Sunday, October 30, 2011

The knees AKA the Frames

Okay, its been way too long since the last post--too much living not enough blogging!

The boat has really been moving along since last winter and there is much to report here--I will try to get a bunch of posts together here quickly, so those of you who have been following along will have much to read and see....

I started out calling these side knees as defined on the plans and was subsequently informed that they are often referred to as the frames. But regardless of what you want to call them, they were really difficult to make and to install.

Our plans called for all of the frames with the exception of the two at the front of the boat to be parallel with the bottom planking and the transom. To do this each of the frames must be canted such that it is not perpendicular to the side planking. As a result there are lots of bevels to plane and cut.

Trial and error fitting

I started by making rough models of each frame prior to committing the angles to oak. But even so each frame went back and forth, back and forth, and back and forth many more times between the bench vise and the boat for custom fitting—hand planning the angle on the plank side, hand cutting out the notch for the chine, and chiseling out the curve for the quarter round. After much work and when I thought that I finally had each frame looking good I cut the final bottom angle on each frame in two directions, across the boat and front to back (a compound cut).

Marking Gunwale position
 Installation would have happened much sooner than it did but, it was a two person job, one to hold the frame in place and the other to drill the pilot holes for the screws. In fact the whole installation of the frames was the subject of much debate on the Wooden Boat Forum when I posted about attaching the planks to the frames. It seems that Atkins has drawn the frames in this boat somewhat uniquely in comparison to what other designer do. Atkins’ frame is narrow at the top and quite wide at the bottom, whereas most frames tend to be more or less the same width throughout their entire length. Consequently the traditional method of securing the planks to the frames is to use copper rivets. But in my case that would necessitate having a 6-7” rivet for the bottom plank. Since I didn’t have rivets that long nor a small diameter drill bit that long this was not an option for me. Instead, I choose to run a couple of 3” screws up through the planking into the bottom of each frame. The concern here of course is that the end grain of the frames is not very strong and will fail over time. I had speculated that Atkins must have drawn the frames the way he did (wide at the bottom and touching the bottom planks) to add additional strength to the craft. This belief was confirmed by another member of the Wooden Boat Forum who noted that the plans he had to another Atkin’s design had a note that indicated the frames were to be attached to the bottom planking. Regardless one of the keys to making a chined boat strong is to ensure that the chine is screwed into the frames (note all the grain is perpendicular to the screws and that is very strong).

I did not however want to screw every plank to the each frame as this would mean that I would have to fill each of these holes and I was concerned about the holding strength of our thin planks once they were counterbored to hold a screw. So I settled on a plan to use ringed copper nails to attach the planks and made the arrangements to procure enough to secure each plank to each of the frames. However, I experienced nothing but grief with the installation of the first nail.  I must not have drilled the hole deep enough (although I was at the extent of my drill bit) and as soon as the copper nail struck the oak without a pilot hole it promptly bent over.  Extraction of a ringed nail is virtually impossible, so I cut it off close to the surface and went away wondering what to do now.

Plugs already in place
Eventually, a light blub went off and made me wonder what I was thinking all along.  What I realized was that because I would be putting the screw through a strake lap my wood thickness was actually doubled.  I would have lots of wood to countersink the screws and plug them.  So I merrily proceeded to drill and fasten each plank to the frames. 

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