BUILDING A FLEXIBLE BAIDARKA

Friedrich Vetterlein

1998

Dedicated to Jochen


Introduction

Today, you can find an abundant number of kayak types easily available for purchase - so why build a boat yourself? I feel that following the generation of a boat from a bundle of wooden strips to a usable kayak is satisfaction per se, especially when it is based on the beautiful designs of historical kayaks developed by the Eskimos in arctic regions, and when the project culminates in the view of a completed frame, as you can see in the figure above. In addition, I was especially stimulated by the idea of introducing a special feature, forgotten today but evidently used in ancient kayaks, a deliberately generated flexibility of the boat's hull. The first part of my contribution (chapters Nr. 1-5) focusses especially on the latter aspect from a more theoretical point of view, including the considerations of how to construct the movable parts of the boat. In chapter Nr. 6, I describe the procedure I actually applied finally - perhaps a stimulation to those who enjoy imparting their own "spirit" on objects of their personal use.


1. Arguments in Favor of a Flexible Kayak

When nowadays you ask a sailor, a kayaker or a surfer whether a flexible hull might be advantageous, they all will agree that a flexible boat is more sluggish and has a lower maximum speed than a corresponding rigid one. A racing boat which has become flexible, or "soft" is considered a spoilt one.
Given this point of view, it is amazing to note that hull flexibility is being increasingly regarded as a potentially desirable quality by those who study native boats from the coasts around the Aleut Islands and South Alaska, considered to be among the most dangerous areas in the world. In these areas the native Eskimos developed a special type of kayak which, by general agreement, is considered to be extremely well adapted to their difficult local conditions.
Specialists in these baidarkas (so named by the Aleut) noticed a particular feature which led them to conclude that flexibility had been incorporated deliberately into these boats (J. Lubischer, ref. 6). Special bony joints, incorporated between the wooden elements of Aleut baidarkas, allow small, smooth movements at these junctions to occur. Different explanations have been proposed in order to explain the significance of this stuctural feature.
The simplest explanation is the assumption that they had been introduced in order to reduce creaking noises during exposure of the boat to external stress (J. Heath cited by G.Dyson, ref. 5, page 23). There are, however, some arguments in favor of additional specific functional benefits that such ivory joints might have had.
As pointed out by G.Dyson (ref. 5, page 22ff.), one functional benefit may have been an improved ability to cope with dynamic stress caused by oncoming seawaves or waves coming from behind. When the boat was hit by a sudden, strong wave, flexibility in the construction induced by incorporation of the bony joints may have helped to minimize the impact. In addition, when the hull was exposed to extreme tension due to passing the trough, a construction as light as that of the native Aleuts, could have withstood such forces only by reducing the hull's stiffness.
As a third advantage to hull flexibility, early ethnologists and recently J. Lubischer (ref. 6) proposed that speed may have been the most important factor. "The waves represent more water that has to be accelerated out of the way, at the expense of the kayak's own momentum" (G. Dyson, ref. 5), an effect which is expected to be greater in a rigid boat as compared to a kayak which is able to bend over the waves. The effect of absorbing frictional energy due to bending of the hull was thought to have dampened the boat's speed, which then could have been minimized by incorporating the friction-reducing ivory bearings in the baidarka skeleton.
Quite a different (fourth) aspect of hull flexibility concerns resonant behaviour, as extensively discussed and analysed by G. Dyson (ref. 5). When the waves attain a certain frequency they may cause resonant swinging of a flexibly built hull. This effect is generally considered to be disadvantageous since it imposes extrem bending forces to the hull. The effect of this mechanism might have been lessened by using joints which allow the different pieces to move with only modest frictional resistance.
With respect to hull flexibility there appears to be a fifth, quite different role not adequately considered in the cited reports. Flexibility might play a great role in laterally stabilizing the boat. In contrast to a rigid one, when a flexible boat is running perpendicular into a wave or when it is exposed to waves coming from behind, its hull may not only withstand the bending forces much better as pointed out above, it will also gain improved lateral stability as explained in the following section.
In the classical calculation of a ship's uprighting forces, the torque is determined by the interaction between the center of gravity and the center of bouyancy (as determined by the geometrical center of the immersed part of the boat). As a simplified rule, the torque is determined from the cross section of the boat amidships. In reality, however, it is the bouyancy of each section of the hull that contributes to uprighting during tilting. The narrow parts of the boat near stem and stern, however, have no uprighting effects due to their more or less round shapes. The shape of stem and stern gains importance, however, when the boat is running into a wave or is taken up by a wave coming from aft. In a rigid boat, immersion near the ends increases while that of the center part decreases correspondingly. In this way the influence of the broad, stabilizing mid part, and thus its overall lateral stability is reduced.
In contrast, the stability of a flexible boat is not disturbed when it may bend into the trough of the wave. Stem and stern bend upwards and prevent the mid part from becoming lifted upwards. This maintains its stabilizing efficiency. In addition, bending causes the center of gravity to shift downwards in relation to the boat's axis of rotation, which also improves its static stability - similar to laying in a hammock.
When the boat passes the crest of the wave, it bends in the inverse direction. The mechanism which stabilizes a boat in this situation applies to both flexible and rigid ones. When on the crest of the wave, stem and stern are lifted partially out of the water and correspondingly, the midpart is immersed deaper. This increases the role of the broad centerpart in regaining stability.

