Another look at drafting- does it help both the leader and the follower?

[blueplanetsurf]

Ok, I did a little "research" (google)  and found this on bicycle drafting:

http://www.deskeng.com/articles/aaabey.htm

Quote:
As the riders continually progress toward the front of the paceline, take a short turn at the front, and then soft pedal as they drift to the rear of the line, the average drag coefficient of a TTT rider (assuming a constant rate of rider rotation and ignoring the effect of dropping back) is around 27% lower than experienced by an individual rider. Perhaps the most surprising conclusion is that, despite the full force of the oncoming air, the lead rider experiences lower drag than if he were riding an ITT at the same speed. The drag coefficient of the leading TTT rider is 0.277, while that of an individual rider is 0.285. This occurs because the second place rider reduces the influence of the lead rider's wake, increasing his base pressure and consequently reducing the drag force.

[SUPracer.]

Physics aside, the whole psychology of having someone right on your tail will make most people paddle faster than they could if they were on their own.

So I reckon it'd benefit the lead paddler, at least indirectly.

[catdailey]

Lol SUPracer...for sure! And blue planet, that's really cool, but again, it might apply only to air resistance and not to water?  (remember, cyclists in a TT or good pace line are very very close together, unlike long SUP's where the actual bodies are vertical and spaced far apart). Brun0? Anyone?

Oh, and from years of racing...cycling, speed skating; drafting is as much a part of the sport as an other, but there's an etiquette to it. Sort of like taking a SUP into a surf only zone, I guess. Violate at your peril and be known to all as a shameless wheel sucker 

[Kaihoe]

Hi Cat

From my experience yes the lead board can get a push from a following board, I have experienced it but only rarely.

The time I can really remember they guy behind me was on a 14 while I was on a 12'6". We where grinding into the wind and chop and I could feel a push ( just like you get in a racing car). I would only feel the push I sometimes. My thinking at the time was that we where effectively getting a long waterline, but he had to be in exactly the right place to make it work.

Given this I wouldn't be surprised if some research did prove that the lead board gets a small advantage. After all when your in someone's draft you can always feel the advantage.

[PonoBill]

Doesn't work for downwinding, and Bill it's pretty hard for you to right behind me when you're in front of me.

[surf monkey]

BlueplanetSurf

Your just the guy to put it to the test. Looking forward to it.

Have a guy heart rate monitor, gps, mapped out course hold a pace then at a certain mark have a faster paddler come in on his draft, then add more guys to see if pace picks up? Or something like that.

Good luck.

[surf monkey]

Could also have them all start out drafting then drop off the draft and see what happens.

[gorgebob]

Drafting pushing into the wind is huge. I doubt the lead paddler has much of a bubble push being 14' feet ahead. The lead boat does leave a small slug trail path. The smooth trailing bit of water. The water in the slug trail follows the board for a slight distance. Having your bow in the slug trail which is smoother than open water and already moving forward has to help out . When a slight cross wind hits the lead paddler it is tough to drop the slug trail for the wind buffer off the side. When I am trying to lock on to a lead paddler I find the fastest way is to  follow the smooth slug trail path up to their stern.

[surf monkey]

Never herd the term slug trail? I Like it.  I look at it as the wake of the board and when drafting I always thought I was riding the wave the board makes the closer to the lead board the bigger the wave / more of a ride.  As you come up on a person you see the bow wave  coming off at a angle to the lead paddler you get on this wave to help you close the gap to the tail of the lead boards wave coming off the stern of the board in front you is pushing a hole in the water kind of making the wave and that's the one to ride.

So i think the lead paddler always pushes that same amount of water making the wave and not sure if the stern wave drag transfers to the second paddler.But on a good draft the boards touch that's making the longer water line? The longer water lines are faster? So confused.

[blueplanetsurf]

I think the dynamics of drafting in water are similar to those in the air although water is much thicker so drag is created at lower speeds.  I think the dynamics that happen on the race cars in the video are essentially the same as what happens in the water.
Bascially, drag through the water is created in the front of the board, where the water is split, hull drag along the sides and bottom of the board, and by the turbulent eddies behind the board.
When you are drafting behind someone, you use less energy splitting the water in the front.  When someone is drafting behind you, the theory is that there is less turbulence behind you as the board behind you essentially lengthens your waterline, so you are also benefiting, although this benefit is much smaller.  I agree that an etiquette in sportsmanlike drafting still needs to be developed in SUP racing, like in biking, where the leader is systematically rotated.  A group drafting in this manner should have a significant speed advantage over a solo paddler in a flat water race and a lead pack should be able to pull away from the main pack, much like in the Tour de France.
I made a very simplified sketch of water passing a single board vs. water passing two boards to illustrate the water flow (think of the water flow lines as the yellow lines in the race car video).

