Re: Is external rudder a good idea?
I was reading about Black Feathers the other day, the Cal 20 that successfully sailed the solo transpac in 2008.
His rudder failed a good ways out (fiberglass fatigue, supposedly) and he was able to replace it easily with some wrangling. He pulled the broken rudder out of the gudgeons and was able to wrestle the old wooden one back into place, allowing him to continue on as normal. The process didn't sound easy, but it was accomplished without a jury rig or serious issue.
Granted, the Cal 20 is basically a big dinghy and most of the larger boats I've looked at with outboard rudders would be substantially more challenging to repair at sea if something failed (e.g. Flicka, Westsail). It still seems to me that, even with those boats that have massive rudders in place off the transom, if something failed, the repair would be easier than dealing with something run through the hull.
It's also easier to inspect, which is a big plus to me.