I have a 22 foot by 22 inch tandem (same basic specs as Tandem Sport) and have just returned from a trip to Tasmania. My "standard" rudder works fine in most conditions, more as a trim tab than something that will make it turn in its own length. Where we had trouble was in serious downwind conditions. When catching a wind wave the nose of the boat dropped, and the stern raised too high out of the water for the rudder to have its desired effect. The same thing happens when we land on a surf beach. In those conditions an underslung rudder has obvious advantages.

I am going to build a longer blade just for these conditions. Tassie can be very windy, and I noticed that the locals had bigger/ longer blades than our crew. I posted a couple of pics on the "trips" section, although we wern't taking photos in the conditions I described!

All the best

Matt