Question: When paddling in a SUP race with a 1mp current moving everyone about 1mph faster is drafting as effective when compared to a  SUP race without a current?

Greg,  USA

Now that is an interesting question. if we were planning our our Chattajack  strategy we would be asking the same thing. The key word here is ask. Once you get thinking about this question, you realize that there is significant physics at play. We consulted with Douglas Dommermuth who is an endurance athlete and theoretical wave physicist to help with the answer.

When we approached Doug our bet was that

1. The real key to this situation is the head wind. Remember, even if there was no wind that day, there is a head wind created by the boards moving with the current and because of your paddling? As the headwind increases the value of drafting increases. Therefore, the faster  you go, the more important drafting becomes. Considering the headwind, drafting is more important the faster you go so drafting is as important with a current.

2. The current, however, would limit the effects of the draft because if you are going at 6 mph and the current is 1mph, someone drafting off of you would be like drafting at a 1mph slower speed and the value of the draft would be minimized.

Doug agreed and said that, theoretically, that is correct and that is probably all there is to this answer. We then got into a long discussion about an issue that may (possibly?)  come into play in some situations. Before you get too wrapped up in this discussion we warn you in advance you may sink 20 minutes of your life into this post and not have much to show for it. Nonetheless, for anyone really interested in paddling and why it is so hard to go fast on a paddle board, it will probably be worth it.

Doug advised us that if a group of paddlers could (theoretically) get their board to plane because of the increased speed of the group, you would be left behind with no real hope of catching on. To understand planing and why this may (or more likely may not) happen,  you need to understand wave drag and the other factors that slow you down.

As a general rule, drag is comprised of several factors as follows:

• Wind drag of the person paddling. This is accounted for by a drag coefficient that is comprised of:
  • “skin friction drag” which is the drag coefficient between the wind and your skin/clothing/etc. It is proportional to the density of the air and the the relative difference between the wind speed and the speed of the paddle boarder squared.
  • “form drag” which is the drag created by your body shape. The projected area of the paddle boarder is important (e.g. Kai Lenny has less drag than Chuck Patterson and you will have less drag if you wear tight fitting clothes – don’t wear baggy clothes if you want to go faster)
  • Wind drag is proportional to the relative difference between the wind speed and the speed of the paddle boarder squared.
  • NOTE: drafting reduces the skin and form drag of the paddler of the person drafting
• Water drag associated with your paddle board. It is roughly calculated as follows:
  • “skin friction drag” – is a function of the wetted surface of the board
  • “form drag” is  related to the shape of the board
  • The total drag associated with water is also proportional to the the density of water and the relative difference between the water current and the speed of the board squared.
• Wave drag: as your board travels through the water it creates a waves. This is called wave making resistance. At slow speeds the displacement of the board moving through the water will form waves. At higher speeds, the board will plane on top of the water but before you can plane you have to get over the wave drag hump. Once you get over that hump, your board will plane and your speed will increase because there is less drag. It is interesting to note that the wave drag is generally near a maximum where the length of the wave that is formed is the same length as the length of the board  (more on this in a minute).
• Additional resistance: there is a slop factor in these equations that factors in things like surface chop, waves caused by surrounding vessels, etc.
  • Paddle boarders constantly surge forward and backward under the action of the paddling.   It takes work to accelerate the mass of your body and board.   Depending on the shape of your board, there is also an added mass term that is related to the kinetic energy of the water that is itself undergoing acceleration.   Basically, you want a light board with minimal projected area so as not to impart too much momentum to the water.

Putting all of this into practice, equations are used to determine the performance of a paddleboard. Historically, real fluid effects for a streamline shape like a paddleboard are accounted for using a drag coefficient for the skin friction that calculated based on the Reynolds number.   A factor is then added to account for additional form drag that may be present.

As you can already see, our empirical  model of a paddleboarder is getting rather complicated. To determine the effects of drafting on a river with current, you need to consider,  wind drag of the person, viscous resistance of the board, wave-making resistance of the board, unsteady effects due to surging, and lift and drag models of the paddles and fins.Lets look at a quick (and overly simplified) equation to demonstrate how these factors are used by scientists who tackle these problems.

To evaluate how critical wave drag is to your (potential) success consider that every board in every condition has a critical speed at which it can start planing. This speed is related to the wave drag because the wave drag tends to be the largest when the speed (U) =sqrt(g L / (2 pi)), where g=acceleration of gravity (9.80665m/s^2), L = length of the board in meters, and pi=3.14159.  For a 14  foot board, L=4.267m and U=2.581m/s or 5.773 mph. This means that your critical speed is somewhere around 5.773  mph.

Remember, this is a very rough approximation but will get you in the general ballpark.   How you get there is irrelevant. If you get there because you catch a wave, because you can paddle that fast, or if the pack in front of your helps you get there (e.g. drafting helps you get up to speed faster) once you hit this critical speed you have the potential to start to plane. It might be of interest to also note that you absolutely need to get over that speed in order to start planing AND some boards will start planing earlier than other after you get over the critical speed.

It cannot be overstated that most of the time when we are paddling we are not planing unless we are riding a bump because we simply cannot paddle fast enough to overcome the forces of wave drag .  Paddleboard speed is limited by wetted surface area which is associated with skin friction (and wind drag). As a general rule paddleboards climb up the steep portion of the wave resistance curve but do not get over that hump unless we start surfing waves. Moreover, in order to plane, you must sustain this speed in order to plane.

What this means for you if you are drafting in a race with a current, if the group ahead of you can as a group (remember the group is more powerful than the sum of it’s parts and can generally go faster than you as a solo paddler) can (somehow theoretically) get up to the critical speed to overcome wave drag before you do, they are going to jump ahead of you and you will be completely hosed.

One thing to consider is that the current will not assist any of you in getting up to planing speed. For example, lets say that you are just sitting there in a current moving at 10mph which is  well above your hull speed – you will not be planing. You will be wallowing in the water moving at 10mph.

The bottom line on all of this is that drafting is important as it reduces drag.

1. If it is just you and another paddler paddling with a current, it is as important as if there is no current.
2. If, you are paddling behind a group it is more important because the group can take turns and their collective pool of energy will likely be greater than yours.
3. Finally, if there just happens to be some set of environmental factors related to current, tailwind, chop, etc, that allows the group to get up to planing speed before you do, you will be off the back, hosed, and likely have no hope of ever catching on.

We do not know what that set of conditions would be, or if they ever happen in the real world but, just like the rest of this article, that is where our mental masturbation brought us and where we are going to leave you. The math is getting too complicated for us to continue.