Could anybody explain the reasoning of using outriggers to support the rig on some single handed ocean going boats - such as Sill.

Guessing that the wider platform gives a number of benefits :

1, Being wider and with a "prodder", some of the shroud load can be converted into sideways compression load instead of tension "up" load. 

2, Gets the side rigging/shrouds out of the way and so you can have more room around the boat for running rigging / lines etc

3, Further benefit of having the side rigging/shrouds further away from the boat you create a larger set of triangles to move the load around and generally provide more stability - this may also help with some of the boats that use wing masts (maybe)

Just my late night Sunday guesses...

How much movement is built in to them - could you make them hydrolic etc.?

Most keelboat and offshore rules don't allow you to adjust rig tension while racing, as it is thought likely to cause accidents. I don't know enough about the Open 60 rule to say whether this applies to them. 

I was wondering if the wide base that this system provides maens no need for spreaders and hence a rotating mast can be used.  We really need an expert to clear this up. 

Meanwhile - yes 49ers are allowed to adjust rig tension whilst afloat but make no provision for it, so you have to take out a block and tackle and take up the tension temporarily on a trapeze wire - not a very safe idea and probably never invisaged by the designer.  If you want to see a boat where the rig tension is really played about with whist sailing have a look at the RS400 or for a more extreme example have a look at a Scorpion or Merlin.

having deck spreaders reduces the downwards pressure at the hounds, and it is changed into more of a sideways tension. This in turn letsa the mast rotate easier than with normal rigging arrangement.

To make it easier imagine sticking a multijull rig on a mono, the deck spreader replace the outriggers.

hope this made sense.

Rig changes are allowed whilst racing on the Melges24 and Etchells (i think), and generally boats like the farr40 and kerr11.3 will change thier rig between races (I know I have to do it if the wind changes more than 5 knots)

Interesting statement: what class are you talking about? On my last keelboat (swept back single spreaders) I'd definitely say a looser forestay is easier to helm to. Tight gives good pointing but a very narrow groove, loose means you have to foot more but the jib works over a wider angle e.g. easier in a chop.

Ignore my las t post, confused myself a bit there. Should have been loose forestay easier to steer to as it hass a larger groove at which speed is high, but tight is fast for pointing but difficult to keep in groove.

Exactly as Stefan says.

should read before I post.