I have a 8.5m Folding tri with twin rudders that I race with the 8.5's in Auckland, I will try and add to the debate below:
Not necessarily true. Many if not most large ocean racing tris fly the mainhull a lot, and only use a single central rudder.
*** Which large ocean racing tris have only one rudder - I cannot actually think of any?? Ormas, MOD70's F40 Tris all have twin rudders.
All full sized 8.5 tri's in the class currently have twin rudders.
More drag (potentially at least in some instances) -
*** as you can guarantee the rudders will be fully immersed then you can make them smaller than a big single one which comes in and out of the water.
If you get the main hull out then drag significantly less. Side note: Main hull flying more a factor of float volume than sail area - at some
stage you will have enough wind but if you have pissy little floats and a heavy boat like an F24 you will just bury the float rather than fly
the hull. I'm not 100% sure flying a hull is terribly fast but skimming the centre hull is and every now and then while skimming you will get a
gust and the centre hull will lift up a meter or so it would not be nice to have a single rudder when this happens.
More complication (due to the linkages required and all that.) -
More stuff to rig/derig (due to the linkages required and all that.)
*** yes is a major pain but the whole trailer folding thing is a pain so I leave it unfolded at a marina now
Potentially poorer helm feel/tightness (due to the linkages required and all that.)
*** Don't believe so. Usually with a big single rudder you end up with huge long wiggly extensions and I think it is better than that regime
Fun to put back down if it kicks up. (some sort of gear to do it remotely perhaps, which adds to #2 & 3 above)
*** Yes also great for snagging marks if you round too close angled forward rudder means they will never let go either
And each one has to be large enough and strong enough to do the whole job.
*** Yes, slightly mitigated by the fact they can be smaller
So, more weight.
That's a lotta minuses, and not one single plus.
*** The plus is you can keep pushing hard (i.e what happens when racing or you will loose) without going out of control at high speed -
the F85 floats are big enough (from what Ian says similar to what I have) that with or without the foils you will get the main hull
flying (accidental or deliberate) and then things could get really exciting.