Why should hydrodynamics be a concern when spearfishing or freediving?

By Max Tapper
20/03/2022

First let’s set the base of the experience

We define the front section of a diver as the diamond shape where

“D1” is an average diver the shoulder width (0.55m) and “H1” (0.3m) the diver thickness.

“S1” is the surface of that diamond.

S1= (D1xH1)/2

S1 =.55x .30/2 = 0.0825 m² is the surface of the diver when in the most efficient position.


When swimming down we admit for the experiment that the finning technique is symmetrical on the pull and the push. The freediver or the spearo follow a perfect vertical. His speed will be fixed at 1m-s . We won’t consider the type of suit or other gear used and we voluntarily ignore other forces that may apply to the diver such gravity, buoyancy (archimede force) and other friction forces as it will make calculation too complicated.

Ok, this is our experiment conditions set.

Now, we calculate more surface / cross sections

“H1”, then increases by twice the resting width to H2 = 0.6m during fins strokes.

Which give us a new surface S2 = 0.165 m²

When exaggerating the finning stroke we will consider a diver width of H3 = 0.8m.

Which give us a new surface S3 = 0.22 m²

Alright, we have a set of 3 surfaces S1, S2 and S3 that we will use in the next chapter so remember them.

A little fluid dynamics physics to help us understand what is going on with our freediver and spearfisher

In fluid dynamics, drag (air resistance, friction, fluid resistance) is a force acting opposite to the relative motion of any moving object with respect to the surrounding fluid.

https://en.wikipedia.org/wiki/Drag_(physics)

Resistance or drag force Fⅆ in a fluid is dependent of few factors:

  • 𝓅 is the fluid density (air = 1.225 kg/m³ ; sea water = 1023.6 kg/m³)
  • 𝓋 is the speed of the object in metre per second (in our case 1m.s⁻¹)
  • А is the cross section area in m² (in our case, S1,S2,S3)
  • Cᴅ is the drag coefficient (here is 0.8 for a diamond shape cross section)

Our final equation is Fⅆ = 1/2 𝓅𝓋ACᴅ

Step one of the experiment

Freediver or Spearo drag force Fⅆ  in Newton, falling through the Air in a perfect vertical:

Fⅆ1 = 0.040425N

Fⅆ2= 0.08085N

Fⅆ3= 0.1078N

Freediver or Spearo drag force Fⅆ ‘ in Newton, falling through the sea water in a perfect vertical

Fⅆ1’ = 33.78 N

Fⅆ2’= 67.56 N

Fⅆ3’= 90.07 N

Step two, thing we can notice is the ratio Fⅆ ‘/Fⅆ

Water Fⅆ3’= 90.07 / Air Fd3= 0.1078N for the same surface S3 the drag force ratio is equal to: RFⅆ3’/Fd3 = 90.07/0.1078 = 835.5

It is amazing to think the water drag is more than 800 times greater than the air drag!!

To put this result in perspective we could say “moving our diver in the water requires the same force as moving half of an average car in the air.”

Step three, let’s make it difficult or not for our freediver?

Now let’s add an elbow sticking out which is a common mistake in freediving and spearfishing when you do your equalisation procedure.

We assume here that the elbow is represented by an isosceles triangle of base 0.2m and height 0.3m ,

that gives us Selbow = .3 x .2 / 2 = 0.03m2


Let’s add this new surface to our worst scenario S3’ of a diver doing super wide kicks.

We use S3= 0.22 m²

So,  S3 +  Selbow = 0.22+0.03 = 0.25m2

Now Fⅆ elbow 3’ = 102.36N

The force/effort necessary to move your body through the water will have to increase by almost 15% while you maintain the same speed. Let’s see it like this, now you have to push 115kg instead of 100kg.

Finally, understanding the results

Best free falling position

Correct body position

Bad body position

Water drag is an invisible barrier for all freedivers and spearfishers. This exercise was set for a diver diving vertically but it could easily be transferable for a spearfisherman swimming on the surface against an increasing tide flow. As we saw in the equation the velocity of the object is to be considered too. The drag force is proportional to velocity in slow-speed flow and it is the square of the velocity for high speed-flow.

As a freediver / spearo our fuel is the O2 we store in our body. This oxygen burns more or less quickly during our dive and it is mainly used when we are moving.

It is simple, with the result we have now shown you to understand “that pushing 62kg of water is easier than pushing 90kg and even more easier than 102kg”.

Working on your hydrodynamics and trying to be streamlined could significantly improve your dive time and subsequently your diving depth for our friends freedivers and for us spearos it will increase your bottom time giving us the chance to catch that fish without taking the risk of pushing your dive to the red line.

So keep that head in a neutral position, and bring your elbow/speargun/arms close to your body and finally train to dive vertically.

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