<p>As is explained briefly <a href="https://aviation.stackexchange.com/questions/8907/why-are-there-so-many-different-propeller-shapes">here on Aviation.SE</a> and <a href="https://physics.stackexchange.com/questions/233331/why-is-an-airplane-propeller-so-different-from-a-boat-propeller-in-shape">here on Physics.SE</a>, the differences in chord length (<em>and other properties of the propellers like diameter and pitch</em>) are mostly derived from the fact that water is ~800x denser than air and results in different optimal properties for highly efficient propellers.</p>

<p>Boat propellers generally are constrained in diameter to limit the draft (<em>maximum depth</em>) of the vessel and thus would have to spin faster to generate the required thrust. This results in large pressure gradients across the surface of the propeller which cause <a href="https://en.wikipedia.org/wiki/Cavitation" rel="nofollow noreferrer">cavitation</a> losses. These pressure gradients are reduced by increased blade count and chord length, which provides more surface area for the water react against and allows the prop to be spun slower.</p>

<p><a href="https://i.sstatic.net/UFE9F.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/UFE9F.png" alt="cavitating boat prop"></a> (<em><a href="https://en.wikipedia.org/wiki/Cavitation" rel="nofollow noreferrer">cit.</a></em>)</p>

<p><em>Image of cavitation on a nautical propeller, where large pressure gradients across the propeller surface cause bubbles of low-pressure water which drain efficiency and can result in damage to the propeller upon collapse.</em></p>

<p>Similarly with desk fans, which are limited in diameter by the available space, and speed (to reduce noise) so blade area is increased to move more air (efficiency isn't as much of a concern). </p>

<p>Multirotors don't have the same diameter limits (<em>having the tips not meet or exceed Mach 1 is the concern</em>) removing the need for high chord lengths.</p>

<p><a href="https://www.reddit.com/r/AskEngineers/comments/93nrbu/why_do_tidal_turbine_blades_look_more_like/" rel="nofollow noreferrer">Relevant Reddit discussion</a></p>

CC BY-SA 4.0
2020-05-07T14:46:29.260Z
A better way to look at this is that lots of fat blades are just what you have to do get enough thrust when you're limited by diameter. Also, another reason for limiting diameter on small boats is keeping the blades strong enough to handle hitting things.
Robin Bennett
CC BY-SA 4.0
2020-05-07T16:18:15.807Z
@RobinBennett That's what I meant to say, did I not?
ifconfig
CC BY-SA 4.0
2020-05-08T12:55:39.653Z
While this is a good answer regarding boat propellers, it doesn't answer the question also asked by the OP about why table fans also use propellers with a high chord length, even though they're pushing air, not water.
Ilmari Karonen
CC BY-SA 4.0
2020-05-09T17:21:52.063Z
@ifconfig - to my mind this answer reads as if boats use short blades to avoid cavitation. I guess it could be read either way, which is confusing.
Robin Bennett
CC BY-SA 4.0
2020-05-09T17:24:38.360Z
@IlmariKaronen - fans are also limited in diameter and speed (for noise). They don't really care about efficiency but they do want to move lots of air. Big, fat blades move more air but have large tip losses because their tips are so big.
Robin Bennett
CC BY-SA 4.0
2020-05-09T17:34:36.920Z
@RobinBennett What's the correct interpretation? I thought they're small to minimize draft and spin slowly with large chords for cavitation reduction.
ifconfig
CC BY-SA 4.0
2020-05-10T10:14:37.127Z
@ifconfig - you're absolutely right, and that's a better way to phrase it. As it is, I think your answer could be mis-interpreted as "Boat propellers generally are smaller in diameter to ... avoid cavitation losses". Would you mind if I edited your answer to clarify cause and effect?
Robin Bennett
CC BY-SA 4.0
2020-05-10T17:12:51.893Z
@RobinBennett No I would not mind at all. I'm curious what you'll change.
ifconfig
CC BY-SA 4.0
2020-05-11T10:28:28.947Z
@ifconfig - I've had a try - what do you think?
Robin Bennett
CC BY-SA 4.0
2020-05-07T05:31:15.963Z
2020-05-11T11:06:14.600Z
2020-05-11T11:06:14.600Z
ifconfig
<h2>Overview</h2>

