What paddles better—length or volume?

Update: I join Rob Case on his podcast discussing this and more (YouTube)

A friend asked me to make a surfboard that paddles well but also duck dives. I offered to make a thin 8-foot board, but in his mind, short and thick was the winning combination. I ended up making him a thick fish. It was what he wanted, but it was not what I was tasked with.

So I started asking around: What do surfers think is more important for good paddling–length or volume? Turns out there’s no majority consensus.

It’s obviously length–or is it?

Boatbuilding theory (related to hull speed) would suggest that a longer vessel glides through water more efficiently. In the case of surfboards, this principle might not transfer directly. Unlike boats, a lot of the payload (our body) is hanging off the vessel. Also, paddling speeds might be slow enough that hull speed principles won’t matter.

Additionally, just from observing surfers you might think longboards always paddle better than shortboards. But longboards always have more volume. So which variable is contributing more?

Most of the surfer's body is submerged

So I decided to put this question to the test. I made the same board in three lengths: 6-foot, 7-foot, and 8-foot, each with the same volume of 42.5 liters. All other variables–outline, rocker, bottom contours and rails–were kept constant with the exception of thickness, so that I could have the same volume despite the boards having different lengths.

If paddle power is all about volume, the performance of each board would be the same. If length matters more, they would perform differently.

All three boards have 42.5L volume

What does better paddling even mean?

I teamed up with professional paddle coach Rob Case to design and conduct this paddle study in a carefully controlled environment.

We tried to specify what it would mean for a board to paddle well. There’s the paddle out and the paddling for position in the lineup, and there’s also paddling to catch a wave. Positioning is the most important thing for wave catching, as Rob has demonstrated previously, because we can never paddle fast enough to match the speed of the wave.

For all other purposes, paddling well means using fewer strokes to cover the same distance–in other words, it’s about efficiency. Rob’s teachings show that you can increase efficiency by training your paddling technique (www.surfingpaddling.com).

To determine paddling efficiency, we decided to measure the number of strokes per minute at a constant speed. Rob has an Endless Pool–a kind of treadmill for swimmers–which allowed us to create a controlled environment with a constant speed of water flow.

Guinea pigs in the pool

We recruited 21 surfers and asked them to paddle each board for a minute in the

Endless Pool, with a 5- to 10-minute break in between sessions, so that we could count total strokes. The water flow speed was set to the maximum speed of 3.6 mph (1:02 per 100 meters). On a surfboard, this pace was challenging but not overpowering.

Testers gave differing feedback immediately following the session, before we had compiled and analyzed all the data. Some thought the 8-foot board was much more efficient, while others had the best impression of the 7-foot board. Everyone agreed that the 7-footer was easier to paddle than the 6-footer.

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Longer boards paddle more efficiently

The data tells a pretty clear story. Across the board, increasing board length meant an increase in efficiency (fewer arm strokes per second). Equivalently, more efficient paddling means more distance covered per stroke.

Analyzing the data by relative efficiency change, we can see that, on average, the 7-foot board paddled about 5% more efficiently than the 6-footer, while the 8-foot board was about 5% more efficient than the 7-footer. Increasing length by about 15% gives about 5% more paddle efficiency as measured in stroke rate. This suggests that good paddle performance is directly related to board length.

What about speed?

In order to get in optimal position and get closer to the speed of the wave, we often do a quick sprint paddle. How much better would a longer board fare if the volume stays constant?

We had access to a flat body of water with two markers 9.7 meters (10.5 yards) apart. We selected this distance knowing that true sprint paddling rarely goes over longer distances than that. We asked the same 21 surfers to paddle between the markers as quickly as they could, accelerating from a dead stop and blowing past the finish line. Starting from a dead stop comes closer to the turn-and-burn situation surfers encounter in the lineup. For our experiment, this meant that our speed measurements included acceleration as well. Each surfer had two trials and their time was averaged across those two trials.

On average, length is beneficial in this setting as well. Going from 6 feet to 7 feet brought a 5% improvement for our test cohort, while the average difference for adding another foot was only 2.5%.

Volume surely has to matter though

While summarizing our experiment, we found that researchers at Cal State San Marcos performed a similar experiment in the Endless Pool with same-length boards that varied in thickness, i.e. volume. They found that stroke rate was constant, independent of volume.

However, they measured exertion via heart rate and oxygen consumption. While stroke rate remained constant, higher volume boards required less physiological energy expenditure.

Paddle speed is a combination of distance per stroke and stroke per minute. If speed and stroke rate stay constant, what could those volume-dependent exertion variations be explained by? It could be due to reduced drag forces. As the higher volume board lifts the surfer further out of the water, there is less drag and one paddle stroke will require less energy expenditure.

Our study suggests that the forward-motion efficiency of that one stroke then depends on board length. The effects of volume and length could be thought of as independent contributions to paddle performance.

Let’s take a bicycle as an analogy. How far one turn of the crank will get you depends on what gear you are in or how big your wheels are. To go at a constant speed requires fewer revolutions on a larger wheel than on a smaller wheel.

If you kept the wheel size constant but deflated the tires, you would still need the same revolutions to get to the same speed but you would have to work harder because of increased friction.

Getting a longer board is like getting larger wheels while increasing board volume is like inflating your tires.

Foam is your friend–but spread it out!

For me, it’s settled: If you ask me to make you a board that paddles well, I’ll give you a longer board with ample volume. If you want a board that paddles well and also duck dives, I’ll make it thinner. But at the end of the day the customer is always right–if you want a thick fish, that’s what I’ll make.

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Thank you!

A huge thank you to all the study participants who graciously donated their time!

Thank you to Caroline Belkoura and Bonnie Tsui for reviewing and editing this piece.