Heat-forming a strake for a clinker-built boat
A ‘schouw’ or scow is a clinker-built boat which primarily derives its ‘bond’ and strength from broad and heavy strakes. The timber frames of a scow are relatively light. Heat-forming the strake is necessary in order to obtain the correct curvature of the timber. Before the strake is heat-formed it is already sawn to the correct width. Sawing to length is only undertaken after it has been heat formed. During the heat-forming of thick strakes the timber is kept constantly wet and is heated regularly and at the same time placed under tension using weights, as a result of which it bends in the required direction. This process needs time. The heat-forming of a thick, wide, long strake can take a number of hours.
Materials and tools
A strong frame (preferably on a platform with wheels), a heavy-duty rack with a number of strong bars, large gas burner, fire basket, coal box, firewood, water hose with spray head, gluing clamps, heavy weights (blocks of iron or stone) on ropes, 1 m plank as extension piece, 2 m lath as a measure, pencil or chalk, length of reinforcing iron, strong plank or steel beam and a number of smaller planks.
- Before placing the strake to be heat-formed onto the rack, first of all light the fire in the top fire basket which is fastened to the rack by a bar that is inserted through it (image 4). The fire serves to evenly pre-heat the strake.
- Heat-forming a strake is undertaken by two people. First of all, place the strake with the side that you want to bend first onto the rack. Place the other side on the frame. Clamp a strong gluing clamp onto this side, on the end of the strake, and suspend a few heavy weights from it (image 1). Place a gluing clamp across the strake protruding from the frame in order to prevent the strake sliding backwards.
- Using a gluing clamp, attach a length of plank approximately 1 m long onto the side of the strake that is on the rack in order to form an extension piece. Clamp another gluing clamp onto the front of this extension piece and suspend a few heavy weights from it (image 3). In this way you will also be able to bend the forward-most section of the strake.
- Now place a lath or plank of approximately 2 m long against the plank acting as the extension piece. Using a pencil or chalk, mark a line where both planks touch each other (image 5). When the strake bends during heat-forming you will be able to tell from the mark the number of centimetres by which the strake has curved downwards.
- The real heat-forming can now start. To do this, thoroughly wet the top and underside of the strake using a water hose with spray head. Now heat the strake on the top and underside evenly using the gas burner (image 2). Heat the section of strake that is on and just behind the top bar of the rack. Move the gas flame back and forth over the entire width of the strake and along a length of 20 to 30 centimetres. Do not keep the flame in one place for too long otherwise the timber will discolour and ultimately set alight. The correct distance from the burner to the timber is approximately 10 centimetres (image 9). Remember that bending a thick strake takes time; bending is not quick and requires patience and time.
- Regularly check how far the strake has bent. Do this by comparing the height of the extension piece (and later the height of the actual strake) with the mark on the measuring lath (image 6). Make a new mark in order to give yourself a new measuring point.
- Remember that the structure of the strake timber is not the same throughout. There can be hard and soft areas in the timber. Be careful with knots because these make the wood react differently compared to a normal piece of timber. If there are cracks in the strake you will have to fix there using a small plank.
- Using a length of iron reinforcing bar that has been bent to the correct curvature (by forming this to match the shape of the location where the strake is to be fitted) check that the correct curvature has been reached (image 7).
- When the forward-most part of the strake has achieved sufficient curvature, slide the strake 20 to 30 centimetres further through to the side of the rack. The easiest way of doing this is to turn the top bar in the rack (image 10). Two people, one on each side of the strake, can also push the strake a little forward.
- Now thoroughly wet the area to be heated, on the top (image 12) and on the underside (image 13). Check that there is sufficient heat coming from the fire basket; add more firewood if necessary (image 18).
- Heat the wet timber once again using the gas flame (image 11), spray the timber regularly with water (image 20) and after a period of time check the curvature using the length of iron reinforcing bar (image 14) and on the basis of the marks on the measuring lath (image 15).
- Heat-forming can cause the timber to distort. You can correct this as follows. Secure a strong length of wood or steel bar (image 23) crossways onto the strake such that it protrudes from the side that you want to curve down further. To protect the strake, first of all place a few small planks of timber (image 22) underneath the steel bar (image 24) because oak turns a blue colour when it comes into contact with iron and water. Secure the bar using a gluing clamp (image 25) and suspend a weight from it (image 26). Continue keeping the timber wet and continue heat-forming (image 31). You can of course also use this procedure if it is your specific intention to allow the strake to warp (twist).
- With a critical eye, check both the front (image 27) and the side (image 30) of the strake to see whether the strake has been re-straightened sufficiently and whether both sides of the strake are back to the same height.
- Repeat these operations until the strake has achieved the correct curvature. Remember that you can still bend the strake further but you can’t lessen a bend once it has been made.
Strake fixed to frame
Heating the wet strake
Strake extension piece weighted down with heavy weights
Rack with fire basket and coal box
Mark on measuring plank
Mark on measuring lath
Checking the curvature using a length of iron reinforcing bar
Correct distance between burner and strake
Moving the strake
Heating the second section of the strake
Spraying the top of the strake with water...
...and the underside
Check the curvature using a length of iron reinforcing bar...
...and by the marks on the measuring lath
Fire burning well in the fire box
Keep spraying the strake
Place small planks under the steel bar...
Gluing clamp on steel bar
Weight on bar
Check from the front...
...and from the side of the strake
Keep wet and continue to heat-form