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SUP, the Venturi effect and what this means for wind (and water) flow.

During the past few weeks we’ve been talking about wind and typical SUP scenarios where breeze plays its part – for better or worse. The wind is perhaps one of the most prohibiting factors for stand up paddle boarding. But not all situations are obvious – hence these articles. Some situations with wind in the mix could also present far greater SUP safety risks than just putting the kybosh on your paddling fun.

For this installment, we’re talking about the Venturi effect and its influence on wind and water flow.

What’s the Venturi effect?

In layman’s terms, the Venturi effect is where a liquid or gas comes in contact with a narrowing. If you think of an hourglass the constricted section could be replicated by a mountain valley or where two landmasses come closer together with water at the base.

Wind and water flow can both be squeezed through this funnel and thereby increase their speed. This squeeze can greatly change what you feel on the water and in the air.

If you want a scientific explanation of the Venturi effect then hit this link –

https://en.wikipedia.org/wiki/Venturi_effect

The Venturi effect on wind and SUP.

You may have all the weather forecast information you need for your SUP adventures ahead. The planets have aligned and it’s all looking peachy. Winds are light, the sun’s out and you’re good to go. Coming back to something else we talk about often is interpreting this data for your chosen paddling area. Knowing your SUP area is half the battle…

Setting off it’s all going swimmingly until you round the next bend. Suddenly, seemingly out of nowhere, you’re hit by a strong gust of wind which is quickly followed by another. And so on. What was idyllic paddling has now turned into an arduous slog straight in the wind. But why?

Even with light breeze a squeeze can increase the wind strength depending on how much of a funnel there is. In the described instance the direction of wind is most likely being forced between two landmasses thereby giving it more power. The gradient, true wind, may still be light and perfect for SUP. But in the area you’re paddling the scenario is much different.

The below diagram shows what’s being described.

A deep valley could accelerate the wind if it’s flowing directly between the two land masses. Even if the gradient wind, high up, remains light. Water flow can also be squeezed.

And for even more of a visual example check out this video featuring river windsurfing under a bridge close to Vienna. Here, riders rely on wind being squeezed between the bridge pylons to create enough puff to sail. Go too far down or upwind and there’s nothing!

Water flow.

The Venturi effect isn’t just limited to wind. It can manipulate water flow also – both inland and coastal. Anywhere there’s a squeeze water flow will increase. Rivers, estuaries and so on are all at the mercy.

With water flow, there are also other factors such as tide. If the tidal flow is running in the same direction as the squeeze then you’re getting even more oomph and will travel even faster across the water’s surface. It may eventually slow down but only when you’ve exited the bottleneck.

The squeeze here can happen around the estuary mouth with water flow increasing in both directions, depending on the tide. Tidal flow will also exacerbate the squeeze even further.

With this in mind you can appreciate how Venturi effects such as the one described above could help you cover more ground when paddling. As long as you’re aware! If not, then this type of situation can also be extremely hazardous.

Know before you go and interpret.

Yep, it’s that word again: interpret. Being able to interpret the information you glean before setting off on your paddles is so important. Having a good understanding of weather, wind, tides and so on is vital. Then applying that info to your chosen area will keep you safe and make sure everything remains fun.

Don’t forget to check out the previous articles we’ve published about wind in this series via the links below.

The effects of wind and tide on stand up paddle boarding (a simple guide).

Common wind scenarios and how they can affect stand up paddle boarding.

Interpreting weather for your chosen SUP spot.

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