The Dutch barges have this system. The main features are that the pivot points for the frame should be aligned with the mast pivot. A substantial tabernacle (mast pivot), is also a good idea.
I have this system for my boat, but the A frame is removable and consists of two spinnaker poles hooked onto a ring at the apex, and the other ends clip onto saddles on the deck. I use the mainsheet tackle for the lowering, but you could also use a winch.

My son had a permanent "A" frame on his 34 ft boat to lower the 40' rig. It worked perfectly. The rig was a double inline spreader with forward and aft shrouds. The cap shroud chain plates where rigidly extended to the the tabernacle hinge level so it kept the tension on the inline shrouds as the mast lowered to the stern. Only the forward lower shrouds and the forestay had to be released.

The same for my 40' rig( inline shrouds) but I use spinnaker poles for my "A" frame. I made some removable brackets that fasten the poles to the toe rail and a fitting for the other end to join the poles, attach the forestay and fasten the block& tackle. My chain plate extenders are able to pivot at the deck height so I need to rig a brace to them so they are forced to pivot at the turnbuckle toggle,,at tabernacle pivot height. A couple of good quality tie down straps work good for this.

Depending on what rig you have may have to customize the apparatus. Aft shrouds with swept back spreaders will go slack as you lower the rig so you may need something to stop the rig from swinging over the side. If you plan to travel under those Freo bridges regularly, some sort of permanent apparatus will make the job quicker and safer.

Thank you everyone for the information, much appreciated. I believe I'm going to use a Highfield lever similar to the below. and incorporate an attachment point for the "A" frame and blocks. Does anyone have a recommendation on where to buy a Highfield lever like this? Alternatively, I will fabricate one myself.

rumblefish said...
Buy them here
www.almasts.com.au/rigging-fittings/

Thanks rumblefish, have sent them an email last night and awaiting reply.

Check out this guy:

http://www.skymaze.com.au/Marine%20Rigging.html

Eddie replaced my mast a couple of years ago. He does some great work and gives excellent service.

One idea I've been meaning to try is a spinnaker pole from an attachment on the mast (maybe near the lower spreader) to the top of a modified stanchion that is in-line with the pivot point on the mast. This should work to stop the side to side movement of the mast in a swell.

I've rigged a temporary A-frame for my boat but when it is in place it does get in the way a little bit. Keep that in mind when designing your system. The foredeck can get quite crowded with sheets, furling lines, mooring lines etc.

One other invention to share: I have mounted a remote controlled 12v 4wd winch on the compression post of my boat inside the cabin where it stays dry. I pass the winch line out through the forward hatch to a turning block at the base of the forestay then up to the A-frame close to where the A-frame connects with the forestay. Once the highfield is released the winchline takes the strain. I can then stand on the cabin top with the remote control and guide and lower the mast in a civilised manner, singlehanded.

Now we just need to petition the council for a wave attenuation barrier on either side of the bridges so we can do this operation without impact from the stinkboats.