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
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)
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
Copyright © 1996 by Redwing Designs. All rights reserved.