All these considerations led me to the idea of building a kayak with hull flexibility. Since this feature was evidently incorporated into those native Eskimo boats used under the difficult maritime conditions of Alaska's coasts, it was logical to use a traditonal model from this area as an example. The following description refers to a two-hole kayak found near Kodiak Island which was analyzed by John Heath in 1962; the plan is published as a drawing in the article "Arctic skin boats" by H. I. Chapelle (ref. 2) and available on an enlarged scale from the Washington State Historical Society and Museum (plan BC-69, Fig 179, page 197; address: Ship Plans, Division of Transportation, Room 5010 NMAH, Mrc 628, Smithonian Institution, Washington, D.C. 20560, USA).
The final result was a semi-replica rather than an exact copy of the traditional boat with respect to shape and material. First, I was forced to introduce changes into the cockpits since they were not large enough when using the original dimensions. Second, it soon became obvious that it would be impossible to get original materials like leather as a covering or baleen for lashings. Therefore I looked for alternative, appropriate modern materials of the boat's skin and interconnections between the wooden pieces which should maintain the beauty of the native Eskimo kayak while attaining a predictable flexibility and strength which whould enable practical use of the final product.


2. General Considerations Concerning Flexible Constructions

Before describing the construction actually applied, I want to reflect on some general problems inherent in flexible constructions. In building a non-flexible hull, the intention is to join together all elements of the boat as fast as possible. In a flexible construction, however, it has to be considered that the external bending forces will induce movements in virtually all parts of the boat and, in addition, these movements occur to quite a different degree within the boat's skeleton. This point is of special significance since it implies that the elastic property of each element, the joints and the wooden pieces themselves, has to be adapted to this specific demand. The aim is to construct the boat in such a way that on bending of the entire hull, each element is subjected to approximately the same percentage of its maximum bending tolerance. When this is not considered and the single pieces are simply connected somewhat loosely together, the following effects will result.
When the bending tolerances of the individual elements differ dramatically, bending of the entire body affects that element first which is weakest and - the general problem of flexible constructions - this particular part of the body is subjected to indeed the entire, undampened strength of the external force. Its maximum bending tolerance is soon surpassed and it will break. The same effect then applies to the next weakest piece, and so on.
Inadequately distributed elastic tolerances give rise to an additional effect, which complicates matters further. When the external force has caused the weakest element to break, the construction gives way somewhat due to a partial loss of stability. When this occurs, the external force's kinetic energy affects the next element in addition to the static bending energy.

A simple experiment shall illustrate this mechanism. Take two wooden rods of equal size, structure and quality. Coat one of them with fiberglass/epoxy resin. Tie cords at the ends of the rods and fix the ends of the cords at firm points like a swing. Now load the rods at their midpoints with gradually increasing weights (take a bucket and fill it slowly with sand or water). Determine the weight needed to break the rods.
In my experiment the uncoated rod withstood a weight of 48 kg; that of the coated one only 34 kg! Why did the coating reduce the bending tolerance? The effect may be explained by the differing elastic properties of both rods which allow kinetic energy to develop to quite a different degree. When the weight begins to load the coated rod, the entire weight acts exclusively on the fiberglass coating. Due to its much greater stiffness, the wood does not support any of the weight in this phase. At a certain load the fiberglass coating tears, now allowing the weight to drop by a certain degree. In this state two factors gain importance. The wooden rod is forced to bend especially in that small section where the coating has torn (and where the bending tolerance is soon surpassed) while the remaining parts don't bend due to their still intact coating. Another cause of the observed difference results from the fact that the weight gains kinetic energy during the falling movement which adds to the static load of the weight itself. Both effects are expected to have caused the greater resistance to a load in the uncoated rod as compared to the fiberglass coated one.


The earlier considerations and the result of the latter experiment reinforce the necessity of building a flexible construction such that each element contributes to the support of an external force with about the same percentage of its specific bending tolerance during different amounts of force applied.


3. Considerations on Distribution and Function of Joints in a Flexible Kayak Skeleton

I intended to construct the hull in such a way that its stem and stern would be able to bend by about 10 cm up and down, respectively. Gunwales, deckbeams, stem, and stern should be joined firmly together since they are the basic constructional elements of the kajak and behave more or less as a freely swinging body. However, the deck line has to be able to shorten and the keelson line to lengthen during bending upwards and vice versa. In order to maintain this effect in the completed frame, the following connections had to be introduced (Fig. 1):

  1. ~longitudinal, sliding joints between keelson and stem/stern;
  2. similar (though smaller) longitudinal, sliding joints between deckstringer and deckbeams, and
  3. rotational joints between ribs and hull stringers/gunwales.