[Waves4supper]

Personally I wouldn't go for the theory that the two boards would work together to create a longer waterline, and thus "surf their own bow/stern wave system", to avoid riding up against the bow wave, which is what you start doing once you go over "hull speed" (I hate the term because it's basically wrong and a simplification). But in any case, riding up against the bow wave is the reason for the exponential character of the resistance curve in this speed range. Also the reason why you go only marginally faster in a sprint, even if you double the power you put into your paddle.
I think for the "longer waterline effect", the boards would have to be coupled mechanically as one stiff body.

What could be a possibility is a phenomenon similar to what a bulbous bow does on a ship. The underwater body creates a "negative" bow wave which coincides with the actual hull's  "positive" bow wave, thus reducing its size. The result is that less of a bow wave is generated, which means less energy is put into the wave system, and thus less resistance, fuel consumption, etc.

Now if the bow wave (high pressure area / "positive wave") of the following paddler coincides with the depression at the stern wave of the leading paddler, you could have a similar phenomenon, and the resistance of the leading paddler could actually be reduced.

I think the effects are so small and hard to measure, that you can't really test it with real people on open water with weather effects, fatigue, etc.

The options I see for someone interested enough to get to the bottom of this are:

- Do a CFD calculation (computational fluid dynamics) for both scenarios (single board / two boards)  - which can be very expensive, but which is also very accurate these days.

- Do a towing test. Put some ballast on each of the boards, then go to a channel on a windless day and tow them, either from a boat or (preferably) from a bicycle on the shore or something. With two dynamometers (fishing scales if you're on a shoestring), you can measure the resistance of each of the boards. Do it many times at different speeds and different weights using 1/2 of the times a single board and 1/2 of the times two boards in tandem, and you could see whether there's a statistically relevant reduction in resistance on the leader board.

I doubt anyone would go through all this trouble, as there's not much you can do with this information (except maybe feel better when someone is drafting behind you).

Bruno

[catdailey]

Thanks Brun0! That was more like the info I thought might be out there from someone who understood water dynamics better than I. I doubted very much that correlations could be made between air flow over bicycles/cars and water flow around/under/over/behind a board.  Road surface doesn't displace like water surface (also makes for nastier crashes)  Thanks for you thoughts...

[blueplanetsurf]

I think for the "longer waterline effect", the boards would have to be coupled mechanically as one stiff body.

Why?  How would the coupling change the water flow?

[Waves4supper]

I think for the "longer waterline effect", the boards would have to be coupled mechanically as one stiff body.

Why?  How would the coupling change the water flow?

Hi Blueplanetsurf,

It would change how the two boards are positioned relative to the (not flat!) waterline.

The concept of "hull speed" comes from the relation between the length of the board and the distance between bow and stern wave.  If the negative stern wave (an area of low pressure) coincides with a peak of the bowwave system (thus one full wavelenght away from the bow (or two, or three), they will to some extent cancel each other out. Again like the bulbous bow: less waves generated means less resistance.

If the low pressure zone behind the stern coincides with a trough of the bow wave (e.g. at 0.5 or 1.5 times the wave length behind the peak of the bow wave), the two wave systems will add up and you generate more waves.
In a boat, you can experience this as the running trim increases and you have the feeling of running up against the bow wave. If your engine has enough horses, you throttle up to increase the speed and the running trim will reduce again (a hard-chine hull form will start planing at that point if all is well).

In the hypothetical case that both boards would somehow be mechanically linked (stiff), the bow of the second board would keep the stern of the first up and things would look different. I think in that case, you will have some sort of "doubling of the waterline"-effect.

But, I  think you'd get some concerned looks if you'd start coupling two boards during a race though 

PS - I hate using the term hull speed as it's an incorrect simplification. People tend to see it as some magical barrier, while there is still a lot to gain by a optimizing hull shapes, even if the length-over-all is fixed.

[DavidJohn]

Maybe they should make all race boards with a deep swallow tail shape so you can really draft someone close and help join the two boards into one waterline that might help both paddlers.. 

DJ