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Last edited 09/2013                        

Kayak Speed Basics, the Twinhull Advantages, and the Principles of W Kayak Design


How Fast Is The W500 Kayak?

Before you attempt to read this 5,000 word technical article, please note that the answer to the simple question “How fast is the 11’4″ long W500 kayak?” is this:
-”The W500 kayak is faster than any fishing kayak that’s shorter than 15′ , and it's as fast as any touring kayak that’s 13′ long“. 
The difference stems from the fact that touring kayaks are faster than fishing kayaks.
These numbers were compiled from data gathered in real world tests and observations made by numerous W kayakers since 2009, when the W500 series was introduced.
This article explains the technical principles that enable this performance.

Abstract

Anglers need their boats to be stable, and that includes most people who fish from kayaks – although some of them may not openly admit it.
The phrase ‘fast kayak’ evokes the image of a long and slender hull, and most people sense that a kayak can be either fast or stable – never both, which does not prevent many kayak manufacturers from ignoring this basic trade-off in monohull design and claiming that their fishing kayaks are both stable and fast…
We maintain that a fishing kayak should be stable enough to allow its user to paddle and cast in full confidence while standing in it, and we’re able to prove that our W fishing kayaks largely surpass anything that was imaginable so far when stability is concerned (watch our demo videos)
Our 11’4″ long W500 kayak is reported to be as fast as a 13′ long touring kayak, which may appear to be a contradiction to those who are not familiar with naval design, especially with the hydrodynamic science of it, or with recent years’ speed achievements of multi-hulled (I.E. catamarans and trimarans) sailing and power boats.
The purpose of this article is to present the principles and advantages of the W boat concept in the context of its application in the design of small paddle crafts such as canoes and kayaks.  It discusses the main points in the hydrodynamics and hydrostatics of twinhull kayaks of the W type, as well as ergonomic and biomechanic considerations.
More technical information is available in our U.S. utility patent No. 6,871,608

What Makes a Kayak Faster?

Statistically, multihulls are faster than monohulls.  Their higher stability helps to increase their seaworthiness, but there are other factors that contribute to creating this advantage, including the reduced wetted beam whose benefit can exceed the loss resulting from higher skin friction.
When human powered boats are considered, ergonomics and biomechanic factors play a crucial role in determining real life performance including speed.
Generally speaking, the speed of a boat is the result of the power propelling it forward (effective propulsion) and the resistance of the water to this effort.
You can generate power with a motor, a sail or the human body.
The displacement of a boat creates many types of resistance, all of which except Frictional Resistance (‘skin friction’) are included in the term ‘Residual Resistance’ (RR).
The faster the boat goes the more the Residual Resistance becomes the main problem to overcome.

The Froude Number and the Practical Meaning of ‘Hull Speed’

In order to understand this complex subject we must first present it a simplified form: The main effort in overcoming Residual resistance consists of
1. ‘Pushing’ water up and aside from the bow, and
2. ‘Pulling’ the boat away from the water behind the stern, that is overcoming a ‘suction’ effect.
A longer boat (longer waterline) will keep the water from filling back that space for a longer time.  This means that a long boat could go faster than a shorter boat before that significant increase in residual resistance occurs.  When this happens a big wave can be seen coming from the stern, and a second big wave is formed at the bow, and from that moment on the boat seems to be moving between the crests of these two waves.
William Froude showed that the speed of waves in knots = 1.34 x L^1/2  where L is the boat’s length in feet.
Froude discovered that as the boat’s speed increases the number of waves along the hull decreases until the boat moves between a big wave at the bow and a big wave at the stern.  From this point increasing the boat’s speed becomes much more difficult, or in other words the boat reached its ‘Hull Speed’.
A boat 100% longer than another will have a nominal hull speed that’s about 42% higher (0.42 linear correlation).  For example:  the hull speed of a 20 ft boat is 6 knots and that of a 10 ft boat is 4.23 knots.
However, the longer boat could generate 100% more skin friction (Fr) and consequently moving it at its higher hull speed will require adding more than 42% in power.

