Monohull spreaders are set at 90 degrees to the mast whereas a catamaran has to have backswept spreaders. The reason is that, on a monohull, there is a backstay and using this, plus the intermediates you can get a nice pre-bend in the mast (the pre-bend is to flatten out the main sail and allow for better performance). On a catamaran with no back stay, you need to use the back swept spreaders and the diamonds to pre-bend the mast. The reason I point this out is because on a catamaran, if you want to broad reach or run, the mainsail cannot be let out all the way because the backswept spreader tips could punch holes in the fabric. On a monohull, the spreaders are at 90 degrees so you can let the main and the boom out much further which is, of course, much more effective. This is one of the reasons it is better to broad reach and tack downwind on a catamaran.