I did not want to delete the picture of the ironclad models, so I added a sketch of a tube engine that could be inserted into holes cut through the planks that support models of this kind. The sketch does not indicate what type of tubing would be used, but the absence of a coil might provide a clue that the tubing would be square or rectangular. There is also a sketch of a circle, square and rectangle with approximately equal contained areas. This sketch might indicate why square tube engines do not require coils. I believe the difference is in the way hot air would flow around tubes with different cross sections. Laminar flow around round tubing and something similar to splashing against the flat surfaces of square or rectangular tubing. Rectangular tubing would have a more favorable ratio of heated surface to contained air or water volume than round or square tubing. It would be harder to bend sideways, so the sketch was based on tubing with just up and down bends. This would be a modular engine. One might be all that would be required in a long narrow boat like the pop-pop Clermont Pete and Dick have discussed, For larger or wider models a stack of engines would be required.