Here's a simple way to add a third seat to your canoe. Why you'd ever want one is something of a mystery to me, though if you only paddle quiet lakes with an extra kid in the boat, this might be useful. The problem with a third seat is that it puts the weight of the extra person a bit higher in the canoe than if that person were sitting on the bottom. The higher the weight, the more tipsy the canoe, but if you've got a bit boat, this extra seat should give your middle passenger a high enough position that he or she can paddle comfortably rather than sitting in the bottom like a big sack of rocks.
Take out the center thwart so you'll have room for the seat and your legs. You'll want the weight of the third seat passenger balanced at the center of the canoe.
Drill holes for the screws that are large enough for the 4 inch long sheet metal screws you are going to use. Make sure they are the same distance apart (7 inches for an 18 by 10 inch seat. These holes will allow you to screw into the center of the hardwood dowels.
You can use 2x2s if you stick with hardwood for strength. Use a 1 to 1-1/2 inch hardwood dowel otherwise. Use screws with washers to fasten the dowels. in place. The washer distributes the binding force across more hull area and reduce wear and potential damage to the hull.
Dab a little epoxy or butyl sealant on the ends of the dowels and position them so the holes are at the center of the dowels. When you cut the length of the dowels, make them as exact as you can. You don't want them to push the sides out or pull the sides in and distort the shape of the canoe. Predrill holes into the ends of the dowels once they are in place.
Dab a little butyl rubber sealant over the holes and then drive the screws into the ends of the dowels to hold them in place. If your canoe is aluminum or wood, you should have plenty of side strength to handle the weight on the seat. If you've got a plastic, fiberglass or composite boat, you may need a brace. more on that later.
The half inch marine plywood seat should be at least 18 inches wide for comfort if you have that much width to work with. If you don't you should really rethink adding another seat cause buddy, if your boat is that narrow, you are going over if you add more weight that high in the boat. Use a router to round the top edges to avoid splintering and chafing you skin when you sit on it.
Turn the seat over and screw 1 by 1 strips to the front and back edges of the seat (the long sides). Glue them down first, then predrill and add 1 1/4 inch screws along the length of the strips.
Once you get all the screws into the 1x1 strips, use the router to round the edges so they won't chafe your legs while paddling in the kneeling position (which you should if you've got a third paddler. It keeps the weight of your legs lower so the boat is more stable.
Set the canoe seat over the top of the dowels. The two 1x1 strips should fit snugly outside the dowel to prevent fore and aft movement of the seat. This view shows the dowels without the canoe. Of course, when you do this part, you've already got the dowels in place across the gunwales.
Once the seat is in place, Drill holes through the seat down into the dowels. Don't put more than about 5 holes into the dowel to avoid splitting or weakening them. Screw 3/4 to 1 inch screws into the dowel to secure the seat in place. Use butyl sealant in the holes before you drive the screws in to seal them against water. Then stain and apply 3 to 6 coats of marine spar varnish to protect the wood from getting wet. If you want to be especially thorough, varnish the dowels and seat separately before attaching them. You'll have better water protection that way and the seat will last for years and years.
If you're afraid your boat won't be able to handle the weight on its sides, especially for light plastic, fiberglass or composite boats, add a center support by simply cutting a 2 by 6 and setting it lengthwise under the center of the seat. Screw the top end in place through the seat. Attach the lower end with epoxy or, if the canoe hull will accept fiberglassing, just fiberglass the entire support and base to hold it in place. If the canoe is fiberglass, you can always fiberglass the entire seat assembly and glass around where it attaches to the hull to strengthen the seat.
This, will of course, increase the weight of the canoe, but if you've got one big enough for a third seat, it may not matter much.
A third seat may be just what you need for paddling around the old pond. I wouldn't want to use one in whitewater, but not everyone does rough water anyway. A canoe rental on a quiet lake could probably add seats like this for the comfort of those threesomes that want to take out a boat and not have someone get his or her butt wet by sitting on the floor of the boat.
Also, this seat design works if you want to replace an uncomfortable seat or add a seat to a thwarts only canoe like the one I learned to paddle in at summer camp.
Have fun and remember. It's your boat. Do what you want to it just so you don't sink her.
Tom King - Tyler, TX
