The Ultimate Sea Kayak [52]

By Tim Dillenbeck

Introduction

This story is describing the process of developing a new sea kayak design. But let me first introduce myself.

I am a Naval Architect as profession and have for the last 3 years developed a great interest in sea kayaking. I built a wood kayak in 2001 and have paddled many different kayaks during these years. I am currently a grade 4 sea kayaker (NSWSKC Club proficiency grading). With this background I felt confident that I would have the ability to design an optimised kayak design, meeting certain design criterias and subsequently build a prototype to prove the design. This project started early 2002 and the prototype kayak was launched late February 2003. The kayak met or exceeded all expectations and was received very positively by many experienced paddlers. In the Conclusion below, I am also discussing the possibility of commercialisation of this project.

Design Input

In all cases of Naval Architect design, it’s important to have a clear idea what we want to achieve. Speed, Weight, Stability, Comfort & Safety, Manoeuvrability & Steering, Sea keeping capabilities and ability to Surf are items to consider. A fast hard tracking kayak may not be very manoeuvrable, a lightweight kayak may not have the structural strength suitable for expedition kayaking, a highly manoeuvrable kayak may be difficult to stay on course in rough seas without a rudder. Therefore one kayak can not do everything, the designer has to make some choices and compromises. Below I have outlined some of the key thoughts as design input for this project:

Speed

A sea kayak should have a relatively good overall hull speed in various conditions. It is really difficult to paddle with a team of strong paddlers using fast kayaks and not be able to keep up. Many designers argue about waterline length, wetted surface areas, etc but those numbers are important but do not cover all aspects of speed. Once you get out to sea, the kayak’s ability to handle waves, wind and increased loading conditions will have a dramatic impact on the ability to sustain a high cruising speed. It’s also important to decide what your typical paddle speed would be in order to optimise the hull resistance to that design point. A racing kayak may be optimised for 12 km/h hull speed and will subsequently not be performing at its best at lower speeds whilst in this case I selected as design input slightly over 8 km/h as being a typical fast sea kayak cruising speed. That means that firstly the still water parameters are to be considered but also Prismatic Coefficient (PC*) and other characteristics regarding the hull shape need to be taken into account such as:

  • The hull shape should be such that it keeps the pitching movement in head sea to a minimum. A lot of pitching movements is loss of energy = loss of speed. Normally very fine ends at bow and stern will increase pitching.
  • If the bow is partly submerged in head sea or in surf, the wetted surface area momentarily increases dramatically which leads to increased hull resistance. Means to keep the bow as dry as possible in all conditions is of importance.
  • In open following sea, the kayak needs to have three key performance characteristics: a) track well to stay on course, b) ability to pick up waves for a quick surf and c) not to nose dive, in particular in heavy loaded condition. A design successfully dealing with a), b) and c) will have superior speed characteristics.

Weight

In general, additional weight is additional drag. However, normally the total weight of kayak, paddler and ‘cargo’ will exceed 100 kg so a 3 kg heavier kayak is only affecting the overall weight scenario by 3%. Considerations must be given to the kayak’s structural strength and practical issues such as lifting the kayak onto the roof-rack of your car. Also weight distribution is an issue, weight high up or in kayak ends is undesirable. Weight low down increases stability and weight near centre as opposed to ends will reduce pitching, i.e. increase ability of speed.

Stability

Stability of kayaks is another area of constant discussions. My view is simple. The initial stability should be sufficiently low to enable the kayaker to easily lean as desired for turning and control the kayak’s roll angle in beam sea (too high initial stability may cause uncomfortable rolling in beam sea). On the other hand, the kayak shall not feel ‘tippy’ to the point where the kayaker constantly must brace with the paddle to stay upright. High secondary stability is desirable at relatively high heel angle (20 to 25 deg).

This will assist in overall sea-worthiness, keeping the heel angle at a leaned turn and improved ability for easy Eskimo roll.

Comfort & safety

Comfort is of outmost importance as a kayaker could spend many hours and in some cases up to a day in the kayak. Comfort is related to some key areas:

  • Seating arrangement should be provided with soft seat, back rest, thigh/hip support, knee bracing under deck and foot rests. All items should be adjustable in order to accommodate various sizes of paddlers. It would be desirable to have key items such as back rest and foot rest adjustable whilst the kayaker is sitting in the kayak. During an extensive paddle, it could be nice to be able to adjust your seating position.
  • Day hatch to have easy access to clothes, food, safety gear etc and a place to hold small items on deck such as a towing line in addition to the traditional straps on foredeck for sea chart.
  • Cockpit size is also an area of many different opinions but my view is: large enough to step in and out of without beaching or paddle bracing but tight enough (including knee braces) to ensure secure sitting position in case of rough conditions (roll in surf etc).

