Jet vs. Prop

There are quite a few yachts in the 50'-150' size range, maybe even larger, that use waterjet drives. Lars(AMG) tells me that the Hamilton and KaMeWa drives used on yachts have the buckets for reverse just like the smaller jet drives. I would think the Captains of those yachts would use that feature mainly for docking. Can you imagine what destruction would occur inside the yacht at higher speeds if the buckets were brought down quickly??

Here's a 64' custom yacht that uses waterjets.

www.advancedyachts.com

Click on the flashing picture, then at the bottom click on Prout Powercats. It's the Panther 64.

Aside from the shallower draft as mentioned earlier, jets have two major disadvantages to props.
1. Jets are usually around 40% less efficient at putting the power to the water compared to props
2. Low speed manueverability, you really have to pour on the power to maneuver the boat in docking situations.
and then the aforementioned cost issue

I don't know how I've missed this thread! Too many thing to do around here I guess. Capt. J... welcome aboard! I'm curious, how did you arrive at the stats of jet pumps being 40% less efficient than conventional props?

Thank you for taking the time to type this and post it Kevin. Much obliged!

I'd like to add there are several variations of water-jets, including mixed-flow and axial-flow pumps. There are also several different jet-pump manufacturers, in which some are not as efficient as others (not naming brands).

This was good side-by-side comparison, however... it can be misleading to the reader who doesn't understand the difference in pump technologies and brands. Although the powerplants and hulls were identical in this test, you would likely find that if another pump was utilized, the results may have been different (good or bad).

Although I haven't had the opportunity to test the current crop of jet-pumps on the same platform, I've studied the different designs and formed my own conclusions, based on my experience with them.

You might have heard the name "Lipps" in the yachting world recently, or in reading the equipment list of some of the new "jet-yachts" recently launched (such as the M-140). There's a reason why an unheard of manufacturer has made quick in-roads into the industry, against long term established names.

I didn't want to make one of our latest new members (Capt. J) uncomfortable by challenging his post. I'm sure he was just relaying information he gathered elsewhere. However, nothing could be further from the truth. Properly designed jet-pumps are absolute models of concentrated thrust. Quite simply, a ducted prop is more efficient than a non-ducted (or shrouded) prop, unless ofcouse... the drag created by the shroud outweighs the benefits of encapsulation. This isn't the case with a jet-pump on boats.

This being said... I have to clarify one thing. Application is critical!!! There are applications better for pumps and application better for props. In depends entirely on the operating parameters of the craft. An offshore race boat would be the wrong application for a pump, just like a Bravo drive would be the wrong application for yacht.

Good point Kevin. Thanks for bringing this up. The weight differential is due to the pumps. They're not light! You've got a lot more under the hull than traditional shafts, props and rudders. This weight is often amplified by the vacuum created by jet-pumps at operating speeds, giving the ride of a heavier boat (which is good).

Here the math becomes more clear... the pumps weigh significantly more than traditional drive components AND it's creating enormous vacuum (downforce=artifical weight), yet it is still producing comparable speeds to traditional shafted props. The pumps we developed used graphite reinforced nylon for some internal components (to minimize weight). In addition, plastics of this nature are also impervious to corrosion, rust, etc, but they can't be used for every component.

It has been known for years except in a few cases where the hull is specifically designed for jet power and everything is just right. Look at the Azimut for example, at 1800 rpm's the propellor version is doing 30 knots and the jet version is doing 18 knots. At 2100 the propellor version is doing 40 knots while the jet powered version is doing 30 knots. I worked on a boat that had mtu 16v-2000's and rated cruise by MTU is 1950rpms. I don't know of many owners that would want to cruise at 2100 rpms in an effort to maintain any speed at the sacrifice of voiding the warranty on a $250,000 diesel. Unless a jet is spinning at the speed it is designed to run at, it is very in-efficient below that because the housing is designed for a certain amount of water to flow through it and the size of the nozzle is designed according to that. Another reason a 9' waverunner with 155hp only does 65 mph. Another reason a 70hp outboard is re-rated around 40hp when a jet lower unit is installed. Jets are used on high speed megayachts many times because a turbine(s) are used to reach that high speed vigure but are only used 1% of the time. When they are not in use, the jets do not have the type of drag that a propeller would. Meanwhile a turbine is not used all of the time because it is not nearly as economical as a piston driven diesel.

Good points, but you're missing the point...

You'll notice as speed and RPM's increased, the efficiency of the pump increased exponentially and quickly caught up to the propped hull. The pump was only just beginning to come into its spectrum. To me, this indicated the pump may not have been the best choice for the operating speeds of this hull. That doesn't mean it's a less efficient propulsion system.

Also, referencing the outboard reduction... you need to examine the jet-drives that are adapted to these lower units. They are centrifugal pumps. There are no similarities between this and what we're discussing.

Just one additional remark regarding the pumps/jets .

Aren't these more susceptible to damage because of debris floating on the water ? Where I live (ostend - belgium) we used to have the passenger version of the High-Power tri-marans depivted in another thread on this forum and it was very known to loose an engine from time to time during passage because of debris on the water. This light not be a big issue in the Carribean or Meditteran, but it is in the channel.

IMHO a combination of both would offer the best solution. I don't know if there has been a study regarding this but a combination of twin-props and 1 or 2 jets would offer both low- and hogh-speed advantages.

<15 knts : Props only -> jet dis-engaged.
15 - 20 knts : props and jets
> 20 knts (1) : Jets only -> propellors dis-engaged (lifted into the hull or free-revving. Last option very unefficient because of drag-created).
> 20 knts (2) : All engaged. This is a most common variant on very fast super-yachts (ie ECO, Millenium, ...).

In a 3 engine combination (1) should work. A torque-applying gearing system similar to a limitted slip-differential should perform this task perfectly. Only you need one to handle the power-and-load of these yachts. This would reduce noise and vibrations. Should the option be to use diesel-eletric motors, it might be even better to keep all engines running and build a combination as above using these types of engines and a converted "diesel-electric jet-drive".

These are just random idea's I'm having here after reading through the tread. Pls forgive me if there is some nonsense in this...

I've got an idea on how to combine a propellor and a jet-propulsion system on 1 engine with the possibility to engage either or both, just need some time to get it drawn.

The reason the jets are not as efficient at lower speeds is because they have to be designed to achieve 2350 rpms at full throttle and the impellor has to have the correct pitch to allow the engine to achieve full throttle rpm, also the pump nozzle has to be large enough for that volume of water. Therefore at cruise rpm 1950, the jet is not nearly as efficient as a propellor. A jet works great with a turbine because a turbine is designed to spin a certain rpm all of the time. When a pump is combined with a diesel engine it is not an efficient system because of the nature of the diesel engine.