Originally Posted by Auspicious
We've been through this discussion on CF a number of times. I've pretty well given up. I remember in Fluid Dynamics under Prof Jacques Hadler (a leading light in propeller design) doing the calculations for this very question. Then we trooped down to the water flow tunnel for testing. Less drag with the prop fixed. It seemed counter intuitive so a bunch of us trooped down to Prof Hadler's office. He pointed out that our calculations showed less drag with the prop fixed. We agreed. He pointed that the water tunnel tests showed less drag with the prop fixed. We agreed. He suggested we needed to work on our intuition.
There are no dumb questions, only stupid answers.
Prof Jacques Hadler did you (and science in general) a disservice. First of all he likely considered your questions as dumb and gave you a stupid answer. Perhaps
he was uncomfortable about being challenged.
A better answer might have been "OK, challenge the calculations and see why they don't agree up your intuition. Make an hypothesis that matches your intuition and test it". Science works best when challenged and many great scientists have been proved wrong later on. There is nothing wrong with that, in fact it is a good thing; it is how we learn about the world. Now I don't know what the professor was trying to prove until I read his work but I do know it doesn't match other findings in the case of a fixed pitch three bladed sail boat either being locked or allowed to freely rotate with the transmission in neutral.
There have been several considered studies mentioned in various CF threads that disagree with what you have posted about Jacques Hadler's conclusion. Good science needs others to test the findings and if they can't be consistently repeated then at least one of the findings is incorrect.
Try this for a thought experiment
You are sailing along nicely on a light day with flat water doing say 4 knots in your 35' sailing boat with your three blade fixed pitch prop freely rotating (transmission in neutral). The prop is say doing 100 rpm and everything (boat speed, wind
, seas, prop rpm etc) is stable. Energy input
(from the wind in the sails) equals the energy lost
by heeling, prop drag and hull
drag etc and the boat speed is constant
Now start the engine and engage forward and bring the prop rpm up to 1000. Intuition would suggest the boat speed would increase and science will tell you why. You have added energy into the system by way of the diesel
being converted by the engine into the increased shaft rotation. The prop now delivering additional thrust causing the increase in boat speed. Straightforward really.
OK, now shut the engine down and put the transmission back into neutral and wait until the system stabilises and the boat speed drops back to 4 knots and the prop rpm returns to 100.
Now apply some braking force on the shaft until the prop rpm reduces to 10 rpm. Does the boat speed up or slow down?
Intuition will suggest you are extracting energy from the system
. There is increased turbulence behind the prop and there is heat being generated by the shaft brake. You know that it takes energy to create the additional turbulence and to slow the shaft down. Where does that energy come from? The only source available was the wind in the sails
and as that hasn't changed, the amount of energy to move the boat along must have decreased at the rate as the amount of energy consumed by the shaft brake and additional turbulence. It follows the boat speed must decrease. If there is less energy now available to move the boat, the boat speed must decrease.
Take this to the logical conclusion which is when the prop is locked stationary, the energy required to keep the prop stationary must be subtracted from the total energy in the system. Boat speed must decrease.
Some argue that once the prop is locked there is no energy required to keep it locked but clearly that is a fallacy. If it were so, then holding the prop shaft stationary could be done with a finger and a thumb. Clearly not so!
Now the same outcome can be demonstrated by a vector analysis of the forces involved but I will leave that for another day!!!!!!