Construction problem - installing centerboatd trunk (pics)

[guillemot]

Hello all,

I took a friend's advice back in 2001 when I started this project and made my swampscott dory's bottom out of 5 narrow boards instead of 3 wide ones as J. Gardner spelled out. Now I have run into a problem.

I cut the hole for the centerboard and cut back the frames to allow for the bedlogs to fit, but the board that runs down the center is very narrow after the hole was cut, and I don't think that I should be bolting the bedlogs to it. This photo shows the problem:

http://www.imagestation.com/picture/sraid170/pd1f71254842c208cf52178fbb0d076cb/f3efdb20.jpg

That's one plank that runs down the center. The line in the foreground is just a pencil centerline.

The frames will butt directly against the bedlogs. Here you can see how little of the bedlog will overlap the adjacent bottom plank on each side (about 1"). The way things are set up, the bolts that hold the bedlogs down will be a) close to the edge of the center plank and b) in a very narrow board that is otherwise unsupported and not connected to its neighbor for the length of the centerboard:

http://www.imagestation.com/picture/sraid170/p2f3c81e2a7ee672e27576a04e0911333/f3efdafc.jpg

Here's half of the CB trunk under assembly showing one bedlog, one side and the endposts fastened. The endposts are just hanging over the edge of the shop bench here:
http://www.imagestation.com/picture/sraid170/p5876e4c0427cc72cd5254d329e6a0f07/f3efdaf0.jpg

What do you all think about making this more secure? Can I bolt the trunk down? The bedlogs do each straddle two planks. If I bed them in 5200 would that provide enough adhesion to keep the bottom together or do I need a mechanical solition? All help appreciated!!! Thanks.

Jeff

[ 05-29-2005, 04:36 PM: Message edited by: guillemot ]

[Jay Greer]

The trick will be to not create a split line on the bottom planks when you fasten the logs down. Staggering the patten will help for sure. Since this is not a large boat, I see nothing wrong with running screw fastenings through the bottom planks and into the logs. However, I feel that there is too little surface for fastenings on the narrow over lap plank. The 5200 will hold till hell freezes over and the frame butts will give added strength. Using slightly wider logs and nibbing them into the frame heels would allow a better overlap if it really is a point of concern. Draining would then need addressing. In times past, the logs would have been set down on white lead and candle wicking.

[guillemot]

Jay,

Thanks for the reply. I'm not sure what you mean by "split lines". Can you explain? Thanks again.

J

[Bob Smalser]

You create a split line when you run your fastener holes in a dead straight line as opposed to staggering them.

I don't see any cleats. How are the bottom planks held together?

If cleated, I'd also widen the bed logs at the base so I could stagger the #14 screws between the center bottom plank and the outboard plank. I'd treat the clunkiness of a too-thick bedlog by milling a cove in its upper edge.

If there's rocker in the bottom, make sure you scribe-fit the bedlogs accurately for minumum strain on the fasteners.

[ 05-29-2005, 06:43 PM: Message edited by: Bob Smalser ]

[guillemot]

Yes, Bob, the bottom is held together with cleats, and the bedlogs are cut to fit the rocker of the bottom. You can see one in foreground of the top photo. I'd like to "get away with this" without remaking the bedlogs a third time if the collective deems it wise. I've always said that a job worth doing is worth doing twice, but this would be a third time (had a problem with the first bedlogs, these are the second).

If I fasten the bedlogs to the butted frames with some bronze angle and fasten the thwarts to the top of the trunk to take the torque load, plus bed the bedlogs generously with 5200 and stagger my screws, plus stagger the outer oak renewable bottom planks from the inner pine ones, will that be strong enough?

[Bob Smalser]

The big problem isn't reinforcing the frames or fastening the trunk, it's reinforcing the joint between the center bottom plank and the two immediately outboard of it. What's left of your center plank is too narrow and as a consequence, too flexible to support the considerable torsional stresses of the trunk over the long haul, even if well-mortised into the thwarts.

I wouldn't have to remake the trunk or bedlogs to widen them sufficiently to fasten into those outboard planks. I could lay up additional face wood to them using epoxy and screws or even use your bronze angle stock to make that critical connection.

Any applied false bottom isn't structural and shouldn't be treated as such.

[ 05-29-2005, 09:20 PM: Message edited by: Bob Smalser ]

[Bob Smalser]

If the pine bottom planks were edge glued together... They aren't.

And if they were, they'd crack, as the 30+ inch wide bottom is too big an expanse of solid wood to be crosspinned by cleats and frames.

One way to make a 3-board dory bottom out of narrow stock is to lay them up with short bronze drifts and epoxy into 3 wider boards....then you still have two caulked seams to absorb seasonal expandion and contraction.

[ 05-29-2005, 09:59 PM: Message edited by: Bob Smalser ]

[LazyJack]

Yup got it.

I re-read the post and took a more careful look...