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Manufacturing of the bottom part

bottom Ready bottom was kept in a water ditch before expanding -- it had to stay fresh and wet.

Pine tree for the bottom was some 55-60 cm thick at the stump, to be 45cm at the 4m height, where the future boat is going to have its middle.

Fresh trunk can weight about one ton, so it had to be roughly hollowed and shaped right on the spot, to become lighter and managable. Then it has to be thoroughly measured and marked, outer surface has to be faired from grooves and bulges. As it is going to be hollowed 13-15mm thin, even some 1-2 millimeter deep groove (nearly invisible for naked eye), running for several meters along wood grains, makes quite a hazardous weaker belt, where cracks will appear; one have to feel for such grooves with hand -- they are better palpable then visible. Several centimeters of spare wood are to be left at the ends -- stems will be finally shaped only after expanding. There are two known ways to control wall thickness during hollowing and fine inside finishing:

  • 1) Drilling of a plenty of small (5-6mm) holes in chess order from outside, and plugging them with wooden plugs of certain length, equal to the thickness of the future walls. One can see these holes appearing gradually when hollowing the inside, and finish the surface flash with inner faces of the plugs. (Plugs can be made darker with tar, for example, for better visibility). It is rather time consuming way, holes must not be more then 20cm apart for adequate accuracy, so over 200 holes and plugs were made in the bottom piece. This way was common in northern Russia and Siberia, but no holes and plugs were found in preserved parts of the original Tuna boat.
  • 2) Poking the wall through with a thin awl, whose length corresponds to desirable wall thickness. This way is quicker (and therefore, more accurate), but it is only applicable for wood soft enough and walls thin enough. Anyway, it works with fresh pine 15mm thick and aspen up to 30mm. It was described by Nikkilä in Finland, and was most probably used in the original boat.
Both ways were tried in this reconstruction, and both were found feasible; neither holes/plugs affect noticeably the strength of hull, nor awl holes are visible after the awl is withdrawn. Awl way is better with Tuna boat thin material dimensions.

Ready (before expanding) bottom was ~6.3 m long, 45cm in diameter amidships. Its thickness was ~15mm below, gradually reducing to 12-13mm at the edges. Space between these edges at the top was only 20 cm, so the circumference was 110-115cm amidships. It weighted less then 100kg, and could be carried for 0.5km by four people without problems. The whole work of its manufacturing took about 2 weeks.

trunk Natural grooves and bend are exaggerated on this picture.

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