Someone who owns another trimaran asked me about my mooring setup, so I documented it for him in an email, and I'm reposting it here in case anyone wants to borrow or adapt any of these ideas for your own situation: Mine is a 12m tri with 28 foot beam, and strong U-bolt attachments for the bridle legs at the forward outer ends of each cross-beam.

Here's what I do:
I use a bridle plate like the Taylor Made Bridle-Plate-Mooring-System for Multihulls w/Safety-Shackle.

I use a 3/4" US-made galvanized shackle to attach the plate to the top of the mooring chain which runs through the center of the mooring ball. Do NOT use a type of mooring ball that has a steel rod running through the center -- the rod rusts out within a few years and the rust is hidden inside -- becomes the weak link. The mooring ball should have a pipe that allows the chain top run through, with a bolt at the top that you put through the chain to secure it.

The other 3 shackles (for each leg) are 5/8". I recommend you avoid Chinese shackles on any anchor or mooring tackle -- they're sold on price and I view this as a critical application not worth saving a few $ for the increased risk.

My center bridle leg is backup only. It's a 15-20 foot long 1-inch diameter Mooring Pendant (Yale Maxi-Moor II or equivalent). I recommend the 20-foot length because it gives excess to wrap your cleat and adjust length to slightly slack with the side legs taking the load.

Each side leg is 20-feet of 5/8" 3-strand nylon with a thimble spliced at the bridle plate end, where it's shackled and pin is seized. At the outer end of each crossbeam splice in a Wichard #2317 snap hook. I've been using these since 2001 and have never had a problem nor have they shown any deformation.

Each leg of the bridle should have flotation so that the rope can't sink beneath the mooring ball (when the wind goes slack) and wrap beneath the chain. I use inexpensive closed-cell foam pipe insulation from Home Depot to float the mooring bridle lines. You can buy slit (split) insulation that's easy to apply, and it also keeps the growth off the lines. I use 3M electrical tape every foot or two to secure it, and use a different color for each leg so it's easy to sort them out if they cross each other. I use a pick-up stick with float attached to the loop on the end of the center pendent.

Also, your mooring ball should be sized so that the center of the ball is slightly below the waterline -- so the rope will go over (not under) when the slack line is pulled again. There are cylindrical shaped mooring floats now and I like those better than balls, because they float more vertical and keep the shackles and mooring plate away from the hull. The cylindrical floats also allow you to coil the pendent so if you leave for an extended period they don't accumulate growth. (You can do that for the center pendant but there's no room for two or three, so I remove them and just leave the center one if we plan to go for more a week or more.)