how to built a mailbox blower

[drhans]

i have seen a lot of huge blowers adapted to outboard engines on a boat, redirecting the water flow downwards to cleanup an underwater area full of sand.
from what i know this system is called mailbox system, does anyone have any more details on how these systems work and how to built one ?

any help is appreciated, thank you very much

 

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[bucketofguts]

search mailbox/blower in shipwrecks! good luck! b