Some very good articles appeared in the short-lived magazine Steamboats and Modern Steam Launches, in particular "Balance is the Magic Word", which appeared in the September/October 1961 issue.  Its wisdom is well worth repeating here.

We have noted that 1/2 horsepower will move a fine (length to beam ration of 4 to 5) 1000 pound hull, at hull speed.  A broad (length to beam ratio of 2 to 3) hull of 1000 pounds will require one horsepower.
Hull speed is defined as 1.3 times the square root of the waterline length of the boat's hull.
Beyond this speed, excessive amounts of energy are required to move the boat.

Five to ten square feet of boiler heat transfer surface will be required for each horsepower.  A good water tube boiler will be close to five, while a poor fire tube boiler will be more like ten.  Common usage to describe the quantity of steam produced in a boiler is the actual weight of water from which the steam was evaporated.  Forty pounds of steam per hour per horsepower should supply a good single engine or a typically worn compound.
To replace this water in the boiler, which is converted to steam, 5 gallons of water per horsepower will have to be replaced each hour, thus determining the capacity of the feed water pump.  It should always be borne in mind that getting water into the boiler under all conditions and at all times is  extremely important.  For that reason we always recommend at least three means of getting water into the boiler.  Common methods are a piston force pump driven by the engine, a hand piston force pump, and an injector.  The latter two being used when the engine cannot be turned.

The condenser should be about one sq. ft. per horsepower if mounted inside the hull and 1/2 sq. ft. per hp. If mounted outside the hull.
Note that there is a distinct disadvantage in using a condenser which is more than one- third larger  in diameter than the engine exhaust.  In the case of a large condenser pipe diameter, the steam will pass right down the center of the condenser and make little or no contact with the cooling surface.  Of course , the use of a small condenser pipe size will increase back pressure and choke the exhaust.  The choking effect of many elbows, bends, and other obstructions in the engine exhaust line is also to be avoided.  Using a piece of high quality heater hose leaving the engine exhaust in a nice smooth sweep to the condenser is a plus.

Most newcomers to steam are surprised to learn about the large amount of heat required to run a steam engine--40,000 Btu per hour for each horsepower.  That means that a typical 3 hp (120,000Btu) engine requires a fire that would heat a home quite nicely

during a New England winter and even at that, needing to run only about the half time.        Now for the really Bad News: 
One pound of dry hardwood burns to produce 8,000 Btu.  One pound of good steam coal yields 12,500 Btu. And one pound of oil yields 19,000 Btu.  Even worse, one pound of cut timber (fireplace wood) with some bark still in place may yield 3,000 to 5,000 Btu.  So we see that the fuel required to run our steam engine is considerable. 

These simple devices cause more confusion than any other piece of boat equipment.  The basic law of physics that explains how propellers work is-- There is an equal and opposite reaction to any given force.
Relating this to our situation:  The boat moves forward in direct response and relation to the amount of water and speed of the water   driven aft by the propeller.  Since the water is a fluid and not a solid, the propeller has to turn more than it would advance in a solid.  This is called "slip".  A slip of 20 to 25 % is a good starting place for a propeller.  This is not exactly correct, but a good starting place.
For example:  What should the propeller pitch be for a boat with a hull speed of 6 knots and an engine that will produce 400 rpm? 
Distance equals shaft rpm times pitch, or the pitch equals the distance divided by the rpm.  Let's apply what we have learned about slip, so using 25%, the theoretical speed will be 8 knots.  Thus the pitch will 8 X 6080, divided by 400 X 60 (which is speed in feet per hour divided by rpm in revolutions per hour) or two feet.  Thus, a pitch of 24" will be appropriate.  The diameter of the propeller can best be determined by reference to published charts.  Lacking this input,  it is generally best to copy what has been done successfully before.  Copying from just one other boat with similar lines and characteristics is not as good a copying from several similar boats.  This is called "research".  It provides a good place to begin trial and error.