 
Fig.1. Distribution of joints introduced into the baidarka skeleton. All numbered interconnections were built from the elastomer Sikaflex 260® joints Nr. 2, 3, 8, and 9 are made lamellar, Nr. 1 and 10 as sliding interconnections. Arrows indicate direction of movements when stem and stern bend upwards (the opposite direction on bending downwards).
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During the sliding movements between deckstringers and deckbeams (point 2), the bending energy should transfer from the stem and the stern via the deckstringer to the cockpit coamings. In this way the cockpits should act as circular springs which absorb oncoming impulses. This principle required the lateral supports of the coamings to be movable, too.
The question arose as to whether joints should also be introduced into the keelson. All surveyers of historical baidarkas have stressed the point that natives always built the keelson in three separate pieces, in contrast to the gunwales each of which was evidently made of one plank always. There appears to be general agreement that this division contributed to flexibility. However, I don't believe that the ability of the boat to bend up and down was the fundamental purpose behind the split construction. As pointed out above, the keelson line has to be able to shorten and to lengthen during overall bending. The bending forces to which the keelson is exposed during this process are rather small and a one-piece keelson should withstand such forces without any problem. The configuration of the keelson joints demonstrated by Lubischer (Fig. 3 and 4 in ref. 6), might at best have served to absorb any sudden impulses hitting the boat from underneath, but it appears rather unlikely that they were constructed in order to allow lengthening and shortening. When the keelson is bent upwards, the rectangular parts of the joint touch each other and impede further flexing upwards. Instead, I believe the keelson's split construction is linked with the problem of maintaining lateral symmetry in a wooden frame. A great problem of unsealed wood is its tendency to warp with age and with changes in environmental humidity. When the pair of gunwales is cut from one piece of wood with the grain arranged symmetrically, as described by W. Brink (ref. 1), the negative effects of warping are overcome. In the completed kayak, warping of such gunwales will cause some flexion of the boat up or down but no asymmetrical deviation will occur. This principle of reciprocal compensation cannot be applied to the keelson, however. It appears to be nearly impossible to get a piece of wood that stays in line over time along a distance of 5 to 7 m. However, this aim is presumably much easier to attain when the latter element is divided into three separate pieces. Changes in the linearity of any one piece will at least in part be compensated by its neighbouring piece.
For this reason I decided to use wood which had been glued together in two layers. This greatly reduces warping, and I did not introduce joints into the keelson. Though I initially prepared three separate pieces, I ultimately glued them firmly together.


4. Construction of the Joints

The classical procedure for attaining a movable, yet extremely firm connection between wooden pieces is to tie them together. A certain disadvantage of this method is the possibility that the joint's position may slightly dislocate and that creaking noises may occur during later bending of the boat. My experiences with silicone rubber led me to the idea of testing whether such material might also allow the construction of a mobile connection.

To test the adhesive strength, I placed two pieces of wood parallel to each other at a distance of about 2 mm and bridged the cleft with different elastomeres. While one piece was kept firmly fixed, the other one was gradually weighted down (by fixing a bucket at the lower piece and gradually filling it with water); in this way, the interconnection was exposed to sheer stress of increasing intensity. The weight pulling down the lower piece was determined and expressed as weight per connected area. Most of the boat's interconnections could be expected to experience forces of similar magnitude during use. The different elastomers tested enabled interconnections of greatly differing strengths. I obtained the best results with the polyurethane glue Sikaflex 260® (Sika GmbH, Hamburg, FRG). With this material I determined a maximal tolerable load of 25kg/cm2.


Next the question arose as to how to apply the glue between the wooden pieces in such a way that it did not restrict the previously described principle of hull flexibility.
 
 
Fig. 2. Connection between hull stringer and rib.
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The conditions were different depending on the respective joint; they will be described in order of increasing complexity.
A. Rib-hullstringer interconnections: The interconnections between ribs and hullstringers proved to be the simplest ones (Fig. 2). Simply filling the space between the wooden pieces with the elastomer still permitted the necessary movements to occur at these points. When maintaining a mean distance of 3 mm between these pieces (and at least 1 mm at the non-parallel interfaces near stem and stern), the requisite slight circular movements at these joints could easily occur.
B. Deckstringer-deckbeam interconnections: It soon became clear that the completely filling the space between
 
