H25 split backstay
Much as I hate to enable anyone to have a bimini (and next will you ask how to raise the boom or recut the mainsail; and then how to change to mid-boom sheeting? :naughty
I changed to a split backstay on my boat primarily for fitting a backstay adjuster -- and found it also lends better access to the motor (which has to be steered in reverse as we all know).
Take off the single standing backstay. Lay it out completely straight on the ground. Measure the width between the two new chainplates. Lay this out on the ground perpendicular to the backstay at the bottom end. Then calculate how far up you want the two legs to go and measure the new distances for the legs. Remember to take into account the distances used up for pins, toggles, and so on. Calculate all TB lengths at half extension. Then take it to your friendly neighborhood sailmaker or rigger to have a new one made up (do NOT attempt to swage this by hand). You will find machine-swage to be the cheapest way to go and your friendly neighborhood sailmaker will be grateful for the work. Mine cost $80. I bought the fish plate and the TBs and the chainplates.
The backstay adjuster I used is the CS Johnson one with 4 wheels and an eye for the tackle. CS Johnson stipulate a 1.5:1 ratio between the height of the fish plate and the width of the chainplates. If you are not planning on an adjuster now but might install one in future, make it like this. My fish plate is about 6 or 7 ft up.
The CPs can be installed on the transom; and going with a 1.5:1 proportion you can approximate (half) this angle when you mount them. It is not absolutely vital to get it exactly right; bolts will be bolts and will take up the slack for a slight misalignment. Use hex-head bolts, not all-threaded machine screws; they are uglier but better for sheer loads. Bed them in 5200. I later chose to not use the chainplates (and sold them on eBay) and installed U-bolts using the last hole of the toerail and a hefty SS angle for a backing plate. I'm a big fan of U-bolts, provided they are really 316 SS. They are strong, cheap, easy to install, easy to bed properly, and look good. (The ones I specified for the Cherubini 44s are rated at 16,000 lbs each-- 2 of them would lift the whole boat. Mine are 1/4" and rated at like 2,000. The H25 weighs 4,000.)
My upper backstay was 1/8" but I upgraded to 5/32" all round. The two lower legs remain at 1/8". They are both fitted with bronze 1/4"-pin (open-body!) TBs.
(The 'fish plate' is that triangular metal thing where the three parts of the split backstay meet. My brother first called it a 'fish plate' --based on a Cherubini Boat Co shop joke-- in about 1977. Ever since then everyone else has been calling it a 'fish plate' because I named it that on my drawing sent to the fabricators in 1980 and it got round to the spar makers. Now CSJ and others refer to it as a 'fish plate' in their parts catalogues.)
I do not know an easy way to explain the trigonometry for how you can calculate the space you will have between the legs as they ascend above the cockpit. (I attempted to calculate this for the forward mizzen intermediates on the 'new' C44 'D' rig and fortunately was not around when they actually rigged that boat in front of the owner!) But if you are planning for a (dreaded) bimini top, call your friendly neighborhood sailmaker or canvas-cover guy after the backstay is reassembled on the boat and he will take the measurements and let you know how it will fit. He may even be able to guide you in calculating how high to make the legs in order to have satisfactory room between them at the back of the cockpit. But with a tiller-steered boat this is less important than if you had a wheel.
(I don't have either right now but I prefer a boom awning and am having one made for next season.)
