Hull and frames design for 'hogging'
Usually there is no construction drawing available for old wooden boats. To still be able to get a line plan of the hull and frames down on paper the main dimensions of the boat have to be obtained, even if the boat is usually weakened and deformed.
When an old wooden boat has to be restored the shape of the boat has often sagged due to the weakening of the old wood. Before you can make a start on the restoration you have to determine the original shape of the boat. If there are no known construction drawings, line plans or specifications for the boat then you shall have to derive the most important dimensions of the shape from the collapsed boat.
The bilge of the old boat is often curved and the bow and the stem of the boat have sagged (image 5); this is called a ‘hog’ or ‘hogging’. As a result of this the longitudinal lines of the boat have become bulged. The sheer strakes of the boat have often sagged outwards because the original transverse bracing, which gives the boat its strength, has disappeared and the bulkheads have usually been demolished or become weakened. To determine the original shape and transfer this to a drawing you proceed with the following steps:
- First you support the hull properly using strong frames and blocks of timber so that you can make the boat level at the position of the mast frame (image 2).
- After this you level the bilge by supporting it with wooden wedges. To keep the bilge in place whilst levelling it you can place props to the roof if the boat is in a low shed. If that is not possible then you can 'ballast' the boat by temporarily placing large barrels of water in the bilge.
- You then place a centre line using a piece of rope. By using a perpendicular line in the forward and aft part of the boat (image 4) you can check whether the boat has twisted. This piece of rope is called a 'plumb line'. You use the centre line to be able to determine the angles of the frames and the hull. It is handy to leave the plumb line in place during the restoration in order to keep checking for twisting.
- When talking about "level" we are referring to 90 degrees from the centre line.
- The main dimensions for the shape of the hull are determined in the boat to be restored. At the location of the forepeak bulkhead, the main rib or mast frame (the most important and broadest frame, which also indicates the width or beam of the boat) (image 1) and the after-peak bulkhead you make a template from a large sheet of plywood of approximately 18 mm thick. By noting down dimensions on the plywood sheets and/or by repeatedly bonding small strips of plywood onto the large sheet so that the other end of the strip touches the inside of the skin of the boat, you will determine the shape and the dimensions of the boat.
- At the position of the main rib you make a basic template of the frame from a sheet of plywood. The underside of the template runs level with the bilge. After that, the template follows the inside of the strakes up to the wash strake.
- The fact that the distance from the bilge to the inside of the rubbing strake remains the same offers a clue, even if the boat has sagged. You can also lift that distance ‘up’ slightly.
- When the bilge is level and the centre line has been determined and the main dimensions have been determined at the position of the main rib you can draw out the lines of the boat on paper. The design of the lines plan for the hull and the frames therefore forms the basis of the restoration (image 3).
The position of the mast frame
The bilge is level
Lines plan is the basis of the design
Forward centre line starting point
Stern or after end is often sagging down