Hull speed is just another term taken into consideration in the process of designing a boat, and taken out of a broader context it is meaningless:  If you made your house watertight and put it in the water it would have a higher hull speed than the world’s fastest paddle sports boat just because it is longer… It does not mean the house would actually be a fast vessel.
Hull speed is by no means a final limitation on speed, and it’s very common for boats, including human powered ones to go faster than their hull speed.

Different Strategies for Increasing Boat Speed

1.    Add power:  With a strong engine and a big budget for fuel you don’t have to worry too much about the energy spent on going faster than your ‘hull speed’.  The same goes for a stable sailing boat with lots of sail power.
If you want to add power to a human powered boat you need to find a way to add more groups of muscles to the propulsion effort by offering the user/s a better posture i.e. biomechanic improvements, and/or means to reduce discomfort and fatigue i.e. ergonomic improvements.
2.    Add length:  That’s applying a ‘delaying’ strategy – You delay the occurrence of the steep increase in residual resistance by paying in increased frictional resistance that you get from having a longer hull.  This strategy is good as long as you have the additional power needed to overcome the additional friction.   Another problem you’d have to deal with is a decrease in your boat’s maneuverability, which is more of a problem in human powered boats where the additional power needed for maneuvering is taken away from propulsion.
3.    Reduce residual resistance:  A good strategy for a human powered boat with only human muscles for propulsion.  Very thin racing canoes and kayaks generate relatively little residual resistance even after when they go at speeds that are higher than their hull speed -  This is why they create relatively small waves.
4.    Reduce hull surface:  This is pretty obvious, since it reduces frictional resistance. However, some kayak designers seem to be either unaware of this strategy or ignore it by adding rather useless tunnels, skegs and scuppers in their kayaks' hulls, and by outfitting them with various types of outriggers that add little real-world stability (secondary stability) while increasing both frictional resistance and residual resistance.

The boat’s ‘fineness’, often described by its Length to Beam ratio (L/B) at waterline is most useful for predicting its speed:  An ICF K1 racing kayak has an L/B of 11:1.   This kind of boats have low displacement and are very ‘fine’, which makes it possible to paddle them at up to twice their hull speed.

Speed in Human Powered Boats, Including Kayaks

Adding power for propulsion is not always practical in canoes and kayaks. However, it’s good to keep in mind that a boat offering a better paddling position, improved stability and control, and the comfort of being able to reduce fatigue and prevent injury by changing positions adds to the paddler’s effective propulsion and therefore may achieve and sustain higher speed.
The Comfort factor and the ability to sustain the physical effort over a longer period of time with less fatigue and no injury pertains to Ergonomics, and the effective power available per paddle stroke pertains to biomechanics

Making the kayak longer is good for as long as increasing surface area does not end up in slowing you down.
Reducing Residual resistance is severely limited by the width of the person sitting in the boat but why sit inside the hull?…
-Rowing shells are faster than racing kayaks not only due to their great length, but also due to the fact the rower sits on top a hull that’s narrower than his waist – A rowing shell’s L/B is much higher than that of any racing kayak.

Displacement/Length (D/L)

“High speeds for canoes are only made possible through their having excellent Displacement/Length ratios and narrow beams. The two combine to produce very small waves which are the major resistance at speeds above S/L 1.34.”
-John Winters, “The Shape of the Canoe”

The smaller the D/L the faster the boat-

  • For a W kayak and a canoe or traditional (monohull) kayak of the same volume, with the canoe or kayak being twice longer than the W boat, the Displacement/Length for each of the W boat hulls and the canoe/kayak is the same.
  • For a W kayak and canoe or monohull kayak of the same volume and length, the Displacement/Length for each of the W boat hulls is 1/2 that of the canoe or kayak.

