I was surprised the other day when an acquaintance voiced a passing opinion that laminating was one of the harder jobs in building a boat, it shouldn’t be. Even though I have never laminated deck beams for a boat, I have laminated other things and the process is quite straightforward. As with most jobs all you need is the right tools – which in this case is an accurately made jig.
“It’s all right for him”, I heard someone mutter, “he has all the right tools”. In fact no I don’t, it’s a struggle when you have to cut 2” timber, even pine, with a jigsaw rather than the bandsaw, which I don’t own. The cut is not at right angles to the base as the unsupported blade bends and this then takes another 40 minutes of planning and checking with a set square to put the problem right – further adjustments then need to be made to ensure both faces of the jig mate with no gaps. But patience and effort reward the conscientious and the job gets eventually done correctly.
I had to buy quite a bit more wood to strengthen my boatshed to bear the weight of the hull when it was turned over onto the trailer. Bit by bit this wood has been used for a number of jobs, the 3” x 3” was used for fence posts when the fence blew down 3 months ago. Some of the 8” x 2” will be used for this jig. The first job is to calculate the curvature wanted and the spring back. When wood is bent it has a tendency to try and straighten out again, in the past the formula I have used is:
Y = X / n2
Where Y is the spring back
X is the height of the chord
n is the number of laminations (in my case 4, each 0.5” high)
I want 6’ 5” headroom to the plywood roof in the centre, to allow for some boxing in of cables and lights, with 6’ at the sides, that makes about a 5” curvature height. I recon if I allow about 5 ½ “ that should just about be right, lets see
Y = 5.5 / (4 x 4) = .34375
Yes almost spot on, the spring back should be about 1/3"
So, first job is bend one of the laminations to the right curvature and use it to mark the jig shape, my wood is 8” wide so I will need an extra piece added to strengthen the outer part of the jig. At this point best to mention that although I am using my saw bench as a work space I definitely will not be gluing up on it, because no matter how careful you are the glue gets everywhere.
Two halve of the jig cut, trimmed to square and an extra strengthening piece added to the right of the picture. As you can see I added 5 guides, when these were screwed on it took a mallet to separate the halves, but after rubbing the articulating faces with a piece of candle they moved more freely.
These locating blocks perform a number of functions.
- Keep the faces in line
- Act as feet to raise the jig off the floor
- Form a level base to hold the laminations
At this point having successfully made one of those, I now made one of these.
The next part is relatively simple, having made the jig – USE IT.
The easiest way is to place it on the floor open to a 6” gap. Glue up your laminations vertically on top of the jig, then wrap them in clingfilm filched from the kitchen, place the wood in the jig ready to be clamped shut. The clingfilm is most important if you want to take the jig apart again and reuse it.
The frames on the boat are 2’ 10” apart so I intend to put beams midway at about 17” with a ½” ply covering and a layer of fibreglass for protection, waterproofing and added stiffness. This will require 12 beams. So making one per day will take a week and a half. Another added bonus in having the beams so close together is that I can router handholds in alternate beams without weakening the structure too much. It will still need to take the weight of a dinghy and possibly two or three people at the same time.
Having had a good night’s sleep I was eager to see if my calculations were correct, so early next morning I undid the clamps and this is what I found.
Laid on top of the jig for measurement the spring back is exactly as predicted one third of an inch.
Testing is a critical part of any system, whether it be software or, in this case, hardware. I generally recon if it will take my weight it will be OK. At present the beam is a bit long as it was cut from a 10’ foot length, for the rest I will trim the extra 2’ before cutting the strips, that way I have a piece that can be used elsewhere.
This deck beam will sit and wait for its siblings to be made, when all 12 are completed they will be machined and the underside edges turned over with the router. One modification I may make having seen how much resin was required, is to use a cheaper one shot pre-catalysed resin, or a woodworking adhesive, for the others, as these will be inside the boat.
Click HERE for a list of articles by Mike Machnicki