Monday, May 9, 2011

Pram build - Step 5 - Hull pieces and epoxy fun

Drafting and cutting out the hull pieces was an aspect of this project I wasn't looking forward to. There is very little on the plans around the hull pieces. While it is possible to draft out the pieces, that would take a long time and would be difficult to do accurately. I decided to cheat instead... and it worked!

First things first, splicing the plywood. Unless you can get 10' sheets of plywood (we don't get it around here), you are going to need to splice together two pieces. The easiest way to do this is to simply butt the pieces together, tape and epoxy one side, wait for it to dry, fill and tape the other side and you're done.

Since I didn't plan this part in advance, and I didn't want to wait for the epoxy to cure, I decided to use a temporary method and join the pieces with a piece of 1x4 screwed to one side. Later on, once the pieces were assembled, I re-did the butt joint properly with epoxy and tape and removed the butt block.

Note: If you search for "scarf joint" on Google you will find all sorts of complicated ways to join plywood. I've used the tape method with a lot of success. I'm not sure why people bother with cutting delicate angles and other complicated methods...

Once I had my joined plywood these are my cheater steps that I took to cut out the hull pieces:

  • First I cut the plywood in half (and 10' long) so that I had 2 sheets of 2x10. The two bottom pieces fit on a single 8' sheet. 

  • For each piece, I simply took the 2' wide sheet and temporarily screwed it onto my frame pieces. I crawled under the piece and traced out the shape of the frame on the plywood. I unscrewed the piece and used a jigsaw to cut it out. Each piece was screwed back in place. I repeated this process for each of the 4 pieces. It worked great!  I used screws throughout the project, even though the author recommends nails. I prefer working with screws, no better reason. 

  • Once all of the pieces were screwed in I used thickened epoxy to fill in any gaps that remained between the pieces. Each joint was then fiberglassed with 4" tape and epoxy. 

A note on epoxy, for thickening epoxy I use a variety of materials. I do like to use the fancy West System fillers when it comes time to fair or do small fillings. For everything else I usually use sawdust. It's cheaper and does a good job filling but it's a little harder to make smooth.

Pram build - Step 4 - Installing chines and keelson

Installation of the chines and keelson is a little bit finicky but isn't too difficult.

I used spruce 1x6x10 as stock and ripped them into 1x1x10 strips.

These are the steps:

  • Trace notches onto bulkheads in each position shown on the plans. I fully notched the bow and transom pieces as well since those notches would be covered with fiberglass tape (and paint) later. This would make life easier now without affecting the finished look. If you are doing clear finish you will probably want to only notch halfway into the bow and stern pieces so that the ends aren't sticking out.
  • Next I used a jigsaw to notch out each piece. 

  • I dry fit each spruce strip to make sure that it was fitting well and adjusted as required.

  • Finally I epoxy glued each strip into place.

  • When the glue had dried I planed each of the strips so that they were flush with the adjacent form piece. At the bow the amount of planing required was more significant. 

  • Finally I adjusted each of the forms slightly to bring everything into proper alignment so that it looked reasonably fair along each line. In retrospect this would probably be easier to do before gluing the pieces on but there was enough play in the strips to achieve fairness.