-But the W kayak has a more important advantage:

The Decisive Gain From Reducing the Wet Beam (Waterline Hull Width  )

Residual Resistance is a complex phenomenon affected by a number of variables of which the wet beam is the greatest factor.  A popular article on canoe [and kayak] design offers a simplified formula that closely approximates experimental results according to which Residual Resistance (Rr) varies as the square of the Beam (B) and the first power of Length (L): Rr = B^2L.
Consider the following:  A molecule of water pushed by the bow will follow the path of least resistance until it is out of the hull’s way.  In this course it will push other molecules that have been pushed aside before, and those molecules will push others that were pushed before, and so on.
In addition, thin hulls are generally more streamlined than wide ones: They have a more gradual adverse pressure gradient and enable delaying flow separation thus reducing drag from the bow wake, which is especially important at speeds higher than hull speed.
Rr is also affected by negative pressure exerted of curved objects moving in fluid (Bernoulli Effect)- The higher the curvature and speed the higher the negative pressure (drag) -A wider beam means a higher curvature in the horizontal plane.
See: “ON THE SUBJECT OF HIGH SPEED MONOHULLS” by Daniel Stavisky, 10/2003

Since reducing the wet beam is beneficial in more than one way, its effect is particularly important, especially at speeds close the to the boat’s hull speed and above that.

When designing the cross section of a hull in a twinhull boat the beam size is no longer a given constraint.

Given a certain beam a semi-circular cross section offers minimal girth, hence minimal surface area, and therefore minimum Frictional resistance.  Because of human constraints (Beam to Draft ratio) a good kayak with a mid ship cross section surface of slightly above 50 square inches will have a non optimal girth slightly over 30″ long.
But the beam of each of a twinhull hulls is not a given constraint, and we are free to design any type of cross section we want, according to what is best, which may not necessarily be the absolute minimum in skin friction:  The same cross section surface of 50 square inches can be divided in two equal surfaces of a little above 25 square inches each, with each having a girth about 15″ long – This is possible if the Beam to Draft ratio of each of the smaller new hulls is 1:1.  The price to be paid in this case will be a certain increase in the boat’s total surface area, but the gain will be a huge decrease in Residual resistance (see formula for Rr):
A 100 liters ICF K1 racing kayak is 220″ long and has a 20″ beam.  The residual resistance for it will be 20^2 x 220 = 88,000.
According to the same formula, a 100 liters, 10 ft long twinhull boat with each hull 5 1/2″ wide at waterline will generate residual resistance equal to 2 x (5 1/2 ^2) x 120 = 7,260.  That is 91.75% less residual resistance than for the ICF K1 racing kayak.
A 100 liters, 220″ long twin-hull boat with 5″ wide hulls will generate 87.5% less residual resistance than a comparable ICF K1.

On the other hand, adopting an “optimal” shape in terms of skin friction would result in two hulls each having a beam of about an 8″, a 4″ draft and less than 13″ girth.  The combined girth of these two hulls will be 10% smaller than the girth of a traditional fast monohull kayak.  This means that the a total surface area of a twinhull boat does not necessarily have to be much bigger than that of a comparable traditional kayak. Consequently, a beam size of 5″ to 8″ will be between the optimum Beam to Draft ratio and the optimal Beam length, which is a promising range of possibilities.

Having two hulls instead of one increases the kayak’s stability, which is always good for speed.

But will the increase in wet surface as a result of having two hulls nullify all these achievements?

Surface Friction and Frictional Resistance (Fr)

“With most kayaks the transition from 4 to 5 knots marks the transition between skin friction being the most significant factor and wave-induced [I.E. Residual Resistance] drag being the most significant factor.”
Kayak Review Info,  Sea Kayaker Magazine – 2004
Note: Sea kayaks and racing kayaks reviewed in those tests are characterized (among other things) by being long and having narrow beams, usually between 20″- 24″. “Chubbier” (lower L/B) kayaks start generating high Residual Resistance at lower speed.

Note: A typical sit-in or SOT fishing kayak is 36″ wide (3′) and 156″ long (13′), which gives it a sub-optimal L/B of 4.3 – and a claim for the title ‘Barge’. In fact, such monohull fishing kayaks are hardly suitable for longer trips, simply because their low speed makes them harder to paddle than the faster touring kayaks and W fishing kayaks.

The following formula can be used to calculate Frictional Resistance:  Rf  = C  x  Cf  x  Sw  x  V^2  where:
Rf = Resistance in pounds
C =  Constant for fresh water or salt water
Cf = Coefficient of friction
Sw = Wet surface
V = Velocity in ft/sec

It’s easy to see that any change in Wet Surface (Sw) will result in a proportional change in the total Frictional Resistance (Rf).
Practically, this near-linear correlation counter affects the sub linear improvement in hull speed achieved by increasing the boat’s length.

