BUILDING A BUILDING FRAME
February 21, 2021
Now that the hull panels for the bottom of the boat are stitched together and hanging from the tie beams of our shop, the bulkheads can be erected so that the bottom panels can be lowered onto them, followed by wrapping the other six panels around the bulkheads and stitching them together. It is essential when erecting the bulkheads to position them precisely in relation to one another, exactly as they should be in the finished boat. Doing so ensures the hull panels will fit nicely around the bulkheads and form the hull shape the designer envisioned.
The first step in getting the bulkheads properly positioned was to build a building frame. A building frame is essentially a sturdy, rigid, square framework on which to mount the bulkheads. The building frame for Burnett was actually the very first thing we started building when the project began - the big, long table we used to scarf our plywood and cut out the hull panels was designed so that, when those tasks were completed, the legs could be knocked off and the top of the table dropped to the shop floor where it would become the start of our building frame.
The scarfing table becomes the building frame, and defies gravity in the process.
To help make sure we could setup Burnett's twelve bulkheads with their required separation, we placed cross members in the building frame. These cross members are where vertical boards that hold up the bulkheads get fastened to the building frame when it is time to "stand things up". To achieve the desired 1/16" precision for their placement, we used a long steel tape measure to locate where the cross members should be fastened to the stringers that run the length of the building frame (the steel tape doesn't stretch the way a plastic one would). Our timber framing experience taught us that a great way to mark measurements is to use a knife blade - a lot more precise than using a pencil!
The cross member for Burnett's Bulkhead #10. The knife mark on the right is from when the scarfing table was first constructed. After a couple months in our warm shop, our hardware store kiln "dried" spruce boards had shrunk significantly, which meant we had to reposition our cross members.
Squaring Things Up
A building frame is like a foundation for a house; if you get it square and level, it makes future steps in the building process much easier because you have reliable reference points to measure from. I have never seen a 40' board that would be straight enough for a building frame that long, so we settled for some reasonably straight boards, knowing we would need to put some diagonal bracing into the framework to get things in line. Even if the boards were perfectly straight, the bracing is a good idea to make the frame rigid; it would be no fun to get all of the bulkheads perfectly positioned, only to have them sent out of whack when someone accidentally kicked the building frame.
The diagonal bracing, which was put in place using a string line as a visual guide for what is perfectly straight, squares up the building frame and holds it that way.
Leveling off the Frame
To level off the building frame we used a laser level. The laser level we used shoots a beam of focused light perpendicular to the force of gravity, and is accurate to 1/8" over 30'. With our building frame sitting directly on the concrete slab of our shop, we measured down from the beam of light to the top of our building frame in enough places to find the highest point on the frame. We then tapped wedges underneath the building frame at low spots to bring them all up level with the highest point.
Using a laser level set at a base station to locate low spots and raise them with wedges. Wedges are a great tool for fine-tuning gaps.
Locking the Building Frame in Place
Continuing with the house foundation analogy, having a solid foundation prevents the structure from deforming through time due to movement of its underpinnings. The hull of Burnett will weigh somewhere in the neighborhood of 10,000 pounds when it is ready to come off the building frame, which warrants a fairly robust structure holding it up and in place. To secure our building frame we bolted it into the concrete slab floor of our shop and sprayed foam underneath the frame where it didn't contact the slab. The foam expanded to fill the gaps and adhered to the concrete and wood once it dried, locking things in place.
Concrete bolts anchor the building frame securely to the shop floor.
Spray foam filling a gap between the building frame and the floor. This gap was created when wedging the frame up to level it off.
Finishing Things off with a String Line
The last step in getting the building frame ready to receive the bulkheads was to pull a string line down the middle of the frame perpendicular to the cross-members. This string line marks one of the most important reference lines of a boat: the center line. Looking down at the deck of a boat from the sky, the center line would split the boat into two symmetrical halves, port and starboard. The position of each bulkhead was measured from this reference line.
A triangular notch in the plywood that the string line rests on worked as a keyway to hold the string in position, and as a reference to get the string back to the exact same spot when we broke it.
I lied. The final step in setting up the building frame was to consume eleven Lucky Lagers and place the empty cans underneath the string line between each of the cross members where bulkheads will be located. The red thread we used for the string line is great in that it is light, stretchy, and strong - and can therefore be pulled taut and not have any sag. But it is incredibly difficult to see, and it's a horrible feeling when you trip on the string and break your carefully placed reference line. The beer cans worked very well as a visual reminder to step over the string line. We only broke the line once when setting up the bulkheads, compared to the tens of times I have tripped over string lines when building previous boats. I should have drank more Luckys during those projects.
The building frame, ready to receive bulkheads. The fir 2x6's to the left will be the upright members that get attached to the bulkheads.
With the Lucky's consumed and in place we are now ready for the excitement of standing things up - erecting the bulkheads on the building frame! Give us a minute to sober up before we get into that.