At the Miami show this year I took a look at JetPac, a relatively new patented marine propulsion system. I didn't spend that much time in their booth, but I done a little research since I got home, and I present here some quotes from their literature and website that might deserve more discussion. I would particularly be interested in Carl's observations as he appears to have lots of experience with 'water jets'.
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Excerpts from their paper "A Short Treatise on Design Considerations of the OPS Jet Propulsion System":
Most engine failures are due to over gearing or overpropping, my estimate is that over 90% of boats are over-propped after delivery, most due to overweight, dirty bottoms or bad center of gravity (“C of G”). Many problems are caused by the owners preference for high speed at lower RPM....
If the engines have the capacity to be overloaded (as in all prop driven boats) a simple C of G change (like a large load in the front of a bowrider) can grenade the engine in a short period of time, with no verifiable installation fault This results in a full warrantee problem, dissatisfaction, expense and ruined reputation.
Although we have regularly experienced similar performance with equally powered planing boats, conventional inboard jet boats have a bow down attitude that robs them of a considerable advantage.
Observing this has made it hard to understand the prevailing opinion that jet drives are less efficient than propellers. This opinion has been accepted as gospel for many years based on an analysis “Hydrodynamics Aspects of Internal Pump Jet Propulsion” (University of Michigan, 1964) H.C. Kim claimed the efficiency from a water jet is far less then that of a well-designed propeller system. Kim’s analysis was even reproduced in the 1988 version of “Principals of Naval Architecture” by the Society of Naval Architects and Marine Engineers (SNAME). This analysis was incomplete and the resulting data misleading.
A study in 1992 conducted by Naval Architects Donald Blount and Robert Bartee dispelled Kim’s analysis in “Design of Propulsion Systems for High speed Craft” (Marine Technology, SNAME, Oct. 1997). This analysis revealed that a propeller-driven boat will have a hull efficiency of 92%, while the water-jet driven boat will have a hull efficiency of 110% at speeds over 25 knots.
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 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).
3) The lower thrust line also makes the boat more stable by cutting down the boat teeter caused by directional changes of the nozzle and this would reduce wandering at all speeds.
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.
Generally, the correct size of jet is not fitted to a boat......
Mercury and OMC have been working on jets for more than 30 years, and virtually all their experimentation, to my knowledge, has been done on inboard/outboard gasoline direct drive, (small diameter, high RPM, high pressure units) or two cycle outboard type jets. These approaches are unacceptable to us, because, it has been established over many years, that a larger diameter, slower turning, low-pressure jet performs much more efficiently.
The further aft the C of G, the faster the boat. This is a major part of the outboards performance advantage (a bracket increases this advantage). Because the outboard is completely over the transom, in a bracket design, the passenger’s location is further aft also, further enhancing the performance.
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Excerts from another portion of their website speaking to the 'advantages':
a)...under Reliability discussion
It starts with the use of an automotive engine because most of us find that our automobile is ready to go anytime we need to use it.
A propeller puts a heavier load on the engine if the boat is heavier and can easily overload the engine leading to premature engine failure. A water jet protects the engine from changes in boat weight. It presents a predictable load to the engine and that load does NOT change with changes in the boat.
Water jet systems typically are more reliable than propeller systems because they are less complex and the engines are protected from overloads more adequately. This advantage can be lost if the water jet components are made of aluminum and are more subject to erosion, corrosion and wear. Two cycle inboard water jet systems are made of aluminum and use an outboard power head through a complex drive system leading to degraded reliability. JetPac™ water jet components are ALL 316 stainless steel and are highly resistant to erosion and corrosion. The JetPac™ is designed to provide reliability.
b)...under Top Speed discussion
...you might have to accept poor acceleration characteristics at low speed and some difficulty getting the boat on plane to be sure of reaching top speed. One drive setup (propeller, gear ratio, etc) usually cannot achieve maximum performance across the full range of speeds in a specific boat.
If top speed is your ultimate priority you may want to consider an outboard or I/O drive system because there are very limited choices in water jets for top speed.
JetPac™ can be an attractive choice for you if your family goes with you in the boat or it is a commercial boat. We have compromised top speed (usually by a few mph) to give you excellent acceleration, powerful towing capability, very attractive fuel economy, and high reliability.
The two cycle water jet and the diesel inboard water jet do not perform quite as well because the engine and water jet weight are ahead of the transom degrading the performance of the hull on plane
c)....under Acceleration heading
A water jet does not permit water to escape off the tips of the blade. Any water that goes in the front of the water jet must come out the nozzle. That makes the water jet more efficient. 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
d)....under Handling heading
Water jets also vary considerably in handling. Two cycle water jets are not as responsive as any of the other systems because the small diameter water jet operating at high rpm and pressure does not move enough water to provide crisp response to steering changes. Inboard water jets, while having larger diameter jets and moving more water, have a disadvantage because the steering nozzle is usually at or just behind the boat transom. This does not give it the same steering leverage as an I/O or outboard where the propeller is usually 24 inches behind the transom providing more steering leverage
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I've chosen these excerpts above as I hi-lited them in my reading of the subject matter. There is the full text of these discussions at
Sword Marine's website
Interestingly I was initially attracted to their technology as a result of seeing a illustration that indicated they utilized a 'kevlar belt drive' item that I have long thought applicable to marine drives,
ref on my website