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Traditional Maritime Skills :: Heat-forming a strake for a carvel-built boat

Heat-forming a strake for a carvel-built boat

Introduction

Before it was possible to start the restoration of the 'Vabel' the original shape of the smack had to be determined. The boat had sagged considerably and had a hog: the fore and aft ends of the boat were sagging down and the washing strake had bulged outwards. The boat was brought back into shape using tie rods. The garboard could then be replaced, after which a new mast frame was made. The first seven strakes could then be replaced on both sides of the keel. The frame-timbers could then be replaced one-by-one, starting with the floor timbers, which are also called the bilge frames. After this, the knees and the futtocks could be replaced. This was followed by restoration of the wash strake. Then it was the turn of the deck, followed by the further superstructure.

A smack is a carvel-built boat that mainly gets its cohesion and strength from the heavy frame arrangement. The strakes of the smack are relatively narrow and thin. It is necessary to heat-form the strakes in order to obtain the required curvature in the timber. For this purpose the strake is first rough sawn to the correct width. The plank is only sawn to length after heat-forming, which involves the plank being made wet, heated and bent with the help of weights.

Procedure

  • Place the plank on two racks or frames. The frame with the part of the strake to be heat-formed is to be higher by approximately 1.20 to 1.50 m so that you can have good access to the top and the underside and have a good view of both sides of the plank (image 3).

  • Clamp the extension lath (lever) to the end of the plank using a gluing clamp. Weight the extension by placing a weight on it or suspending a weight from it. This weight can be a heavy stone or a piece of steel for example (image 2).

  • On the other end of the strake, on the low frame, place a heavy counterweight (image 5) to hold the strake in position (image 4).

  • Bend a piece of reinforcing iron rod into the correct curve to match the curvature of the position into which the strake is to be fitted. The iron rod is used to see whether the plank has already reached the correct curvature (image 15).

  • Spray the underside and top of the strake on the high frame end with plenty of water using the hosepipe. Constantly repeat the spraying (image 6).

  • Heat the wet timber of the strake on the underside and top using a large gas burner (image 8).

  • Position the long lath that serves as the measuring lath at an angle against the forward position of the strake. Place a mark where the ‘measuring lath’ touches the strake (image 11). When the strake is heated this will lower because of the curvature being formed (image 10). This can be seen readily by the line on the measuring lath (image 13).

  • Now you can alternately spray and heat the strake, especially on the underside of the strake. The plank will gradually start to bend (image 14). The underside of the plank discolours because of the heating, often from becoming darker through to becoming black (image 19). This is planed off before the strake is fitted.

  • The entire process of soaking the plank, heating it and bending can take a number of hours depending on the length of the strake, the amount of curvature, the thickness and the width of the plank and on the type of timber being used.

  • Once the forward part of the strake starts to bend sufficiently you slide the plank forward over the frames slightly further (image 16). Reposition the weights if necessary (image 22). Also reposition the measuring lath to a new ‘measuring point’ and re-mark where the strake and the measuring lath touch each other.

  • Repeat the process of soaking and heating.

  • Check the required curvature using a piece of curved reinforcing iron rod.

  • Attention (1) – As a result of the heating the strake can sometimes ‘twist’. In other words the plank not only adopts the required curvature but also starts to rotate. This can be corrected by clamping a small transverse lath to the strake using a gluing clamp, onto which you suspend a weight. As a result of this the ‘twist’ is counteracted so that the plank becomes straight again (image 20).

  • Attention (2) – If you are too impatient to get the plank to bend then this may cause splits or the plank to break. Heat-forming timber takes time – and therefore patience. Therefore make sure that you do not keep the flame of the burner on the same spot for too long and that you do not put too much weight on the plank or the lever.

  • Attention (3) – Once a plank has been heat-formed in one direction it cannot be bent back again. Do not therefore make the curve too large. Once the curvature in the plank appears to be roughly correct then remove the plank from the frames and try it up against the boat frames at the position in which it is to be fitted. The plank can be bent further but it cannot be bent back again.

  • Only saw off the ends of the plank after heat-forming. This means that you will be able to saw the plank to the correct length and will not end up with a plank that is too short as a result of too much timber having been cut off (image 21).
 
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Tools Required

  • Two racks or frames on which the planks can be placed
  • Burner
  • Hosepipe
  • Gluing clamps
  • Weights
  • Pencil
  • Saw (handsaw, jigsaw or circular saw) (image 1).

Materials Required

  • Lath of 1 m length as extension
  • Lath of 2 m as measuring lath
  • Piece of reinforcing bar with the correct curvature

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