Speaking Good Boat, Part II (Kayak Hull Speed and Beyond) - by John Winters
"Kayak Hull Speed", like prismatic coefficient, is a much loved
phrase by pundits and paddling "experts". "This kayak has a
high hull speed." (few ever have a "low" hull speed) or
"We were paddling at hull speed." are commonly used to imply that
"hull speed" is a limit to displacement speeds and bloody fast at
that. The more daring suggest that planning lies just the other side of the
magic number. You know better, or will when you finish reading this.
The great pioneer of hydrodynamics, William Froude, coined the phrase
"hull speed" when he discovered that extraordinary amounts of
power were needed to propel the ships he was testing any faster than
in knots. It was, for him, a practical but not an absolute limit. The speed
corresponds to the speed of a wave having the same length as the effective
waterline length of the hull. To see why the resistance grew so rapidly we
must first know that there are two major types of waves formed by a kayak -
transverse and diagonal. We can ignore the diagonal waves that have only
minor impact on resistance and concentrate on the transverse system. Figure
1 shows the wave systems as viewed from above. At this point things get a
bit more complicated because a transverse waves are created at the bow and
at the stern. As kayak speed increases so do the wave lengths created and at
some point the length of the bow wave will match the length of the kayak and
its crest will coincide with the first crest of the stern wave. When two
waves coincide in this manner their heights are additive as shown in Figure
2 and resistance increases accordingly. Since wave size is a function of
displacement, heavy kayaks make big waves and light kayaks make small ones.
Additionally, the longer the kayak, the faster it can go before the two waves
coincide. Hence the common wisdom that long kayaks are "faster"
than short kayaks which is perfectly true to a point. The "point"
is that small light kayaks make such small waves that they are easily driven
beyond "hull speed" and long light kayaks have higher wetted
surface that offsets the reduction in wave making resistance.
You will recall that the bow wave lengthens with increased speed. Suppose
you have enough power to get the bow wave crest aft of the stern. If you
can, an interesting thing happens. The trough of the bow wave coincides with
the crest of the stern wave and begins to cancel it out as in Figure 3. The
result is reduced wave making resistance. Once past "hull speed"
wave making resistance increases very slowly and, can even drop while
frictional resistance continues to increase. Since a shorter kayak has less
wetted surface than longer version it is apparent that there are times when
a shorter kayak is faster.
An interesting phenomenon is the change in trim as speed increases. As
the trough of the bow wave moves aft, the stern sinks into the hole and the
bow rises. Some writers have said that it is this "hill" of water
that the kayak must climb and attribute the "hill" to the increased
resistance. A little common sense will clear this up. How do you climb a
wave that is being constantly created by the bow? As fast as you climb it a
new one is being created in front of you. One can just as easily lift
oneself by his own boot straps. Some eighty years ago Admiral Taylor the
great naval architect explained that the change of trim, was a symptom of
speed, not an obstacle. Few kayak designers have read Taylor's classic text
book on naval architecture, "Speed and Power of Ships" and can be
forgiven for not knowing this important fact.
So what happens if the kayak does start to level off? Isn't that planing?
Regrettably, not always. For a kayak to plane its center of gravity must lift
bodily from the effect of dynamic forces on the bottom. It takes an enormous
amount of power to do this (Imagine lifting a weight equal to yourself and
the kayak and then imagine how difficult it is to do it by paddling!). No one
has yet demonstrated planing in a canoe or kayak despite the claims. Any
reduction in resistance at high speeds is due to wave cancellation and not
wave size reduction due to the reduced displacement that accompanies planing.
So, why did Froude screw things up with his "hull Speed"
business? Well, he didn't. At least not for people who read the fine print.
What Froude said was that wavemaking resistance increased rapidly as hull
speed was approached. He did not say that hull speed was the limit to
displacement speeds. He just didn't have the power or light construction we
have today to make it an issue (nor was he much concerned about the
resistance of native kayaks) Today modern ships, kayaks, and canoes are
light enough or have enough power to easily surpass hull speed. In fact, we
regularly test sea kayaks at S/L 1.5 and sprint kayaks and canoes can top
So how should we use the term "hull speed" when speaking Boat?
That's easy. We shouldn't. And, when others do, just point out that
"hull speed" is a term of convenience referring to the speed at
which the bow wave length and kayak length are the same and that it doesn't
have any real significance for kayaks of low displacement length ratios like
kayaks. It should be enough to establish yourself as an expert in nine out
of ten kayaking conversations.
Copyright © 1996 by Redwing Designs. All rights reserved.