Posted by: ukhbbr | September 15, 2008

Beginner’s Clunky Ply – Strong Back

Paul Fisher said 3×1 screwed into a T beam would work well, ideally with old dry wood that had finished all its twisting.
I don’t have anything like that so the best I can do is get 3×1 planed which has been stored indoors at the local builders merchant, using the straightest I can find. Any comments?
Having said that it will have to be stored outside on the grass under a poly cover…..unless I build an extension to our 12ft conservatory. Or I might build a rainproof cover along one side of our house and the fence. It would be about 5 feet wide.
Thinking a bit more the strong back might have to be moved. I know that’s not ideal but I can think of ways of realigning it – string through holes in each stage or a laser level.
 
S
12/09/2008
 
Umm, yes quite a challenge you have got there. I, personally don’t like the T strongback – I just think there are so many ways that it can twist out of shape. Modern timber is such diabolical quality that I would be very loathe to follow Paul’s recommendation. All my canoes have been built on a ladder type strongback built out of L girders for lightness and great strength and stability. I think the T strongback comes from across the pond where it is universal for the Cedar/ canvas canoe build and I don’t think the need for absolute level is so important – largely because the planks at bow and stern are not finally brought in permanent alignment until the boat is off the moulds so there is scope for fudging.
 
Once you have got the backbone (hog and stems) securely fixed the ladder frame becomes quite rigid, triangulation I expect. I did put legs on wheels at one end of the frame with the other clamped in one of those £10 workbenches so that I could move the thing around – even with 12′ width in the workshop there were still times when  ……
 
The advantage of the L girders is that you can use 5×1/2 floorboard stock and join lengths with a butt strap to extend it. the short leg of the L is something like 2×1 screwed and glued inside the top of the side plank. Not sure if I make sense – attached some snaps to hopefully clarify.
 
N
 
L Girder frame upside down showing simple construction.

L Girder frame upside down showing simple construction.

L Girder frame right way up showing cantelievered supports
L Girder frame right way up showing cantelievered supports

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Responses

  1. Hello All, Jack Chippendale recommends using a box construction. When I built my 16ft double-ender, I did so on a 12x4ftx18ins (the last dimension may have been only 12″) box made out of 2×1″ and cheapo thin ply. It was amazingly rigid and, with the final cross bracing on the underside, can be accurate to within one or two mm in terms of twist.
    Tony Waller


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