<p>The most efficient propeller is single bladed, has infinitely narrow chord, has infinitely thin airfoil, has an infinitely long blade, spins infinitely slowly, and requires infinite torque. So at the risk of stating the obvious, propeller design is a study of the tradeoffs and compromises required to make something practical. Here's a list of some compromises:</p>

<ul>
<li>length: landing gear height (or water depth) restricts how long a blade can be. </li>
<li>material constraints: thicker airfoils resist forces better. Stronger materials allow for thinner airfoils while still meeting required blade strength</li>
<li>power-plant constraints: the torque required to spin the propeller should match the motor/engine's efficiency and power curves.</li>
<li>number of blades: more blades equal more thrust for a same diameter</li>
<li>increased activity factor (aka blade width): wider blades produce more thrust for the same diameter</li>
<li>balancing: one prop is very hard to balance without causing lots of drag by the opposing mass.</li>
<li>vibration: three props give fewer vibrations than two</li>
<li>sound pollution: smaller props spinning more slowly are more pleasing on the ear</li>
<li>consumer perception: consumers like to see big blades in their fans, regardless of real-world efficiency and performance.</li>
<li>density of the fluid: 

<ul>
<li>dense fluids such as water can cause cavitation, which can quickly  damage the propeller. One solution is to change tip shape so as to minimize cavitation likelihood.</li>
<li>on the opposite scale, with low-density fluids such as air, the blade tip can spin so quickly it stalls.</li>
</ul></li>
</ul>

<h2>As it relates to drones</h2>

<p>Rotorcraft are a little special, so let's break out by type:</p>

<h3>Vertical propulsion</h3>

<p>Multirotor drones and helicopters have tons of space compared to boats, and so don't particularly need to restrict their diameter. They choose their rotors based primarily on power-plant performance.</p>

<h3>Longitudinal propulsion</h3>

<p>Fixed-wing drones with landing gear obey the same rules as full-scale aircraft. If  the prop is going to hit the ground on takeoff it's not very useful!</p>

<p>Fixed-wing drones without landing gear, e.g. hand-launch gliders, can use very large folding propellers. These are chosen similarly to multirotors, where it's about efficient power-plant matching.</p>

<p>As pointed out by @RobinBennett, "You don't want it stalled when the aircraft is stationary, so there's a maximum pitch angle - and that sets the maximum speed. If you want to go faster you need to spin the prop faster, and if you've got a fixed amount of power, you need a smaller prop just to spin it faster."</p>

CC BY-SA 4.0
2020-05-07T14:53:39.133Z
The other major compromise (and possibly bigger than all the others) is that props need to work across a speed range. You don't want it stalled when the aircraft is stationary, so there's a maximum pitch angle - and that sets the maximum speed. If you want to go faster you need to spin the prop faster, and if you've got a fixed amount of power, you need a smaller prop just to spin it faster.
Robin Bennett
CC BY-SA 4.0
2020-05-07T16:12:20.633Z
Good point, updated answer.
Kenn Sebesta
CC BY-SA 4.0
2020-05-08T15:10:57.647Z
"You don't want it stalled when the aircraft is stationary". F5B models, are short burst, high powered, high speed glider type models, and the props have a lot of pitch (for the high speed). These props will stall when the glider is stationary, but it's not an issue since the models are thrown when launching, and the excessive amount of power means there is still some thrust even when the prop is stalled.
rcgldr
CC BY-SA 4.0
2020-05-08T17:48:11.333Z
That's good to know. I don't think it changes the answer, though, as the stall characteristic still is not desirable. It is instead a result of other compromises. BTW, the tubercles (aka sawtooth) profile for F5D blades is pretty cool!
Kenn Sebesta
CC BY-SA 4.0
2020-05-07T10:46:34.177Z
2020-05-08T12:50:30.537Z
2020-05-08T12:50:30.537Z
Kenn Sebesta
<p>Almost all propellers used for drones have very thin chord length compared to the propellers used for things like table fans or boats which seem to have blades with much longer chord length. What is the reason that drones most often use propellers with blades that have thinner chord length?</p>

