Thanks to everyone for the help and the links!
Just to clarify things, I already have the hull for this model, and have installed the R/C and running gear. It's just awaiting final "sea trials" (to check the ballast + watertightness) before I fix the deck on and start building the superstructure, fittings etc.
I've already tested it in the bath, but need to check it in a larger area of water; the neighbour's swimming pool which I usually use for testing boats is currently emptied for repairs, and I don't want to risk it in the rather muddy and deep local pond until the "seaworthiness" of the hull is fully tested.
There is a splendid book which should be available from the liabrary called "the steam Yachts" or maybe "the great Steam yachts" with loads of photos of steam yachts owned by the Vanderbilts and similar.
Is "Salt-Water Palaces" the book you're thinking of? If so, I have this book, it's where I got the photos + plans of "Maid of Honour" from. I picked this particular yacht as my subject as the book had a (basic) set of plans for it, the hull shape and length/breadth ratio is very close to the second-hand hull I have, and with an approx. 2.5 foot model hull, the scale works out at almost exactly 1/48. (The real ship is 126', not 1/192' - my mistake!)
It's a large(ish), ocean-going steam yacht, not a river steam launch like "Topaz" - but I'd definitely like to build a model steam launch one day, perhaps with a working steam plant. I have a set of plans for the launch "Eva" which is now preserved at the River & Rowing Museum, Henley on Thames. I need to find a suitable hull first, though!
Here's the plan I'm using - rather basic, but good enough for a "based on" model rather than an exact replica.