Engine Box Origami 1.0

Today's post is about flat panel construction. While there's been a fair bit of un-reported filling, sanding, and icebox-making going on these last months, here I share some quick thoughts on making an engine box using the dado & bend technique.

Step one is to make a nice panel.  This one was laid up on the vacuum table at 400gsm per side of 1/2" pvc core which I perforated with the light-duty CNC.  Fiber was split between +/-45 and 0/90.  The bottom side is laid up on a piece of 7mil mylar which yields a shiny surface.  The top is peel ply.  

Inside the boat, I modeled the new box with door skin and hot glue per the usual technique.  The design incorporates a structural center arch, to support the cockpit floor, a large removable front panel giving full access to the Yanmar, and a smaller removable hatch in the back to access the gear box and shaft seal.

To get an accurate model, I decided that I had to set the motor on its new mounts.  In the end, I am really struggling for space, so every millimeter I can reclaim with a good plan is worth the effort.  I sized the box to allow 2" of breathing room all around the motor.  If you look closely, you can see the 2" think block of pink foam that I hot glued to the main pulley hub to act as a spacer.  

It's a big effort to cycle the motor in and out of the boat since I can't do it in the garage.  Once again, I ripped the front of the garage apart and somehow pulled it off.

At the shop, I digitized the walls of the door skin model and went to work in Rhino, first to make the main form, then to flatten via Rhino's brilliant developable surface command.  To make curved corners using the dado trick, look at the length of the inner skin and the outer skin as you go around the corner.  This is pretty easy in CAD software.  This will tell you how much length you need to remove from the inner skin in total via grooves.  For an arc, the first and last groove should be exactly half the width of the middle ones if you are doing it right.  Here a 3" radius means 6 grooves, the middle ones ~1/8", and the first and last at ~1/16" wide.

Having a nice set of parts makes the initial assembly quick and easy.  As I cut the hatches and the main frame separately, I was worried about getting them to align nicely after glue-up.  To deal with this problem on the main engine lid, I pre-assembled everything with door skins, hot glue, and eventually fasteners.  Faking everything together as a single panel, I buttered up my grooves and jigged her up to cure in position.  The masking tape is just there to keep the hot glue off the final part.

Once the basic form was cured, I strengthened things up with some DB90 covering the dados.

With the basic parts all sorted on the bench, the real work starts when you glue and tab everything in the boat.  That's the long and boring part which I may pickup in a second post.  Pic here shows a the base installed, and a little extra uni getting bagged on floor to ceiling to support the forward cockpit floor.