The myth of length

It is often said that the speed of a kayak increases as the length increases. This is largely a myth.

If you look at the total resistance on a kayak hull, you will see that two phenomena dominate. Friction resistance is related directly to wetted surface area. Wave making resistance is related to waterline length (see hull speed). Note that these two things change differently with length.

For a given displacement, if you increase the length of the hull, you increase its wetted surface and hence its friction resistance. You also decrease the wavemaking resistance. Therefore, you reduce wavemaking at the expense of increasing friction. See Optimal length.

What does this mean?

If you are paddling at full speed all the time, wavemaking resistance dominates and using a longer kayak is beneficial. However, if you are an inherently slow paddler and prefer a more pedestrian pace, then increasing the kayak length will not make you faster. In fact, it will make your paddling less pleasant. The reason is that at lower speeds, friction is significant and using a longer kayak means fighting more friction to achieve the same speed.

Increasing the length of the kayak gives a kayak a higher potential top speed. However, you're going to work for that speed. If you don't want that extra work, a shorter kayak will be preferable. In fact, the shorter kayak will also provide better turning and handling than an equivalent longer kayak. Don't choose the longest kayak - choose the right length of kayak.