Why is an airplane propeller so different from a boat propeller in shape? Examples:
Airplane propeller 
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Boat propeller 
Source:http://www.nauticexpo.com
It is obvious that the airplane propeller is drastically different from the boat propeller in shape. At first glance the former has very narrow blades while the latter has much wider ones. Moreover, the blades of the airplane propeller all taper to rather sharp tips, which is a VERY striking contrast to those of the boat propeller.
So why the difference? What are the physical reasons accounting for this striking contrast? Does it mainly result from the difference between the weight of the air and that of the water?
 A: Draft and cavitation.
Ships cannot afford to have big propeller diameters, they have to make do with the smallest diameter available in order to stay within the draft of the ships' hull. They operate in a medium which is 800 times denser than air, and one important concern is to avoid cavitation. This again means to limit suction peaks and leads to very high blade chords. The high blade count and chord results in a very large activity ratio so the pressure forces have enough area to act upon to generate the desired thrust.
Compare a ship's propeller to that of submarines: Here the single blades are more slender and the propeller looks like a cross between a ship's and an airplane's propeller (see the picture below for an example).

Submarine propeller. Its mounting position on the centerline gives it more space for a bigger diameter. Minimum draft is not a concern, so the propeller blades are less stubby. Their sweep distributes the cutting through the wake of the rudders over time, which helps a lot to reduce noise.