Safety equipment should be incorporated in the design and be in line with the highest sea kayak standards (safety lines, pump, provisions for towing, etc).

Manoeuvrability & Steering

RUDDER or NO RUDDER, what a great topic and again there are many different views. My belief is very basic. Firstly, a kayak design should not be such that a rudder is needed to compensate for lack of tracking or weather cocking (turning into or away from the wind in strong beam wind conditions). Therefore, a kayak should be able to be comfortably handled in most conditions without a rudder. However, I firmly believe that even the best kayak needs correction strokes in strong following sea and very often I have noticed speed loss from even the best paddlers. For long paddles in heavy sea condition, a rudder will assist in saving energy by keeping the kayak on course and eliminate correction strokes. Also for those keen in using sails, a rudder is desirable.

The rudder design is a challenge by itself. Fixed rudder (non retractable) are prone to damage and most standard retractable rudders are hydro dynamically not well designed and they are quite exposed in ‘up’ position causing windage drag, possible injury to people and exposed to possible damage if rolled onto the beach.

The challenge for the new project was to come up with a better solution.

Manoeuvrability of the kayak without use of rudder is important. Although the kayak should track well paddling straight forward, a superior turning ability when leaning the kayak is desirable. Avoiding difficult situations by having good control of the kayak (rock hopping, trapped in narrow waters, etc) is a safety issue and it’s also more fun to paddle a lively kayak with a ‘sporty’ feel.

Sea keeping capabilities

This topic had been addressed to a large extent under Speed & Stability. In addition, it’s important for a true sea kayak to have enough reserve buoyancy to cope with expedition packing load (can be an additional 40 kg or more) without demonstrating poor behaviour in rough sea conditions.

Ability to handle surf

This topic is seen by many as a play issue and yes, it’s fun to surf with sea kayaks. However, the key issue here is speed and safety.

Surfing in open sea is discussed under Speed, but the real test is the approach to a beach through the surf. The ability to surf in way of a breaking wave with minimum tendencies of nosediving or undesirable broaching is a safety issue.

Development

As input to the design, I studied many existing kayak designs and conducted extensive research work regarding designs, hull shapes, resistance calculations and general articles from kayak enthusiasts.

The kayak was designed starting with a lines plan in order to define the hull shape and providing the basis for hydrodynamic calculations. The waterline length was early determined from resistance calculations and choice of the Prismatic Coefficient (PC*). The importance of Waterline Length should not be over exaggerated as the benefits are quite small. For example, the theoretical top speed for 4.75 m WL is 9.8 km/h and 5.0 m WL is 10 km/h. Optimum PC is 0.5 to 0.6.

The selected optimum design speed was 8 km/h (see above) and the PC is taken from the following table. Max speed is when ‘speed/square root of length’ is about 1.34 and this equates to about 10 km/h for a 5 m WL length kayak.

From the table, a PC of 0.53 is selected as optimum for the chosen cruising speed.

The centre of buoyancy (LCB) is another important decision to make. This will result in Fish-form if LCB is well forward of mid ship and Swedeform if LCB is well aft of midships. There is lots of literature discussing this issue but although Fish-form theoretically gives lower hull resistance you will find problems with tracking. Practically when considering sea worthiness and manoeuvrability, a neutral or slight Swedeform seems to be the best choice.

To compare some kayaks on the market (% of WL length aft of midship, the higher positive number the more Swedeform):

Model WL length aft of mid ship
Mariner XL 5.0
Coho 0.8
Chesapeake 17 0.6
Guillemot Night Heron 2.1

My choice was 2.5% of WL length aft of mid ship.

The final issue is the amount of ‘rocker’. The are no clear rules for that and the designer must select the curve of the keel line based on experience and comparing with existing designs of known performance. It has a lot to do with tracking and manoeuvrability as well as stability. A straight keel kayak is more ‘tippy’ than a high rocker kayak with the same beam.

Once these basic parameters are selected and the displacement of the kayak is established (113 kg displacement used as design point) the shape is quite well determined and the challenge is to ensure that the lines plan is modified until it meets all the design criteria.

The final check is stability. Very little seems to have been published about actual kayak stability. However, I found published stability curves of Guillemot Kayaks assuming a 200 lb paddler with a pre-determined position of the centre of mass. By comparing calculations I found that this prototype kayak would have initial stability (from 0 to 12 deg Heel) very similar to the average Guillemot but the max stability was peaking around 19 deg heel for the Guillemot and 24 deg for the prototype kayak.

