Q and A: Stone Age of Swimpacking.

In this blog I am addressing more frequent questions that people often ask me and later will discuss some ideas that I have how to decrease the drag from a bag. But first:

Bag towing update:

Recently I have tried a new technique for bag towing that worked much better then the one I suggested before (ankle mount). This time I have tried to tie single rope on a tightening knot to my waste and the other end to the bag. The rope had 3 floaties made out of pool noodle above my heals to prevent the rope tangling in my feet. This setup completely freed up my feet adding a bit of power and prevented rocking of the bag (increased efficiency). I will be using this setup in future.

Question: Why does the bag not sink?

This was one of the first questions that we wanted to answer when me and Martin first went for a swimpacking day-trip. We stuffed our dry bags with everything that we would bring to a multi day trip: clothes, food, 1 L of water, hammock and a tarp, and thrown it into the water. To our amazement, the bags were floating on the surface of the water as if they were inflated with only air. Did we put something in the bags that would cause them to float? In fact, no matter what you put, as long as the total density of the stuff in the bag is smaller then that of the water and the bag is not leaky – the bag will stay afloat.

After filling the bag with all the stuff for the trip and compressing it to the point when you start hearing the stitches of the bag cracking (don’t do it), the weight of the bag was ~ 12 kg. Dividing that by the volume of the bag, 35 L in my case, you get total density of the bag is 0.34 kg/L. The density of water is ~1 kg/L which is much denser then the bag, thus the bag floats and even has a huge margin of flotation to support the swimmer on the water.

You must make sure that your bag doesn’t leak, because if it does it will start filling up with dense water and the density of clothes added to the density of the water will cause the total density of the bag to be greater of water, so your bag will go to the bottom. Thus it is important to keep the bag water tight which is second most popular question I get.

Question: How to keep the bag water tight?

So far, I haven’t found a producer of dry bags that would openly claim that their bags are truly dry when thrown into water for hours, and they are right. All bags that I have tried leak after half-hour in water.

I found an easy but temporary solution to this issue: I double bag my stuff. I use a big storage Zip-lock bag inside the dry bag. The dry bag serves as protective layer and is responsible for structural integrity of the setup while the inner liner protects the contents of the bag from water. This setup is far from ideal because the outer bag manages to collect quite a bit of water inside adding to the weight of the bag. Closing the inner Zip-lock is also a pain because it takes 5 minutes each time you take something out of the bag. Also, the inner liner is not very reliable and server maximum of 2 trips (about 4 days), so I usually take a spare one with me in case the first leaks.

In future I am planning to make a more permanent fix by attaching the zip-lock mechanism to the top opening of the bag (without the inner liner). This will make it more reliable and less time consuming then an inner bag because the watertight lock will be much shorter and much easier to close.

Question: How to stay hydrated and sated and 'navigated' on the water without support

Swimpacking journeys take many hours of swimming per day, during which hydration and calorie intake is an inevitable necessity. If you are swimming long distance with no support crew, you must carry food and water in your dry bag. Now imagine that you feel like it is time for a sip of water. You have two options: you either open the dry bag while in the water to get your bottle, or you open your dry bag after you get to the shore. First option is probably a bad idea, because it will likely end with a sunk dry bag. The second option could be too time consuming and not always an option if you, for example, are swimming to an island and there is no shore to get out on (which you shouldn’t do without support anyway, but who am I to teach you). Same problem arises if you need to eat or check with GPS your progress or you need to check the heading on the map.

One of the solutions is to have a water bottle, water-friendly food (jell tubes) and water-resistant GPS/smartphone outside of the bag, in the pocket attached to the side of the dry bag that faces upward during the swim. I haven't tried this myself yet, but my plan is to take a piece of mosquito net-like fabric and glue it onto the dry bag. The access to the pocket would be through a zipper. This way all things that I might need during the swim are accessible without opening the main compartment.

Question: How to minimize drag from the bag.

One may ask, what is the point of towing a heavy bag that will slow you down so much that you will only be able to swim half of what you can normally swim in one day? If that is the case, then 2-day swimpacking trips are non-sense. A more hydrodynamic bag might allow to swim much further then a normal dry bag and allow for even longer swims.

According to my calculations, the bag full of stuff for an overnight swim slows me down on 0.2 km/hr if my average speed without a bag is 2.3 km/hr. The force that the bag applies on my ankles during the swim is very tangible. I am not an expert in fluid dynamics, but I understand that at least three variables are responsible for drag during the movement through the water that can be optimized: 

1.     Area of cross section of the bag. Imagine that you tie an umbrella to your feet and start swimming. If you open the umbrella, it will cause huge drag, and you likely will not be able to move. Whereas if it is closed, you will feel almost no drag.

2.     Surface area of the contact between the object and water. Try dragging a long strip of fabric through the water and then folding it many times and putting it in a small bag after which trying to drag it again. The drag will be much bigger when the strip is unfolded.

