First, measure the depth of your canoe from gunwhale to keel. Allow 3-4 inches more for maneuver room for each canoe. If you're doing a 6 boat trailer, then it will be 3 boats high. Figure the depth of two boats plus 6-8 inches, plus however far the upright will stick down to where it will be welded to the frame.
Once you know how long your uprights and cross trees are, you can buy the steel - 2/4's for uprights and 2x2's for cross trees. Have them cut to the length you want. You'll need 2 uprights and the same number of cross trees as you have boats. A six canoe trailer will need 6 cross trees.
Cut the steel once it is all marked. If you don't have the equipment, you can get everything cut at a local welding shop. Tell the steel cutter what you're doing with it. Sometimes a second pair of eyes can spot trouble before it gets made into a permanent mistake. You can't uncut steel. Lay out the cross trees and clamp them in place on the two uprights. Mark where the two angle braces (if you use them) and the lengthwise cross-members attach. Test fit the pieces in the places they go. Make sure everything lines up before you weld it together. Correct any problems, measurements or alignments. Don't forget the gusset plates. Carry the trailer and clamped pieces to the welder or weld them up yourself if you have the skill. Because the trailer is made of steel, it will be heavier than an aluminum or factory-made trailer. A good recommendation is limiting it to six places. An eight-place trailer may be a little top heavy when fully loaded and could strain the boat trailer.
Weld the two uprights in position as shown in the picture. The uprights should divide each canoe into approximately thirds. Get a good welder to do the job if you aren't one. The welds have to be very very solid. Next weld the cross trees to the uprights. The top one can be welded on top of the upright. Weld gusset plates to the joints between the cross trees and uprights to provide extra support. Measure twice, cut once. Have your welder check your work to make sure everything lines up properly.
Measure the distance between the front cross member and the trailer hitch. Allow at least 1/3 the length of your longest canoe (6 feet for an 18-foot canoe) plus another couple of feet to allow the vehicle to turn without the canoes digging into the back of the vehicle. If your trailer hitch is too short, you'll have to cut it off and extend it to an adequate length. Your welder can do that easily when he's welding the pieces together.
Screw the lid to it's hinges and hasp as shown in the picture. You'll probably want to lock the box while the trailer is left unattended. Some people make the box vertical because you don't have to unload a canoe to open it if you put the door on the end. The problem with that is that when you're driving on narrow roads or it's windy, the wind blows sideways against the broad expanse of plywood and against the big light canoes and can make your trailer dance around dangerously. For safety, lay the box flat. It's not that hard to unload the bottom canoe to get to paddles and life jackets.
When the uprights and cross trees are welded in place, paint the trailer with primer, then a good paint for metal in whatever color you choose. Brighter colors help people to see you when approaching from the rear.
- Used boat trailer
- Six tube steel cross-members - 6 to 8 foot, 2 x 2 inch (consult with your steel supplier to recommend the proper thickness of the pipe walls)
- Two tube steel uprights - 6 to 8 foot, 2 x 4 inch
- Two tube steel lengthwise cross-members - 8 to 10 feet long (depends on trailer length)
- 1/8-inch steel plate large enough to cut six 5-inch sided triangles
- Four steel strips - 1.5 feet long 1/2 inch wide x 1/4 inch thick (optional) Two marine-grade 1/2 inch 4 x 8 plywood sheets >
- Welding machine and supplies
- Grinder and disk assortment
- Two or three large hinges and hasp
- 1/2" bolts with nuts and washers, 3 to 4 inches long