Jet vs. Prop

Hi Wilson,

On Jetboats there are probably as many ideas as there are jetboats...
Very short, and this you already know, but anyway a jetboat has less draft and is (when you learnt) easier to handle in tight corners. It often gives less vibrations and a quiter ride as well.

Less good fuel economy in certain yachts and speeds, more costly to buy new, sometimes tricky to learn how to handle and a reputation of sliding sideways in following seas at lower speeds. (A keel can reduce this).

Like with the famous Hinckley you can get a joystick steering which is easy to learn. But even Hinckley has props as an option on their new 55´so it is clear that people have different opinions. Why don´t you talk to them about it? http://www.hinckleyyachts.com/

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.

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?

In a recent issue of one of the yacht mags there's an article about the new Azimut. I'll post the details about issue, magazine name, model, size etc when I get home tonight (cause I can't remember them now), but the reason I'm mentioning this is that the writers actually got to do a "side by side" comparison of the 2 versions of the same yacht using the different styles of propulsion. It's an interesting read as they do a breakdown of where each type is better or worse.

Boat International USA - May/June 2005 Issue - Page 61

The review is of the Azimut 86S.

Instead of retyping the entire article I'm going to see if I can get out my scanner and scan it instead.

To avoid trouble for the Forums, note that this article is Copyrighted and "can not be reproduced without prior written permission..."

But you can probably tell the conclusion of their findings in your own words...

Yeah, I know the issues involved with posting it to the forum. I was actually going to email it (the scan) privately to Wilson so he'd have the full details, and then I'd just post the basics here for the sake of discussion.

Ok, here are the 'cliff notes' to the article comparing the two drive options of the Azimut 86S (taken from the above mentioned magazine):

Option 1: Two five-bladed props and Arneson drive transmission powered by twin 2000hp MTUs
-Min spd: 8.5 knots @ 875 RPM
-13 knots @ 1200 RPM
-15 knots @ 1400 RPM, has not lifted from water yet, sound level in saloon remaining below 68 db
-16.5 knots @ 1600 RPM hull lifts out of water then accelerates to 20.4 knots
-1800 RPM 2nd turbo cuts in and average spd is 34 knots with noise level 73 db at the helm or 75 db in the owners cabin
-40.3 knots @ 2100 RPM (flaps at 4 deg.) produces 80 db in the owners cabin and a slight vibration
-full throttle = 2350 RPM yields 45.2 knots with 80 db in the saloon and 84 db in the owners cabin

Option 2: twin Hamilton turbines with same powerplants as above
-joystick operation enables one to move the yacht almost any which way you please, spin 360 degrees, any direction, move diagonally even, and has a minimum spd of 3.5 knots
-4.7 knots @ 600 RPM
-1000 RPM noise level in the saloon is only 54 db
-10 knots @ 1200 RPM
-11.2 knots @ 1400 RPM
-13 knots @ 1600 RPM (which is 36% total power)
-2nd turbo engaged at 1800 RPM using 55% power, rises out of water at 18 knots yet noise is just 63 db at helm and no vibration whatsoever
-30 knots @ 2100 RPM using 74% power
-40.5 knots @ 2300 RPM (91% power) with just 76 db
-at 100% power exceeds 43 knots in calm sea, reaches 45 knots once electronically stabilized for optimal trim
-stop in just 5 seconds time in three times her own length by reversing the waterjet thrust

Conclusion: Although slightly slower (1.5 knots) the waterjet propulsion is without a doubt much more comfortable, easier to handle, and significantly quieter, never exceeding 76 db. Also the draft is shallower.

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.

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.

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.