How long is a long kayak? How wide is a wide kayak? How fast is a fast one? Who cares?
If kayak symposium conversation is an indication, paddlers care. Nevertheless, paddlers persist in using hopelessly inadequate terminology to describe boat characteristics. "Fast", "not fast", "wide", "narrow", "stable", "tippy" are but a few of the terms that have nebulous or different meanings to different people. Naval architects long ago recognized that something better was needed, and devised terminology uniquely suited for talking about boats. Unfortunately, they kept it to themselves. While the uninitiated were mumbling around in an indefinite verbal haze, naval architects were nattering about coefficients and dimensional ratios. Most canoe and kayak designers avoided the issue by implying that small boat design was an arcane art form understood by only a few and unintelligible to mere mortals.
So at the risk of being drummed out of the Society of Highly Secretive and Mystical Kayak Designers, I will break ranks with my brethern and reveal hitherto inviolate secrets. Master this terminology and you too can speak "Boat" with the most erudite of paddling sophists. In fact, you might become better at it than some who came by their memberships under dubious circumstances.
Let us start with: How long is "long"? Builders like to use overall length because it sounds better to people who want lots for their money. The length that counts is the waterline length because it is the prime factor in boat performance. For kayaks, this is usually a lot less than the overall length. Some traditional designs have as much as two feet of skinny boat hanging out over the ocean. It looks pretty, but it doesn't do much. People who speak good "Boat" always say waterline length.
That's a good start, but waterline length doesn't tell the whole story. We also need to know how fine the boat is. For this we need a ratio, and the important ratio is between the waterline length and the displacement. (Displacement is the total weight of the boat and its contents.) Why displacement? Because boat resistance is heavily influenced by how easily water is pushed apart and drawn back to fill the hole left by the passing boat. The longer the boat relative to its displacement, the easier it will be to drive through the water.
To give us a nice neat number for comparative purposes, naval architects divide the volume of displacement by the length (on the water, remember?) cubed, i.e. V/(0.1L)^3. There is something neat about this ratio. It doesn't matter what units you use (metric, imperial, or Biblical) . So long as the units are consistent, the number is always the same. Such formulas are are called non-dimensional, and are useful in a world that cannot agree on how long anything should be and how to measure it.
This number is called the fatness ratio and ranges from 0.63 for long, light boats to 1.8 for short, heavy ones. The average loaded touring kayak has a fatness ratio around 1.4 to 1.4.
Now, how wide is "wide"? For this we abandon the usual maximum beam measurement for the same reason we abandoned overall length. The important part is in the water so the critical measurement is the waterline beam. So, is a 24 inch kayak wide? Well, yes, if it is only 12 feet long. On the other hand, 24 inches isn't very wide at all for a 17 foot long boat. What we want then is another ratio, and this time its the ratio between length and beam, or L/B.
The typical range for kayaks is about 11.0 for sprint racing kayaks, to 6.0 for the stubby little boats designed for the terminally frightened parent. The higher the number, the narrower the boat.
Something we hear a lot from builders is how low the wetted surface is on their boats. This is important because low wetted surface means low resistance. But how low is "low"? Here the ratio is the wetted surface area divided by cube root of the volume of displacement squared. All the mathematical manipulation is to provide a non-dimensional number. The formula looks like this: S/V^2/3 where S is the surface area and V is the volume of displacement. A low wetted surface ratio is around 8.0: a high one is 9.5.
We hear a lot about high volume and low volume boats. Here I will step on some toes. The terms are meaningless. What counts is the designed displacement, or how much weight the boat was designed to carry. A properly designed kayak will have enough volume to carry the people and gear without emulating a submarine. Having more is no advantage and having less is poor design. Unless one is partial to squirt boats, it is difficult to design a sea kayak that won't have enough room for more gear than any well-heeled paddler should own.
Is there a magic number for all this? No, there isn't. All one needs to know is the designed displacement. If it fits you and your gear then it's right. If it doesn't, it isn't. so the magic words are "designed displacement", not "high or low volume". Hand-in-hand with volume is "depth". Kayaks with a lot of depth are supposed to be high volume and those with less are supposed to be low volume. Unfortunately, depth is a poor measure of internal volume. The cross-sectional shape of the deck (elliptical, pyramidal, hyperbolic, or parabolic) has a greater influence on volume. In fact, depth doesn't tell much about anything. The nice thing about this is that you don't have to know any new words.
So, master the words and formulas and you are well on your way to being bilingual.