In this instructable you will be adding ribs to form the hull of your kayak. This involves bending wood with steam or hot water. Although this may sound like a daunting prospect, it isn't. There's a total of 24 ribs to bend. If you have no experience bending wood when you start, you will be quite good by the time you're done.
Step 1: Materials
Ribs are best made out of hardwoods like oak. Softwoods like pine or spruce don't bend nearly as well. Elm bends even better than oak, although you're not likely to find it at most lumberyards. Ash is also a good bending wood.
I like to get my oak boards in 3 inch widths, 5 to 8 foot long. I pick through the boards to find ones that have flat grain that runs straight for the whole length of the board. The longer the board, the less likely it is that its grain is going to be straight for the whole length of the board. The boards should also be free of any irregularities in the grain.
Ribs need to be about 8 inches longer than the width of the gunwales where they are being inserted. For estimation purposes figure an average of 24 inches per rib. Lets say you buy boards that are 5 feet long and 2-1/2 inches wide. Your ribs will be 1/4 inch thick, so you can get about 7 sticks out of each board assuming you lose 1/8 inch per cut. Each of those sticks will give you two ribs, so each board will give you 14 ribs. Allowing for breakage means that you will probably need three of those 5 foot boards to make all the ribs for your boat.
Step 2: Tools
Steambox for heating the ribs
Gloves for protecting your hands from hot wood
Saw for trimming the ribs
Clamps for holding the ribs in place on the boat
Step 3: Cutting Rib Blanks
The easiest way to make rib blanks is to run your board through a table saw, cutting it up into strips 1/4 inch thick by 3/4 inches wide.
Ribs bend most easily and with the least amount of breakage if you cut your blanks so they have vertical grain. To do that, start out with a plank that has flat grain. The slices you cut off the flat grained plank will have vertical grain.
Step 4: Soak the Rib Blanks in Water
Ribs need to soak in water for about 3 days before you bend them. I made soaking trough out of a section of rain gutter with the ends capped off. I soak the rib blanks full length and do not cut them up into shorter sections until it's time to steam.
Step 5: Cut Up the Rib Blanks
When you're ready to steam the ribs, pull the blanks out of the soaking trough and cut them up into individual ribs. Length of the rib should be the width of the gunwales plus two hand breadths. You could of course cut all the ribs ahead of time, but it's a good idea to make sure that gunwale width plus 2 hand widths is long enough by bending a trial rib first.
Step 6: Round the Edges of the Rib Blanks
Round the edges of the ribs with a spoke shave. One or two passes with the spokeshave is sufficient. Rounding the edges makes the ribs less susceptible to splitting. It also makes them more comfortable to sit on.
Step 7: Mark the Bend Position
With the rib blank laying across the gunwales, make a mark on the rib two fingers width in from the edge of the gunwales on both sides. This is roughly where you want the major bends of the rib to be. The section between the marks should be more or less flat to slightly arched.
Step 8: Rib Shape
Rib shape in the middle of the boat near the cockpit should be slightly arched in the middle with a sharper bend toward the sides.
As you proceed toward the bow, the hull will get narrower and taller so that the ribs start becoming more arched in the center.
As you move toward the stern of the boat, the hull becomes flatter so the ribs tend to stay flat in the middle with the bends concentrated near the edges until the very end.
In general, the flatter the ribs are in the middle, the flatter the bottom of the kayak will be and the more stable it will feel.
Ribs that are more uniformly arched throughout will give you a more rounded hull that is less stable but slightly faster and livelier than a flat bottomed hull.
If you are building your first kayak or don't have much paddling experience go for the more flat hull.
Step 9: Rib Steaming
You will need to steam your ribs to get them hot enough to make the wood plastic enough to bend. See my instructable on how to make a steam box for details.
Step 10: Bend Ribs With Hot Water
If you don't feel like going to the trouble of making a steambox, you can also bend wood by using hot water. Bring water to a boil in a kettle and then ladle it over the rib repeatedly. After about a minute or so, the rib should be pliable enough to bend.
Step 11: Rib Bending Strategy
Start with a rib near the middle of the boat in the cockpit area. Put it in the steam box and let it steam for about 5 minutes. 5 minutes will be long enough. As you get into the swing of things, you might find out that you will need to steam them even less than 5 minutes. Exact time depends on your wood, how wet it is and how hot your steambox is.
