Originally Posted by jmschmidt
I guess nobody wants to believe that a free-wheeling prop produces more total drag, both parasite and induced, than a stopped one or a feathered one, but it's true. What do these people think makes the prop free-wheel if not the energy it is extracting from the forward inertia of the boat under sail thereby slowing the boat for an given condition? Counterintuitive maybe, but true nevertheless.
I dunno about counterintuitive but the main reason why nobody wants to believe that a freewheeling prop produces more drag than stopped one is simple - because it isn't true
As many other have posted, look at the empirical evidence already referenced in this thread and others of the same ilk.
However, if you need a rational and bullet proof theoretical explanation, please read on. By the way, you won't need to know any fancy maths or understand complicated formulas, just a basic understanding of first principles of physics.
First, let's get some basic physics dealt with.
Energy can't be created or destroyed, just changed from one form to another.
the forces on a sailboat can be resolved into 6 vectors (2 for each axis).
When the vectors are equal and opposite, velocity remains constant, when they are not, acceleration (positive or negative) occurs.
OK, next look at our situation; we only have two conditions to consider
1. The prop is allowed to rotate as freely as it can be given the confines of the gearbox
being placed into neutral - we will call this freewheeling for the purposes of this discussion.
2 The pro is held stationary by either a shaft lock of by placing the gearbox into gear
(ahead or astern) and thus having the engine compression
preventing it from rotating - we will call this locked.
These are the only two conditions available to us. Note, we are not
considering feathering or multi pitch
Now comes the explanation…
We only need to consider two of the six vectors: thrust and drag. Any forces that cause the the boat to move forward can be resolved into one vector - thrust. Any forces that hinder this forward motion can be summed together and becomes drag. When the thrust and drag are equal, the velocity of the boat remains constant. However, if some force acts on the boat to increase the thrust vector, the boat will accelerate until the drag increases and balances out the new thrust vector. It will then remain at this new velocity until something else changes. Of course, the reverse is also true.
So in our simple explanation, we have a sailing boat complete with a fixed bladed prop and and prop shaft. However, instead of a gearbox and engine, just consider the inside end of the shaft is fitted with a simple crank than can be turned by hand. I assure you that this arrangement acts in a similar manner as a gearbox and engine as far as the forces in question is considered.
Provided nothing prevents the crank from turning, the prop, shaft and crank will start to rotate as soon as we get some boat speed happening. For our purposes, we will say this occurs around 3 kts STW.
So assume we are sailing along at 5 kts with steady breeze, the prop will be rotating - for our example, let's say it is doing 30 RPM
in a clockwise direction.
It is freewheeling in essence. If our velocity is constant (5 kts), then the thrust and drag vectors must be equal and opposite.
Now put you hand on the crank and by pure muscle power, speed up the rotation to say 60 RPM
in the same clockwise direction. Clearly this is going to take some effort. So what happens to this extra energy that has been introduced into our steady system
. Just as clearly, the prop is now going to be providing some additional force on the boat - in fact, it will increase the thrust. So now the thrust is greater than the drag and the boat will accelerate to a new and increased velocity. It will continue to accelerate until the drag caused by the extra speed balances out the the new thrust vector. At this point, the velocity will remain constant again but will now be be say 6 kts. While you keep turning the crank with the same vigour, the boat will now be doing 6 kts.
When you think about it, this what occurs when you start the engine while motor
OK you get tired and let the crank go, the boat will slow down back to 5 kts and the prop and crank will be freewheeling again.
For any one still reading, this is all quite intuitive and surely non-controversial.
Now lets say, you put your hand back on the crank and apply some effort in the opposite direction and slow the crank down to say 15 RPM. Again, this will take some effort but in the opposite direction as before. So what happens to this energy that has now been placed into the system
. It is not hard to understand that it must be adding to the drag. Remember that the effort to speed up the crank added to the thrust, the effort to slow the crank is opposite and therefore must be added to the drag vector. Really, this is intuitive once you think about it. So if the drag vector is now increased compared to the thrust vector, the boat must slow down until the vectors equalise. This occurs as the slower motion decreases the other drag effects until the total drag vector equals that of the existing thrust. For this argument, let's say the new velocity is 4 kts. Once again, if you let go the handle, the energy you were applying to slow down the prop will be removed from the system and the prop RPM will increase back to 30 RPM as will the boat speed increase back to 5 kts.
This demonstrates that forcing the prop to slow down from it's natural freewheeling speed cause the boat to slow down. It doesn't take much imagination to see that applying even more force to slow it down to a standstill MUST decrease the boat speed even further.
Likewise, releasing it from a locked position and allowing it rotate freely, must increase the boat speed.
Remember, it is all about the total energy in the system. Adding energy in one direction will cause an increase in the thrust vector and adding the same amount of energy in a opposite diction will increase the drag vector.
Thus when compared to a freewheeling prop, starting the engine (adding energy) will make the boat go faster while adding energy in the other direction direction (locking the prop) will slow the boat.
Of course, chucking the prop and shaft and all that engine gear
into the briny depths will get rid of more drag and the boat will go faster.
And oh, I almost forgot, drag is always proportional to speed; it isn't a fixed quantity for any particular boat.