Monday, November 29, 2010

So much to do

Yesterday proved to be the most productive day of the long holiday weekend. Although I didn't touch No. 3 except to finish cleaning it out, I made significant progress on No. 2. She's shifted onto a "tipping beam" transverse stabilizer, her rub rails and gunwales are all off, and her sternpost is 90% unfastened, the only thing holding it in place being those pesky keel bolts. I probably pulled hundreds of screws, and in the process of digging out bungs, actually turning the screwdriver (of which I bent two) and otherwise scraping, pulling, cutting and banging my hands are quite knocked up. All the small cuts and scrapes have combined to produce slightly swollen and very sore appendages. But as my old man used to say at times like these - "what are you doing in the bathroom day and night? Why don't you get out of there and give someone else a chance?!"

I cannot be expected to compensate for your lack of movie viewing nor google skills.

I've been getting many emails asking questions about the Monomoys. I fully realize that without being at the Dockyard on a regular basis, trying to follow all that's going on can be difficult and confusing. There is a simple solution to that (hint, hint). Nevertheless, I appologize for not being able to get back to everyone sooner, and for those whom I haven't contacted, I hope I answer some of your questions in the blog.

To answer a few of those in-depth, and hit some of the general questions on tangent, here's a summary of wooden boat structre, and maintaining the shape of the boat while replacing parts - which I refer to during the construction process as 'stabilizing' the hull. Stability afloat is a totally different beast altogether, which I'll discuss later on.

First, you cannot evaluate a wooden boat's strength and integrity by conventional static 'beam' theory, as you would a building or a girder. Rather, treat it like a basket. The individual parts are rather slight, the strength being derived from the sum of the parts in concert rather than taken one at a time. Imagine a boat shaped basket. It flexes and shifts, with some parts under compression and some in tension, depending on the force acting on the strucutre. Many are under both forces simultaneously, acting in different directions. As you remove straws from the basket, the structure gets more wobbly, and might lose some of its shape. And therein lays the problem of stabilizing the structure while removing critical pieces.

The majority of the hull's strength comes from the hull planking. Modern steel ships are longitudinally framed, meaning that the largest and most numerous structural members run the length of the ship. That idea is no different in wooden boats, including the Monomoys. In the case of small wooden boats such as ours, the planks are 'tailored' to very specific shapes that when bound together form the shape of the hull. They press - or are meant to press - against each other such that they resist the various forces that twist and bend the hull. In the case of our Monomoys, we are dealing with carvel planking, where each plank butts up to its neighbors. This is different from lapstrake or clinker planking, in which the lower edge of each plank rests on the upper part of the plank below it, similar to clapboard siding.

Binding the planks together are another very important but much smaller set of structural members called frames. Most people might call these 'ribs' because of what they resemble. But contrary to what many might think, their primary purpose is not to provide the transverse curve of the hull, but rather to follow the curve created by the planking and pull the planks together side-to-side. Executed properly, the edges of the planks then press against each other, creating a friction-fit of sorts that is further bolstered by tightening during caulking. Removing frames means that the planks begin to lose their connection to their neighbors and risk losing the shape that the carefully tailored planks hold when bound close together.

The keel in our boats functions as a sort of 'king plank', being only two to three times thicker than the planks themselves. The top of the keel rests flush with the inside of the planking, and the planks butt up to it on each side. The frames bolt directly onto the top of the keel, which eliminates the complex 'floors' used in other types of boats. But this poses a unique problem - remove the keel and the force of both sides pushing down has nowhere to go except into the frames. I don't trust the old frames to hold that load, and so removal of the keel and any of the surrounding strakes of planking means replacing the frames first.

The stem and sternpost connect to the keel, but they function primarily to attach the ends of the planking. Remember the basket? Well, cut a slot in each end of the basket and what happens? The sides will pull apart. We have the same issue with these boats, but this is mitigated by the fact that we left the thwarts in place to keep them together. Those will probably be augmented or replaced with beams run from sheer to sheer (top plank to top plank) as we progress. But for now, the hull has proven stable with both stem and sternpost detached.


Lesson over, here's how we're progressing on the boats:

No. 2 - remove stem and sternpost, make new parts and install. Reattach old planking. Reef the seams - removing all old caulking. Working every other frame, replace all frames, tightening the seams by means of straps tightened around the outside of the hull as the planks are re-fastened. Replace old keelsens with new. Go back and replace any damaged planks, including both garboards. Replace ALL butt blocks. Recaulk all seams. Replace necessary interior joinery.

No. 3 - begin with reefing the seams and re-framing, proceeding every other frame and tightening up the seams. Remove stem and sternpost, cleanup scarfs and re-install. Replace keelsens with new. Recaulk all seams. Replace necessary interior joinery.


I know all of this doesn't answer every question, but I hope it sheds some light on some of the major points.

More to follow, including some commentary on the completed preparations for this weekend's event.


No comments: