Re: [PSUBS-MAILIST] propulsion

The point I was making has to do with realized thrust, as it is felt in the vehicle and utilized by the pilot. The 36 X 36 bronze props we used weighed 175#, but we had a ramping circuit in the motor controllers that would allow you to switch from forward to reverse at whatever power you wanted and the system would do the rest. It was a very short delay, all things considered.

I've had eight ton subs trimmed neutral that moved along very nicely near the bottom with the motor turning so slowly that I could actually see the prop making little ticks forward as the SCR fired! And on the other end of the spectrum, I could bring that same boat to a dead stop in half a length, or turn it 360 degrees on the front of its skids just by goosing that big wheel with no other inputs. And the boat would stop from over one knot in less than a boat length.

Hi Vance. You're right. I'm using Madison Ave numbers for thrust and won't really know how much until I get them wet.

As for the low speed maneuvering from a big, single prop.....it is more efficient ( in power used ) but I would think also less precise.

Another thing to consider is the ability to stop rotation and then reverse it. A larger prop will take more time to come to a stop, and require more time and power to start. I think a short burst of power applied to a large prop would be less efficient than the same power applied to a small one.

My guess is that the efficiency ( thrust per watt ) on a larger prop is realized only after reaching the optimum level for that particular prop.

Start-up and reversing to stop, or " bumping" the prop to move just a little can't be efficient, and the larger the prop....the more energy is used to start it.

Anything that spins has a gyro effect. The larger it is, the more power it takes to get it going.

If we wanted to move just 6 inches and stop, and do that 20 times, it seems like it would be tough with a single big prop.

A small prop could be bumped with much less power consumed.

As for steering, you're right again. Trying to steer with just thrusters would be difficult to hold a straight heading, and the constant throttling up and down would take lots more power and waste a lot. Air or hydraulic rudder and dive planes are the obvious power savers.

Let me know what you think. Frank D.