fithah4...Thanks.
Yes, graphs would certainly be helpful, several actually. They do exist and could be extracted/copied from various sources. I'm sure a good search engine would find quite a few useful examples of Propeller Curves with corresponding engine speed/hp curves for a variety of propulsion plant types. In the end, though, I think that is probably far beyond the potential, or even the possibility, of producing a technically complete and correct model to incorporate as a MOD in our beloved subsims. "Tweaking" certain files, as was done in MODs such as "Improved Ship Physics", is probably as close as the game engine itself will allow.
Steve...hello again and yes, you're correct.
Hydrodynamic drag is the biggest factor, but it's not the only one and it is also not "consistent". The drag factor typically used in hull/propulsion plant design is actually the speed-to-length ratio, or speed in knots divided by the square root of waterline length. As that ratio increases, the propeller horsepower required may increase as the 3.5 to 4th power of the speed in knots. Other factors that enter into the problem on a day-to-day basis include bottom fouling, actual displacement vs design displacement, trim, list, sea conditions, head winds, etc.
Also, I referred to the "ideal" propeller curve, which is a cube-law curve. Although we've been coming closer in recent years, there still is no "ideal" propeller; funny things like "slip" keep us from getting there. So, the relationship between a real propeller's rpm and its horsepower to drive the ship, is not an ideal curve. The "ideal" curve (Propeller Law) is used in design work, but the performance characteristics of the ship's propulsion plant aren't even known for certain until they are finally determined at Sea Trials.
AND...if you want to add one more complication directly related to diesel engines, there's BMEP (Brake Mean Effective Pressure). But, I've probably added far too much "confusion" to the basic discussion as it is, so we'll forego that (for now at least).