A smaller wet beam is better since it reduces the hull’s proportional surface area:  S/V ^ 2/3 where
S =  Surface area and
V =  The boat’s volume
An optimal Beam to Draft ratio for an elliptical mid ship (monohull) cross section is about 2:1, but we cannot expect a monohull kayak to come close to having such ratio because of the user’s sitting position.  A fast traditional kayak would usually have a Beam to Draft ratio higher than 4:1.   This means that the monohull kayak’s surface area is far from the optimum for its volume, and the further a solution is far from being optimal the easier it would be to conceive a better one…

However the hulls of a twinhull boat are not limited by the ‘Sitting-Inside’  position constraint, and therefor can be designed to have an optimal wet Beam to Draft ratio.  For example: when fully loaded the B/D of each hull will be optimal in terms of residual resistance and with less load the B/D will approach 2:1, which is the best in terms of frictional resistance.
A range of practical solutions stretching between two optima is certainly good news for designers -
Since the Length to Beam ratio for the hull of a twinhull boat is superior to that of a monohull kayak, it is possible to make the twinhull boat shorter than a monohull having the same displacement.  Eventually all this enables designing a twinhull boat with a surface area not much bigger than that of a fast monohull kayak with a similar volume.
Also, Turbulence (non laminar flow) at the bow and the stern is a considerable source of Frictional resistance in non optimal hulls, but it is much smaller in ultra thin hulls.  This means that in the case of a twinhull boat a bigger surface area can increase surface friction by less than a full 1:1 factor.
Note: Ultra thin catamaran hulls don’t look like thinner versions of kayak hulls, and those of you who would like play with hull design software and test their ability to design W kayaks should remember that such hulls have much higher Prismatic coefficient (Cp),  Block coefficient (Cb) and Waterplane coefficient (Cwp) than kayak hulls have, or more simply- they are much ‘fuller’.

In an article on monohulls and multihulls, Tuck and Lazauskas found that for ships with an ideal Length to Beam ratio (over 40:1) and ideal Beam to Draft ratio the Residual resistance can be reduced to less than 10% of the Total resistance.  Tuck and Lazauskas emphasize that those are optimal numbers achieved in a theoretical exercise under unrealistic conditions, and expect results for realistic boats under various constraints to be considerably different.  In the case of paddle sports boats those figures imply that an optimum monohull kayak would be around 27 feet long and 8 inches wide, which is not even imaginable.

Designers of fast canoes and kayaks (e.g. sea kayaks, racing kayaks and canoes) have noticed that a gradual increase in surface friction of up to 50% can sometimes stay unnoticed by the user. This could imply that Frictional resistance (Fr) is worth less consideration than Residual resistance (Rr) in the design of fast kayaks, canoes etc.
Another fact worth remembering is that the importance of residual resistance vs. that of frictional resistance increases at higher speeds.

Sensible Design in View of Required Performance – The ‘Optimum Shape’ for the Real World

The most comprehensive source of information on kayak speed available is the series of tow tank tests conducted over a decade ago for Sea Kayaker Magazine.
The tests’ findings are interesting in the context of ‘Real World Paddling’:

1. The Rudder Factor

Most of the trials were run with rudders retracted, however the trials run with rudders deployed revealed that rudders created a significant amount of drag.
The magazine decided not to use the figures recorded with rudders since rudders help counter yaw and can be very effective in keeping a boat on course while the paddler focuses on straight ahead paddling, and the the benefit of rudders in real life conditions could outweigh the disadvantage of the drag they create.

2. The Waves Factor

The towing tanks tests were conducted both in flat water and in waves.
The results recorded in waves had dramatic differences from those recorded in flat water due to Pitching and Rolling problems.
The magazine decided not to include those results because of the difficulty in testing dozens of kayaks of different lengths in different types of waves.