Fig. 3. Procedure for building the lamellar joints (No. 2, 3, 4, 8, and 9 in Fig. 1). 1.: Front view of the packed glass plates; 1a. and 1b.: side view showing laterally the spacers and medially the elastomer which is pressed into the medial clefts, then covered with a plastic foil. 2. and 3.: After polymerization the glass plates are removed, the length of the lamellae trimmed to 6 mm, a thin layer of the elastomer applied to the surface of the wooden pieces which are to be joined, and the pieces gently pressed together.
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Fig. 4. Construction of the sliding joints (No. 1 and 10 in Fig. 1). Cross sectional view during polymerisation of the elastomer (left) and in the finished state (right); for lateral view of the final joint see Fig. 5 and 6.
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deckstringer and deckbeam with the glue, even when maintaining a distance of 5-6 mm, made longitudinal shifts between these elements impossible.
The reason is that the elastic molecules of the rubber may only lengthen and shorten when it is possible to slide against neighbouring molecules. To take this effect into account, I experienced with lamellar structures. The free space between lamellae is sufficient to allow the differences in extension of the rubber to occur, since each lamella does not touch the neighbouring ones. The dimensions of the lamellar connections and a technique to produce them with simple equipment are demonstrated in Fig 3.
C. Keelson-stem and keelson-stern interconnections: With the laminar construction, sliding is possible along only limited distances. Longer distances necessitate a widening of the cleft which in turn causes the joint to become unstable. The distance by which stem and stern have to shift parallel to the keelson was too long (10-12 mm) to connect them with laminar structures. Therefore an alternative method had to be found. What I ended up applying was a sliding joint. The elastomer was applied for a length of 12 cm at the stem and the entire length of the stern. The glue was first put onto one side and then immediately covered with a stiff plastic foil which was bent in such a way as to form a cylinder section (Fig. 4). After the polymerisation process was completed, the opposite side was treated in the same way, taking the convex side as a model to form the concave, opposite one. Finally two plastic foils were placed between the rubber. Outside the joint, two holes were made in the foil and some glue attached in order to prevent the foil from drifting. The longitudinal, sliding movements occur between these two plastic foils. I used an especially resistant material (Hostaflon® ET foil, 0.1 mm thickness, Nowofol, D-83313 Siegsdorf). The joints were held together with cords (Fig. 5 and 6).
 
     
 
Fig. 5. Stem of the baidarka skeleton (upside down) showing the joint between stem and keelson. Fig. 6. Stern of the baidarka (upside down) showing the joint between stern and keelson.


5. Selecting an Adequate Covering

Before describing the process I followed in building the baidarka, I want to share some considerations regarding selection of an adequate covering. I reflected at length on this point. The skin had to be sufficiently firm; I didn't want to be constantly afraid of a stone or anything else that might cause a hole in the covering during use of the boat. It also had to be applicable in such a way that folds could be avoided, a precondition which soon led to the exclusion of finished materials. Finally, it had to correspond with the historical appearance of the entire construction. Although attractive, I soon rejected the idea of using leather since no adequate material was available and even if it had been, its durability is inadequate. Only the use of textiles proved to be realistic, which had to be impregnated after the sewing process.
Textiles I had available were 1.) a canvas-like, firmly woven cotton (450 g/m2), 2.) a linen fabric (315 g/m2), 3.) a polyester fabric (230 g/m2), and 4.) a nylon textile offered by G. Dyson (510 g/m2). The cotton and linen textiles had the best aesthetic appearances, the others looked somewhat "artificial" and synthetic.
The most important characteristic, however, appeared to be the material's mechanical strength. In order to evaluate this feature, I tested its resistance to tearing. If a sharp object such as a rough stone or a shell comes into contact with the boat's skin and penetrates the surface, the object will continue to tear at the fibers and extend the initial hole into a tear. This effect appears to correspond best with the forces to which the fibers are exposed when the fabric as a whole is torn apart. Therefore, I prepared samples of about 20 cm 10 cm and made a longitudinal incision of about 5 cm in each. One corner was fixed with a tie at a firm point; a second string was fixed at the other corner. Then I stood on a pair of scales which I had placed exactly below the fixing point and, while looking at the scale, I pulled at the free string until the fabric began to tear. The final value was calculated by subtracting my body weight from the weight indicated on the scale at that point.
The following data (refering to uncoated fabric) were obtained: 1.) the canvas-like cotton: 9 kg, 2.) the linen fabric: 16.5 kg, 3.) the polyethylene material: 6 kg, and 4.) the nylon fabric: more than 80 kg.
Before definitively selecting a material, I had to choose an appropriate impregnation. G. Dyson experienced the most success with a neoprene coating. However, it was impractical to have his material sent because of the flammable solvent. Therefore, I had to run my own experiments. I tested the following agents: 1.) linseed oil, 2.) linseed oil primer, 3.) Latex primer, 4.) Acrylic primer, 5.) Coelan ``Bootsbeschichtung®, 6.) and 7.) two ``GummilösungenÔÔ, neoprene-based: Patex®, Henkel, and the polychloroprene kautschuk, GXGQ 8802, Conti-Tech, Breslauerstr.14, D-37154 Northeim, Germany.
The first finding I made was that these liquids penetrated the fabrics to significantly varying degrees. The low-viscosity solutions 1, 2, and 4 penetrated completely, agents 3 and 5 to a lesser degree and 6 and 7 to the lowest degree. The latter compounds formed a layer in direct contact with the material, however, they could not diffuse into the closely packed bundles of fibers.
On first sight, the latter effect appeared to be disadvantageous but testing the tensile strength of the impregnated tissues revealed an unexpected result: The strength of the materials treated with the completely penetrating liquids declined greatly. For example, the canvas' resistance to tearing was reduced from 9 to 4 kg, that of the nylon fabric from over 80 kg to 40 kg after impregnation with Coelan. On the other hand, application of the neoprene coating to the nylon yielded a result no less than 80 kg.
The most plausible explanation is the following: In the untreated fabric, the tearing force applied causes a shifting in the arrangement of the fibers at the point of incision such that tearing can only occur when a greater number of fibers are torn at a point in time. In contrast, when the fabric is impregnated, such a shift in the weaving cannot occur. At a point in time the external force affects only a few fibers, which tear under much less effort, one after the other.
A different characteristic of a boat's skin is its resistance to abrasion. In principle, this feature depends on the elastic properties of the material: An elastic material yields when it comes in contact with abrasive particles and the latter elements lose their energy. In contrast, firm material is affected by the undamped energy of the abrasive object. The effect is most evident when sanding a piece of rubber and a piece of varnished wood - the differences in the amount of material sloughed off are exteme.
Regrettably, I did not find a method for quantifying this factor simply. On a qualitative basis resistance to abrasion was expected to be best in the elastic materials, the Neoprene-based elastomeres (Patex and GXGQ). Linseed oil and linseed-oil based varnish are much less elastic. When exposed to greater deforming forces, they become plastic, retract no more and soon tear. This characteristic applies to Latex primer as well and is even worse in this respect. Based on these findings and the observation that the material doesn't undergo a reduction in resistance to tearing, I decided to use one of the neoprene-based glues. Since only the compound GXGQ formed a smooth surface when dry, I finally used the latter material.


