Preventer
Sailing downwind really requires a preventer to stop the main from an unwanted gybe.
The mass of the sail and boom puts a lot of strain on the attachment points. I have rigged a Dyneema line under the boom that connects to a bungy near the goose neck. This line has some loops to which I can connect a heavy duty 4:1 tackle. I hook it up to the cleats in the middle of the boat. To avoid shock loads, I have rigged a rubber shock absorber. Also I have rigged a shock absorber on my mainsheet. Works really well.
The advantage of using the mid-cleat is that you also pull the boom downwards, thus controling the twist in the mainsail. By pulling it downwards, I can ease the mainsail more without touching the spreaders. Adds a fraction more speed to the boat.
What size fiddle block w/ cam & becket did you use? Not sure what the working load is for the boom in this case.
The disadvantage of using the midship cleat is the safety aspect. This arrangement produces massive loads in the event of a crash gybe running downwind, loads way beyond what your equipment will handle. To learn why boom preventers should always be led to a strong point at the bows and then back to the cockpit for tension control, see Maritine New Zealand’s excellent accident report and analysis of cause in the double fatality aboard sy Platino dated 25 July 2016. Also in the report is the proper way to rig a preventer. Hope this helps.
preventer