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The lowdown on buoyancy in a wave pool

Written by Chris Klein

There is so much theory tossed back and forth about why surfing in freshwater feels so different than it does in salted waves. We did some digging to mythbust Fresh vs. Salt buoyancy rumors and answer some basic questions.

Guess what we found out? Yes, salt water is more buoyant than freshwater. But it’s a bit more complicated than that. Did you factor in temperature and salt density?

Editor’s Note: This article originally appeared on WavePoolMag on December 23, 2019. We are reposting it as we’ve received more inquiries regarding this topic.

Thank you Canada. Here’s a one-minute visual on the density differences between cold water, hot water and saltwater

Saltwater has more molecules, so it’s denser

One cubic foot of saltwater and freshwater do not weigh the same. The saltwater sample would be 64.1 pounds while freshwater a slightly less 62.4 pounds. Why? Because of the added weight of the extra salt.

Water loves to bond with salt. The h20 molecules cluster around the salt molecules, and the result is that saltwater has more molecules overall than freshwater. When you’ve added more weight to that cubic foot of water (the salt), you are producing a denser type of water. More density means you will float higher up – but more (but more on that in a bit).

The world’s oceans are about five percent salt. The Dead Sea, however, is roughly 25 percent salt. Consequently, the Dead Sea water is denser than your average seawater. You can float on your back in the super-salty brine of the Dead Sea with very little effort.

Another example is cargo ships. When a boat stocked with containers moves from saltwater to freshwater inlets and upriver, it will sit lower in the water. Captains have to factor this to compensate their cargo loads for such changes in buoyancy.

A simple rule of thumb is: More salt equals more buoyancy for surfers and surfboards.

Koby Abberton and Eneko Acero had to rubber up for their mid-winter Wavegarden session. The result? The cold water is denser (and therefore provided more buoyancy.) But this was offset by the weight in the surfers’ heavy neoprene.

Cold vs warm water: Density!

Densities of salted and unsalted water change depending on just how much salt or sediment is in the water you’re surfing. We’ve heard big-wave chargers say a cold water wipeout at Maverick’s packs more of a punch than a warm water beating at Waimea Bay.

Water molecules speed up and spread out when heated. Their expansion means they occupy a larger area, and therefore warm water is less dense. Cold water on the other hand, with slower and less expansive molecular movement, is denser. Water reaches its maximum density at around 40 degrees Fahrenheit.

The denser a liquid is, the higher you will float in it. For example, you will float more in an über dense fluid like liquid mercury, than you would in plain tap water. Although contrary to how it would seem, this means that cold saltwater is more buoyant than warm salt water.

Tropical waters tend to have more salt content than cold water regions so they would be more buoyant than cold water strictly due to salt content. This, despite the mass-density question. Yes. Salt content is a bigger factor in buoyancy than temperature.

Another factor is that if you’re surfing cold water you’ll be wearing some rubber. Surfing wetsuits will give you more buoyancy depending on their thickness. For example, a 5/4 hooded fullsuit will allow you to bob around on the surface while a 2/2 spring will not give you much flotation at all.

But remember, if the wave is good enough and you’re going fast enough, floatation doesn’t really factor in, as proven by Lucas Fink at Kelly’s wave

What do I do now? Go surfing

Many pros adjust their quivers for wave pool sessions. These pros turn to Expanded Polystyrene (EPS) construction in their surfboards more often than Polyurethane (PU) because EPS is lighter in weight, which gives the board more buoyancy.

EPS also has a different flex and feel. While the lighter weight tends to chatter in choppy surf, wave pools don’t suffer afternoon onshore winds. EPS is often described as being springy and twitchy.

Don’t forget to consider whether you’ll be adding weight (and thereby decreasing your buoyancy) by wearing a wetsuit. Boardshorts are blissful as you don’t need to bust out the calculator when dialing in quiver sizes and board constructions.

So while the wave settings at your local pool will vary, buoyancy characteristics of freshwater when compared to saltwater, will remain the same.

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