|MadSci Network: Physics|
That's a good question, and there's the simple answer, and the more complicated answer for it.
Short answer: The world speed record for skis is 250.7 km/hr. The world speed record for a snowboard is 201.9 km/hr. I'm assuming that the record holders had something close to optimum conditions and premium equipment, so skis are just faster.
The complicated answer is... well, more complicated. Let's break down the individual factors:
1) Pressure. Pressure is indeed greater when gliding on an edge than when gliding flat. However, there is an optimum amount of pressure that would make you go the fastest. You only need just enough to melt a layer of water between your ski/board and the snow. More pressure than that will increase the friction, and slow you down. Way too much pressure will dig down into the snow and slow you down. Cleats or spikes in your shoes will concentrate all of your weight onto a few pressure spots, but this improves traction. Advantage: It depends.
2) Direction. To get your best speed, you'll point your board/skis straight downhill, lean forward, and go for it. At this point, you are gliding flat. So all the calculations and math you did to design your board based on edge contact go out the window. Typically, snowboarders can't stand like this for too long, because they are unstable. They will have to carve using the edge of their board to be able to realign the board with the direction that they are going. Skiers do the same, but with two skis, they don't have to carve as much to maintain direction and balance. So skiers can go straight downhill, while boarders have to slalom more. Advantage: skiers.
3) Snow Type. Skiers are more stable on powder than ice, but can go dangerously on ice. Boarders are also more stable on powder than ice, but will crash very quickly if they try to go 250 km/hr on ice. Advantage: skiers.
4) Length/shape. Skis are typically longer than boards. This is because they are separated and anchored to the foot more securely than with board bindings. Also, skiers can use their other foot and their two poles to gain leverage to move/twist/rotate/tilt one ski. Boarders, since both feet are anchored to the board, must use their balance, inertia, and the mass of the rest of their bodies to adjust the board position. Simply put, a snowboarder with a board the length of your average ski will not have the necessary steering control, and will probably crash. Longer skis typically go faster than short skis, so if you guesstimate the width of a board as equal to the width of both skis put side by side, skis will go faster than a board because they're longer. Advantage skiers. (By the way, the idea that long skis go faster than short ones is a good example of why pressure cannot stand alone as a determining factor.)
I hope this helps, and I hope you wear a helmet on the slopes. (Especially if you're going for the 250 km/hr record out there.)
European Space Agency - Space technology on the slopes
History of the Snowboard
ABCs of snowboarding - Snow types
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