[YachtForums]

Originally Posted by Kevin

One last detail I forgot to put in the original post: The prop driven 86S weighs 60.6 tons while the waterjet driven version is 2.2 tons heavier (62.8).

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.

[Capt J]

Originally Posted by YachtForums

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?

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.

[YachtForums]

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.

[Steven H]

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.

[Capt J]

Originally Posted by YachtForums

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. There are no similarities between this and what we're discussing.

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.

[YachtForums]

Good points and I agree with you, except...

When pumps enter their designed operating spectrum, they will excel... to a certain threshold. After this point, they will become less efficient due to aeration and lacking the vacuum to bring water up to the intake gullet at higher speeds. As speed increases and the hull developes more lift, (riding higher on the water), pumps are prone to inducing more air, resulting in a reduction in vacuum. This is also compounded by water trying to by-pass the intake gullet.

This has been my point all along... it's not that jet pumps are less efficient. They are actually more efficient in many applications, especially when the craft will be operating at or near their optimum output.

I can tell you this.... there ARE configurations that are more efficient for pumps than what we see commercially, for example surface piercing pumps. These are pumps that are placed partially beneath the hull's surface. In this scenario, water doesn't have to make an abrupt turn up to the impeller (via the intake gullet), but rather... water encounters the impeller directly as it passes under the hull.

[Codger]

Couple of pages of relevant info.
http://www.wartsila.com/en,shippower... 70,,1600.htm

http://www.rodriquez.it/fastferry/tmv84.php

[Dan Evans]

I really like seeing the side by side comparison on the Azimuts. Thank you Kevin for remembering that article and sharing it. It has been a few years since anyone has posted anything on this thread, I am hoping to revive it...

Originally Posted by YachtForums

When pumps enter their designed operating spectrum, they will excel... to a certain threshold. After this point, they will become less efficient due to aeration and lacking the vacuum to bring water up to the intake gullet at higher speeds. As speed increases and the hull developes more lift, (riding higher on the water), pumps are prone to inducing more air, resulting in a reduction in vacuum. This is also compounded by water trying to by-pass the intake gullet.

I was thinking about that little cavitation problem that jets encounter at higher revs. Unless I am missing something this is because of cavitation and because of when the hull rises out of the water air can make its way into the gullet. Does this apply to larger yachts, say the 100'-150' range? I could see this happening if the yacht was traveling very fast or in very rough seas, but in normal conditions would air getting under the hull and into the pump be a concern?

Some of what I have heard as far as surface drives go is that they are designed to run only half submerged so if the entire prop is submerged you are overloading your engine. So in rough seas and at slow speeds when the drives can be more than half submerged you can potentially overload your engine. Jets protect your engine from this, correct?


Another jet vs. prop argument I was thinking about was underwater debris. I dont think I need to mention the damage that has been done to props, but what about jets? Other than sucking up sand do they tend to get damaged by other debris?

As a closing comment, am I the only one who is nervous when jumping off a swimming platform on a boat equipped with surface drives?

Dan