Without more kayaks to compare with it was hard to know how good that is but the indications of a solid secondary stability seemed to be there.

A 1:5 scale model was built and tested in a swimming pool to confirm the waterline at various loads, behaviour in head sea and turning ability when leaning. The model test was not scientifically measured, it was rather a visual check that the kayak did what it was designed to do. As far as could be seen, the model performed very well. In particular, the turning ability in a heeled condition was impressive.

Building The Prototype

The kayak was built using 5 mm thick high density closed cell foam panels. The foam panels were carefully lofted, cut and stitched together in a similar fashion as the well proven plywood stitch & glue technique. The hull was finally covered with 1 to (in some places) up to 4 layers of 6 oz fibreglass cloth using epoxy resin on both sides creating very light weight and stiff sandwich panels.

The deck and bulkheads were made in a similar way. Valley hatches were fitted, oval for rear hatch and 12” round for day hatch and front hatch. This is very much in line with well proven practice in many commercial kayaks today.

When the hull and deck was fully fibreglassed and completed during construction the hull weighed 8 kg and deck 4 kg. I was actually paddling the bare hull (without deck) as a last check before completing construction. But still, with hull fairing, painting, deck fittings, seating, rudder, foot pedals, electric pump & battery, and those heavy Valley hatches, etc it still ended up around 23 kg. About 4 kg more than expected. It was good that I selected the designed waterline at 113 kg because that meant that I had plenty up my sleeve.

The aft end of the kayak was provided with a fully balanced high performance custom made rudder being positioned under the hull in operation and ‘garaged’ in a well arrangement when in up position. This rudder provides very effective steering in all conditions and is well protected in up position.

Tim Dillenbeck's Ultimate Sea kayak

Testing

Testing a kayak is somewhat difficult. All paddlers have different personal views what is good and what is bad. I have tried to do some basic testing to verify that the kayak meets the pre-determined design criteria.

It’s proven to be a fast kayak. I have now paddled together with some very strong paddlers using well know commercial kayaks and it seems to be performing very well against other fast kayaks, even those with considerably longer waterline length. The kayak does excel in rough conditions which was one of the key design points.

It’s also very stable and it turned out exactly as expected and behaved as indicated in the design input. Although the kayak is lively and easy to turn and roll, it feels very stable in all conditions.

I have paddled in strong wind conditions without rudder at various angles to the wind. The kayak is extremely neutral and does not weathercock into or away from the wind. However, it’s still easy with a moderate lean and a sweep stroke to alter the course in any direction. I have seen kayaks having great difficulty heading into the wind in strong conditions and that can be quite dangerous but this one has certainly none of those dangerous characteristics.

Due to the fact that the kayak is ‘semi’ hard chine and has quite a bit of rocker, it does turn very well when leaning and carving a turn. The reality was just as demonstrated by the model in the pool!

The most fun is to paddle this kayak in rough conditions. It really feels safe and it does stay on top of the waves and very seldom would you experience the front of the kayak under water. Particularly in the surf, it takes off easily, the front of the kayak is not submerged and it tracks wonderfully even without the rudder due to its moderate fixed skeg arrangement.

The rudder is a bit unique and requires very small pedal movements for turning. Due to its light weightiness (the whole rudder assembly floats) the rudder has to be locked in down position and care must be taken not to ground that kayak whilst the rudder is down. This may seem as a draw back (and I guess it is) but because that kayak handles so well without a rudder, it’s advisable to pull it up in areas where grounding could be expected.

Conclusion

Personally I am very thrilled and excited about the success of this project. It does prove a point, and that is that it is worthwhile to really spend the time to determine the design input and expectations and then carefully use the best principles to design and model test before progressing with construction of the prototype. At this stage there is not one point or issue I feel I should have done different or better (this may come later!).

As I had some difficulties to find a sea kayak that did all the things I wanted in early 2002, I started this project as a hobby just to play around with some designs and build something for myself. The whole project ended up far much more involved than originally planned and a boat/kayak fabricator could probably now see the possibilities for commercialisation of this kayak. I personally would not be in a position to consider fabrication on a larger scale but I have in the meantime protected the design and certain design features are patent pending.


* Note: Prismatic Coefficient (PC) is a non dimensional number indicating the relative fullness of the vessels ends. For a kayak, a low PC below 0.50 indicates that the volume is concentrated amidships and the ends have a fine entry whilst a high PC above 0.60 indicate a more distributed volume and fuller ends.

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