3. ‘Fish vs. Swede’ or ‘Seaworthiness vs. Theoretical Speed’

Kayak designers seem to agree that while the ‘Swede’ form for a kayak (where the greatest beam at waterline is aft of the Center of Gravity- CG) is faster on flat water due to its lower (horizontal) angle of penetration, the ‘Fish’ form (where the greatest beam at waterline is forward of the CG) is more seaworthy as it reduces the the kayak’s tendency to pearl and broach.
-This article was published in SeaKayaker Magazine

Tow Tank Tests vs. Real World Tests

While these considerations may be relevant (though far from decisive) when testing speed performance within a specific kayak category (e.g. ‘Sea kayaks’) they would significantly distort the picture when applied to cross-category comparisons (e.g. monohull kayak vs. W kayak): In the real world (e.g. ocean) even the fastest kayaks must be paddled with rudders (or skegs), otherwise their low directional stability (yaw problem) decreases their effective speed by too much, while even the 10 ft long (short..) W Kayak boat does not require a rudder because catamarans track better than monohulls.
Furthermore, in the real world the kayaker is required to pay attention to the rudder as well as to use his body to manipulate it.  These cognitive and physical resources are drawn for the same pool the kayaker uses for propelling his boat.  Consequently, the kayaker’s power that’s available for propulsion is reduced.
As for waves, which are given in the real world, it is widely accepted that the less stable a boat the less seaworthy it is.  Since the W boat concept offers better stability and control in both hydrostatic and biomechanic terms the ‘Wave Factor’ should be included in the discussion as favorable to the W kayak concept. Considering both Rudder and Waves factors combined it is safe to conclude that the theoretical real-world speed of sea kayaks and other fast kayaks is in average 20%-25% lower than that indicated by the flat water tow-tank results.

In one of the articles recommended in this page E.O. Tuck and L. Lazauskas offer the results of an elaborate, theoretical comparative study on the drag created by ships of 1, 100, and 10,000 tons with monohull, catamaran and trimaran designs.
Their two main conclusions seem to be:
1. Optimum (extra long) monohulls are always better than optimum catamarans or trimarans of the same total displacement, from the point of view of total calm water drag alone, unless there are restrictions on the ship geometry.
2.  The inclusion of further restrictions is of greater importance. Further constraints, such as on maximum length or minimum beam arise inevitably from commercial, structural, safety, sea keeping, or sporting requirements.  When these constraints are imposed, the ship proportions will return to the more conventional range, but at a price in terms of increased total drag.

This optimal world excludes sailing boats since they are moved by wind, which makes them heel, and generates waves that further destabilize them.  The solution to this problem is a keel, which considerably enlarges the boat’s wetted surface area and makes the hull non optimal for this article. The stability of motorized monohulls can be increased using ballast, but that also increases the total wet surface area and places any monohulls outside the definition of ‘optimal’ according to this article.
therefore, there are no real world examples for an absolute speed advantage of displacement monohulls over multihulls.

Tuck and Lazauskas found that a 40:1 Length to Beam ratio is optimal for speed, and with such ratio Residual resistance counts for only 10% of the Total resistance to the boat.  Moreover, they allowed for the monohulls a Beam to Draft ratio of 2:1, which is not a realistic one for canoes and kayaks, which is closer to 4:1.  Considering the L/B ratio of an ICF K1 racing kayak is merely 11:1, it is clear that the constraints imposed on the design of small paddle sports boats are severe, and the actual performance of such boats in terms of speed is therefore very different from that of Tuck and Lazauskas’ optimal boats navigating in straight lines in an ideal environment, under no constraint other than their volume.

A canoe or kayak’s volume is given before starting its design:  It is dictated by the weight of the user(s), the gear carried and the boat itself, the user being the most important factor.  The user’s power, skill and endurance are other severe limitations.
The boat’s required performance is measured mainly in terms of speed, stability and control.
The monohull kayak design offers a less than optimal solution for allocating the boat’s ‘asset’, which is its projected volume:

Nearly all the monohull kayak’s buoyancy is concentrated along its longitudinal axis (center line), where it contributes close to nothing in terms of lateral stability.
The monohull kayak’s wet sides contribute little lateral stability at a price of a large surface area and a big increase in residual resistance that limit speed.  The monohull’s above waterline sides offer some secondary stability but at a price of a decrease in directional stability (i.e. yaw) as the waterplane cross section of a monohull tilting sideways is no longer symmetrical in the longitudinal direction, that is relatively to the boat’s direction of progress.

