I had a similar experience with my shearwater 16 and the stock seat.  I took an extra foam seat top piece (my wife didn't use the stock seat on her shearwater so I had an extra) and glued it on top to make the seat extra thick.  I then carved out a nice friendly butt print (60 grit sandpaper mostly). I can go 6+ hours a day with no problem now.  I think the extra inch of elevation, and the deep butt print that supports my thighs was the key.

I highly advocate experimentation and customization.  You will get it dialed in.

I don't have the boat, but have a couple and for a while tried to go with the stock seats and after a while switched up and went to a good outfitter shop and spent some money on a nice seat.   It is worth it!!   I was even using a wooden seat in one of mine trying to be all natural and cool looking but after a while I figured it was better to be comfortable.   Can't recall the name of the seat but there are many nice ones to pick from.

A lot of people have given this seat great reviews-

http://www.redfishkayak.com/seats.htm

I have been having a similar problem for several years. First, I replaced the thin seat in my Merganser with one that I carved out of 3" foam. That didn't work. I then modified my seat several times to reduce pressure on the sciatic nerves. That didn't work, either.

I now think the problem may come from constant pressure against the foot pegs. Yesterday I visited the CLC booth at the WoodenBoat show and bought another piece of 3" foam. I plan to make a foot pad on the front bulkhead instead of the foot pegs. I did this on my Shearwater 17 and found it much more comfortable than the pegs.

A friend of mine, who uses the redfish seats in all his boats, suffers the same problem, so I don't think it's the seat. BTW, I use a CLC seat as a pad on my wooden desk chair.

-Wes

I've been using the Happy Bottom seat in all my boats and it lives up to it's name. I also tend to leave the foot pegs a little forward for a more relaxed position.

Dan

Try putting a pair of rolled up towels under your thighs between the end of your seat and your knees.  make them thick enough to provide some support.  if the experiment works then buy some minicell foam and carve yourself some thigh supports.  I find this works well in conjuntion with the longer pegs.

Have you tried shortening the pegs?  I have the same problem in my skin on frame, needed to shorten the pegs to correct.

My personal favourite is still the custom carved seat, find that 6 hour + comfy :)

I used a rolled up eva sleeping mat to support upper legs when I used to kayak. Haven't had th problem since swapping over to surfskis. I always assumed it was because my upper legs were supported to some extent by the leg hump used in surfski cockpits in conjunction with increased leg drive. However, on the weekend I took my pax20 surkski for pre-finish testing and had dead leg within 20 minutes of paddling. I am using a clc happy bottom seat with the hip pads removed in the ski. I solved the problem by adding a 30mm packer underneath the rear of the seat giving it a rake. 

Worth a try .

I have the exact same problem as Craig, but mine starts after 1 hour. In both of my Shearwaters, I used an NRS "mild" Hot Seat, with an additional 3/4" layer of mincell.I tend to think that the problem starts with pressure on my tailbones, even though I sculpted the minicell with corresponding depressions.

Already planning my next build, a Night Heron. Any reviews of the Creature Comfort Seat? Will it fit in a NH? Does its use preclude the installation of hip braces?

I have a Redfish custom seat in my Shearwater 14 and I love it (see the link in Hickory's post, above). 

I just came back from the West Coast Wooden Kayak Rendevous in Port Townsend.  I currently have a flat, rather hard foam bottomed seat installed in our Wood Duck 12 in an attempt to get a high-backed seat for my husband's bad back.  That seat made my legs start to hurt when I paddled it for about an hour last week.  At the Rendevous there was a Wood Duck 10 with one of the Redfish seats; I borrowed the seat to try in my Wood Duck - what an amazing difference to paddle.  It was like a new boat.  Very comfortable, and so much more control because of the snug fit with the built in hip padding.  We were trying to figure out why it was so much more comfortable and decided that one reason was the coutoured (butt-shaped) minicel bottom spread out the pressure so it wasn't all on one spot. 

Sounds like Jim 777 has a scuplted seat and still has problems, so maybe that's not it, but I have never had a problem with the Redfish seats.  If there is any way you could try boats with different seats, it would be a great idea to see what works best for you.

I'm using the stock kit seat in a chesapeake 18 and had been happy with it up until recently. I paddled a 6 mile course in June supporting the potomac river swim and my legs felt fine. Then I paddle a 6.5 mile trip this past weekend and my legs, especially my left leg began to hurt about 1/2 way into the paddle.  Only change is a rudder installation and my footbrace position is about an inch lower and was adjusted back a bit from where I had it before. Plus our pace was quite a bit faster on this recent outing. So I'm going to move the footbraces forward a couple of inches and see what happens. 

I think you might have found a solution ludwigd. Let us know how it works out.

My footbraces were already extended to the max, but I was able to move them another inch by cutting off part of the nylon pedal that slides on the racks. I know, it's hard to explain, but it's even harder to post photos on this site to show how I did it. It seems to have helped ease the sciatic pain.

-Wes

I looked at the Redfish seat web page. It looks like a very good, custom fit seat, but it never says WHICH firmness of minicell its made of-is it that rock-hard stuff that my tailbones will go right through, or the softer kind?

Jim 777, good question about the minicel type.  I guess I've only ever seen one firmness.  I just went out and did the squish test (squeeze between my fingers) on my Redfish seat and the gray minicel foam that came in my CLC Shearwater kit.  Seemed the same firmness to me.  If you want a definitive answer, you can e-mail Joe and ask him; he's very helpful.