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Note from Kayarchy: This is from Grant Glazer's "Hints & Tips" pages.
Because those pages have been offline for some years, I've reposted this one to keep it accesssible.
You should be able to spend as much time in the kayak as you can in a car seat. There is one major difference - you don't want the back higher then the cockpit rim. Not only will it interfere with rolling but it will also wear out your spray skirt by constant rubbing. A high seat-back is also unnecessary since you should have a straight back or a slight forward lean when paddling. Lumber support is all that is needed and is more comfortable. The other design features of a car seat do apply. It should be:
When planning out the seat make sure it is as low as you can get it. Most of your weight is concentrated at the back of seat. If you sit too high in the kayak, it can greatly affect the stability of the kayak. Even a 1cm adjustment can make a large difference.
You can make the seat out of fiberglass but closed cell foam is much easier to use and comes ready padded. I have yet to find a good cheap supply of thick foam in NZ but most kayak shops either stock it or will get it in for you. 1/2 a square meter should be enough. Other sources of supply include an old boogie board, art packaging, or a cheap backpackers mattress from The Warehouse. This last option will require you to glue up layers to get the right thickness.
Once you have enough material, there are two ways of shaping the seat. You can heat the foam to 140 deg Cel then sit on it. The softened foam will shape to a perfect inverse image of your rear, and will retain its shape once cooled. You should be wearing your normal paddling outfit when you do this. For some reason sitting on 140deg's kinda' put me off this method. So I went for option 2, carving the foam.
Here are a few great web sites that explains this process better then I could:
[GRANT'S OTHER TWO LINKS ARE DEAD]
Although I will add a couple of hints to the above process:
Drill the Foam
I found the easiest way of removing large quantities of foam was with a steel brush drill bit. Make sure the foam is unable to move about by jamming it between pieces of wood which are clamped to the work bench. Do not use any pressure at all and hold the drill with both hands. This is a VERY messy process, but a vacuum cleaner quickly tidies up the foam dust.
Grate and Sand it
A small SureForm or Surform is ideal for fine tuning the shape. Once the rough shape has been created, try it out for fit and comfort. It is usable as is but why have a ugly seat when you have spent so much time on the kayak's finish? Sand the seat with sandpaper to get a velvet finish. I normally start with 80 grit and finish up with 300 . When sanding the seat don't use too much pressure since it will tear the foam. It also helps if the paper is has curved edges to avoid accidentally cutting the foam. A final smooth finish can be obtained by waving a heat gun over it. Practice on a scrap piece first and don't hold it to close or on a high heat setting. Once the foam starts to darken in colour and get shiny, stop - otherwise your new seat will end up being a blob of plastic.
Try, try and try again
While making the seat, keep trying it out in your kayak with hip pads and backrest in place. Sit in the kayak for at least an hour to check if your feet fall asleep. You can read a book or watch TV Just make sure nobody pops around to visit. When trying out the pictured seat I had a problem with the sleepy feet syndrome that I was determined to solve.( The problem turned out to be that my pump, which I store between the hip pads and hull, was causing a very slight pressure against my leg. ) I had the kayak on a mattress in my lounge when some work mates turned up. I still haven't been able to convince them that I don't tuck my kayak up at night to go to sleep, and no I don't read it bedtime stories!
Colour it in - The Two Tone seat
Thick foam normally only comes in a couple of colours - Gray and Green. But The Warehouse, Doyles, Rebel Sports, and other discount stores sell foam Backpackers mattresses in a variety of colours. To get a two tone seat like in the picture above you have to inlay the thinner backpackers mattress into the thick foam:
If you are going to try the air cushion idea, then you will need to make sure it is completely air tight (thus the sealant). Any gap will suck in water which is impossible to get rid off. It will also make rude noises whenever you sit on it! The seat back and hip pads are made the same way.
It is also worth while carving some knee hooks, padding under the deck and the rudder pedals. If you paddle barefooted then add some on the hull underneath the pedals.
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