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Traditional Maritime Skills :: Heat Forming a Strake

Heat Forming a Strake

Introduction

Creating a curve in a sawn plank so that this (strake) fits onto the boat skin.

What steps does a boat builder have to take if he wishes to make a curved and twisted strake from a sawn straight plank by locally heating the plank?

What tools, instruments and machines does he require for this?

This is based on a ship that is already on the stocks. This example is a "hengst" fishing boat but it can relate to any wooden boat. The bottom/bilge and topside strakes have already been fitted. The strake to be burned now and fitted on the ribs is a sheer strake. The size of the plank has been determined by measuring using the method described in "measuring a strake". This plank now has to be curved and given a twist. This will be achieved by locally heating the plank. The curvature will be achieved quicker and the twist will be achieved by using weights. The tools required are present in the workshop. Heat forming  is undertaken outside, next to the shed, where the necessary equipment is also ready.

The end result shall be the starboard sheer strake, like the sheer strake already fitted on the port side.

Tools, implements and machines for heat forming a plank into a strake for a boat

We shall first look at the implements, machines and tools required. The successive steps for heat forming will then be explained.

Heat forming rack
The heat forming rack consists of a heat source and a rack with adjustable bars on which the strake will rest. A simple rack with a single point of support is shown in the left picture. The right picture shows a rack that is used for heat forming longer strakes. In this example the rack on the left is used.

A heat source
Various heat sources are possible. Traditionally a fire was made from reeds. Reeds were available in abundance on the riverbanks where the yard was located and reeds also burn very evenly. People now use gas, coal, timber or infrared as a heat source. Combinations are possible, for example a wood-burning fire combined with a gas burner. When building a wooden boat approximately two-thirds of the timber that is bought is lost during sawing. The surplus timber is used as an energy source for feeding the fire.

A number of other essential hand implements and tools are required for the operations...

A rod
The rod is an iron rod that is used to determine the required curvature on the boat by matching the rod to the existing ribs. The iron rod must be such that it can be curved by hand and retain its shape once it has been bent.

Weights
Weights are used to speed up the bending process and for setting the twist. Whatever is to hand is used for the weights.
The photograph on the left shows a weight from the gym, the photograph on the right a piece of railway track and an  unidentifiable piece of metal.

Extension(s)
If required, an extension is used to increase the effect of the weight and thus speed up the bending.

Clamps and brackets
Clamps are used to secure the weights, or the weights and the extension, to the strake to be heat formed. Brackets and clamps are also used during twisting. Applying a twist to a plank is also known as "making a snake".

Water
Water is used to attempt to prevent the timber from becoming very hot and burning.

Measuring stick
The progress of the bending is recorded using the identification marks on the measuring stick.

Plank
The size of the plank to be heat formed is determined using the method described in: 'measuring a strake’. To ensure ideal conditions for the heat forming the timber, oak in this case, must have a relative humidity of between 18 and 20%.

Procedure

Successive steps in the heat forming process until the plank is bent into a strake that can be fitted:

  • Place the plank which is still straight above the seat of the fire. The plank is placed on the rack so that the seat of the fire is located where the plank will be bent first.

  • Fit the weights. Heavy weights are attached to the underside of the plank so that this end remains on the ground and the curvature will be formed in the inclined section. To speed up the bending process an extension is attached to the top end, with extra weights on the end of this extension.

  • Position the measuring stick. A measuring stick is positioned vertically against the end of the strake in order to record progress.

  • Light the fire. The fire is lit using the off-cuts from the plank. Other potential heat sources are:
    • gas
    • infra-red
    • coal
  • Control the heat. The fire needs to be controlled in such a way that the underside of the plank does not smoulder or ignite. Gassing of the timber should be avoided. By spraying with water the heat is controlled and maintained at the required level. The majority of cooling is undertaken on the top side of the plank but the underside of the plank can also be sprayed if  necessary in the event of potential gassing of the timber.

  • Recording progress. To obtain the required curvature the bending process must be monitored precisely. By applying systematic indications on the measuring stick it is possible to record the bending progress. The rod, which imitates the shape of the boat skin, determines when the curvature is sufficient.

  • Applying the twist. The previous operations have given the plank a curvature. If a twist is required, as in this example, a number of additional operations have to be carried out during the heat forming process. The twist is achieved by placing weights on the end of the plank (offset from the centre line of the plank) during heat forming. In this way the plank will adopt a downward bend on the side with the weights.

  • Recording twist progress. The progress of the twist can also be monitored accurately using a measuring stick.

  • Fitting the heat-formed and shaped plank to the boat
 
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Tools Required

  • Heat forming rack
  • A heat source
  • A rod
  • Weights
  • Extension(s)
  • Clamps and brackets
  • Water
  • Measuring stick

Materials Required

  • Plank

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