<p>Common drone propeller shape:</p>

<p><a href="https://i.sstatic.net/neX7x.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/neX7x.png" alt="drone propeller"></a></p>

<p>Common table fan or boat propeller shape:</p>

<p><a href="https://i.sstatic.net/CTWm7.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/CTWm7.png" alt="Boat propeller"></a></p>

CC BY-SA 4.0
2020-05-08T15:43:22.253Z
This [similar question](https://aviation.stackexchange.com/questions/67177) explains the issues related to household fans.
rcgldr
<multirotor><propellers><comparison>
Why do drone propellers have blades with thin chords?
Jacob B
# Why do drone propellers have blades with thin chords?

(Asked by: username_0 on 2020-05-07)

Almost all propellers used for drones have very thin chord length compared to the propellers used for things like table fans or boats which seem to have blades with much longer chord length. What is the reason that drones most often use propellers with blades that have thinner chord length?

Common drone propeller shape:

[![drone propeller](https://i.sstatic.net/neX7x.png)](https://i.sstatic.net/neX7x.png)

Common table fan or boat propeller shape:

[![Boat propeller](https://i.sstatic.net/CTWm7.png)](https://i.sstatic.net/CTWm7.png)

**Tags:** multirotor, propellers, comparison

> **Comments:**
> **username_1**: This [similar question](https://aviation.stackexchange.com/questions/67177) explains the issues related to household fans.  

---

## 2 answers

### Answer by username_5 on 2020-05-07. Score: 12 | Accepted answer

## Overview

The most efficient propeller is single bladed, has infinitely narrow chord, has infinitely thin airfoil, has an infinitely long blade, spins infinitely slowly, and requires infinite torque. So at the risk of stating the obvious, propeller design is a study of the tradeoffs and compromises required to make something practical. Here's a list of some compromises:

* length: landing gear height (or water depth) restricts how long a blade can be.
* material constraints: thicker airfoils resist forces better. Stronger materials allow for thinner airfoils while still meeting required blade strength
* power-plant constraints: the torque required to spin the propeller should match the motor/engine's efficiency and power curves.
* number of blades: more blades equal more thrust for a same diameter
* increased activity factor (aka blade width): wider blades produce more thrust for the same diameter
* balancing: one prop is very hard to balance without causing lots of drag by the opposing mass.
* vibration: three props give fewer vibrations than two
* sound pollution: smaller props spinning more slowly are more pleasing on the ear
* consumer perception: consumers like to see big blades in their fans, regardless of real-world efficiency and performance.
* density of the fluid:
  + dense fluids such as water can cause cavitation, which can quickly damage the propeller. One solution is to change tip shape so as to minimize cavitation likelihood.
  + on the opposite scale, with low-density fluids such as air, the blade tip can spin so quickly it stalls.

## As it relates to drones

Rotorcraft are a little special, so let's break out by type:

### Vertical propulsion

Multirotor drones and helicopters have tons of space compared to boats, and so don't particularly need to restrict their diameter. They choose their rotors based primarily on power-plant performance.

### Longitudinal propulsion

Fixed-wing drones with landing gear obey the same rules as full-scale aircraft. If the prop is going to hit the ground on takeoff it's not very useful!

Fixed-wing drones without landing gear, e.g. hand-launch gliders, can use very large folding propellers. These are chosen similarly to multirotors, where it's about efficient power-plant matching.

As pointed out by @RobinBennett, "You don't want it stalled when the aircraft is stationary, so there's a maximum pitch angle - and that sets the maximum speed. If you want to go faster you need to spin the prop faster, and if you've got a fixed amount of power, you need a smaller prop just to spin it faster."

