As an ex Brit that has built a couple of these Springers over in the US of A, may i make a few comments on the 'provisional' UK rules?
1/ Why change the existing rules, except for imperial to metric?..
2/ The metric hull measurement compared to the imperial hull is to within mm's, i dont think the difference is that noticable.
3/ A superstructure is good, but not essential. There is a Springer in the US that just has a 'golf green' as a 'top!. The fun is creating a superstructure that is to your liking and yet is recognisable.
4/ Motor should be whatever you have spare in your bits box. A Springer at speed acts like a submarine.. so a faster Springer can be a dissadvantage!. As long as you dont exceed the permitted voltage (see other comments) Gear reduction is allowed in the US, as long as the motor / gearbox etc, results in a single, non steerable 3 bladed prop.
5/ 1 1/2 inch prop or closest metric equvalent.. either is allowable
6/ As I've mentioned before here, a Springer, to bring it down to the waterline takes two 6v 4.5 ah SLA batteries.. and some additional ballast (mine needs an additional 10 oz!) to get one to sit at the waterline. Two 6v 4.5ah batts weighs in at 1.8kg / 3.6lb compared to less than a lb for a 7.2v batt pack.... so be prepared to add at least 3lb of lead if you go the 7.2v route!..
6v SLA's will also give you longer running times by the way.
7/ Rudders / korts have been discussed over on the US RC Groups site..
For those that havnt been there..http://www.rcgroups.com/forums/showthread.php?t=522762
.. there are some 46 pages on Springers!.
..and even though a kort is possible within the 4 sq inch rule / 25 Sq mm., its impracticle (sp) because it ends up less than 3/4" / 19mm wide.. and is virtually ineffective.
8/ More than one rudder is allowable, however they should not exceed the 4 sq inch rule / 25 Sq mm (in total area) rule. As my Springer turns almost on itself with a 'standard' rudder, i think more than one rudder may actually be a disadvantage, except when going astern...
I hope my comments are taken as constructive.
I'm looking forwards to seeing some of your 'builds'!..