|John, someone else's measurements are not likely to do you much good. Although these boats come out of the same molds and are in theory exactly the same, the actual truth is that each boat is a unique individual. Bulkhead placement varies slightly from boat to boat, the exact size and placement of the support beam varies, etc. So, the measurements of my 27 Mk III will be at least several fractions of an inch off of the proper measurements for your boat.
A number of years ago, I had to replace the compression post in Carriden, because the original laminated plywood post had rotted at the bottom. Like you, I used a jack to push up the support beam across the top of the cabin. When I had the support beam completely straight, I measured the distance from the cabin sole to the underside of the support beam and the deckhead beneath the tabernacle. I managed to source a piece of solid teak from Exotic Woods in Burlington, which I then cut and shaped to my exact measurements, mimicking the general shape and profile of the original post. I then slipped it into place, tapping it into final position because I had cut it to be a snug fit. This post has served me for over a dozen years now without any problems.
The reason that the original post had rotted was that water which came in through the forehatch, or was spilled in the head, would sit on the cabin sole, having no suitable drainage because it was blocked by the teak sole in the main cabin. To eliminate this problem, I created a small limber hole in the bottom of the main bulkhead, more or less on the centre-line of the boat. I coated the insides of this limber hole with epoxy so that water could not penetrate the bulkhead itself. Water now drains through the limber hole into the locker under the forward port settee, where it then has a clear and straight path into the bilge. Problem solved. I also cut away the portion of the teak sole under the compression post, so that the new post is resting directly on the fiberglass sole.
Marcus from Carriden
Mk III, Hull #847