Speaking Good Boat, Part I - by John Winters

How long is a long kayak? How wide is a wide kayak? How fast is a fast kayak? Who cares? If kayak symposium conversation is an indication, kayak paddlers care. Nevertheless, kayak paddlers persist in using hopelessly inadequate terminology to describe kayak characteristics. "Fast", "not fast", "wide", "narrow", "stable", "tippy" are but a few 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 kayaks. 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 kayak 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 brethren and reveal hitherto inviolate secrets. Master this terminology and you too can speak "Kayak" 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 kayak performance. For kayaks, this is usually a lot less than the overall length. Some traditional designs have as much as two feet of skinny kayak hanging out over the ocean. It looks pretty but it doesn't do much. people who speak good kayak always say waterline length.

That's a good start but waterline length doesn't tell us the whole story. We also need to know how fine the kayak 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 kayak and its contents). Why displacement? Because kayak resistance is heavily influenced by how easily water is pushed apart and drawn back in to fill the hole left by the passing kayak. The longer the kayak is 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.

There is something neat about this equation. It doesn't matter what set of units you use (metric, English, or Biblical). So long as the units are consistent the number is always the same. Such formulas are called non-dimensional formulas and are useful in a world that cannot agree on how long anything should be or even how to measure it.

The number is called the fatness ratio and ranges from .63 for long light kayaks to 1.8 for short heavy ones. The average loaded touring kayak is around 1.3 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 waterline beam. So, is a 24" kayak wide? Well, yes, if it is only 12' long. On the other hand, 24" isn't very wide at all for 17' long kayak. What we want then is another ratio and this time it's 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 kayaks designed for the terminally frightened parent. The higher the number the narrower the kayak.

Something we hear a lot from builders is how low the wetted surface is on their kayaks. 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 the cube root of volume displacement squared. All the mathematical manipulation is to provide a non dimensional number. The formula looks like this;

where S is the surface area and  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 kayaks. Here I will step on some toes. The terms are meaningless. What counts is the designed displacement or, how much weight the kayak 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 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 bi-lingual.

Copyright © 1996 by Redwing Designs. All rights reserved.