6. Final Construction of the Baidarka

After describing some general physical principles and methods to solve the specific problems that arise when a construction needs to be flexible and firm at the same time, the following section will deal with the procedure I actually performed. As outlined above, my model was the two-hole baidarka, drawn and described by J. Heath (ref. 2). In principle, the method of construction should be applicable to other types of baidarkas as well.
In the event that somebody wants to build a boat using the description given here, it should be noted that some details applied and described on the following pages were not essential, but yet proved to be quite helpful. (E.g., I used a helling and moulds as primary supports of the frame). Also different materials may be substituted for those used here, according to local availability. However, the designer should consider the strength, weight and aesthetic appearance of the materials. The reader is especially referred to the descriptions of e.g. Ch. Cunningham (ref. 3) and W. Brink (ref. 1) who describe the building of a one-hole Greenland-style kayak and an Aleut baidarka, respectively. They give excellent instructions in great detail in the handling and preparation of wood and coverings. They apply the traditional method of tied junctions for connecting the wooden pieces. In contrast to the present approach, however, they do not focus on hull flexibility as is done here.

6.1. The Plan:

The first procedure I did was to transfer the plan I had obtained from the Washington State Historical Society and Museum to an exact 1:10 scale by photocopy, which greatly facilitated all of the following procedures (Fig. 7).

 
Fig. 7. Plan of the Kodiak baidarka as drawn by J. Heath with the present modifications included (see text). Arrows indicate positions of the deckbeams and cockpit supports, respectively. Reproduction of the basic plan by courtesy of the Smithsonian Institution, NMAH / Transportation.
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Next I selected five cross sections (section Nos. 3, 6, 10, 14, and 17) to prepare moulds. The diameter of the hullstringers (1.0 cm) and that of the keelson (3.0 cm) had to be subtracted from the outlines of the cross sections (Fig. 8).
 
 
Fig. 8. Plan of cross sections (see Fig.7) and of the moulds. Numbers correspond with No. of section planes in the original drawing of J. Heath. They had the following distances from top of stem: 91 cm (No. 3); 205 cm (No. 6); 317 cm (No. 10); 437 cm (No. 14); 544 cm (No. 17).
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In the course of building the boat I realized that I had to introduce some changes into the plan. First, there is evidently a slight error in the cross-sectional view which regrettably, I realized rather late. The point where plane 14 crosses hull stringer No. 3 had to be shifted outwards. The correct distance can be taken from the top view of the longitudinal plan which indeed shows no change in the course of the 3rd hullstringer towards the stern of the boat (Fig. 7). Second, I omitted hullstringer No. 5 which appears to be located too close to the gunwales and which is indeed lacking in the left cross sectional view. Third, the size of the cockpits proved to be too small. In order to interfere with the boat's view as little as possible, I only extended the longitudinal diameter to 63 cm, thus giving an oval shape to the coaming. Later, even this measure proved to be insufficient for an easy entrance into and - more importantly - exit from the boat. As a remedy, I replaced the 3rd and 6th deckbeams with pairs of cockpit supports, similar to those regularly provided for support of the coamings at their largest diameters.
The position of the ribs are not drawn into the plan. I placed them at a distance of 13 cm and indicated their position in the plan of the boat's top view. In this way, I could take off the exact point where the ribs and the gunwales interconnect.

