I'm 6'1" and weight got up to 240 and I was fine in my Chess 17.  (I'm down to 215 now!!).  I have to believe you'll be alright in the Chess 18.  I have big skier, bicycler thighs and cut off the upper thigh braces/keyhole shape on the cockpit coaming during initial construction, making it a regular egg-shape.  I put foam (the foam not used on side thigh braces, which were also left out of the build) under the inside of the coaming at about the location where the keyhole parts were cut off and it is a natural fit for where my upper thighs need bracing as is.  I left off the thigh brace plates on either side of the seat, I think the boat is structurally sound without them, and it makes it easier to store a water bottle or coat behind your butt and still be able to reach it. You probably won't be eskimo rolling your Chess 18 or running any whitewater, so unless you going to take on big ocean waves I can't imagine missing the side thigh braces.   

My Chess 17 has the kit supplied backrest backband and the Hot Seat Grande bottom seat on top of a piece of 1" closed cell foam - this seat set up works fine, but I much prefer the Creature Comfort seat that I put in my wife's Sheawater Sport.  The Creature Comfort is very comfortable on the butt, and the solid back does not ride up or down like a backband. I looked, the Creature Comfort spec says the backrest is about 12" high, compared to 16" for the sit-on-top seat you are looking at. Without having a countoured sit on top seating area under the sit on top seat pad, it looks to me like you might like to put the sit on top seat on top of a Hot Seat Grande or shaped peice of foam to get some butt contour down there, but it looks like that would be a good option if done that way.

Recap recommendations: Get rid of the keyhole upper thigh braces and side thigh braces during the build.  Choose a Creature Comfort or try a Sit-on-Top seat with A Hot Seat Grande or some sort of extra countoured pad between the sit on top seat botton and the bottom of the boat.

And I'm betting you'll find the kit very easy to build if you done a plan build.  You might have to clean the garage and maybe get things set up on the diagonal to be able to get around both sides of the hull during the build.

Good luck, report back what works!

Hi, 

i would echo a lot of what Bubblehead said but would add the following:

a) 20 feet should be plenty.  not a lot of room to walk around the ends, but i have routinely built boats in a garage with little more than a foot available.   you don't typically spend a lot of time standing directly across from an end, and when you need to, you can typically open the garage door, put the boat on a diagnal or push it up against one wall to create more space on one side.  that said, if your garage is dirty, i always begin my projects (which i build in a garage) with a cleaning/decluttering of my garage.

b) no difference in build from the 16.  just bigger/longer and at least one more scarf or puzzle joint becuase they cut the panel from 8 foot sheets.

c) if you are 280 you are over the weight range of 270 lbs paddler weight in the CLC information/specs for the boat.   i would consult with clc to get their perspective.  there is clearly a safety factor they build into their specs.....and fwiw, being a bit heavier usually is not going to create a handling problem or adverse stability problem.  but it depends on the hull and clc would be best to advise you.  if i had to guess, you will find it harder to turn and it will track straighter slightly overloaded.  these boats when fully loaded still have a lot of freeboard, so you won't sink it.  it rated for a total payload of 350 lbs.   the paddler weight recommendation typically is more about fit and managing the center of gravity.  350 lbs in the hull will have a lower center of gravity then a person of 350 lbs....so a heavier person (over the recomended paddler weight) may make the boat more tender if you start to get out over the side more than you expect.

d) i think  bubblehead did a great job of describing seat options.

my only other thought is to check out the builders club https://www.clcboats.com/builders_club/search/  and see if there is an 18 close by that you can try.  when i have had guests at the upper range of the boats i have let them use, usually, weight was not the problem, but that they could not fit into the boats coaming, or did not have enough leg/knee room or it was simply too tight.  the chesapeake 18's cockpit/coaming opening is no bigger than the  chesapeake 14, 16 or 17.  they use all the same pre-cut parts.  i find towards the upper end of the wieght range, the 31X17 inch coaming opening is the more problematic measurement than the rest of the boat...its the only piece of the boat that didn't scale up with the increase in size.

fwiw....you can build a custom coaming and make it larger, its a bit of a pain to do, but not that difficult....just more work.  if i had to guess, this is where you may have a problem.  most of my larger statured friends i paddle with have boats with larger coamining dimensions.

hope that was helpful

h

   Might want to consider carving a foam seat,the seating area is flat on these boats and most premade seats don't give enough thigh support.