Thursday, 24 May 2012

Forward Hatch

By far the biggest challenge so far was the forward hatch.  The designer John Welsford had designed a hatch that was traditional in design from what I have seen on the internet.  It had a hinged joint on the forward end (ie couldn't be forced open by waves from the front) with the interior higher than the join between the outside surfaces.  This meant that any water getting through the from the outside would hit the inside layer, which was higher, run down into the void and then back outside through drain holes.

The interior I decided to make from clear Hoop Pine, with the outside from a medium weight Australian hardwood.  The space between the interior and exterior was 15mm, created by spacers of 20mm x 15mm clear hoop pine.  The curved top of the hatch was three layers of 3mm ply bent to shape.

Below is the dry fit of the interior which I started with.  Interior dimensions of the hatch are 600mm x 600mm.  You can see the hoop pine spacer will be secured with stainless steel screws and epoxy.

Below the spacers will have the deck come through right up against the inside surface.  This will require a little bit of cutting as I cheated.  The designer's plan called for some very nice hardwood corner pieces cut to fit on an angle with the inside and outside surfaces coming in to meet them.

I looked at it an could only see me stuffing them up, so I decided that I would simply make two of the inside pieces butt up against the outside at ninety degrees and I would screw through to hold it together until the epoxy set.  I had to put a bit of thought into it and it will become apparent later how it goes together.

Below you can see the spacers epoxied (with glue powder) and screwed to the outside of the inner layer, along with the exterior epoxy coated.  These surfaces will come in contact with any water that makes it through the join between the top and bottom halves.  Epoxy should protect them hopefully.  

The two rectangular ones in the middle are the ends with the small one being forward, the sides show the slope which matches the deck camber.

Below is a close up showing the counter sunk drill holes for joining (top left and bottom right partly obscured) along with the join.  You will note a large gap that indicates my measuring mistake.  I later filled it with more thickened epoxy.

Once dry I epoxied, screwed and clamped the inside together.  Later the next day I came out to admire my work and found that I had stuck on one of the end pieces upside down.  Do NOT panic and rush thinking the epoxy will instantly set.  I had a problem with one part and in the rush stuck one of the four pieces on the wrong way up.

When you find a mistake, don't rush into trying to fix it either.  I almost lost it and tossed the whole thing onto the concrete but a few deep breaths, a hacksaw blade along the joint and through the screw holding it and I got the offending end off.  Put it right way up, drill a new hole as the old one still had the bottom half of a screw in it and all was well.

I then measured up the hardwood for the bottom of the outside and epoxied the inside surfaces which would complete the drainage void.  See below:

These were then screwed and epoxied onto the spacers and the screw holds filled with thickened epoxy.

Below is a view from above looking down into the drainage void.  Note the gaps at the corners, which I had made to fit some left over 40mm x 40mm hardwood which were off cuts from the first few layers of the tiller (see earlier post).  You can see two of them in the bottom right of the picture near my right foot.

Before I fitted the corner blocks I wanted to make the top half so that I could "adjust" the corners to cover up my poor woodworking skills if necessary.

Below you can see the hardwood cut and sitting for epoxy glueing and screwing.  You will see that the curve cut in the front and rear ends is the same, but the height is different.  Welsford's design called for a building jig to be made and the ply moulded prior to fitting.  I was too lazy so decided to try to use the hardwood itself to provide the shape.

My biggest concern was that the base would be out of square too much for the top to fit.

Unlike the bottom half, the top could not be screwed together and support itself due to the modifications I had made to the design.  There was nothing for it but to epoxy and fillet it together and see how it went.

Thankfully it worked and through careful checking of diagonals and adjustment after clamping it was fine.  I then shaped the top of the wooden corners on the lid and stuck the bottom hardwood corners on.  They were a few mm off but some sanding of sharp edges and they looked good.

Then it was time to fit the curved top of the lid.  Firstly three layers of 3mm ply were cut to size and holes drilled for the curve.

 It was then epoxied between the layers and screwed onto the hardwood with the screws on about 50mm centres.

The top was then covered in two layers of epoxy after sanding.

I gave it a flip and put more epoxy on the inside including fillets along the edges and joins for strength.  I decided not to glass the top, time will tell if I should have or not but it seems to be very strong.

Then the standard two coats of oil based undercoat/sealer and two coats of exterior grade house paint.  The inside was not epoxied, nor was the exterior hardwood.  Only the joints along with the top curve inside and out and the void.  In addition the top was painted the same blue as the tender.

In the photo below if you look closely at the outside bottom you will see two dark dots.  These are the 10mm drilled drain holes which go on a downward angle from the inside void to the outside.  Each had epoxy on a cotton bud rubbed along the inside surface, but it is hardwood and that may have been more for my sense of mind than any protection of the wood. 

Below shows the removable hinges from Whitworths Brisbane.

Below: Inside are adjustable stainless steel latches.

Below if you look along the bottom edge of the top half you will see rubber sealing strip which provides the seal between the top and bottom when compressed by the latches.  It will be a while until I can find out if it is waterproof!!

Lastly put in an adjustable hatch support that holds it open.

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