Reducing a monohull’s wet beam in order to increase speed decreases lateral stability, which has a negative effect on speed and comfort.
To be ‘fine’ a monohull needs to be excessively long, which requires more effort for propulsion and maneuvering. Tuck and Lazauskas found that for speeds roughly above 1.5 hull speed optimum catamarans are about 25% shorter than optimum monohulls.
The low sitting position in a monohull kayak is wasteful in terms of paddler’s energy since a small and relatively weak group of muscles in the shoulders, chest and back has to provide most of the propulsion and control efforts, while other, more powerful and better fit parts of the body are largely prevented from sharing the load and increasing available power.
Sitting low also makes it more difficult to make the paddle move in parallel to the hull and at a close distance from it.  Instead, the natural movement of the blade is more in parallel to the water surface, in a curved course at a distance from the boat.  This leads to high energy loss as a result of the difference in speed between the paddle’s tip and the part that’s closer to the shaft, and because the paddler needs to put more effort in keeping directional stability.
Since the paddle moves at a low angle relatively to the water surface the difference in resistance between the blade’s low (more submerged) and high parts creates an unwanted rotational effect with the shaft acting as axis. Overcoming this problem is achieved by a combination of the paddler’s continuous effort (‘technique’) and an asymmetrical, thin (less full) and consequently less efficient design of the blade.

Most fast kayaks (and canoes) have hard chines that increase their wet surface i.e. further distance them from an ‘ideal’ shape in speed terms.
Looking at the findings in Tuck and Lazauskas’ article it seems that in average an optimal catamaran generates roughly 15% more Total resistance than an optimal monohull of the same volume.  But real life monohull kayaks and canoes cannot be considered being even close to optimal according to this article, while real life twinhull boats are not limited by the constraints imposed on monohull boat design, and therefore can be made to be closer to the theoretical optimum catamaran design.

11’4″ Long W Kayak Model vs. a Longer Monohull Kayak – Speed Comparison

The speed advantage of the 11’4″ long W500 is limited to canoes and touring kayaks in its size category, that is about up to 13′ in length, and to longer canoes and kayaks with very wide beams (e.g. typical fishing kayaks).
This can be explained by the very steep increase in Rr as function of speed above the hull speed, which is typical to wide-beam monohull canoes and kayaks, compared to a milder increase in Rr under those circumstances in ultra thin hulls such as those of the W1.
Fast canoes and kayaks with very long and narrow hulls (high L/B) are faster than the 10′ W1 in most cases.
These findings basically correspond to the observed average 25% speed advantage that multihulls have over comparable [displacement] monohulls (i.e. similar displacement and length) in the sailing and motorboat worlds.
An additional explanation to this relative speed advantage of the W500 is its improved biomechanic and ergonomic design, which enables the paddler to allocate more power more effectively than the traditional monohull kayak does.

The Potential of the W Kayak Concept

Statistically, multihulls are 25% faster than comparable monohulls in the world of yachting, powerboats and sailing.  This could give the reader an idea of the potential of twinhull paddle sports boats but it’s not necessarily a final limit:
The improvement in stability and hydrodynamics is relative to the effect of the constraints of the basic [displacement] monohull form.  The relatively wide beam and difficult paddling posture imposed by traditional kayaks may be more significant limitations than propulsion constraints imposed by monohull designs in larger boats.  Paddlers’ complaints about leg and back pains induced by the traditional paddling postures are strong indications to a general and serious ergonomic problem that impacts both well being and paddling speed.  Narrow monohull canoes and kayaks can sometime be slower than wider and more stable ones simply because their instability makes them too difficult to paddle in some cases.

