Downwind foil speed: the bump is faster than the set

Crest speed vs set speed

Roughly 2 : 1 in deep water.

A 5-second bump

Its crest travels at about 28 km/h, or 2:08 per km.

Riding at 45°

About 1.4 × crest speed, on the same bump.

Where to sit

High on the front face, where the water is rising.

Foil choice

A frontwing that flies efficiently at the speed the bumps and your line demand.

You get onto a good bump, it takes off without you, and you're left pumping across flat water while the energy disappears downwind.

That's not a technique problem. It's how open-water swell moves, and it's useful once you know it.

Why does the bump keep leaving you behind?

Swell doesn't arrive one wave at a time. It travels in groups, the sets, and in deep water the crests inside a group move at about double the group's speed. Each crest is born at the back of the set, grows as it moves forward through it, and dies away at the front.

So the crest you catch was always going to leave. It's heading for the front of the set, and a fresh one is already building behind you.

So the useful thing to follow isn't a single wave. It's the set, and the crests it keeps making.

How fast are the bumps?

In deep water, wave speed depends only on the period, the seconds between one crest and the next. Longer period, faster wave.

Riding at 2:00 per km is 30 km/h, about the crest speed of a 5-second bump. Wind-driven bumps inside a bay mostly run between 3 and 6 seconds. That's why a bay run and an open-ocean run feel like different sports on the same wind.

Why should you never ride dead downwind?

Pointed straight down the swell, the swell's own speed is your ceiling. Turn across it and you can hold the same crest while travelling further and faster.

Holding a crest that moves at speed c while you ride at angle θ to its direction takes c ÷ cos θ:

• 30°: 1.15 × crest speed
• 45°: 1.41 × crest speed
• 60°: 2 × crest speed

Take a 5-second bump with its crest at about 28 km/h. On a 45° line you're doing about 40 km/h, roughly 1:31 per km, and the bump itself hasn't sped up at all.

The angle also puts you on better water. As a wave passes, the water on its front face is moving upward, which adds lift on top of the downhill slope. On the back face it's moving down. Going straight over the top of a bump means climbing and losing lift at the same time. Crossing its back at an angle costs much less.

How fast does your foil need to fly?

Put the two numbers together. The period gives you the crest speed, and your angle multiplies it. A 4-second bump ridden straight asks for about 22 km/h. A 6-second bump held at 45° asks for about 48.

Every frontwing has a band where it flies efficiently: low enough to stay up in the lulls, high enough to hold the crest without hanging on at its limit. More area and a high-lift section move that band down. A thinner, lower-drag section moves it up. The job is to choose the frontwing whose band sits around the speeds your water actually asks for.

Short, small bumps are the hardest case, because they move slowly and carry little energy. That's where a frontwing that glides well at low speed pays for itself. A longer fuselage and a gentler stabilizer extend that glide, and the build wizard keeps the connection right when you mix parts.

Where to start

The physics shows you where the energy is. Hours on the water teach you what to do with it.

Frontwings by aspect and size are on the frontwings page. For the lever behind the frontwing, see why the fuselage decides how your hydrofoil rides. For choosing a frontwing on more than area, see span, profile and area.