Originally Posted by kitten
My twopenneth (as I understand it)
Taking water as a constant volume noncompressable liquid (which it's not, but it is close)
A propeller mostly drags itself through the water by creating low pressure on the front of the blade which is why cavitation (lowering water pressure below vapor pressure) destroys drive.
A jet takes in low velocity water and raises it's pressure and speeds it up by restricting it's area, ejecting it out the back producing drive via the inertia of the column of water behind it, put the column under water and most of the drive is lost, which is partly the reason jets can lose in the rough. (try it with a hose pipe in a bucket of water. End of the hose in the air it will push, end of the hose in the water it don’t so much)
Cheers for that Kitten.
Nice bit of explaining. In the context of the forces used then the water is constant volume
got that one.
Find the hosepipe analogy very helpful and appreciate that something about the density? extra opposing force? when submerged means the jet might be less powerful? So maybe slower? in rough seas.
Nice bit of tug video Alex. Were they trying to spring that tug onto the dock or just testing its strength?
(Reason I ask: I've done a few approaches like that myself)
(Given loadsa and a choice of boats, a deluxe fitted-out tug -none of your North Sea Boot Camp style outfits RW
- would be a strong choice for me
Or an ice breaker