Centerboard case for cross planked skiff

[Mathews]

Me again. I'm building, as my first boat, the crabbing skiff shown as Plate 9 on p. 74 of Chappell's Boatbuilding. This is an 18-1/2' double-ended, flat-bottomed skiff being built traditionally, including cross planked bottom (7/8" white cedar) nailed into white oak chine logs with si-bronze ring nails. Maximum beam is not much more than 3'. The bed log for the CB case is around 6' long, 7 or 8" wide, and 1-1/2" thick, white oak. Case sides will be white cedar, tongue-in-groove, screwed to logs (from the inside) and posts (from the outside) and drifted or throughbolted together. Logs will be bedded in Sikaflex 292.

What is the order of construction? I want to split the bed log into two halves lengthwise for ease of construction. I will use ring nails and several carriage bolts from underneath. The plans show no batten keel or anything similar inside or on the bottom, and it would be difficult to add anything like that at this point (other than maybe a shorter batten on the underside below the CB trunk). I feel like I need to fasten the bed logs to the bottom before I cut the slot, but then there is going to be all that sikaflex gumming up the works. Also I need to fasten the lower side boards to the logs before assembling the logs because of access. I will paint the surfaces with red lead before assembly.

Has anyone built in this configuration that could suggest the order in which I proceed? Do I need any bedding compound in the tongue-and-groove joints if they are primed with red lead?

[Tom Dugan]

Well, fools rush in, so here I go....

Gary,

Centerboard trunks are pre-assembled along with their logs before they're dropped into the boat. The posts are left long so they can be cut flush with the bottom once everything's locked in place. Obviously, this means cutting the slot before anything is spooged in place. Red lead and spooge the trunk pieces as they're assembled. I don't recall seeing T&G used for the trunk, but I know I'd bed the T&G, but with some care. You don't want a rough inside surface to hang up your centerboard.

Now hopefully someone who actually knows what they're talking about can follow up. http://media5.hypernet.com/~dick/ubb/biggrin.gif

-T

[Thad]

I think you have pretty well answered your own question. In this construction it is the lower edges of the side planks that give the line of the bottom sweep, so the bottom planking must go on so you can spile for the logs. As you are fastening the trunk sides to the logs from the inside, this has to be done before the logs are fastened to the bottom. The posts (or head ledges) should go through the bottom, so they can not be installed until after the slot is cut. You have to fit the logs, marking where they go on the bottom so you can drill for the log fasteners from the inside before drilling back from the outside. Besides marking where they go you might well drill through the bottom boards at the corners of your slot so they will be located on the outside. You will assemble your trunk sides and then fasten the logs to the bottom. The sikaflex is not traditional and is messy. If you are committed to Sikaflex you could put a thread of cotton on the slot side of the log and the beads of Sikaflex on the other side, possibly avoiding the mess. When the trunk sides are in place then you can cut the slot, install the posts, and carry on. I like to rivet or otherwise bolt the posts in the trunk.

[Mathews]

The main concern I have is structural. Now that the boat is planked and the bottom in, it seems like I will be creating a weak spot by cutting several feet down the middle of the bottom and nailing/bolting in the trunk. There will be a thwart tied into the case at the upper aft end, and the plans show no keel or batten, so should I be concerned? I know my question was about construction sequence, but I guess I am also concerned that even if I cut the slot last, the trunk is still tied to nothing but cedar cross planking. But that's what the plans show.

Also, what would be the traditional way of bedding this so as to avoid sikaflex? If it involves white lead paste, could someone please tell me where I get this mythical substance? Kirby Paint?

[ishmael]

No expert here, but why not add a plank keel? It would be nice to have something else to bolt the bedlogs to than just the bottom plank ends, and it will stiffen the whole bottom too. Sometimes, working boats were knocked together pretty rough and ready, so maybe that's why the drawing dosen't show this 'nicety'.

[Dale Harvey]

Stick with the Sika, and as reccomended lay in a piece of heavy cotten twine, so you don't squeeze to much Sika out when you tighten the assembly to the bottom. Let it set a couple of days at least before torquing the fastenings down. I might run the bedlogs by the slot as far as the middle of the next unslotted crossplank fore and aft, and put a fillerpiece the same thickness as the headlogs between them, thru bolted or rivited. I would also consider a foredeck with its after beam affixed to the top front of the case. An inner false keel/ mast step running clear up to the stem and toed into the bedlogs wouldn't hurt. I tend to overbuild things though.

[Thad]

White lead paste possibly with a wick on cotton would have be the traditional bedding. The key to the design as drawn in in the extension of the bed logs past the slot and their solid fastening to full bottom boards. Now, I have looked at Chapelle's drawing with my loupe and it appears to me that the log is drawn as a 1 1/2" x 7" plank, 4' 11" long, with the slot cut in it. It appears that the head ledges come down to the log, not through it. This raises a number of structural questions. Traditional sharpie construction involves much longer logs for the case, extending their support for the middle of the boat. I find the structure on this case near minimal. The boat is quite flat through these sections. If the sides of the trunk were well fastened to the log (from below before installation in my view) then they would carry the head ledges and the whole would carry the bottom. It is interesting how successful an integrated structure of wood can be.