3 ms·

Related thread from a few months ago discussed the Sharrow propeller mentioned in this article, which applies the same concept to boats: https://news.ycombinato

Related thread from a few months ago discussed the Sharrow propeller mentioned in this article, which applies the same concept to boats: https://news.ycombinator.com/item?id=33949895 https://news.ycombinator.com/item?id=33949895 The difference in geometry for air vs. water propellers has a lot of complicated reasons but above all the fluid density of water is about 1000x air: ~1000kg/m3 for water vs ~1kg/m3 for air. Air propellers are optimized for very high-speed operation which is needed to produce any significant thrust in low-density fluid. Boat propellers are very "built up" and physically sturdy since they are used in low-speed operation with high torque. I have seen people use RC airplane propellers in water for small autonomous boats -- They do work but look very comical (think 10x the size of a boat propeller, running at 1/10 intended speed) Cavitation effects -- microscopic bursts of vacuum at low-pressure boundaries along the propeller -- come into play much sooner than with air propellers.

someweirdperson 4y ago
> Cavitation effects -- microscopic bursts of vacuum at low-pressure boundaries along the propeller -- come into play much sooner than with air propellers. In liquids vapor-bubbles of the liquid (not vacuum) are caused by low pressure (or high temperature, boiling), and cavitation is the collapse of the vapor back to liquid state when pressure rises (or temperature drops). Propellers in air do have issues with the speed of sound, but that's a different matter than cavitation in liquids.
- chrisdalke 4y ago
  Thanks for the corrections: So the bubbles are not a vacuum but low-pressure vapor and cavitation is the name for the collapse, not the bubble itself. And this can only happen in liquids where there is a potential phase transition. I've got a little marine knowledge but my physics is lacking so I appreciate the clarification! I assumed cavitation had similar parallels in air propellers with high tip speeds (as you approach the speed of sound) but seems that is a different effect?
  - aqfamnzc 4y ago
    I would say it's still a vacuum in the more common meaning of the word: A reduction in pressure, not necessarily a complete vacuum.

someweirdperson 4y ago

> Cavitation effects -- microscopic bursts of vacuum at low-pressure boundaries along the propeller -- come into play much sooner than with air propellers. In liquids vapor-bubbles of the liquid (not vacuum) are caused by low pressure (or high temperature, boiling), and cavitation is the collapse of the vapor back to liquid state when pressure rises (or temperature drops). Propellers in air do have issues with the speed of sound, but that's a different matter than cavitation in liquids.

chrisdalke 4y ago
Thanks for the corrections: So the bubbles are not a vacuum but low-pressure vapor and cavitation is the name for the collapse, not the bubble itself. And this can only happen in liquids where there is a potential phase transition. I've got a little marine knowledge but my physics is lacking so I appreciate the clarification! I assumed cavitation had similar parallels in air propellers with high tip speeds (as you approach the speed of sound) but seems that is a different effect?
- aqfamnzc 4y ago
  I would say it's still a vacuum in the more common meaning of the word: A reduction in pressure, not necessarily a complete vacuum.