Re: Prop Slip Questions
whether a two-bladed set-up or a single blade, isn't the effective pitch for a given blade or blade set close to the average of pitch for the entire blade area weighted by the relative distance each portion of the blade area travels in a revolution?
for the engineering / science dork types in the crowd: before adding inefficiencies:
My naive understanding is that the counter rotating props most important advantage is that it allows far less of the energy / force to be wasted torquing the boat one direction or the other since the rotational force vectors that act on the boat are canceled by being split roughly half one way and half the other.
The other advantage / consideration that is closely related (or interrelated), is that to be truly effective, the front blade must be designed to best prepare the water stream for the second blade and the second blade must be designed to most efficiently couple with / make use of the water stream that has already passed over the front blade.
For example, the water flow that the second blade experiences, will already be rotating somewhat in the opposite direction then the second blade is turning. It will also be moving much much faster astern, relative to the boat, than it was when it contacted the front propeller (as a result of the force acting on the water in opposition to the force driving the boat forward, equal and opposite reaction, etc...). As a result, the rear blade requires less surface area per rotational (whether you lose this through blade width, diameter or some combination) to act on the same amount of water as the front prop. In addition, since the water is not moving when it contacts the first blade, and is moving astern when it contacts the rear blade the rear blade requires a higher actual pitch to give the same effective pitch as the front blade - at a given speed. The rear blade also very likely requires a higher ramp rate to minimize cavitation.
This assumes that that the two props are counter rotating at the same rpm, and is somewhat of an oversimplication do to a constantly changing Reynolds number over the blade(s) surface area, regional transitions between laminar and turbulent flow, etc, etc... (in other words, hard core fluid dynamics sux... and I'm not going to dig in far enough to do the required math at this stage in my life!)
I would think another potential advantage is that in balancing overall performance, the blades can be designed to "meet" at projected wot / peak horsepower yielding the highest top end, and also skewed individually to disproportionately effect different speed ranges on the bottom end... allowing for both increases in top speed and in low end or mid range... actually designing props to do this would be somewhere between an engineering marvel and a crap shoot...
The name of the game is converting as much of the rotational force (torque) at a given rpm into force in the forward direction, to do so without producing cavitation any more than necessary and to do so most effectively over the widest range of (prop) rpm ranges. This balanced with the minimizing drag of both the gear case and especially created by the blade surface area....
In theory, with a perfectly designed prop set a counterrotating dual prop drive should be both faster and more efficient... assuming no mechanical losses, etc, etc...
I'm still learning here.... and so may not be spot on on everything, but I'm starting to get a better handle on it...