6.2. Helling and Moulds:

As mentioned above, I used a helling to facilitate the construction. In principle, it consisted of four planks screwed together in such a way that they formed a firm block of 15 cm 15 cm 400 cm. Before mounting the moulds, I placed all plates on top of each other and drilled small holes (4 mm diameter) through all moulds at corresponding points (Fig. 8). I drilled 6 holes, two in the plane of symmetry, and two pairs, 1.5 cm distant to this line at the lower and upper margin of the moulds, respectively. After having mounted the plates to the helling by the use of rods, their correct position could be controlled by shining a light through the respective holes. This test could be repeated whenever the question of a shift in the plane of symmetry arose. This was of special importance after the upper part of the boat was completed and the frame turned around in order to enable a better mounting of ribs, hull stringers and keelson.

6.3. Preparation of the Gunwales and Attachment of the Inner Plank to the Moulds:

I had straight-grained pine wood of 3.0 cm 1.2 cm and 220 cm length available, which I could connect to each other by scarfing and gluing with epoxy-resin. Two pairs of these planks of about 600 cm length were prepared for each side. Next, the position of the ribs was taken from the plan, indicated on the gunwales, and incisions of 22 mm (width) 10 mm (hight) 6 mm (depth) cut into the outer side of the inner plank in order to enable the attachement of the ribs to these holes later. The inner planks were tied to the moulds at the respective incisions. No problem arose with respect to bending the divided gunwales into the predetermined shape.

6.4. Cutting the Deckbeams and Fixing them at the Gunwales:

I prepared the deckbeams from solid pieces of fir, according to the dimensions given in the original drawing of J. Heath (2.85 cm 4.1 cm in the mid part, 1.25 cm 5.1 cm at the ends). I placed the deckbeams at the following positions (see also Fig. 7). The following data refer to the distance (cm) from the top of the boat. 1.): 102, 2.): 154, 4.): 261, 5.): 336, 7.): 463, 8.): 513. Deckbeams 3 and 6 were changed to cockpit supports (see below) and were placed at 204 cm and 404 cm. After fitting their free ends exactly to the inner surface of the gunwales, they were fixed with epoxi resin, a median copper screw and two wooden dowels of 4 mm diameter.

6.5. Cutting Stem and Stern and Connecting them with the Gunwales:

The medial parts of stem and stern were made of waterproof plywood, 10 mm thickness. Next, using epoxy I glued solid pieces of lightweight wood onto both sides of these plates. At the stem, the maximum thickness was 8 cm, in order to enable cutting of the specific shape of the stem. Recesses had to be created for the connection with the proximal ends of the gunwales. The shape of the stern was much easier to form. Planks of 10 mm thickness were glued onto both sides of the medial plate and the margins were rounded.
The connection of stem and stern to the gunwales proved to be rather difficult. First I tried to trim the distal ends of the planks such that they fit exactly to the surface of stem and stern. After applying epoxy resin onto the interface, I tightened the connections with screws. Since my sawing technique (by hand) was not correct enough to attain exactly symmetrical oblique sections of the gunwales, I did not succeed in arranging stem and stern accurately in the plane of symmetry. Therefore I changed my procedure as follows: I filled the (just unequal) space with epoxy-resin thickened with microballoons to a creamy consistency. Then I adjusted and attached stem and stern so that they were exactly in the plane of symmetry. This task was made easier by use of the holes in the bulkheads (see above). All pieces were then kept in place until the resin had hardened. I applied the screws at the end of this procedure.

6.6. Preparing and Mounting the Deck Stringer:

I made the deck stringer from straight grained pine wood, (2.2 cm 3.2 cm), the same material used for the keelson. The proximal section of the front part was formed by a separate strip such that the result was a deepening between stem and proximal section of the deck stringer (Fig. 7 and 11). The stringer was connected firmly to the stem and stern with epoxy; to deckbeams 1, 2, 6 and 7 with lamellar connections as described above; and to the cockpit coamings with the elastomer in solid form. All connections between the deck stringer's mid-section and the deckbeams and cockpits were also made with of solid elastomer (see also Fig. 1).

6.7. Building the Cockpit Coaming:

I sewed strips of ash-veneer, 2.5 mm 50 mm so that they formed a length two times the circumference of the coaming. After flattening both ends, I attached one end with clamps to long nails which had been positioned in a plate in the desired oval shape. The second layer was glued with epoxy resin and attached with clamps. The same procedure was applied to glue 4 layers of strips, 20 mm in width around the upper margin of the coaming. (In order to accelerate the latter procedure, I glued all layers in one session - a non-advisable approach!
 
