Yes, you can install two Group 27 batteries in the CP 27

[wes]

Here's how to do it:

1. Buy two Trojan 27-AGM batteries, or batteries of similar size (the Lifeline AGMs will not work but others might). I had a little trouble finding a distributor in my area (NC). Eventually discovered that Batteries Plus Bulbs is a Trojan dealer and was willing to order for me. Cost was around $250 each. The Trojans have 89 Ah of capacity (20 hour rating) so this will result in a 178 Ah bank.

2. Trim off the handle brackets from the sides. Easily done with a hacksaw. This reduces the width of each battery from 12.05" down to about 11.5".

3. The resulting 23" of total battery width fits the 23.5" wide battery compartment with about 1/2" to spare. I slipped in a scrap of plywood between the batteries to prevent any shifting from side to side and to encourage air circulation.

4. Cut some short lengths of 1.5" x 1.5" wood to make blocks to prevent the batteries from shifting fore and aft. I screwed them down with one #10 screw per block.

Photos below -

[deisher6]

Hey Wes:  Professional job. 
Hard to believe that there exists a 30 year old sailboat that has only two wires per terminal! 
Just seems wrong.
Smooth Sailing.
regards charlie

[moonlight]

Theoretically, Group 31's w/ 105Ah should fit anywhere a Group 27 does.

Or, I can build you a LiFePO4 equivalent of a Group 27 size with 200Ah.  Cost 6x as much as your Trojans.  Lasts 6x as long (30 years).  And with 80% depth of discharge instead of 50%, you get 160 usable Amps from each battery (compared to 45 from the Trojans)...

[wes]

I can attest from my own research that not all Group 27s are the same size. The Trojans fit in the CP 27 battery compartment only because they are significantly smaller in width than other group 27 batteries, including the Lifeline ones. This is the main point of my post.

Wes