OK gang: The good news is I was able to replace the exhaust hose, the through-hull looked OK, and all is good to go now. 
Turns out there was 3 mm hole that had eroded through from the interior at the top of the hump just before the connection to the through-hull. When I cut through there was a 2" x 4" oval area that was heavily worn away, with the area ~1/2" around the hole being <1 mm thick (hose starts out ~ 5 mm thick). Although I didn't perform a full inspection of the entire length, there did not seem to be any other areas degraded so badly. Still...
The bad news: For the Mk 2 the hose is 2 INCH ID, and you only need 12 FEET. I think the dimensions Stu quoted (1 5/8" ID X 17 ft) earlier must be for the Mk I. Fortunately there was a marine supply store nearby that had exactly what I needed so avoided having to slice up the 18 foot section I had ordered. Even better, the original supplier I used will take the overlong section back for a full refund.
Pre-work hints:
1. Remove the wheel to make access to the aft locker easier.
2. Pull EVERYTHING out of the aft locker and aft cabin (or at least both mattress sections). It's pretty cramped in there, and you'll want every free cubic inch for maneuvering.
3. Strongly recommend that you take out the aft panel in the rear cabin to really get a good look at the hose routing. This also makes putting the new hose back in place much easier as there is the blower hose and several cable bundles running in that area as well. Doing it blind is a non-starter. NOTE: The steering gear cover is attached to the bulkhead and the overhead so that needs to come off as well to make this happen.
4. Leave the starboard laminated board in place. The center and port panels that support the mattress need to come off to access the muffler and hose.
5. Get FOUR new 316 Stainless hose clamps or something equally as good. IMHO the factory ones were not top of the line. Costs a little more, but you really don't want these corroding away on you.
Getting it done:
1. I found the drain on the muffler box was somewhat corroded and stuck. When I tried to open it and empty the box before commencing work, the entire drain came out. I was able to work it loose, clean it up, apply some teflon lube to the threads, and the reinstall it easily (don't forget the teflon tape). Don't over-tighten or you'll strip out the threads on the box.
2. Start the actual hose removal in the aft locker by removing the two straps holding the hose in place: one just behind the line for the port cockpit drain and the other at the top of the compartment (the fit is so snug that only the first one is really needed later). Take the drain hose off and unscrew the barb to make access to this strap easier. Just don't forget to put it back unless you want a following sea or the water in the cockpit to enter the tube and wash out the locker. NOTE: It is physically possible to get all the way into the locker (provided you are no taller than 5' 9" and 190#) to do some of this, but mostly it's a lot of awkward bending, reaching, and pulling.
3. Go to the aft cabin. The muffler is located on the port side storage area under the "L" of the center floor panel. The hose runs aft from there under the 2' x 3' panel. There is a vinyl covered metal strap holding the hose in here that needs to come off (I opted to not reinstall as it is a B**** to get back - try to insert a screw in a small space, in the blind and upside down using a stubby screwdriver). The hose runs from there under the tray for the aft water tank and then angles to port and up.
4. Getting the very stiff hose off the muffler box outlet is akin to trying to drag a very large snake out of a tight hole and will provide a nice opportunity to exercise your muscles and nautical terminology. Once you get it off pull it about 6" back into the cabin. This will pull the hump down from the top of the locker and make getting it out later far easier.
5. Now go back up to the locker, remove the through-hull clamps and pull the hose off. Muscle the hose out into the open and start pulling until it comes out. This may require some creative persuasion with your feet and hands to pull it roughly parallel to the hull and out from under the water tank tray.
6. When reinstalling I found it easier if I fed the new hose from inside the aft cabin. That way you can make sure it gets to the right place and does not foul on any of the cables or blower hose. Coming in from the aft locker is pretty tough due to poor visibility of the pathway and no room to work.
Total time on job (not counting getting the shorter hose): ~ 2 hours. I could probably have gone faster, but preferred to go slow as this was my first time trying this. Keep that ben-gay handy for afterwards.