The following figure represents the useful potential of the W concept in the design of a wide range of paddle crafts:

Kayak Design: Speed and Stability

The schematic drawing shows the trade-off between Speed and Stability in traditional (monohull) kayaks and canoes (Red line), which limits the performance of any monohull K or C model to the area under this line.
The relationship between Speed and Stability in W kayaks is represented by the Green line.
Contrarily to monohull kayaks and canoes, the W Stability increases as a function of Speed (I.E. longer hulls).
The potential speed advantage of W kayaks is about 25% higher than that of monohull kayaks and canoes of similar weight, volume and length, based on statistics from motorboats and sailing boats, and confirmed by tests run on 3 experimental W kayak models, and two production model – the 10’4″ long W300, and 11’4″ long W500.

The W’s initial potential stability is considerably higher than that of monohull canoes and kayaks – See Demo Movies

The gray areas in the above figure represent models that are either too slow or too unstable to be useful.

“Catamaran-Kayaks” vs. W Kayak – The Differences

Interestingly, while some traditional ‘Catamaran Kayaks’ are more stable than monohull kayaks they are not faster than regular monohull kayaks.  This can be attributed to two factors:

1.  Stability:  The ordinary ‘Catamaran Kayak’ design places the paddler on top of a platform connecting two hulls or pontoons, with his/her legs stretched forward in the typical ‘L’ kayaking position. This elevates the Center of Gravity (CG) of both paddler and boat compared to regular kayaks and SOTs without improving the means available for active balancing and control.
As a result a paddler sitting on top of a traditional ‘catamaran-kayak’ may find himself quite unstable and lacking good means for controlling his/her boat.
The W Kayak is significantly different by the fact the paddler’s legs are not stretched in front of him/her but go deep down into the hulls, and his/her feet rest firmly below waterline at the boat’s lowest point.  This position both lowers the CG as well as offers optimal balancing and control capabilities over the boat.  In fact the W Kayak is more stable than any kayak or canoe- monohull or dual hull.  (Kayak Stability Article)

2.  Power:  Paddling from a higher position is known to improve the paddler’s leverage on the paddle, but only if the paddler benefits from adequate support, which traditional catamaran-kayaks cannot offer.  In comparison, the Riding, and Standing positions offered by the W kayak enable applying powerful paddle strokes similar in strength to those applied by racing and whitewater canoeists who paddle in the kneeling positions.

Limits of the W Kayak Concept

As a result of the user sitting or standing with a foot in each hull, the W  kayak design presents a special problem relatively to normal, larger size twinhull boats (catamarans), which is the small distance between the hulls.  The water flowing in this space generates a higher resistance, especially if the hulls are very long and very close to each other.
However, the two hulls are very narrow (high L/B) and displace a small volume each, and consequently generate very small waves so that practically this limitation seems to have negligible effects.  This potential problem is also dealt with by having the asymmetric hulls divert some of the flow from the space between the hulls.
Tuck and Lazauskas found that in speeds lower than 1 x hull speed the optimum separation  (W/L – Width to Length) is roughly 20-30% from the catamaran’s length, but for speeds between 1 and 2 times the boat’s hull speed there seems to be no optimal W/L.
They also found that in some cases optimum catamarans can generate less resistance than comparable optimum monohulls due to a phenomenon known as Wave Cancellation.

The second generation of W kayaks named the W500 series, was designed with the sides of both hulls facing each other completely straight and flat. This has reduced to a negligible minimum the flow disturbances in the space between the two hulls.

Tests we performed years ago with a 15 ft W boat prototype have shown no significant increase in wave interaction and non laminar flow in the space between its hulls compared to a short (10 ft) model.  This positive phenomenon can be attributed to the decrease in Draft in the longer boat.

More Food For Thought:

Multihull sailboats are heavier than the 'ideal shape' because of their additional structural elements and increased hull surface by volume, while monohull sailboats are heavier than the 'ideal shape' because of their ballast and weighted keels that are required to improve directional and lateral stability.
These structural, static elements represent 'prices' that designers have to 'pay' in order to increase the seaworthiness of their designs.
When it comes to human powered boats the possibilities for adding heavy static elements are very limited, and much of the problems of tracking, balancing and controlling are left to the user/s to deal with dynamically, i.e. by drawing on the power of their muscles and their attention, and consequently, pay a price in speed terms.

