, we make enough hydrodynamic lift that we can consider ourselves planing. To plane is where all shortboard surfers and windsurfers operate. They need enough speed to LIFT the board and rider out of the water. At this point you don’t want a pintail anymore. You NEED straight rails and the hull should resemble a speedboat or a ski boat more than a kayak hull.

Now, back to those speeds, the “hull speed” of a 17 ft paddleboard or kayak is about 5.5 knots. A 21 ft surfski or solo outrigger is about 6.2 knots. So to breakout and plane we need to be going about 11-12 knots. This is well within the range of surfing.

There’s a long complicated theory with all this, but that’s the way it works out for us. BTW, sprint speeds are right in that 10-11 knot range. And people have played with planing type hulls for the 500m and 1000m races sprinter run. But its VERY marginal to plane. There was a French windsurfer that built a special low-speed planing board some years ago. In the 7-10 knot range his board was unbeatable. Below that, the old style long boards crushed him. And above that speed, the shortboards annihilated him. It always works out this way.

I hope this helps. If you’ve got further questions, drop me a line.

Best regards,

Al Bowers

So OK. Now I know. On to the next question