Executive summary: All else equal, a longer boat will have a greater hull speed. A greater hull speed means less drag and hence greater speed for a given amount of "paddling effort". Hence, longer kayaks can be said to be faster than shorter ones.

The bit you're missing is that boats make waves, and these waves can make it harder to go fast. Selectively cutting out some sentences from here:

A boat displaces its own weight in water. When the boat is moving, it must push that much water out of the way as it goes forward. Each boat creates a bow wave and a stern wave. When a boat reaches "hull speed" the bow and stern waves coincide to make one huge wave system. A heavy boat gets trapped in its own wave system.

Kayaks aren't heavy boats but they generally follow this rule, and generally follow the hull speed rules. If you look up those rules then you will see that the length of the waterline is the crucial factor with a longer waterline giving a higher hull speed. This is because the resistance offered by the standing wave is greater than the friction of a larger wet surface area. If you think about it this makes sense because a primary advantage of water transport is hugely reduced friction compared to ground transportation.

If you aren't convinced yet you can look at the experimental results and pretty charts found here. Their research showed that the longer kayak in their test set had the least drag, but the smallest boat at times was easier to paddle than the medium boat. So in practice a longer kayak will most likely be fastest, but ultimately a better designed but slightly shorter kayak might be faster.

(Just for fun)

Another thing to keep in mind is a longer kayak gives you more room for passengers. Of course not all passengers can fit.

posted over 7 years ago

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Erik‭

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Longer, narrower kayaks out perform shorter, wider kayaks in speed and long distance performance. One of the reasons being is that a longer kayak has more volume, ergo more buoyancy and floats higher. They glide over the surface of the water, cutting the wake in front of them like a knife. Shorter, low volume kayaks sit deeper in the water. They're great for white water and maneuverability, but if you try to take them into open water then they essentially plough through it vs. gliding along the top of it. Friction is not a noticeable factor as long as you maintain the hull of your craft and keep it smooth and waxed.

posted over 7 years ago

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ShemSeger‭

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There is a reason why there are different kayaks: the small ones (a.k.a., whitewater kayaks) are very turnable, and this is exactly what you want in a strong/fast moving current. If things get "white", you must be able to do things like turn on the spot in an instant, with not too much effort. Short, fat kayaks do that splendidly; it is harder to keep them from turning than to turn them. (And other things factor in as well, for example being able to "edge", to do rolls easily and quickly and so on.)

For sea kayaks, the opposite is true. You almost never want to turn in them; you can go wide arcs with no problem. But it is very important that you get from point A to B with minimal effort, as you can get into real, life-threatening danger if you run out of steam far from civilization with no real survival gear with you (think "eskimo in the ice"). Hence, those are designed to go forward very easily, and to resist turning with every stroke. (You also don't want to "edge" as there is not really a fast-changing current in play; and while you certainly want to be able to roll these kayaks as well, it does not matter if it takes you a few seconds longer.)

The physical reason is simple - the fast type is streamlined (more deep than wide, without pronounced edges anywhere). Making few waves or submerged vortices, slicing through the water easily. The turning type is anti-streamlined; i.e., designed to "sit" on the water with minimal submersion, with a wide body to facilitate that, and with special edges designed to catch quick-changing currents.

Hence, yes, if you want to go straight and far (and fast), then long = easier. If you want to be very mobile (turning a lot) and speed is of no concern, then short = easier.

posted over 7 years ago

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AnoE‭

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The classic maximum speed of a displacement vessel is associated with a Froude Number [velocity / sqrt(g*LWL) ] = 0.4.

But if you want velocity in knots and measure water line length in feet (with acceleration due to gravity = 32.2 ft/s²) then Vmax = 0.4 * sqrt(32.2) * 3600 sec/hour / 6076 ft/nautical mile * sqrt(LWL)

or Vmax = 1.34*sqrt(LWL)

The coefficient of 1.34 is the classic value, but values between 1.3 and 1.5 are given which represents a Froude number range of 0.38-0.45. The coefficient is not an approximation for √2 as claimed elsewhere. The reason for the drag increase is due to the interaction of the waves induced by the hull as mentioned by Erik. The reason for the range is because hull design can impact the induced waves.

A planing vessel is not subject to this limit. It is probably easier to plane a shorter craft, but planing by human power, while possible, is not sustainable over any distance.

The wave-induced drag can be overcome with a wave-piercing design. Length is your friend here as well. For a given beam (just enough to fit the paddler) a longer boat will have a smaller leading edge wedge angle and more wave-piercing capability.

None of this means the given paddler has the strength and stamina to reach that point where the wave induced drag spikes. As you state, the longer boat will have more surface area which is more skin friction. It will also tend to be heavier (given the same construction) and displace more water. At some point, the theoretical increase in speed with length is no longer practically achievable. The exact point is different for every paddler.

The other thing to consider is the type of water you will operate in. Kayaking a long boat in high waves will reduce the effective water line as the boat's bow and stern lift out of the water. Kayaking a long boat on flat open water is a dream. Paddling that boat in a quartering wind can be a huge frustration.

Length is the most important variable for straight line speed, but the right length for you may not be the longest you can find!

posted over 7 years ago

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jve‭

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As a general rule, yes

Consider a boat weighing 20kg, a paddler weighing 80kg.

You're now displacing 100litres of water, the way in which you displace that volume has a lot to do with how easily the boat will handle.

• If you choose to displace it as a wide cylinder on a vertical axis you will have high maneuverability and low forward speed.
• If you choose to displace as a narrow cylinder on a horizontal axis (aligned fore/aft) you will have high forward speed and low maneuverability.

Why?

Simply put: Drag

• The wide short boat is primarily suffering from form drag
• The long narrow boat is primarily affected by skin friction

Image from above linked wikipedia (drag) article

What this means in practice is that in a short wide boat, for example a playboat (Length: 168 cm Width: 64 cm Weight: 14 kg), you'll hit a low top speed where a wave has built up in front of and behind you (bow wave and wake) between which you're stuck, you can't break through the wave ahead, you can't break through someone else's wake to get ahead of them. Most of your energy is going into moving water around rather than moving yourself forward.

If you were in a long narrow boat, for example a wavehopper (Length: 400 cm Width: 60 cm Weight: 18 kg), you'd cruise past the playboat while doing a lot less work and barely noticing that you'd cut through the waves, its speed is limited almost entirely by how fast and efficiently you can move your paddle.

From this point on physics SE is probably your friend.

Everything is a compromise. For my purposes the polo boat is a perfect compromise, but then I play polo. In a straight line someone in a sea kayak or any given racing boat is going to be faster, straight lines are boring.

The real answer isn't in asking whether a longer boat is faster, it almost universally is, it's in asking what you want to do with your boat.

posted over 7 years ago

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Separatrix‭

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The narrower the beam, the less paddling resistance the boat will have.

For any given weight capacity, the longer boat will have a narrower beam. (The widest part of the boat, where the paddler sits) The benefits of a narrow beam far outweighs any additional surface drag that might occur from having a longer water line.

One of the ways this is observed is the lower amount of turbulence in the water behind a long, narrow boat. Water turbulence is just wasted paddling energy. Picture how racing shells are designed.

posted over 7 years ago

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Pooneil‭

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My understanding is that with any boat the maximum hull speed possible is equal to 1.4 X square root of the waterline length. This is for powered craft but as you are supplying the power it should still apply.

posted over 7 years ago

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charlesgill‭

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