---some further thoughts.....mostly in response to your question...''If the cabin height rule is removed and maximum P is changed to 295", then what material objections are there to the i550class rules? ''............
..........'to the plans' already leaves a lot of space,,and really what does it mean??....... tolerances that allow the range of variables that'll come out of various garages are already enough range for that ''clever builder to find a better way within the box and change the game'' ...this has definitely been the case for various sabot/el toro classes ,,,fireballs,,505's,,thunderbirds,,sharks,,,anything that's an open build--even the Olympic finns and stars!!..
.......I don't see that either class group's rules allow anything much different in their hull builds,,,apparently even the early,,'to the plan' 'P' measurement was the 295'' that the NA group uses.
...Certainly there's enough variations around poor Tim's boat in sails,,gizmo's,, foil configurations,,to see that there'd be very few boats that would be allowed in a 'to the plans' class.,whatever that is....Even manufacturer one-designs have to deal with some of these issues......I see that the keel bulb is defined differently between classes--the NA rules allow integral ballast,,rather than a strictly designed bulb....nobody's done that,,afaik,,,so perhaps that variable can be removed,but there'd be little way to make integral ballast more effective,,so it's a rather moot point.
Overall,,,whichever rule you look at is absolutely no more than a 'box' rule at best,,,otherwise known as an 'open one design' which allows variations around a theme similar to the classes mentioned above--the only option that would properly address your concerns would be a manufacturer one-design,,,which of course would eliminate -all- boats already built!!!
......'plywood only' would likely hamper the possibilities of T/S finding a production builder ....this would definitely reduce the likelihood of a production builder and class growth. I doubt a production builder of any actual volume is going to want to mess with plywood. ,,and with weight and distribution rules that are already in place,, what's really the problem of a FG build?
..............People just have to accept that my i550 and your i550 and all the other ones out there are still all i550s... They all look a little different... They all have different foils... They are still i550s..... No rule set can both allow a homebuild yet prevent those 'clever craftsmen' from giving it a shot,,,but the boat's short/wide/flat panel configuration is the ULTIMATE EQUALIZER already!!...... time on the water,,away from the fumes,,and more boats sailing and racing will prove this out.