I was just wondering what determines the most efficient number of blades on a propeller/fan… whether it’s in air or water or some other fluid. For instance, you could have 3 blades, but would 4 make it more efficient? What about 5? What about 100? You could keep adding blades, one at a time, until it’s practically solid.
Why do most planes and boats have 3-5 blades on their propellers while jet engines have dozens of blades inside of them? A jet is a turbine rather than a prop, but it’s the same question.
Also, I realize they’re top secret, but the propeller/screw of most modern nuclear submarines have 6-8 blades on them. Why? I’m almost certain that more efficient = more silent, so it wouldn’t be a tradeoff between those two competing interests (cavitation causes both noise and drag, and the wearing down of parts).
you get more potential thrust with more blades, with the tradeoff of somewhat lower efficiency.
a jet engine- whether you’re talking about the compressor stages, or the main fan in a turbofan- is forcing air into a duct, and has to create a lot of pressure. the close spacing of the blades prevents air from “blowing back” towards the front of the engine.
Each blade disturbs the air for the blades that follow it, so the fewer blades the better, in that respect. You can’t have fewer than one blade, so that would be the most efficient from that standpoint (as jz78817 said), although a single blade is often a bit impractical.
The longer the blades, the faster the tips move though. Once the tips start moving at speeds that are close to supersonic, then your efficiency starts to go to hell again, and to compensate for that, now you have to add more blades and make them shorter.
Another factor is that a small number of really long blades may be better for efficiency, but they aren’t always practical. If you took the really long and thin style of propeller blades that you see on wind farm power generators and tried to use that style on something like a little two-seat airplane, you’d end up needing landing gear that are about 30 feet tall to prevent the blades from hitting the pavement when the plane is on the ground (ok, I’m exaggerating a lot but you get the point, I hope). Those sorts of practical size limits also favor a larger number of shorter blades.
By “one” blade I understand that as two blades forming a straight line, like two radii forming a diameter of the circle that is the propeller. Each blade is a radius, as I understand it.
So … why does boat A have a propeller with 3 blades while boat B has one with 4 or 5?
I’ve never seen anything like a propeller with one blade sticking out and a short stubby counterweight on the other side to keep it from wobbling around.
But of course one blade will disturb the least amount of air. But that doesn’t mean it’s most efficient. I doubt it would produce much thrust at all.
What i’m wondering is, say you have a 100hp motor on your boat. You start off with a propeller with two blades (opposite each other). How fast will the boat go? Will adding a 3rd blade make it go faster? What about a 4th? A 5th? 10 blades? 20? At some point the blades will start overlapping each other.
There’s gotta be some number of blades that produces the fastest result (given constant engine power), and a number of blades that, after it, produces diminishing marginal returns. I’m sure all boat engines, and airplane engines w/ propellers, are designed with the number of propeller blades that give the best performance given the power of the specific engine.
The number of blades is just one factor. The radius, the form of the blade, the power driving the propeller, the intended speed of the craft, and many more factors combine to determine the effectiveness of a propeller. The number of blades by itself tells you nothing. A one blade propeller is very efficient because it will deliver the maximum amount of thrust per horsepower but there is a limit to how much thrust can be delivered for a particular size of blade, to get more thrust you need more blades. A particular shape and pitch of a propeller blade will be efficient at moving air or water within a narrow range of speeds through the air or water, to gain more efficiency at slower or faster speeds of travel you need to change the shape and pitch of the blades, not change the number of blades. For any specific set of conditions, and the specific form of blade there is an optimal number of blades. It’s not as simple as more blades are better.
Here’s a picture of one. And it produces a good amount of thrust, nicely matching what the engine can deliver. (I’ve flown this aircraft type - an Alisport Silent motorglider.)
An important aspect to this design is that the propeller “teeters” - it’s free to pivot at the hub, so it can seek a thrust angle that keeps forces in balance and stresses low.
I thought that had been well explained already, but let’s see if this helps:
For an engine producing a certain amount of power, you need a certain amount of propeller to use it all. The fewer blades you have, the more thrust you will get per unit power. However, there are limits to how long you can make a single blade, partly to keep the tip from going supersonic and partly to keep the prop from hitting the ground. At that point you start needing more blades because there is no good alternative.
This makes sense, and it’s a good explanation, but how do they decide how many blades to put on? There has to be some point of diminishing marginal returns.
This kind of question soon leads us to some grad level maths. This gives a good example of the kind of calculations a model aeroplane enthusiast might use to decide on the best propellor configuration.
You could consider it a single blade prop, but that’s not a useful description. Open screws were not very efficient, the concept of bladed propellers was not understood at the time. When the Wright brothers were developing the propeller for their first aircraft they looked at the state of the art for ship propellers and found little there. So the science is only about 100 years old. Just a few decades before that paddle-wheels were the state of the art for watercraft.
Not really, because the leading edge of the screw is stirring up the water for the rest of it. I read many years ago that one such ship actually broke off part of the screw, and surprisingly performed much better with the stub - leading to the development of the propeller.
