Sunday, 6 January 2013

Steaming the Ribs

The oak used here is 'green' (not seasoned) and the high moisture content will help achieving the tight curves required for the kayak.  The steaming process will convert some of the moisture to steam and the ribs will end up considerably drier (and more 'seasoned') once it has been 'cooked' and then bent into it's new shape.

The video below shows the process of steaming and fitting the ribs.

This has to be quite a fast process as the ribs quickly cool and 'set' once removed from the steam box.  Time to move quickly and put some gloves on as the oak comes out scalding hot.

The length gauge (above) has been made to mark the ribs to length and ensure an even transition between the deeper bow and much shallower stern.

Once the ribs are cut to length the ends are thinned.  By doing this the ribs take up a much shallower arch across the bottom (centre of the curve) and then a sharper radius as they form the sides of the kayak and run into the mortice.

The Steam Box (made with 50mm silver backed insulation foam, duct tape and a wallpaper stripper).  It will last for this job!

Each end of the hot oak is first bent around a former to make the grain more flexible.  A leather strap supports the timber while bending at the back.

The rib is then quickly moved across to the kayak deck frame, slotted into the first mortice and then using a timber wrench bent up into an arch and gently pushed into the mortice on the opposite side.

About one in four ribs fail and split at the critical stage.  Nothing else to do here other than start again and cut a new rib to length and thin the ends down.  It is difficult to make spares in advance as it is impossible to know which ribs will fail during the bending process and they are all different lengths.

As soon as the rib is fitted into the frame and while it is is still hot and reasonably flexible (which is only minutes), it is important to sight along the kayak and adjust the profile of the rib so it is symmetrical and running fair with the others.

Slowly the whole rib cage of the kayak comes together.

When the ribs have 'set' they will eventually all need to be removed and sanded before finally being pinned with an oak dowel into the mortices.

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