Making a curvature in a sawn strip of wood so that this rib fits onto the hull of the ship.
What steps does the shipbuilder take if he wants to make a curved rib from a straight piece of sawn wood by heating that strip of wood?
What tools, implements and machines does he require for this?
We shall first look at the implements, machines and tools required.
The successive steps for steaming will then be explained.
Ribs: Ribs have to be bent from straight, sawn strips of wood. Bending is made possible by first heating the strip in a steam tunnel.
After sufficient heating time the required curvature can be made in the strip by bending it on a bending jig. The required curvature is determined by measuring using a curvature jig.
Steam box: The steam box in the example is an almost square longitudinal box.
The steam box is built from strips of plywood that are joined using stainless steel screws. The joints are sealed using silicon. The box is two metres long. The steam box is double-walled, with insulation fitted in the cavity. The disadvantage of a single-wall box is that too much heat will be lost. The insulation material to be used must be moisture resistant in order to maximise the effect of the steam in the box.
For safety reasons a hole is drilled in the middle of the bottom plank of the box in order to discharge excessive steam.
The box is fitted with transverse supporting laths. These laths have a mutual spacing of 15 centimetres.
It is preferable that these supporting laths can be rotated, which makes it easier to insert and remove the strips of wood.
Water tank: This reservoir is the steam generator. In the example a gas bottle is used. A beer barrel or a similar metal tank can serve as an alternative. A flexible connection is fitted to the tank to convey the steam to the steam box.
A filler also has to be fitted in addition to the flexible connection. A stopcock is fitted between the filler and the tank in order to prevent energy loss when the water is being heated to form steam.
A heat source: A heat source will ensure that water in the tank is converted into steam. This steam flows into the steam box via the flexible connection and will heat the wood so that it becomes pliable. In the example the water is heated using a gas burner. Alternatives can be: electricity, coal, timber, etc.
Water: The water is converted to steam in the tank by heating.
Curvature jig: Bending to the required shape can be started once the wood has been heated sufficiently. An initial tool for this is a bending or curvature jig.
The people in the workshop have developed their own device for this, which can be adjusted to any shape by means of sliding laths.
The bending jig is placed up against the skin of the ship and the adjustable transverse laths are slid against the skin with a mutual spacing between them of 10 centimetres. In this way the tops of the laths form a smooth line which shall be the determining factor for bending the steamed ribs.
Bending table: The bending table is similar to and has the same functions as the drawing bench for metal. In the bending table in the photograph slots have been created through the table and these contain moveable locking pins. The movement of the locking pins allows any required curvature to be achieved.
Wood: Two types of wood are being steamed in the example. On the top is ash, which shall be bent to form mast hoops and on the bottom oak which will be bent to form ribs.
Wood selection; Wood that is to be steamed must meet two main requirements:
1) the moisture content must be between 20 and 30%
2) the workpiece is flawless, without knots/burrs and has straight ends
It is also important that the strip or plank has the same thickness along its length.
Place the strips of wood into the steam box; The strips of wood to be bent are placed on the transverse laths. Rotating transverse laths make it considerably easier to position the strips. (image 1)
Fill the water tank with water; (image 2) The tank must be filled sufficiently to allow the steam process to be maintained for a sufficient length of time. A general rule of thumb is that to make a 2.5 cm thick strip of wood very pliable it will have to be steamed for approximately one hour. In the example 10 litres of water was poured into the tank. After the tank is filled the stopcock on the filler is closed to prevent steam from escaping via the filler and to thus ensure that maximum steam enters the steam box.
Place the tank on the burner; The tank is placed on the burner and the heat source is ignited. Steam is produced once the water reaches boiling point. It is from this moment that the time (1 hour for 2.5 centimetre thick strips) can be recorded. If the tank is transparent then it is possible to see whether the water is boiling. An enclosed tank is used in the example and the sound of boiling water is used to determine the moment at which steam has most likely started to be produced. This can be confirmed by opening the stopcock on the filler very slightly. (image 3)
Bend the strips; (image 4) After the steaming process has been allowed to finish the heated strips of wood are removed from the steam box and placed on the bending table. One end of the strip is clamped between two locking pins. (See bending table).
Set the shape using the curvature jig; The curvature jig is used for setting the locking pins on the bending table. (image 5)
Bend the strip; The strip that is now pliable is pushed against the locking pins. (image 6)
Set the final shape; When the end is pushed against the last locking pin the strip is fixed into place by fitting another locking pin on the opposite side. (image 7)
Bend the round mast loop; (image 8) The mast loops are bent from the ash strips. A jig with a notch has been produced for this purpose. In addition to the notch there is also a locking strap. In the example a copper strap has been selected.
Position the strip; One end of the ash strip is pushed into the notch. (image 9)
Bend the strip; (image 10, 11, 12 & 13) The strip is bent around the jig. Once the full bend is complete the strip, protected by the locking strap, is secured using a clamp.