[brian eiland]

Normal inboard jets are made to adapt to engines forward of the water jet. This means the jet drive shaft has to be higher than ideal because of the engine crankshaft height. Although jets should be fitted with a reduction to be efficient, most are fitted directly to the engine. If the jet were fitted as close to the bottom of the boat as possible, efficiency would be much higher….

I don’t know that I fully understand this claim, or its insinuation. It seems to me that in many of the PWC the jet units (and their impellers) are located about as low in the hull as the bottom edge of the impeller would allow?

If the jet were fitted as close to the bottom of the boat as possible, efficiency would be much higher for these reasons:
1) Frictional losses on the inlet and outlet would be less, giving greater efficiency.

2) Jet outlet would be lower on the transom and thrust line would therefore be lowered. (A low thrust line is desirable because it moves the active C of G aft giving less of a nose down attitude to the boat).

4) Inlet size would be reduced; this would enhance the efficiency of the boat by reducing the hook effect caused by putting a large hole in the most critical part of the hull.

Are they claiming that a ‘lower’ jet unit might even be more efficient? Is this to insinuate that the jet unit might actually be lower than the hull bottom?

I can’t believe this would be true, as at that point you have now placed some of the jet unit out in the water flow under the hull’s bottom, and thus increased the drag factor, just as with a conventional prop and strut support. One of the virtues of the jet unit is that it be capable of creating enough ‘suction’ to draw its water supply up into its inlet rather than projecting a drag creating inlet scoop down into the water flow, and/or trying to ‘ram charge’ its inlet, right??

Smaller diameter water jets operate at higher speeds and higher pressures and do not move as much water as larger diameter water jets. A large diameter water jet creates more thrust because it moves much larger volumes of water

I found this observation interesting. While working in SE Asia for a few years in the late 90’s, I was involved with the water-jetting of gas/oil pipelines and fiber optic cable into the seabed at the beach approaches. While most of our other competitive outfits involved with similar work almost all utilized ‘hi-pressure’ flows to facilitate cutting the trenches underwater, our hydraulic specialist designed low-pressure, hi-volume flow systems. Ours were the most efficient at doing the jobs.