Conventional propulsion systems for boats are rather inefficient.
The amount of thrust actually developed is always FAR less than
the amount that should be available from the shaft horsepower
that creates it.|
One of the main reasons why this inefficiency exists is because a spinning propeller imparts a substantial outward radial velocity to the water it is trying to push rearward. The energy expended in causing this outward radial motion is wasted energy. Since the radial distribution of this outward velocity is uniform and symmetric, the net effect is that all the force vectors cancel out and no net thrust comes from it. Only the rearward component of the velocity of the water that is accelerated by the action of the propeller, contributes to the desired thrust.
The structure of the shroud-duct DOES increase the drag in the water. But the exponential horn shape of the interior improves the acoustic impedance match of the propeller throat with the surrounding water. For all practical vehicle speeds, standard Engineering calculations show that the added drag is much less than the extra thrust created by the structure. At extremely high vehicle speeds, the drag does grow to be greater than the added thrust. However, as mentioned above, at all practical watercraft speeds, the benefit MUCH exceeds the added drag.
Appropriate aerodynamic design considerations can reduce the drag. A straight, constant diameter tube would certainly have substantial good effect. But it is generally also beneficial to slightly flare out both ends of the shroud-duct. Such a shape would have the contour of an exponential horn. This shape would correct for any differences in acoustic impedance in the water ahead of the shroud, in the shroud, and behind the shroud.
The value of this concept is in ensuring that ALL of the energy put into the water by the spinning propeller will result in productive thrust, and not wasted as radially ejected water.
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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago