: My idea for a external strongback,please let me know what you think. First
: hurdle,I don't have access to a table saw and don't wish to buy one, so
: plywood is not an option. I'm thinking about using (2) 10 ft pressure
: treated 4x4"s, joined with a 2ft long scarf joint (cut by lumber
: yard). The joint will be screwed & glued and a 5/8" x 3 1/2"
: x 2' long plywood doubler on each side(screwed & glued).
: My plan is to mount (bolt) the strongback onto three B&D "WorkMate"
: benches with the scarf joint on the middle bench.
: I'm hoping the pressure treated wood will not warp in a heated garage keep at
: a min.of 40 Deg.F when I'm not working on the kayak. Please comment, I
: value your knowledge & opinions.

Let's get back to basics: What boat design are you building? Does the designer specify a particular type or size of strongback? If so, then build one the same size--and probalby the same manner, as described in the plans.

Pressure treated wood is not needed if you are keeping the strongback inside, or at least sheltered from rain. The preservative is a poison which kills mold spores and insects (like termites and carpenter ants) which would eat wood which was moist and in contact with the wet ground. If you are building under cover, none of these conditions apply. Get a less-expensive piece of wood. it will work as well, or better. did I mention poison? You need to be careful being around treated wood, and particularly careful around sawdust from treated woods. Inhaling even small quantities of those poisons is not good for your heatlh.

Warping of wood is a combination of the amount of pressure you put on it (weight), the amount of pressure mother nature put on it while the tree was growing, temperature, and moisture. Trees which were severely stressed while they were growing will try to relieve those stresses when they are cut into boards-- and those boards are going to be prime candidates for warping.

The simple way to reduce warping is to bolt together two boards with the grain patterns reversed. Some people will rip a 4x4 in half and then reassemble it, with one half flipped.

Are you sure your lumberyard will do a scarf joint on a 4x4 for you? Why would you want one? In your situation a simple half-lap joint would work fine. You could cut this with a hand saw, circular saw, band saw, sabre saw, etc.

The plywood doubler on the sides of a scarf joint-- or a half lap joint-- seems unnecessary.The joint should be strong enough by itself, and it will be supported on a saw horse, or a table. Besides, I thought you didn't have a saw to cut plywood for a box beam. Where did you get a saw to cut plywood for these reinforcements?

If you have a circular saw, or can buy one (lots of sales for these. A base model can be obtained for under $40 most of the time, and less if on sale.) Then, a cutting guide will be all you need to rip plywood planks from a 4x8 sheet, for a plywood box beam. A straight edged piece of wood and a couple of clamps will serve as a guide. For this application yo can dispense with clamps and use nails driven through your guide and halfway into the plywood to hold the guide in place.

2x4s are pretty cheap (usually under $2.0 each), and two of them are going to be cheaper than a single 4x4. My advice: For a 20-foot-long beam get five 8-foot 2x4s, (This should be cheaper than 4 10-footers. Cut a 4 foot-long piece off of one of these and then stack your pieces like bricks to assemble them into your strongback. The bottom of the stack starts with the 4 foot section on your left, then continues toyour right, in line, with two 8-footers. On top of this you lay (from left to right) two 8-footers and end with the 4-foot section. The overlap is 4 feet on either side of a joint. That gives a very strong joint.

You can use bolts, screws, or 12 d nails to hold the thing together. 16d nails will go through both pieces and you'll end up cutting yourself on the points at some time in the building process. If you want this to be permanent, put some glue or construction adhesive between the boards before nailing them together.

You won't need to bolt the strongback to your workmate tables. The tops on hose serve as clamps or wood vises. Just turn the tables so they are inline with your beam, open the tops a bit more than the thickness of yoru beam, drop the beam in place, and crank the tops tightly against the beam. You really only need two of these, spaced about 12 to 14 feet apart. to keep the middle from sagging you can use a single piece of 2x4 as a prop. Attach it to the side of your beam with a 2 1/2 inch drywall screw and you can remove it if you need to get something around it. If this prop is a bit short then shove shims or wedges under it to bring the middle up to level.

Hope this helps

PGJ