 
Fig. 9. Cockpit of the baidarka showing the modified supports which proved to be necessary for an easy entrance and exit.
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Eliminating gaps between the strips by compressing them led to the formation of new ones at other places! This problem can easily be prevented by waiting for the glue to harden before adding the next layer). Finally the upper margin had to be planed to a half cylinder.
For lateral support of the cockpit coaming I cut pairs of curved wooden pieces which I glued between the coaming and the gunwales with elastomer. Small strips of wood (1 cm 1 cm 50 cm) had to be glued inside the gunwales in order to impede the strong bending that would otherwise occur when these parts were exposed to the body weight during entrance into and exit out of the boat. To support the front section of the cockpit coaming (and as a replacement of deckbeams 3 and 6) I installed pairs of hard wood (ash, in the mean 5.0 cm 1.5 cm 20 cm) between the gunwales and the front part of the coaming (Fig. 9). The medial ends of these pieces had to be joined with epoxy in order to attain a sufficiently firm construction; I connected the lateral ends to the gunwales with elastomer. Having finished these steps, I turned the entire construction upside down, moulds included, in order to mount the remaining parts of the boat. In this step, the use of the measuring holes in the bulkheads were of special value.

6.8. Preparing and Mounting the Hull Stringers and the Keelson:

For hull stringers, I used pine wood which I could buy in strips of 200 cm length, already cut to a half cylinder. They had to be scarfed and glued together with epoxy resin and their edges slightly smoothed by the use of a plane. With respect to the keelson, I used straight grained pine wood, composed of two layers as described above. Three sections were scarfed and glued with epoxy resign. A plane was used to attain the specific cross-sectional shape of the keelson. The finished hull stringers and keelson were tied with cords to the points indicated on the moulds (Fig. 8).
After finishing the latter steps, I connected the keelson to the stem and the stern. This procedure has also already been described above.

6.9. Preparing and Mounting the Ribs:

Since my goal was to restrict the inner space of the boat as little as possible, I wanted to build the ribs rather flat. Therefore, I had to use a relatively firm wood. Again, I used ash, a hard, yet elastic wood (veneer of 2.5 mm thickness). For each rib I prepared two pieces of 20 mm width each which could easily be bent without hot steam. Only the mostly curved ribs (1-5 and 37-39) had to be bent with this method. Even these pieces were easy to prepare, since simply immersing them in water of about 60C for a few minutes was enough to enable proper bending. During drying, the strips were held to their shape with tape.
Mounting was performed as follows (Fig. 2): In the first step the elastomer was applied to the marked area on the hull stringer. A small piece of cardboard (5 mm 5 mm 1 mm), placed just into the elastomer, served as a spacer which prevented direct contact of the wooden pieces that might occur during mounting due to clamping. To prevent the glue from touching the wood outside of the joint, I used tape. The outer layer of the rib was glued first and held in place with small clamps. After polymerization of the elastomer, the overflow glue had to be removed - despite the tape, an ennervating task. It's possible that incomplete filling of the cleft, such that no overflow could occur, would have sufficed also. The inner surface of the outer stripe was sanded, the inner stip attached with epoxy resin and its surface smoothed by sanding. The end result is demonstrated in Fig. 10.

 
Fig. 10. Inside view of the baidarka towards the stem.
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Having completed these steps, the outer plank of the gunwales could be mounted. In order to not interfere with the elastic attachment of the ribs' ends in the gunwales, I used the elastomer for connecting the outer plank of the gunwale with the inner one.
The last step in finishing the skeleton was to make the wood waterproof. For this purpose I applied a thin layer of the epoxy-resin; only one application was necessary. However, this material proved to be less well suited than I had thought - the entire skeleton became less elastic due to the spreading of the resin over the surface of the elastomer joints. Strongly bending the entire boat greatly restored the flexibility and taught me that a varnish which remains soft, e.g. linseed oil, would have been much better. The finished skeleton is shown in Fig. 11.

 
Fig. 11. The finished skeleton of the baidarka.
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6.10. Sewing and Sealing the Covering:

