I am currently building 3 live steam launches from the Victorian and Edwardian eras.
The Chatham Historic Dockyard houses the steam launch Osborne that served the Royal Yacht of the same name, their website has useful photos and blueprints including the general arrangement and the hull lines. If you can't find them I can send them to you. I built a 1/12 scale model several years ago and am currently building a live steam version at 1/10 scale.
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I recently completed a 1/8 scale live steam model of the Windermere launch Bat, which is I believe mouldering away in the Windermere museum suffering from lack of funds. IMHO it is the prettiest launch of any era. The only details I have are from Bonhams auction website, one of which is here, along with my model:
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I am also planning a live steam model at 1/13.5 scale of a Royal Navy Admiral's barge which dates from about 1920. There is a plan for this model published by Conway Maritime Press. There is a good reference book "Steam Picket Boats" which may be available from second-hand bookshops. This is a miniature model that I made about 20 years ago. Another very pretty boat.
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It will be difficult if not impossible to get accurate drawings of steam plants of that era. For live steam installations I use the boiler/engine packages from China (AliExpress is your friend). I selected the boat designs because they can accommodate the steam plants that are available of the market. Each model is about 40" long.
Good luck with your research. I don't think the NMM is a good source for details of this type of vessel. A google search may identify several other websites that are more likely to contain details of launches of the era. I search "images" until I find something relevant and then hope to find a website of plans sites with more details.
