"Strap hum" is an annoying loud humming sound made by boat tie-down straps vibrating in the wind of a moving vehicle.

The first time I experienced this phenomenon was only about 5 minutes after renting my first canoe... It was for a trip through Boquillas Canyon in Big Bend National Park. I was a stone cold rookie and didn't even possess my own boat yet. I had been invited on the trip by Louis Aulbach and Dana Enos, after running into them in several Texas Parks and chatting with them in the campgrounds. So there I was, pulling out of the river outfitter's lot, and as soon as I accelerated over 50 mph, I heard it: hummmmm... What's that? I ignored it, and continued accelerating. At 60 mph it got louder: HUMMMM... It sounded like there was a jet airplane following me down the road. Being the smart guy that I am, I quickly deduced that the noise had to be something to do with the boat, since my car didn't hum like that previously on the drive to the outfitter.

I pulled over, and cinched up all the straps as tight as I could pull them, and got back on the road. HUMMM... It was still there! Not knowing what else to do, I just reduced my speed to the low-hum level, which increased my drive time to Boquillas Canyon. And that's a long drive from Terlingua. 

I have even see professional truckers experiencing this problem, with heavy duty straps securing an object atop a flat bed trailer. The longer the unsupported length of strap, the more likely it is to vibrate. You would think professional truckers would know how to deal with this, but some don't. Perhaps they can't even hear the sound happening from where they sit way forward inside their cab. The noise is probably more obvious for a canoe sitting right over top of your head on your roof.

I've since learned how to avoid that annoying and destructive vibration and sound, and I'll share that secret with you here shortly.

Strap hum can also happen with surf boards, or with objects which have a concave surface where air can flow underneath the strap where it passes over the concavity.

The rapid vibration also produces heat, and the vibration can be so severe that it can buff a groove in the plastic on your canoe. It can strip the paint off your car. It can break down the fibers of the strap and weaken it to the point of failure. It causes strange looks from passing motorists. And it will interfere with you hearing your favorite music on the radio.

What makes a strap vibrate?

The scientific principle for this vibration is called "vortex induced vibration", or VIV for short. As the air flows over the strap, it slows down from friction where it contacts the strap and curves around it, while the surrounding air continues to move at full speed straight ahead. 

This creates vortices, or swirls, in the air on the down-wind side of the strap, as the differing air speeds merge together again. These vortices change the air pressure along the surface of the strap, causing it to be pulled and pushed into the low pressure areas, first one way, then another. As the strap reaches the limit of its stretch, it rebounds back to where it was, like a rubber band. The vortices thus induce a rhythmic motion that we call vibration.

VIV is a very serious matter for engineers. It can even happen to chimneys (video). The most dramatic example is the Tacoma Narrows Bridge disaster (video), which happened in 1940, before the phenomenon was understood.

Why does a vibrating strap hum? 

Vibrations by themselves aren't really "sound". Remember the old philosophical question; “If a tree falls in a forest and no one’s there to hear it fall, does it make a sound?” The vibration of the strap vibrates the air molecules around it, sending out pressure waves through the air, just like tossing a stone into a lake sends out ripples in the water. These vibrations in the air enter our ears, and are interpreted by our nerve centers and brain as "sound". Thus, our bodies detect the vibration and translate it into sensory input. 

The tie-down strap acts just like a plucked guitar string, a struck tuning fork, or the vocal cords in your throat when you talk. When they vibrate, you hear sound. Try tying down your kayak by running a strap across the open cockpit, and listen to the vibration being magnified by the open cavity inside the kayak, like a giant bass guitar.

What can you do to stop the hum?

Now that we understand the phenomenon, I'm finally down to the helpful advice of this story.

Some people will fold up an old towel, or stuff a hunk of foam underneath the exposed strap, to eliminate the vacant space in its length. I used to do that, and lost a few hunks of foam out on the road that way. I got tired of having to pull over and chase down hunks of foam between speeding cars.

There is an even better, easy fix to this nagging problem. Put a half-twist in the exposed length of strap. Yep, that's it. Quick, easy, and it requires no extra gear! The longer the length of strap, the more twists you may need to put in it. Experiment with your tie-down straps and figure out how many twists works best on your vehicle with your tie-down method. Then just repeat your successful formula for each trip.

This twist works by disrupting the symmetry of the air flow over the strap. The wind will now flow around the strap without vibration inducing vortices. Problem solved!

No more strange looks, and you can create your own vibrations by singing along with your favorite music again.

(Click to enlarge)
Straight straps: Hum!		Half-twist straps: No hum

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The author, John Rich