Are longer kayaks faster than shorter ones?

Question

I've been researching which kayak I'd like to buy, and I keep coming across the notion that longer kayaks are faster than shorter ones. I can understand that shorter kayaks have a greater tendency to alter course with each stroke, thus affecting your overall forward velocity, but if you correct for this in your calculations, it seems to me that a longer kayak would have more overall surface area than a shorter one, and would thus be slowed.

It is perhaps the case that this slowing of the longer kayak is not as pronounced as the loss of forward velocity experienced by the shorter kayak. Is there something else physically going on that affects the kayak's performance in the water that I'm not thinking of?

tl;dr: is there any truth to the fact that, all things being equal, a longer kayak will have a higher top speed for a given kayaker, and if so, why?

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Edit

Note that I ask about length specifically. In other words, two boats of identical design and type, with identical beam widths, identical hull materials, and the same pilot.

Answer 1

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.

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(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.

Answer 2

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.

Answer 3

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.

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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.

Answer 4

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.

Answer 5

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.

Answer 6

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.

Answer 7

What is hull speed? Hull speed is the maximum practical speed of a displacement (non-planing) hull. Since a wave's length is proportional to the square of its speed, the wave created by a moving hull will at some speed become longer than the hull's waterline. At this speed the stern of the craft will no longer be supported by any of the following wave crests. You will feel the stern squat into the trough following the bow wave. You will also notice that far greater paddling effort yields little increase in speed because to go faster you must now, in addition to the other forms of resistance, also work against gravity to climb out of the trough.

How is hull speed calculated? (or is it measured?) The formula for the speed of a water wave, 1.34 times the square root of the wavelength (in feet) equals the speed (in knots), is often used, by substituting the crafts waterline length for wave length, to calculate theoretical "hull speed". In reality many other factors including weight, slenderness, and the fullness of the bow and stern are also involved.

Since longer kayaks are potentially faster should I buy a longer kayak so I can paddle faster? The concept of hull speed often leads to the oversimplification "longer is faster". This has caused many paddlers and several designers to buy or build kayaks that are extremely long in order to be faster. However, because longer, like wider, also means more wetted surface, there will be a length where all the available power will be absorbed by the increased friction, before hull speed can be reached. Longer then becomes slower. Unfortunately, the longer kayak will also require more effort at all lower speeds because friction is present at all speeds. The speed advantage of a long waterline is only apparent at top speed. Extremes of length (and then only up to a point) benefit a racing kayaker and few others. If you can't reach hull speed (squat the stern) when paddling hard that kayak is probably too long and/or too wide for you. The above discussion only points out one of the disadvantages of buying a longer kayak, more work. There are several other disadvantages (other things being equal). It will weigh more and be more awkward to carry. It will require more storage room. But most importantly, in strong winds a longer kayak will be more difficult to handle (especially when not gear laden). This is due to the increased windage, the longer lever-arm offered the wind, and generally the slower turning speed of a longer kayak. Not being able to control your kayak in a strong wind could have disastrous consequences.

Answer 8

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!

Answer 9

I think this is wrong, I understand the concept in a very different way. As far as I know "hull speed" is a misnomer/doesn't apply to kayaks at all.

Long kayaks are not faster, short kayaks are faster, let me explain.

Kayak width is the determining factor for speed and efficiency, kayak length is just secondary to the way the kayak has to be built.

Manufacturers typically build their kayaks to hold ~250lbs; if you took a 10 foot kayak and gave it a 24" wide hull like you would find on a touring kayak it would only have a capacity of about 150lbs. The kayak simply has to be 14 foot long if you are going to get that typical target weight capacity of 250-300lbs.

A 22 inch wide kayak has to be 17 feet long to meet 250lbs A 20 inch wide kayak has to be 19 feet long to meet 250lbs capacity.

While certainly less significant than width, a boat that is too long has excess drag because of the excess wetted surface. All things being equal if you took a 100lb kayaker and put them in that 10ft long 24" wide kayak they would outperform an equal arm strength 200lb kayaker in a 14ft long 24" wide kayak because of the extra drag. Drag from wetted surface trumps any benefit you would get from longer length. On flat smooth water shorter kayaks that are tailored to rider weight are simply faster.