Still sorting photos and will try to post soon. If you want them sooner contact me via email.
Turns out there was 3 mm hole that had eroded through from the interior at the top of the hump just before the connection to the through-hull. When I cut through there was a 2" x 4" oval area that was heavily worn away, with the area ~1/2" around the hole being <1 mm thick (hose starts out ~ 5 mm thick). Although I didn't perform a full inspection of the entire length, there did not seem to be any other areas degraded so badly. Still...
The bad news: For the Mk 2 the hose is 2 INCH ID, and you only need 12 FEET. I think the dimensions Stu quoted (1 5/8" ID X 17 ft) earlier must be for the Mk I. Fortunately there was a marine supply store nearby that had exactly what I needed so avoided having to slice up the 18 foot section I had ordered. Even better, the original supplier I used will take the overlong section back for a full refund.
Pre-work hints:
1. Remove the wheel to make access to the aft locker easier.
2. Pull EVERYTHING out of the aft locker and aft cabin (or at least both mattress sections). It's pretty cramped in there, and you'll want every free cubic inch for maneuvering.
3. Strongly recommend that you take out the aft panel in the rear cabin to really get a good look at the hose routing. This also makes putting the new hose back in place much easier as there is the blower hose and several cable bundles running in that area as well. Doing it blind is a non-starter. NOTE: The steering gear cover is attached to the bulkhead and the overhead so that needs to come off as well to make this happen.
4. Leave the starboard laminated board in place. The center and port panels that support the mattress need to come off to access the muffler and hose.
5. Get FOUR new 316 Stainless hose clamps or something equally as good. IMHO the factory ones were not top of the line. Costs a little more, but you really don't want these corroding away on you.
Getting it done:
1. I found the drain on the muffler box was somewhat corroded and stuck. When I tried to open it and empty the box before commencing work, the entire drain came out. I was able to work it loose, clean it up, apply some teflon lube to the threads, and the reinstall it easily (don't forget the teflon tape). Don't over-tighten or you'll strip out the threads on the box.
2. Start the actual hose removal in the aft locker by removing the two straps holding the hose in place: one just behind the line for the port cockpit drain and the other at the top of the compartment (the fit is so snug that only the first one is really needed later). Take the drain hose off and unscrew the barb to make access to this strap easier. Just don't forget to put it back unless you want a following sea or the water in the cockpit to enter the tube and wash out the locker. NOTE: It is physically possible to get all the way into the locker (provided you are no taller than 5' 9" and 190#) to do some of this, but mostly it's a lot of awkward bending, reaching, and pulling.
3. Go to the aft cabin. The muffler is located on the port side storage area under the "L" of the center floor panel. The hose runs aft from there under the 2' x 3' panel. There is a vinyl covered metal strap holding the hose in here that needs to come off (I opted to not reinstall as it is a B**** to get back - try to insert a screw in a small space, in the blind and upside down using a stubby screwdriver). The hose runs from there under the tray for the aft water tank and then angles to port and up.
4. Getting the very stiff hose off the muffler box outlet is akin to trying to drag a very large snake out of a tight hole and will provide a nice opportunity to exercise your muscles and nautical terminology. Once you get it off pull it about 6" back into the cabin. This will pull the hump down from the top of the locker and make getting it out later far easier.
5. Now go back up to the locker, remove the through-hull clamps and pull the hose off. Muscle the hose out into the open and start pulling until it comes out. This may require some creative persuasion with your feet and hands to pull it roughly parallel to the hull and out from under the water tank tray.
6. When reinstalling I found it easier if I fed the new hose from inside the aft cabin. That way you can make sure it gets to the right place and does not foul on any of the cables or blower hose. Coming in from the aft locker is pretty tough due to poor visibility of the pathway and no room to work.
Total time on job (not counting getting the shorter hose): ~ 2 hours. I could probably have gone faster, but preferred to go slow as this was my first time trying this. Keep that ben-gay handy for afterwards.
Still sorting photos and will try to post soon. If you want them sooner contact me via email.
