Originally Posted by Alan Wheeler
Absolutely and it is certainly not a case of Religion. It is pure and simple mechanical logic and the age old rule
applies. Read the Manual!!
I agree - Read the manual for your transmission and engine.
However it's not really mechanical logic, it's physics and while your analogy is interesting the fault lies in the fluids we are dealing with. It's been a while since I did fluid dynamics but air is compressible and for our purposes water
An airfoil has a shape designed to compress the air and bring bernoulli into the game
. As the wing moves through the air it compresses the air on the leading edge and top 1/4 or so of the wing. After that the air accelerates rapidly and creates a low pressure on the top of the wing. This is lift.
There is also a downwash created on the underside of the wing due to angle of attack and that brings newton into play. Aircraft props work the same way.
Putting air aside, there is very little airfoil shape to a boat propeller
. The boat propeller
creates thrust primarily due to Newton.
One way to visuallize this is to imagine the prop stopped and the water
moving. Let's also assume for a moment that we are talking about slow speeds <8kts so the effect of angle of attack of the boat prop can be nulified for the moment. Let's also assume a CW rotating prop.
When the prop is powered the water strikes perpendicular to the the aft side of the prop and is deflected or thrust rearward by the pitch of the prop. A low pitch prop has to turn faster relative to a high pitch prop to push the same amount of water per second.
On the forward side of the prop the water wants to separate and so a balance of pitch and prop speed must be designed to minimize this separation, called cavitation.
If we add back in the forward motion of the boat, it becomes logical that a prop will start by cavitating and "lugging' the engine slightly as the boat starts moving. Eventually the angle of attack of the water moves forward and the cavitation effect is reduced and the prop "unloads" slightly. The speed of the hull
must also be considered when designing a prop as must the weight of the boat as the weight of the boat will be a huge factor in getting the boat moving and the prop unloaded.
High speed boats generally will have a lot of pitch because the angle of attack moves forward so much.
There is more twist at the root of the blade than at the tip, because the speed of the prop at the tip is different (faster) than the speed at the root. So the diameter of the propeller is also considered when designing the shape of the propeller.
Chord length and shape is also factored in as a function of boat speed, engine speed and prop diameter. Low speed props are generally more paddle like with less twist than a high speed prop.
There is another thing happening - the water does not move straight across the prop. It moves outward as well due to the shape of the prop.
At the tip of the blade the water spills off and creates a vortex. This vortex is not usefull and creates a lot of drag on the prop as the "higher pressure water on the backside mixes with the lower pressure water on the front side. The same thing happens on aero wings which is why designers have tried all sorts of things from droop tips, to saw blades, to circular wings to winglets to control the vortices.
So lets freewheel the prop and run the water in reverse.
The water now strikes the front side of the prop - as it is now coming not from the perpendicular but from the front of the boat. The water pushes on the front of the prop and the prop starts to move CW. The faster the boat goes the harder the water pushes and the faster the prop goes.
The prop is changing the direction of the water hitting the front side (the same deflection angle as in forward) but importantly as it moves CW it also has to displace the water on the backside of the prop which is now hitting the rear of the prop perpendicular. The faster it goes the more water has to be both deflected and displaced and drag continues to go up. The vortices also exist as water spills off the tip.
If the prop is stopped the water must be deflected but the back side water does not have to be displaced. The vortices still exist to a lesser degree and it is possible that at a certain speed the back side cavitates and that creates (a lot of) drag. Also no matter how fast you go the angle of attack never changes on the stopped prop. It is always from the front of the prop so the drag curve will definitely look different than a freewheeling prop.
I will not state that a stopped prop is faster. All props are different.
I would surmise that a small prop could freewheel and not make be a big deal, but a large prop and a fast sailing speed freewheeling would be bad In my humble and unscientifically tested opinion.
However if I were going to do 10,000 miles under sail, I'd sure practice with my boat and try to figure it out.
Originally Posted by Alan Wheeler
The MIT article I remember from way back. It actually offered no real information in the end.
They did a great job comparing several 2 and 3 bladed fixed props and a few folding and feathering props under power.
What I had trouble with is that by their own admission they did not test any of the props in a freewheeling condition. Their conlcusions were calculated.