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Mar 02, 2017, 04:56 PM
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Electric ducted fan vs water jet drive

Newbe gyoung here with a question,
If one wanted some basic parts to build their own water jet drive why couldn't they use some ducted fan components as design reference or even for actual use? Their designs are similar, except for the type of media propelled (air vs water)and possibly flow direction? I understand that due to the density of the media more torque would be required to move compariable volumes of each type, as well as the strength of component materials used to move that media.
Am I really that far off in my thinking ??
Thanks Gene
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Mar 02, 2017, 05:28 PM
Steps? What steps?
zozer's Avatar
Yes you are. Too many differences to count, including thrust vectoring, efficient water intake, far different rotor sizing and shape, ducting geometry, waterproofing and motor load. You can try to build your own, but be ready for failure.

The easiest, simplest way is to build a stern tunnel in the boat's bottom just larger than the prop you want to use and run a solid drive shaft through the center, ending 1-2" ahead of the transition. Put a rudder in the end of the tunnel for steering. Kinda like these:

Mar 02, 2017, 05:29 PM
Registered User
dunk911's Avatar
I dont know if it's feasible but if it worked the amount of water you'd displace if you could achieve the correct rpm would be ludicrous.
Mar 02, 2017, 07:39 PM
Registered User
Yeah the mediums are so different. You have to move a ton more air vs water for the same momentum gain.
Mar 02, 2017, 08:30 PM
Just Plane Nutts
AirDOGGe's Avatar
There 's a reason why ducted fans are so much larger than jet drives. You have to move a lot of air to equal the mass needed to propel a boat, so a large duct and fan is required.

Water is much, much denser and heavier than air, so a far smaller column of it can be propelled rearward to produce the same thrust. Also, the water pump drive requires the same amount of power to spin it as the bigger ducted fan because of. A ducted fan would require a HUGE amount of power to spin at high rpms below the surface, and probably destroy itself in the process from the massive loads the blades would endure.

This doesn't even go into issues like proper blade profiles and such. Both ducted fans and water jet drives are basically fans spinning in a tube with minimal blade clearance, but each is optimized for it's specific task and medium to move. They are not interchangeable.
Last edited by AirDOGGe; Mar 02, 2017 at 08:40 PM.

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