Sunday, 15 February 2009

The collapsing cratch

The collapsing cratch
15th Feb 2009

My last entry made reference to my home made collapsing cratch plate and, given the interest in my boating DIY projects, I thought I would take a look at how I made it.

I think that I should point out that one of the greatest loves of my life (apart from Belle of course - thanks for a lovely Valentines day!) is my carpentry. I have been a wood junkie ever since my dad let me start to use / wreck his tools when I was about 6 or 7 - and the scope to do woodwork at Secondary school was a major factor in the planned failure of my 11 plus. As a result I have a well equipped carpentry workshop, and having the right tools makes a big difference to the end results.

So the brief was a cratch that was aesthetically pleasing (no sticking up bits for extra headroom), a glazed front to allow light in and to facilitate a forward view from the cabin and finally it had to be collapsible, so the whole assembly can be removed in nice weather, maximising the usable space in the restricted well deck.

As with any building project, one starts with the foundations. How to attach the cratch to the boat? The front of the well deck has a raised lip so I fashioned an "L" shaped bit of wood which fitted snugly across the top and against the front edge, shaping the ends to match the contours of the boat. I then a made a couple of shaped plates which could be hooked under the gunnels and then screwed into the top plank. The whole thing was then painted up to final gloss coat and fitted to the boat using exterior grab adhesive. This gave me a neat base for the cratch plate itself, and was attached to the boat without having to drill any holes in the metal.

Next came the central front post and top board. It was important to get the lines looking right so the dimensions were worked out by laying the boat pole partially over the well deck to get a height that is consistent with the height of the cabin roof. I then using a set square to get a 90 degree angle up from the foredeck. The foredeck slopes back a bit so this means I ended up with a slight backwards rake, which avoids the rather startled "sit up and beg" look which is often seen.

The top board is a simple pine plank hinged at the rear onto a wooden base plate made from some surplus hardwood work surface. This base plate has some runners attached to reflect the curve of the cabin roof and then attached from under the overhang using four non rusting brass screws. Because the cratch carries the tunnel light I needed to provide a power source. This was carried from the cabin into the mounting block, round the back of the hinge and along the top of the plank. As these sections move the wiring is vulnerable, so heavy duty multicore cabling was used (the yellow sort used on building sites and demanded by the BSC) and this was channeled into a groove routed into the top of the board. It was then fixed onto the groove using silicon and capped off with a thin strip of wood.

The front post was made by laminating (a posh expression for fixing together) two planks. I used a wider, thinner one at he back and a thicker, narrower thicker one for the bit on show at the front. This created a natural rebate for the glazed sections to sit into. To get a flush finish I used exactly the same thickness of timber for the front post and the frames of the glazed sections.

Because it all had to collapse, I cut the post about 2/3rds of the way up and put a hinge on the bottom. This allows the lower part to fold back down and hang flush with the front of the well deck. The top section was then hinged onto the top plank and the light attached to the front. I wanted the two bits to locate back together accurately so I created a rebate using the two bits of timber used in its construction, and added a couple of dowel prongs (male) in the top which fit neatly into two holes drilled in the the base the top bit (female). The pictures will make sense of this explanation!

So now we have a base plate attached to the front deck, the hinged top plank fixed to the cabin roof and the splittable front post connecting the two together, also carrying the tunnel light. You may be thinking "so what does he use for a tunnel light when its all collapsed?". A good question and an easy one to answer. If I need to pass through a tunnel with the cratch removed I simply flip the top plate back over and the top section of the front post drops down, complete with the light.

So that leaves the triangular window sections, possibly the trickiest bit of the job. With the base plate and front posts all working together I measured the width and height and cut the square profile timber to length. N.b. Always make the rule (the vertical bit) go the full length from top to bottom or you will encourage rot where it joins the stile (the horizontal bit). With the two bits of wood lined up on the bench you can find out how long the other side is (the hypotenuse if you remember your maths from school). The three sections are then fixed to each other using simple mortice and tennon joints, glued and screwed through.

To get a nice clean finish I then planed off the leading edge of the "hypotenuses" to conform to the lines of the boat, and provide a good point of attachment for the cratch cover. With the whole window frame fixed together I then routed a rebate in from the front and, after painting the frames, I had my local glazier cut some toughened glass to fit. This was bedded into silicone and held into the rebate with some pre painted square strips of timber, with one corner rounded off.

Finally, the windows needed attaching to the base plate and the front post securely, but in a way that was easy to remove. This was achieved by inserting four dowel pegs into the upright which corresponded with holes drilled on the the post. This has the effect of locking the windows in and stiffening the front post back into a solid unit. The bottom stile sits with is rear edge against a rebate attached to the base plate. Its important to make sure the glazed sections and the base plate don't sit in a puddle of water, so I created a gap by making them rest on a couple of L shaped angle brackets, which lift then up by 3mm and also prevents them sliding forward. I also drilled a hole in the base rail to line up with the outer angle bracket into which I inserted a long and well oiled bolt with a wing nut holding it in place from inside.

The last stage was to decorate it, which was achieved using the traditional harlequin design in colours used elsewhere on the boat plus a powder blue.

The big questions have to be:

Q. Does it work?
A. Yes - like a dream.

Q. How long does it take to disassemble / put back together?
A. About five minutes.

Q. How much did it cost?
A. About £60

Timber £15
Hinges (2) £3
Light Recycled from previous location
Wiring £5
Glass £35
Paint Used left overs

But it's not about the money.

As I said at the start, I an a wood junkie with PVA adhesive flowing through my veins. It's about the challenge of the task, the planning and problem solving, the making do with bits and bobs saved from previous jobs, its about sawdust in my hair and playing with my toys in the shed. Maybe you can now understand why I like Fred Dibnah!


MortimerBones said...

hmm interesting! I wonder whether a lass with a screwdriver and no talent could make one....

hmmm.... I think I might give it a go!

Captain Ahab said...

Give it a go, you know you want to....

English Blogger said...

That looks Bloomin wonderful, I wish I could do stuff like that.... I try to put a screw in and the wall falls down.