Does water temperature affect your surfboard fin performance?

Yes it does.

Here is a subject which has taken me years to wrap my head around. To understand, one needs to grasp the concept of a Reynolds number i.e. the relationship between momentum forces (lift) to viscous forces (drag). The resulting number does not have a dimension (meters or litres for example), but is just a number.  By calculating a Reynolds number one is able to compare foils, and understand which flow regimen we are talking about. (Water vs Air for example)

When water temperature decreases, the viscosity of the water increases. Viscosity can be visualized as 'thickness' , so honey is more viscous than water. Water gets thicker when it gets colder. A drop in temperature increases drag and  turbulence while decreasing the extent of smooth laminar flow.

Garrett misjudged the temperature

A typical example

Shown below is the effect upon a fairly typical single fin foil of change in water temperature from 20 degrees Celsius down to 1 degree Celsius. The speed and chord length in this example are kept constant.

As can be seen the Reynolds number is much lower in cold water, this shows the increase in water viscosity. 

So here are the calculations are specific for this foil.

RAF 27 AIRFOIL (raf27-il) Details
(raf27-il) RAF 27 AIRFOIL RAF-27 airfoil Max thickness 9.8% at 30% chord. Max camber 0% at 0% chord As you can see the temperature drives the viscosity (thickness) and thus the Reynolds number .

Water temperature 20 degrees C

Velocity	m/s	26.843 mph	43.2 kph
Chord width	m	0.19685 ft	2.3622 in
Kinematic Viscosity	m2/s	1.054e-5 ft2/s
Reynolds Number	735,166

Water temperature 1 degree C

Velocity	m/s	26.843 mph	43.2 kph
Chord width	m	0.19685 ft	2.3622 in
Kinematic Viscosity	m2/s	1.769e-5 ft2/s
Reynolds Number	438,009

How does that look on a graph?

The red line in the two graphs below represents the higher water temperature, and the purple line the lower temperature.

As can be seen here in the Cl/alpha graph ( which shows changes in lift vs angle of attack) at lower water temperature the fin will stall earlier (stall is seen where the lift suddenly drops, as angle of attack increases).

In cold water the fin will stall at about 11 degrees aoa and in warm water, at about 14 degrees. this is a significant difference.

at lower water temperature the fin will stall earlier (stall is seen where the lift suddenly drops, as angle of attack increases).

Below we can see Cd/alpha, or drag vs angle of attack. As the angle of attack exceeds 7 degrees the drag produced in cold water soon becomes double that of the drag in warm water, and the difference increases as the angle of attack goes higher.

As the angle of attack exceeds 7 degrees the drag produced in cold water soon becomes double that of the drag in warm water

How does this affect other foil types?

Results vary widely depending upon the kind of fin foil used, and the chord length of the fin. Increasing the chord length of the fin raises the Reynolds number and is one way to overcome the angle of attack limitation seen in cold water. This means that a fin with a longer fore and aft length would suit colder water. (ever wonder why Hawaiians bomb out at cold water spots such as Mavericks? Could be their habitual fin choice) All else being equal however, increasing the chord length will also increase skin friction drag. So a heavy handed base length change without doing the Math might not do the job as expected.

Increasing speed also increases the Reynolds number.

So, increasing speed, water temperature or the chord length of the fin all have the same effect on the Reynolds number. Chord length and water temperature are fixed during any given session. Speed however, constantly changes and the surfboard fin is required to operate over a range of speeds during a ride. A fin which is optimised for low speed will be larger than is necessary at high speed, and vice versa, and a fin which is optimised for warm water might experience stalling issues in cold water, when used at the low end of the speed range. 

The cold water dilemma

Here is the key: Issues are most likely to become apparent in cold water when using high aspect ratio fins with short fore and aft chord lengths. (Remember those over confident Hawaiians ?)

High aspect ratio fin  (warm)

Ideally, when travelling,  a different fin or set of fins will be needed for different water temperatures. the foil section can be changed, but the most obvious way to alter the fins is to use a longer chord in cold water, and a lower aspect ratio fin.

Low aspect ratio fins (cold)

Thanks to airfoiltools.com for the graphs, foil section and Reynolds number calculator.

How do Reynolds numbers change with fin thickness?

Did you like this in depth analysis? Try Single fins and asymmetry 

For a more basic glossary of terms used when talking about surfboard fins .