“Improvements to the monohull design have only increased sailing efficiency about 20% over 100 years, whereas by changing from a monohull to a multihull a much greater increase in sailing efficiency is realized.”
-Richard Boehmer, Naval Architect



W500 kayak front view
Front view of the W500

It is possible to design W hulls that are wider, longer, deeper etc.


Cross section of a W500 fishing kayak
Cross Section of the W500

The user sits, rides or stands in it with each foot resting firmly at the bottom of each of the kayaks' twin hulls, below waterline, for max power, control and stability.
The 11'4" long W500 kayak is just 28.5" wide, and its patented form makes it the world's stablest kayak.

About this Article


We published the first version of this article ten years ago.
At that time, the leading application in kayaks was sea-kayaking (Touring), and kayak fishing was in its infancy.
Stand Up paddling (SUP) was almost nonexistent, and many people opposed the idea of paddling standing.
Few people attempted to outfit their kayaks with feeble electric motors, and no one contemplated the possibility of using powerful outboard gas engines.
Back then, most kayakers believed that the only way to increase a kayak's stability was to make is substantially wider or to outfit it with outriggers - both methods resulting in increased drag and reduced speed and convenience as many contemporary fishing kayak designs prove..
Many people then didn't realize that multi-hulls, and especially catamarans, have revolutionized boating, including kayaking, by offering both more stability and the possibility to reach higher speeds. Nowadays, the dominance of multi-hulled boats in sailing and motorizing races leaves no room for doubt.

Things have changed since then, and today most people are more knowledgeable, aware and open minded than a decade ago.

As we predicted back then, kayak stability and ergonomics have become by far the top priorities of both users and manufacturers.
In contrast, long and narrow traditional style kayaks equipped with tight spray skirts that require their user to have a 'bomb proof' Eskimo Roll technique have fallen out of fashion.

Since 2007, this article was viewed over 33,000 times by 23,000 people, and its popularity hasn't decreased over time.

Special Thanks

Over the years, many people proof-read this article and contributed their comments and questions about it, both in public discussions and in private emails.
Some of them were known kayak designers, others less known ones, and many were kayakers and kayak anglers, of which the majority owns a W kayak from either the discontinued W300 series and/or the W500 series.
All these people deserve our thanks, since without them this article would have been less complete.


What Do W Kayak Users Say About It?


Our Fishing Kayak Blog features hundreds of stories and thousands of pictures contributed by W kayakers.
Our website's Fishing Kayak Reviews section features links to nearly 200 reviews contributed by W kayak owners, including their full name and state.
Note that we seldom publish a review that's not accompanied by pictures.




Resources:

Are Sea Kayaks Seaworthy?

The Evolution of the Kayak

Article on Kayak Stability

'CATDESIGN v1" - Catamaran Design Software Program by Rene Calvo:
http://www.wavewalk.com/CATDESIGNv1.xls

Unconstrained Ships of Minimum Total Drag by E. O. Tuck and L. Lazauskas:
http://www.cyberiad.net/library/multihulls/multipep/multipep.htm

Hydrodynamic Drag of Small Sea Kayaks:
http://www.cyberiad.net/kayak.htm

Curves of Residuary Resistance for Small Power Vessel of about Cp=0.64 and
Comparative Curves for Frictional Resistance:
http://boatdesign.net/forums/attachment.php?attachmentid=383

MICHLET -Calculates the total (viscous + wave) resistance, wave elevations, and bottom pressure signatures of thin monohulls and multihulls: http://www.cyberiad.net/michlet.htm

Catamarans'  WSA http://boatdesign.net/forums/showthread.php?t=938

Multihull Design Considerations for Seaworthiness by John Shuttleworth:
http://www.john-shuttleworth.com/Articles/NESTalk.html

The Seaworthy Seakayak (monohull), by J. Winters

Polynesian Catamarans: http://www.pbs.org/wayfinders/polynesian2.html

Your comments and questions are welcome - CONTACT US









User Manual: Launching, paddling, fishing, recovery Children kayak fishing


This Invention Is Protected By U.S. Utility Patent Number 6871608

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