Jstem,
Thank you for the compliment, yes she's a lot of fun.
On the horsepower formula I was referring to, as Greg wrote, should have more than just the required horsepower to make hull speed. My engine is rated at 10hp.
For a days cruising (12 hours) on good seasoned mixed hardwood firewood, we use about 2' X 7' X stove length. Or around 300 pounds I'd estimate. The whole floor under the pilot house is the wood bunker. We always carry a little extra, I'd sure hate to have to burn my shoes to get home
As for what engine, a simple single or twin non condensing is probably the easiest. There is no perfect engine/steamplant. Some benefits are going to cause some sacrifices and vice versa.
For boilers, the Roberts/Yarrow type water tube boilers are really good steam generators, but they require more tending - smaller water volume. The Large VFT (Iron Chief's boiler)and HFT boilers are the easiest to operate, more stable and forgiving, they do everything more slowly, unfortunately that includes making steam. Monotube and Flash type boilers are the best steam generators, but require some sort of intelligent controls to maintain them, and are best suited to steam car applications. Personally, I think the Ofeldt boiler is the best ever designed in mainstream use, all things considered - Simple, compact, lighter, safer, excellent steam generation, good internal circulation, manageable and relatively inexpensive and easy to build.
-Ron