Your next rib can be on either side of the first rib. Alternate back and forth until you have about 10 ribs in place, then put in all the ribs toward the stern. After that put in ribs toward the bow.
The reason for finishing one end of the boat and not hopping back and forth after you have the first ten ribs in is that the progression of shape in the front and the back of the boat is different and it is easier to get the transition of shape right if you stay on one end of the boat.
When you made the gunwales, you cut rib mortises to within 24 inches of the ends. You will probably find that gunwales at the last rib mortise are so close together that getting a tight enough bend in the middle of the rib is impossible without breaking it. So you can leave the last rib mortise on either end un-occupied.
After you pull a rib out of the steambox, you have a limited amount of time to bend it. Once it cools down, the rib begins to stiffen up. So you need to work fast. Once you have clamped the bent rib to the gunwales, you can still make adjustments to the shape, but the major bending should be done while the rib is right out of the steambox.
Ribs will remain pliable as long as they're wet. As they dry, they stiffen up and settle into their new bent shape.
Step 12: Rib Fairing Strategy
The transition in shape from rib to rib needs to be smooth especially along the line that the two hull stringers are going to follow. The hull stringers will be supported by the ribs and if there are gaps between the ribs and the stringer, the stringer will not follow a smooth curve. So as you start putting in ribs near the ends of the boat, take a long piece of wood and lay it over the ribs where the stringers will be and make sure there are no gaps
Step 13: Rib Trimming Strategy
You will have cut your rib blanks longer than they need to be. Once you have a rib clamped in place you will eventually need to trim it so the ends will fit into the rib mortises. If you only have two clamps or not enough clamps for all the ribs, then you will have to alternately bend, trim and seat your ribs.
Step 14: Bend Your First Rib.
When your first rib blank has steamed long enough, put on your gloves. Grab the next rib you will be working on, slide it into the steambox and pull the first rib out of the steambox. Cover the opening of the steambox back up so you don't lose heat.
Grab the rib on either side of the bend mark and bend it. Then bend the other side. Shove the rib between the gunwales and under the keelson and clamp it to the gunwales. Go to the end of the boat and check to see if the height of both bends is the same and symmetrical. If not, loosen the clamps and adjust the position of the rib to make it symmetrical.
In general, ribs want to arch up in the middle so that you need to push the legs of the ribs up at the sides to flatten out their shape in the middle.
When you are happy with the shape, move on to your next rib.
Step 15: Mark the Rib for Trimming
With the rib clamped in place, mark where it intersects the gunwale.
Also put a mark at the top of the rib at the keelson to indicate which side of the rib faces forward. After you trim the rib to length you will want to insert it with the right side facing forward.
Step 16: Trim the Rib
Trim the rib 5/8 of an inch longer than your mark.
Rib mortises are half an inch deep. We add an eigth of an inch in length to make up for the fact that the rib ends will be farther apart when seated in their mortises.
Step 17: Taper the Rib Ends
Taper the sides of the ribs down to 1/2 inch at the end. Also put a slight taper on the outside face of the rib end.
Step 18: Insert the Rib in Its Mortises
Seat the rib ends in their mortises at an angle, then tilt the rib upright.
Step 19: Bend the Rest of Your Ribs
As you bend each new rib, bend it to fit in with the ribs that are already in place. You will need to keep a good transition of shape from rib to rib as the boat becomes more narrow toward the ends. It isn't enough that each rib is symmetrical, it must also relate well to its neighbors.
Step 20: Put in Ribs Near the Ends of the Boat
Ribs near the ends of the boat are more strongly arched than the ribs in the center of the boat. So they are more likely to break when you try to bend them. Breakage at this point is normal. If your ribs break, shave them down in the center to maybe 1/8 inch thickness and they will be able to handle a tighter bend.
Step 21: All the Ribs Are in Place
You're done with this part of the instructable. Take a breather. You just finished the hardest part of making a kayak.
When we come back, you will be adding chine stringers and deck stringers to your kayak and doing a few other little odds and ends to complete the frame of your kayak.