Originally Posted by nautdesign1

Water jets are essentially ducted fans for the water. The large jets have their inlet geometry, blade configuration etc etc designed for a particular design speed and loading for maximum efficiency. As you might imagine, successful high end builders such as Pershing don’t make this stuff up as they go along. A failure to meet the design speed for a $10-$15 million yacht can be catastrophic and thus extensive CFD and FEE studies are commissioned. For this size of craft and speeds near 50 knots conventional shafts and associated stern gear create untenable drag but also cause huge flow/cavitation problems. They are therefore generally excluded from anything in this class that runs over 45 knots. In the 40-50 knot range, water jets are often similar in efficiency to surface drives so in Europe we see the specification of both. However above 50 knots the water drag (and the increase in vessel displacement resulting from the mass of water entrained within the jet path) make surface drives the more efficient option. Nevertheless, even at speeds up to 70 knots jets are still utilized as they have a proven track record and can handle enormous power. The torque restrictions on existing surface drives still somewhat limit their application on sizable craft.

Hope this helps,

ND1