Discovering cardboard

Building a hard top for this boat has been very much on my mind for at least the last three years. As a typically trained busines school graduate, I thought out detailed processes for each major phase of the work. In most cases, this is a good way to think about complicated projects.

Sometimes, however, it is possible to rely too much on process analysis and to paint oneself into a corner. This is how I came to waste a week and countless auto miles looking for thin plywood. I had planned on using 1/8"  Luan plywood for the new top pattern. One-eighth inch Luan plywood is nowhere to be found.

Eureka!

Then I remembered the big cardboard carton the marine plywood came in. So, today I have learned a lesson about being more flexible. And the project is on the move again.

Cardboard is great stuff for big patterns. It can be easily worked with a box cutter. Mistakes can be fixed with package tape.

How we made the pattern

We tacked a big piece of cardboard on the framework previously built on the boat. Then I followed around the outside of the framework with the point of a pencil compass. As the compass point tracked the framework perimeter, the pencil transfered the structure's curves to the underside the cardboard. The overhang widths on the front, sides and back of the top were penciled onto the cardboard by adjusting the radius on the compass for the desired overhang dimensions. Real boat carpenters call this technique "spiling." Spiling can be tricky, but accurate results can be achieved with practice and care.

So here I am, spiling away. Please note that I have discovered another important fact about making big patterns:

It is not necessary to pattern the whole structure. I need only make a half-pattern. When it comes time to transfer the pattern to the plywood, I will pencil around my half pattern, then flip it over to trace the opposite perimeter.






Here's what the half-pattern looked like after I had cut along my compass tracks. Dia, who is 5'  6", provides a sense of the cardboard cut-out's size.

At this point, the pattern will produce a top that is 10' 2" long and 7' 1" wide.

The pattern, tacked back on the framework for a test fit, suggests how the final product will likely look. The "eyebrow" overhang in the front will be 8". We first planned for a 5" overhang in the back, but after looking at the pattern in place, we increased the rear overhang to match the front--8".




The overhang along the sides of the top was set at 5 1/2". We wanted to extend the side edges far enough so that rain running off the top wouldn't flow inside the cabin coaming, but not so far as to hamper cabin access and movement around the side decks.
    
  
I draped a weighted piece of string over the pattern's edge to simulate a drip line. You may be able to see it better if you click this image to enlarge it. I measured about a 2" clearance from the drip line to the coaming edge. That may not be enough. I might widen the the final top width to increase this offset.


Rain and sawdust

Rain is expected here over the next few days. The pattern will be moved to a dry corner of the garage for a time. Next, I will start the work of joining three 4x8 sheets of marine plywood into one big sheet from which the final top will emerge.