This web-page has a link to another C and C site where you'll find an "Alladin's Cave" of info. Among that stuff is a copy of the original C &C 27 Mk 111 brochure giving the mast height above the water as 41 feet. I've never measured it but have always used this number for clearance under bridges and power lines--- There's a marina nearby which I can't get to with Aragorn --- the power line strung across the entrance there is too low .
Given that the "P" figure for the Mark 111 is 31 feet, if you measure from the gooseneck fitting ( "x" feet)to the mast step, you can easily figure the distance from the mast step down to the waterline .
41 - 31= 10 feet from the gooseneck to the waterline minus "x" = above water height of the hull . Add the draught of 4 ' 6" and you'll find you needed clearance ( except that the hatch cover is a little higher than the mast step and as Dave suggests , you'll need a bit of room under the cradle as well)