3.     The speed of movement of an object relative to the water that it contacts. You may say "duh, if speed is zero, then the drag is zero, but I want to move forward with some speed" It may seem too basic to mention, but you will see bellow that this variable can hide an obscure solution to problem.

When considering the shape of the bag there must be a shape of the bag that is optimized for both surface area and the cross section of the bag. To be able to optimize this on my own I would need 4 years worth of fluid dynamics courses, which I don't have, so I will simply plagiarize from a kayak design. As far as I know, competitive rowers prefer longer, narrower and slicker kayaks. Assuming that a towing bag doesn't need much rolling stability, a longer and narrower design with polished walls is a more optimal design then a conventional dry bag that has ratio of width to length = 15/7 (=2.14). A typical width to length ratio of a race rowing kayak is 70/5 (=14). If using this ratio for a bag and assuming we keep the same volume (35 L), the dimensions of such bag would be: 205 cm long and 15 cm wide - Not a very practical bag, especially if you plan to transport it on your back during the hiking sections.

Optimizing for only one factor usually gives sub-optimal result, specifically from practicality perspective. Let’s include practicality in the equation. Let’s assume that the longest bag that can be transported o land is the maximum length of an expedition backpack. The longest 80 L backpacks that are sold in stores are around 85 cm tall. If taking this length as a limiting practical length of a towing bag, then the diameter of a such bag, that accommodates 35 L, would be 23 cm, with Length/Width ratio = 3.7 – just a bit better then a normal dry bag.

One possible hack that could elongate the bag twice of the longest backpack is to have 2 dry bags stacked in series with a slick connection to prevent turbulence in the water. Once on the land the 2 bags would be detached and tied together in parallel and the straps would be attached. Such configuration would give total length of a bag 85 cm * 2 = 170 cm and width = 16 cm (V = 35 L). This configuration would give Length/Width ratio of 10.6, which is much better then configuration that is twice shorter then the double design.

I am planning to buy some dry bag material and PVC glue to make some experiments. Examples provided above will be the first models that I will try to build. Stay tuned for updates and comparisons.

I have mentioned that the magnitude of drag will depend on speed of movement of an object through the water. Now imagine that the water itself moves relative to the shore. In such case the speed of the bag relative to the moving water can be zero but the bag will still be moving relative to the shore. These movements of water are currents. Open water swimmers are using naturally occurring currents in the ocean to help cover longer distances or swim faster. I will focus on the water currents formed by the swimmer while swimming. When the swimmer moves through the water, they inevitably accelerate the water that they are in contact with, producing various currents. You can see these currents in figure 1. We can utilize these currents to “pull” the bag with us by strategically locating it in the zones with strongest currents.

In my previous blog I mentioned, that there are some setups of bag tying that cause less drag then others. For example, if the bag is located right above your back, the drag will be minimal due to the slowest currents relative to the swimmer (Figure 1). The upper yellow circle shows the spot for most efficient placement of the bag which is a bit bellow the shoulder blades. It is only important for the front tip of the bag to be located in the area of slowest current relative to the swimmer, since the largest drag is caused by the front of the object moving through the water. This positioning of the bag is ideal for smaller bags used for day trips of for safety buoys.

Unfortunately, it is impossible to fit all sizes of bags above the swimmer’s back because the bigger bags tend to sink too deep and interfere with swimmer’s movement. The next zone of slow current is right behind the feet of a swimmer (not shown). Remember that swimming behind another swimmer is much easier then on your own. But it only takes a meter of distance between you and the swimmer in front of you when this effect disappears. Thus, the closer the bag is to the swimmer the less drag will be exerted on the bag. Thus, the shorter the rope that ties the bag to body - the better. 

Figure 1. Speed of water flow around the body of swimmer under water. a) Simulated data. The lines represent the direction of water flow (towards the rare of the swimmer). Blue color represents faster flow relative to the surrounding water (slower relative to the swimmer) – less drag exerted on swimmer, more orange (yellow) color represents slower moving water (faster relative to the body of the swimmer) – higher drag exerted on swimmer. b) Visualization of real water flow around the swimmer’s body using Tufts Method. White worms show the direction of water flow. More wavy worms signify lighter current relative to the body of swimmer – less drag. Source: Arfaoui et all. (2011)

In many sports where speed is a key factor, athletes are trying to decrease the drag from the water, air or road using various technology: aerodynamic bikes, sophisticated bike wheel design, hydrodynamic kayaks, etc. I see no reason why swimpacking could apply technology to make the journeys less physically demanding and would allow for wider range of distances to see more stuff.  

References:

Arfaoui, A., Popa, C.V., Fohanno, S. et al. J Vis (2011) 14: 3. https://doi.org/10.1007/s12650-010-0062-x