No doubt that your approach would work, but I don't think it will look very good cosmetically.  My first approach would be to trim the inner edge of the keyhole in front to see if that gives you enough room.  If not, then I would remove the entire coaming with a heat gun.  That would give you the option of any size/shape you want.  Once happy with the "hole," just add a new coaming and lip.

the great thing about these boats is the ability to customize

i don't think your approach has any challenges other than how to make it look nice and work it into the old coaming (as your are thinking) or removing and building a fresh cockpit as mark suggested.   in other words, i don't think you will have any structural issue...its just about how to meet your 'look nice' standard.

as an idea, you could start with your approach and then if not happy, revert to rebuilding the cockpit in its entirety.

i routinely have made my cockpits a little longer than the spec build for pretty much the same reason....i really like to be able to get my butt in first and then pull my legs in. 

the cleanest look would probably involve cutting a new coaming piece once you have reshaped and built your new risers on the deck.  but i think you could do this without removing all of the old cockpit/risers as mark is suggesting, but just, after removing the existing coaming (just the top piece) extend and add new riser material and put a new coaming on top,

i hope that makes sense

h

   I understand your problem. I cut out the "keyhole" things that are supposed to be thigh braces in the cockpit coaming. It made it easier for my knees to get out...........and in. 

As suggested previously I'd take the coaming off. Move the support below the deck and re-cut the cockpit. Note your skirt likely won't fit after that. 

I have a 40+ year old knee injury and getting in and out of the WD12 is a lot easier than the boats with smaller cockpits. Here's the WD12 coaming on top of a CH16LT which has the same coaming as the CH17.

 Following the advice of the guys above, you could buy the coaming and riser bits for the WD12, remove the original ones and replace them with the ones for the WD12. The main advantage to doing it that way is that the covers and spray skirts for the WD12 would be a perfect fit.

Laszlo

   I like Laszlo's idea except that it looks like I might lose the knee brace area. I use the "keyhole" areas ((which I have glued some foam underneath) while paddling and don't think I'd feel secure without that brace. I have an inflatable kayak that I carry in my RV on "non-paddling" trips (Gumotex Framura) and don't have as solid a place to brace my knees and it doesn't feel as "under control" as the Chesapeake. It will likely be a few weeks before I make a decision and start on this project. Will update when I make some progress. Thanks for the answers. 

i would take laszlo's idea as a guide with respect to defining a new coaming piece..but you can easily create your own new coaming top piece once you sort out the inner dimensions, remove the old coaming and build up the new riser area.

fwiw...i think if all you need is another couple inches so you can get your leg in and width is not an issue, just pencil it out and cut it and test fit.  once you have it right, then build the coaming back up once you sort out the shape.

seems to me the only cutting you need is in the front area.  i would not move the seat location or make the project any bigger than it needs to be.

h

Definitely don't move the seat location. The WD12 coaming position in the photo was for comparison of the opening sizes, not a guideline for where to cut. It was just the position where the WD12 coaming stayed balanced and didn't fall off for the photo.

Building the new coaming to a standard size (WD12 vs. ad-hoc, for example) can be important, though, since as the opening gets larger the need for a spray skirt, spray deck and/or cover becomes greater. The larger hole admits significantly more water and sunlight. If you're good at sewing, you can make your own custom covers as well as your own custom coaming.

Laszlo

   At the risk of mentioning other manufactures both Waters Dancing kayaks in Alberta and Pygmy kayaks make some combings larger than the C-17 but smaller than the WD-12. I don't know if you can buy just the combing from either one though.

I have an older Ches18 with smaller cockpit and ChesDouble with huge cockpits easy for me and my German Shepherd.  My Ches18 is very hard to get my left leg in and out of the past few years.  I am going to purchase a larger coaming.  I also have a large crack in deck a few feet from bow.  Fix both at same time and add some light fiberglass in rub areas. 

cockpits (coaming and spacers) $125

32.5" x 19"

33"x 21"

34" x 18.5"