> **Comments:**
> **username_3**: The other major compromise (and possibly bigger than all the others) is that props need to work across a speed range. You don't want it stalled when the aircraft is stationary, so there's a maximum pitch angle - and that sets the maximum speed. If you want to go faster you need to spin the prop faster, and if you've got a fixed amount of power, you need a smaller prop just to spin it faster.  
> **username_5**: Good point, updated answer.  
> **username_1**: "You don't want it stalled when the aircraft is stationary". F5B models, are short burst, high powered, high speed glider type models, and the props have a lot of pitch (for the high speed). These props will stall when the glider is stationary, but it's not an issue since the models are thrown when launching, and the excessive amount of power means there is still some thrust even when the prop is stalled.  
> **username_5**: That's good to know. I don't think it changes the answer, though, as the stall characteristic still is not desirable. It is instead a result of other compromises. BTW, the tubercles (aka sawtooth) profile for F5D blades is pretty cool!  

---

### Answer by username_2 on 2020-05-07. Score: 5

As is explained briefly [here on Aviation.SE](https://aviation.stackexchange.com/questions/8907/why-are-there-so-many-different-propeller-shapes) and [here on Physics.SE](https://physics.stackexchange.com/questions/233331/why-is-an-airplane-propeller-so-different-from-a-boat-propeller-in-shape), the differences in chord length (*and other properties of the propellers like diameter and pitch*) are mostly derived from the fact that water is ~800x denser than air and results in different optimal properties for highly efficient propellers.

Boat propellers generally are constrained in diameter to limit the draft (*maximum depth*) of the vessel and thus would have to spin faster to generate the required thrust. This results in large pressure gradients across the surface of the propeller which cause [cavitation](https://en.wikipedia.org/wiki/Cavitation) losses. These pressure gradients are reduced by increased blade count and chord length, which provides more surface area for the water react against and allows the prop to be spun slower.

[![cavitating boat prop](https://i.sstatic.net/UFE9F.png)](https://i.sstatic.net/UFE9F.png) (*[cit.](https://en.wikipedia.org/wiki/Cavitation)*)

*Image of cavitation on a nautical propeller, where large pressure gradients across the propeller surface cause bubbles of low-pressure water which drain efficiency and can result in damage to the propeller upon collapse.*

Similarly with desk fans, which are limited in diameter by the available space, and speed (to reduce noise) so blade area is increased to move more air (efficiency isn't as much of a concern).

Multirotors don't have the same diameter limits (*having the tips not meet or exceed Mach 1 is the concern*) removing the need for high chord lengths.

[Relevant Reddit discussion](https://www.reddit.com/r/AskEngineers/comments/93nrbu/why_do_tidal_turbine_blades_look_more_like/)

> **Comments:**
> **username_3**: A better way to look at this is that lots of fat blades are just what you have to do get enough thrust when you're limited by diameter. Also, another reason for limiting diameter on small boats is keeping the blades strong enough to handle hitting things.  
> **username_2**: @RobinBennett That's what I meant to say, did I not?  
> **username_4**: While this is a good answer regarding boat propellers, it doesn't answer the question also asked by the OP about why table fans also use propellers with a high chord length, even though they're pushing air, not water.  
> **username_3**: @username_2 - to my mind this answer reads as if boats use short blades to avoid cavitation. I guess it could be read either way, which is confusing.  
> **username_3**: @IlmariKaronen - fans are also limited in diameter and speed (for noise). They don't really care about efficiency but they do want to move lots of air. Big, fat blades move more air but have large tip losses because their tips are so big.  
> **username_2**: @RobinBennett What's the correct interpretation? I thought they're small to minimize draft and spin slowly with large chords for cavitation reduction.  
> **username_3**: @username_2 - you're absolutely right, and that's a better way to phrase it. As it is, I think your answer could be mis-interpreted as "Boat propellers generally are smaller in diameter to ... avoid cavitation losses". Would you mind if I edited your answer to clarify cause and effect?  
> **username_2**: @RobinBennett No I would not mind at all. I'm curious what you'll change.  
> **username_3**: @username_2 - I've had a try - what do you think?  

---

