Hi Simon!

I'm glad to read you are entering the zone of insane speed of those lobster-like devils! "straight line rooster tail spitting speed machine to honk down the pond" I couldn't agree more with this perception of a rigger!
I have just finished my own scratch-built electric rigger from plans available at http://www.pirespreto.com/newversion/rigger.html

The layout is really simple and the materials you'll need are mainly balsa wood and thin plywood and some epoxy resin for making the structure water resistant. You can get an idea of how it looks like in the pics.
The specs are as follows: length 59 cm, beam 30 cm, weight appx. 1 kg
I am currently using 6 cell NiCd batteries(the design is meant for 7 cells, which is going to happen soon by the addition of 1 cell to each 6er pack) and an LRP 14 turn car motor. The ESC is a car Mtronics Sonic 3 Tempo, modified to be water cooled. The motor has also a water cooling spiral tube and 2 additional short water tubes soldered on the two brush hoods(the latter seems a good idea to avoid the wires from popping off!-things do get hot over there, perhaps the motor is too fast for that usage(too few turns, that is)
Tests on the lake are so far promising, it really gets off the water in a snap. I have tried 2 Graupner props, one with 32mm Diameter and one 45(!)mm. To my surprise it goes faster with the big one, maybe the big prop is working off the water to a considerable extent, thus letting the motor speed up adequately. However, I'm already looking at brushless combos to really spice things up...
What kind of 540 do you plan to use (turns)? What voltage? Do you have any personal experience with brushless motors and ESCs?
Hope you like the boat and decide to give it a try. If you have any questions, don't hesitate.

Andreas