The nylon fabric I ended up using is available in a width sufficient for a complete enclosure of the boat. The sewing method I applied was recommended by G. Dyson (ref. 4): a cross-stitch with two threads. For a needle I used a strong, straight darning needle. I sealed the border of the cut fabric with the GXGQ-glue. A few minutes after its application onto the cut margin of the material, the outer 2 cm of the fabric were folded. The fold stayed in place due to the gluing efficiency of GXGQ - an advantage over the method of cutting the fabric with heat (solding iron). - Adaptation of the material to the boat's shape was excellent; therefore the lengths of the sutures were relatively short and special darts only had to be applied behind the back cockpit.
The application of GXGQ proved to be quite difficult. A prerequisite is that no solvent (toluol) has evaporated before use, otherwise an even coating isn't possible. During the application process, gloves and an adequate gas mask have to be worn. The application has to be repeated several times, as soon as the preceding coat has lost its fluid consistency. Additional applications after several weeks necessitate an intense sanding of the surface. - When the layers are too thick, bubbles develop which have to be carefully removed before the solvent has completely evaporated. Primarily, the color of GXGQ is rather ugly: a mixture of gray and green. However, upon exposure to light, it slowly changes to a light brownish tone which gives the boat quite a natural appearance. Each subsequent coating, however, attains a different level of color; in other words, when it is applied only to circumscribed areas, its extensions remain permanently visible thereafter.
After having finished the coating, I fastened a strip of nylon material around the cockpit coaming. For this purpose, I prepared strips of rubbered nylon, 1.5 cm wide. In the still uncoated material I pulled out some fibers at a distance of 1.5 cm, applied the glue GXGQ and, after it had dried, cut the fabric along the indicated lines. These strips were glued around the cockpit coaming in three steps, each separated by a phase of drying: 1.) attachement of one end in the deepening of the coaming; 2.) gluing the remaining strip while applying constant tension during the hardening process; 3.) gluing the end and the entire external surface of the coaming.

6.11 The Seats:

For seats I used waterproof plywood, of 2.0 mm thickness, which I placed into the boat's interior. In order to prevent the plates from shifting, I glued small pieces of wood (2 cm 1 cm 1.5 cm), onto the underside in such a way that they exactly enclosed the keelson, one pair per free space between the ribs. Each plate was held in place by a screw and a nut. The scew with its head fixed downward on the keelson (by use of a separate piece of wood) was placed into a slot, that was sawed into the plywood along its median line. In order to improve handling of the nut, I enclosed it in a wooden knob. A cushion on the bottom plate and at the cockpit coaming made the seat quite comfortable. I did not prepare spray shirts by myself but had them individually manufactured by a firm offering paddling equipment (Helmi-Sport, Neustadt a.Rbge, Germany). -
The baidarka was ready to launch (Fig. 12)!

 
Fig. 12. The finished baidarka - on return from launching.
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7. Concluding Remarks

When paddling the boat, it was pleasant to note that no blowing noises arose when it passed a wave and its stem reentered the water. It was a nice feeling to realize that the hull's flexing movements in the feet which was due to the fact that on bending up and down, the distance between seat and feet decreases and increases. During these movements no creaking noises became discernible.
Although I have not yet used the boat under extreme conditions, when paddling on a lake in smaller waves, it soon became evident that the boat does not destabilize when its stem or stern is taken up by the waves. The built model of a Kodiak baidarka is relatively broad and one might argue that this feature caused the latter effect. What I believe, however, and what I have confirmed in these first experiences is that it is not so much the width of the boat that has caused the described effect but rather the flexibility-induced prevention of steady \emph{changes} in lateral stability, that occur in a rigid kayak when passing through the waves.
These practically observed and theoretically postulated advantages of a hull's ability to bend when exposed to external stress raises the question why no greater interest exists in the attainment of flexibility for boats other than baidarkas. With respect to improved lateral stability, this effect is supposedly much less important in other boats since it is attained by other means: in sailing keelboats the center of gravity is placed beyond the center of displacement and dinghys and surf boards are stabilized by their relative shortness and extreme flatness. Therefore, I suppose that flexibility as a stabilizing factor remains of special interest primarily in those sea kayaks in which the center of gravity lies above the waterline and which are rather long and narrow. When boats like these light baidarkas are exposed to the especially difficult maritime conditions as those of Alaska's coasts stability appears to be a quality of greatest significance.


Acknowledgement:

I would like to thank my son Thomas for installing the computer text program and for introducing me to its usage. I am indepted to Marie for reviewing the English style and to Anja for scanning the figures and arranging them in the text.


References:

  1. Brinck, W.: The Aleut Kayak. Origins, Construction, and Use of the Traditional Seagoing Baidarka. Ragged mountain Press, Camden, Maine (1995)
  2. Chapelle, H.I.: Arctic skin boats. In: The Bark Canoes and Skin Boats of North America. E.T. Adney and H.I. Chapelle (eds.), Smithsonian Institution, Washington, D.C.pp 174-211 (1964)
  3. Cunningham, Ch.: Building a Greenland kayak. Sea Kayaker, 18-29 (Winter 1992) and 32-63 (Spring 1993)
  4. Dyson, G.: Baidarka. Edmonds, Washington, Alaska Northwest Publishing Company (1986); German Edition: Touristbuch Hannover (1989)
  5. Dyson, G.: Form and function of the baidarka. The framework of design. Occasional Papers of the Baidarka Historical Society, Bellingham, Washington, Nr. 2. (1991)
  6. Lubisher, J.: The baidarka as a living vessel. On the mystery of the Aleut kayak builders. Occasional papers of The Baidarka Historical Society, Nr. 1, Port Moody, British Columbia, Canada (1988)
  7. Zimmerly, D.W.: Qajaq. Kayaks of Siberia and Alaska. Division of State Museums, Juneau, Alaska (1989)


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Friedrich Vetterlein
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