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Quote:

Originally Posted by nautdesign1 (Post 3316177)

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

While some of the thoughts are spot on some are not exactly.

The efficiency of the jet pumps at a very specific point can be efficient, however, over a broad range the jet pumps are not as efficient as they have difficulty in dealing with speed and displacement changes. Typically the jets are used on 35 knot range boats and cease to be uses on things higher than this speed design.

This boat uses the jets for the simple reason that they can operate at a ram seed and this is relevant when the auxiliary power is turned on as they do not have to use a two speed transmission to allow the outboard motors to keep up with the inboard motors.

As for torque limitations, this is why we make 11 different sizes and the largest can handle 38,000 ft'lbs of input torque. Take the TF50 put a 20:1 reduction ratio in the ZF box and you can see why we make them so large..