One of the many upgrades I've done to Belle is the addition of a back stay tensioner. I've been very pleased with the results, and so I've attached photos of the installation for others who may want to do a similar upgrade.
The C&C 25 mk1 has a transom hung rudder, and a cut down section of the transom that the tiller moves in. With end-boom sheeting, the traveler is a curved track that runs the full width of the transom and actually passes over the tiller.
There really is no strong point in the transom that will accomodate the mounting of a pad eye for a more conventional tensioner based on a block and tackle. Such an installation would also tend to block the movement of the mainsheet and traveler. The aft of the cockpit also tends to get a bit "busy" for the helmsman, so I wanted an arrangement that could be tensioned and released from whichever side of the helm was most accessible.
The design uses (2) Garhauer 25-06SL fiddle blocks with cam cleat and becket, and a Garhauer 25 series triple block attached to a stainless fitting holding wire pulleys that straddle the split backstay on the boat. The fiddle blocks are attached to holes that already existed in the cast aluminum fittings at the end of the toe rails.
Reeve in 50 feet or so of 3/8 double braid to complete the tackle, and you get a tensioner with 6:1 purchase, that can be pulled or released from either side of the cockpit, and runs neatly parallel to, and just inside, the existing split backstay wires.
The Garhauer hardware is supurb quality, and surprisingly inexpensive. The cost of the installation was only about US$150, including a couple of needed Wichard long "D" shackles and the line.