Knee tubes, especially in kayaks with ocean cockpits, also provide an extra bracing option for your knees.
When you design your knee tube or shelf, just make sure you don't compromise your ability to get in or out of the kayak. Remember as well that the shelf or tube may affect your ability to perform emergency re-entries on the water. Before you commit to a design, mock up the design in the cockpit using blocks of foam or something and practice with this first.
Knee tubes can be made of plastic, fiberglass, Kevlar or many other materials. They can be glued in or, if made of composite material, molded directly under the deck. Some prefer to use webbing or bungies attached under the deck to hold a knee tube in place, but this doesn't allow for strong bracing. Just make sure that whatever tube you choose is large enough in diameter that you can fit your arm in, make a fist to grab something and still get your arm out!
ABS plastic drain pipe, about 10cm in diameter can be used. Get the thin-walled stuff, as ABS can be heavy. Glue a cap on one end and leave the other open. Other plastic tubes can be used; some folks use large jars, too. You can fashion a removable cap for the open end. A flap of nylon or polyester fabric or mesh can be attached with glue and velcro for a servicable cover.
If you want to make a composite tube, decide on which fabric to use - fiberglass is cheap and easy to work with. Plan on about 5 oz. fabric or more. Two layers of 3 oz. worked well for me. Unless you have resin around, you might as well spend a bit more for good epoxy as it's tougher and, in the good marine brands, less stinky to work with.
You can make a mold with almost anything the right size. Aluminum ducting, the kind that has an openable seam, makes a good mold. Wrap the mold with plastic wrap (like Saran Wrap) so that the resin/epoxy doesn't stick - this is cheaper and easier than messing with PVA mold release.
The easiest way to remove the mold is to cut a slit in it lengthways prior to covering it with FRP. When the FRP is cured, the mold's slit can be compressed closed to make the mold's diameter smaller and removable. If you use the aluminum ducting, then don't interlock the seam to close it - just tape it closed on the inside. Rip off the tape and the aluminum tube can be coiled in on itself to make it smaller. The plastic wrap should peel off easily.
When you wrap the mold with fabric, let a flap hang down over one end to close it. If you want to make the open end a little stiffer and neater, put a bit of string or line (nylon or polyester) about 3-4mm in diameter around the open end and wrap the fabric over it. This will make a lip that is quite stiff compared to just the composite fabric.
BTW, there are two schools of thought when it comes to making FRP things. One school says cover the mold with fabric and then paint with resin/epoxy. Others say to lay out the fabric on a work area, paint on enough resin/epoxy to just wet it out and then drape the wet fabric over the mold. Personally, I prefer the latter for small stuff.
Once you've made the tube, you can glue it in place under the deck. Some folks like to use strips of fiberglass/epoxy and drape one over each end of the tube to hold it in place. Gluing the tube directly with a marine sealant will also work. If you aren't worried about the tube being rigidly installed (as for bracing) then you could use silicone sealant (the kind used for caulking bathtubs and such) - this will allow you to remove it someday... with a little effort.
Another approach is to use the aluminum duct to make a U shaped tube instead of a circle. Open the tube along the seam and fasten it to a scrap of wood as an upside down U. Cover with plastic wrap and drape the epoxy saturated fiberglass so that there is a flange at the bottom - sort of like:
When the composite has cured but within 24 hours, remove it from the mold, put a thin layer of epoxy on the flange and then press the flange against the underside of the deck to glue it in. This will make a larger tube that is very strong for bracing. You can also put the mold directly in the kayak and drape the wet composite over that, gluing the flanges to the deck in one step. Don't forget to sand and clean the deck area first so that the resin/epoxy holds well. BTW - it is easier to do this if you support the kayak upside down while installing the tube and waiting for it to cure.
Once it's all done, cover the knee tube with some minicel for padding, especially if you're using it for bracing.
BTW - one feature that some folks like is to drill a couple of holes at the closed end of the tube so that water drains out.
Some kayak manufacturers offer shelves as an option. They are usually made of plastic and are molded to fit nicely under the deck. You can also buy under-deck bags from various paddle gear makers. You can also make your own with some ease. Personally, I consider under-deck bags to be the same as a shelf.
If you want to make a rigid shelf, then you can follow the description for making kneetubes above and simply change the mold to one that matches what you want your shelf to look like. It is probably easier to make a fabric underdeck bag as a shelf.
Attaching a shelf or underdeck bag can be challenging. Some of the commercial offerings take advantage of the through-deck bolts that are used to hold the deckline fiddlybits in place. You may have to buy slightly longer bolts, but you can use these to attach your shelf. If you make a rigid shelf, just drill holes to match the bolt placements and attach.
Another way to attach the shelf is to glue it in. You can use a good marine sealant or epoxy. This is a good option for rigid shelves but is not as good for fabric bags. The latter are better to make removable, as the fabrics tend to wear out over time.
Webbing straps can be attached to the bolts described above. Fold over the end of the webbing and use a punch to cut a small hole through the webbing. Doubling it at the end will make it stronger and reduce the risk of a tearout. Use a hot nail or a candle flame to melt the webbing around the hole so it doesn't fray. Be advised that webbing straps across the full width of the kayak represent a potential entrapment hazard!
Velcro and similar hook-and-loop materials are used in some cases. However, standard hook-and-loop does not have much strength in tension and gets even less strong when wet. For that reason, it's not a good product to use in tension. Marine grade and industrial types are better - rather than being hook-and-loop, both sides of the material have interlocking bits that are shaped like little mushrooms. However, these products are hard to find and quite expensive.
A better way to use hook-and-loop materials is in shear. To do this, glue D-ring pads to the underside of the deck. Attach strips of webbing to the shelf/bag and run the webbing through the D-ring and back down to the shelf/bag. Use hook-and-loop on the end of the webbing and on the shelf/bag to secure the thing in place. The tension in the webbing will be transfered through the hook-and-loop by shear to the shelf/bag.
Making The Bag
There are several options here. You can buy a commercial drybag of appropriate shape, make a drybag using heat-sealable fabric, buy or make a mesh bag or buy or make a nylon bag. Mesh has the advantage of draining quickly it it gets wet. Coated nylon can be made waterproof with seam sealer, but the closure can be tricky to make waterproof. Nylon fabric with a bit of mesh can make for a bag that won't let little things out too much but still drain well.
Heat sealable fabric is pricy, but has the advantage of not needing much sewing. Most seams are sealed together by heating them with a clothes iron.
Sewing up a nylon, mesh or nylon-and-mesh bag is pretty easy if you can handle a sewing machine. With a little forethought, you can avoid complicated stitching and make all seams straight. Closures can be roll-top, draw-string, zipper or whatever. Zippers are simple and, if you use all plastic, rustproof.