bcooke wrote:
How is density measured? By that I mean what is a 'unit' of density?
Density is mass/volume. Across the pond (and, therefore, in most textbooks) density is referred to as "specific gravity". Standard units are g/cm^3.
Distilled water, by definition, has a density of 1. Salt water is denser than fresh, which is roughly the same as distilled. Sea water is usually 1.03.
bcooke wrote:
By frontal area, do you mean the total area of the propeller 'disk' or is it some fraction thereof? I wonder since the propeller is actually moving if the frontal area is something less than the total area the propeller swings through?
Some fraction thereof. The drag of a spinning prop will be less than the drag of a flat plate of the same diameter.
This is one of the "sticky bits" about determining drag, because not only does the motion of the prop matter, but the thickness of the prop, the curve of the blade, the velocity of the prop's rotation, the amount of air in the water, the turbulence of the water's flow - all of those make a difference in the final answer.
I've never calculated the drag of a boat prop before, and even aircraft props are something I did only as an exercise. (I didn't do planes, my aerospace degree was emphasis on the space.) But in real life, engineers use rounded figures and determine a drag coefficient by experiment - in short, build one, put it in the water, and determine what it is. Then use that number to determine the calculated drag for varying conditions on the assumption the coefficient won't change. (Which is actually not the case, hence one of the reasons why engineers drink.)
If it's critical to know the correct answer at all operating regimes - such as for a spacecraft, or a fighter jet - you spend millions on years of simulation and calculation, and when you're done you
still do a thousand real-world tests and change the answers to match.
bcooke wrote:
Do propellers create any 'induced' drag as well when they are being dragged through the water?
Yes.
Overall, the drag of a prop is not something I would really spend much time worrying about. Even for a racer, I'm willing to bet that the difference in drag between a locked prop inline with the keel and a locked prop perpendicular to it is less than the difference in performance of an extra inch in the foot or leech of a sail. To me it's like the idea of keeping the bottom of the hull glass-smooth; it's less about the performance of the boat and more about the discipline and dedication of the crew.
Hence the reason why I don't sail competitively...