Curiously I am also building a model of Velox but to nominally 1/48 scale plus a bit. The reson for plus a bit is that I am installing a steam engine and just needed greter depth for the boiler and length for the engine room to ensure appropriate stability given the unavoidable high center of gravity of the boiler and engine.
The boiler is a mk2 Cheddar moldels puffing unit (Sadly Cheddar are no longer trading) with my own designed and built in line double actin twin oscilator. I am driving only two shafts as I have no real idea of the power development from the engine and this is a more efficient solutionm than 4 shafts due to frictional losses and propellor efficiencies.
My 1/96 scale Victorian Battleship Canopus is powered by a Cheddar models Puffin and weighs in at about 35 pounds in all. Top speed is perfectly scale and if I let the bolier pressure build to about 40 psi it tears along with a magnificent wake. The engine I have built is nomilaly the same kind of size as a Puffin unit so I am optimistic I will have the same power, but in a lighter, if longer hull so top speed should be adequate, who knows! This is the fun of building something different!
To date I have completed the hull and engine/boiler installation and was mid way working on the fittings, all brass, when I got divorced so until I can get myself set up again progress has ground to a halt. I'm nearly set up again so should shortly be able to make some more progress.
In practice I cannot claim mine to be an exact model of Velox due to the chages I have made to accomodate the power plant but it will be representative of the type of turttle back 27 knotters of the period, and of course I'll finish her in Victorian colours.
Good luck with your model
Geoff