Why don't surfboards fly out the back of a utility truck when driving on the highway?

I have never had it happen myself and every time I throw my boards (shortboards not malibus) in the back of someone's ute, with no cover on the tray, they always assure me they'll be alright( the surfboards) but i never fully believe it so the whole car journey i sit there in constant paranoia whilst on the highway watching the boards in the rearview mirror, they're shaking and definetly being effected by the air current but they never fly out , so I just sit in suspense in the situation.

Why don't the surfboards fly out, even at high speed and crosswinds?

Is it right to call the air current passing around the car mach flow?

I'd appreciate some help with the theory behind this so i can stop driving around with this fear of my boards being ruined!

• Wow: people still use "ute" ? :-) . Yes, you can model the airflow around the car, and as you've observed, there is little to no lift (vertical force) anywhere, and little to no drag either (toss a piece of paper out the window and the drag will stop the paper in its tracks). The biggest risk is that you'll go over a road hump and bounce the boards out. Commented Oct 26, 2015 at 14:28
• What do you mean by mach flow? Is this a supersonic ute? Commented Oct 26, 2015 at 15:22
• Ya know, whether or not a board is ejected, if it's bouncing around it'll collect dings&gouges. You don't want that. ALways tie cargo down. Commented Oct 26, 2015 at 18:28
• For readers in the US, folks in other parts of the world use the word "ute" for a variety of "utility trucks" (including El Camino type vehicles and light pick ups). So, think of a vehicle with an open bed in the back. Commented Oct 30, 2015 at 13:45

A crude model of the UTE is used simulate the pressure field created by the car as it travels through atmospheric air. We must determine the pressure in truck bed because it applies force on the surfboard. The car travels at a constant $$60 ~\text{mph}$$ (an arbitrary, worst-case value).

UPPER: The velocity profile of air flowing around the car.

LOWER: The relative (to atmospheric) pressure profile and surface plot.

The position of the surfboard greatly affects your answer. I assume the surfboard lies flat, entirely within the truck bed (with the tailgate up). For now, we assume horizontal forces on the surfboard are negligible. Summation of vertical forces will determine whether the surfboard will fly out.
Pressures act on the upper and lower surfaces of the surfboard. The relative pressure profile at the bottom of the truck bed (lower surface) is shown below. A similar profile exists along the top of the surfboard. As a simplification, average relative pressures are used to sum forces on the surfboard, where $$P_{ave,lower} = .02 \ ~\text{psi}$$ and $$P_{ave,upper} = .03 \ ~\text{psi}$$.

Various surf websites list short-board dimensions and weights, where $$A_\text{surfboard} \approx 2100 ~\text{in}^2$$ and $$W_{surfboard}\approx 6~\text{lbf}$$. At $$60 \text{mph}$$, the resultant forces due to pressure are: $$F_{lower} = A_{surfboard}P_{lower} \qquad \text{and} \qquad F_{upper} = A_{surfboard}P_{upper}$$ $$\therefore F_{surfboard} = (F_{lower} - F_{upper}) - W_{surfboard} \approx -27~\text{lbf}$$

Therefore, your surfboard is held down by $$\approx 27~\text{lbf}$$- Your surfboard apparently testifies to this! As other answers indicate, there are other considerations. Accelerations and transient effects are unknown. While your surfboard appears to be safe at constant speeds, you should remain wary and assume that it could go flying.

This model illustrates two types of fluid flow (Flow Regimes): Laminar flow characterized by 'smooth' profiles, and Turbulent flow characterized by 'chaotic' profiles. Mach flow exists when a fluid's velocity is greater than its speed of sound ($$c_\text{air}\approx 761 ~\text{mph}$$), creating a shock wave- it is not relevant in this problem. *Note that turbulence and road chatter are likely responsible for the surfboards "shifting".

Flow simulations (CFD) are the most accurate way to solve this type of problem, but since you are interested in theory, consider an idealized (less accurate) model of flow: Bernoulli's equation, $$\Delta [P + \frac {1}{2}\rho v^2 + \rho g z] = 0$$. It relates pressure and velocity so that an increase in velocity decreases pressure. This is seen comparing the flow simulation's velocity and pressure profiles, most apparent on the roof where velocity is high (orange) and pressure is low (green). It is impractical to apply the Bernoulli equation to this problem because neither the velocity or pressure are known as they flow over complex geometry. Additionally, it assumes incompressible fluids and does not account for turbulence.

Sorry but surf boards do fly out. You may not have experienced it personally but keep doing it the way you are and you are bound to experience it.

If you are thinking that as the velocity of the air travels over car it is some how holding the boards in you are incorrect.

"Actually the Bernoulli's Principle states that as fluid velocity increases, static pressure decreases. Static pressure is the pressure felt by an object or person suspended in the fluid and moving with it. It is the pressure felt when air molecules run over the top of your hand with your palm faced down."

Bumps, sharp turns, and extreme winds can send your board flying. Its just the sum of the forces acting on your board and yes they can overcome the force of gravity.

Check out some ways to secure your board. Most of these are for pick up trucks but I think you get the picture now.

http://www.thesurfboardman.com/2011/05/fcs-tailgate-surf-racks.html

https://fortress.wa.gov/ecy/publications/publications/0807030.pdf

http://www.surfing-waves.com/forum/viewtopic.php?f=15&t=6812#wrap

"Transporting surfboards is not something to be taken lightly. Aside from the fact that they are expensive and you don't want it to fall out of your truck; they can also become a hazard to other drivers if it flys out.

I have seen countless boards torn to shreds becuase they were not strapped down properly and found themselves lying on the freeway. A pickup truck is especially tricky becuase many assume you can just tie down your board like any other item you would transport in the bed of your truck."

• The science is Bernoulli's Principle and the sum total of the force vectors opposing gravity if that isn't science, well we will leave there. Not to difficult to see you are taking a big chance and putting others at risk. Commented Oct 29, 2015 at 21:41
• Sorry Joel I was under the impression you had more of a science background and could do this for yourself. I will break out the physics of Gravity and the forces working against it as well as Bernoulli's principle if I have some free time later. By the way I never heard of Mach flow. Commented Oct 29, 2015 at 23:49
• Cheers :) I'm actually a lab technician in biotechnology. I just don't have a physics background . Mach flow is measured by the rate of flow of a liquid or gas across a solid surface I think. Like a aeroplane or outlet on a gas cylinder. I fully understand a car doesn't travel as fast as a plane but the flow is still occurring even it's minimal.
– user96257
Commented Oct 30, 2015 at 0:08
• I think its actually referred to as compressible flow in which the rate of this is given a value called a mach number.
– user96257
Commented Oct 30, 2015 at 0:14
• I did a calculation for Mach Flow for you and it comes out at approximately .09 or 70mph/767mph the speed of the object / the speed of sound. To distinguish between compressible and incompressible flow in air, the Mach number (the ratio of the speed of the flow to the speed of sound) must be greater than about 0.3 (since the density change is greater than 5% in that case) before significant compressibility occurs. You would need .3 x 767 mph or 230 mph before Mach Flow becomes a factor. The Mach number (M) is defined as the ratio of the speed of an object (or of a flow) to the speed of sound. Commented Oct 30, 2015 at 4:18

The ground effect that is used in the F1 cars make a large pressure differential between the upper and the lower board surfaces. If the air can flow freely bellow the surfboard it will fly away, otherwise it is 'glued' to your truck. Be precautious and lock it mechanically.