I'm still thinking about hoist support using the pier and octagon posts. Though I shall probably try to use the doubled stepladders they are very unwieldy for such a small platform. They are also rather heavy when doubled up. Once tied together the whole arrangement then has to be brought upright and the hoists attached.
You would not believe the effort that went into the arrangements shown in these two images.
First the mounting parts were raised to the obs. floor using the chain hoist. A single stepladder was carried up and bridged across the large ladder opening in the obs. floor to support the hoist. Then the second stepladder had to be carried up there manually to prepare for the biggest lift.
The two builder's stepladders were straightened out and their base extensions tied together securely at their tops. The lifting strop was the final addition before the bases of the ladders were brought inboard and their tops pushed up high. Larch boards were placed across opposed pairs of octagon posts to retain the ladder bases within the platform. Finally the ladders were stabilized laterally with four ratchet straps.
When I have previously used the double ladder arrangement as a support for the chain hoist I have been able to move the whole caboodle about on the ground. With the ladders up on a raised platform I don't have any room [at all] for such maneuvers. So the final lift will require a lot of lateral pulling to avoid damaging the pier or the mounting base. Hence the ratchet straps to keep the hoist support upright.
I had managed to lift the PA housing into the base fork but the Dec housing and attached 2" PA shaft is still too heavy for me to manage. Hence all the effort to provide a safe support for the hoist at sufficient height to bring the mounting back together again. I can feel the strain in my back, this morning, after all that heavy lifting. Even the chain hoists [with chains] is a hefty object in its own right. So I shall have to be very careful not to injure my back again.
The secondhand boat winch proved to be unsafe because the ratchet, backstop click was not working properly. The coil spring was actually pulling the ratchet away from the gear wheel! I hung the boat winch from hefty SS carabiners but it really needs fixing to something solid to stop it moving about when winding the handle. The awkward arrangement of the ratchet requires two hand on the winch at all times.
I am glad I decided to use the chain hoist instead. As it provides very safe locking at every moment of the lift [and lower] with very precise control of the height. As mentioned before, I have to carefully avoid causing damage to the scenery with the flailing hand chain. The heavy chains easily mark the aluminium parts of the mounting. A bit disappointing after spending hours smoothing the scrap metal of its construction.
It required a couple of attempts before I could get the mounting on the pier. I didn't have the necessary height until I shortened the strops by looping them. Even then I had to mount the Dec and PA together to be able to push the altitude pivot through the support fork. There is still work to do on the altitude adjustment turnbuckle. The front plate is still sliding forwards.