Created : - - 1996
Last updated : 5 januari 2002
Classification : general publication
Copyright : restricted freeware
Status : Figures need to be recreated
Comments : An article exmplaining the right us of the Froude law for maximum hullspeed
max. hull speed = 1,4 * sq. rt. (hull length)
giving maximum theoretical hull speed should be viewed. Even though many yachts are limited in speed by this law, many modern lightweight high performance boats are not. Or at least far less than the yachts are. This is often overlooked. This article explains why.
The article was found at : http://www.qcckayaks.com/resources/speakboat2.asp and is confirmed by the writings of Frank Bethwaite in his book "High Performance Sailing". Both Frank and his son Julian are well known in dinghy and skiff sailing part of the sailing world and they has gethered alot of knowlegde from their projects in the Australian 18 ft. skiff class. The Bethwaite page can be found at :
http://www.bethwaite.com/bethwaite/index1.htm
"Hull Speed", like prismatic coefficient, is a much loved phrase by pundits and paddling "experts". "This boat 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 boat - 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 boat speed increases so do the wave lengths created and at some point the length of the bow wave will match the length of the boat 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 boats make big waves and light boats make small ones. Additionally, the longer the boat, the faster it can go before the two waves coincide. Hence the common wisdom that long boats are "faster" than short boats which is perfectly true to a point. The "point" is that small light boats make such small waves that they are easily driven beyond "hull speed" and long light boats 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 boat has less wetted surface than longer version it is apparent that there are times when a shorter boat 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 boat 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 boat does start to level off? Isn't that planing? Regrettably, not always. For a boat 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 boat 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 S/L 2.0.
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 boat length are the same and that it doesn't have any real significance for boats 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.
Graciously compiled by Steve Roberts