Jet drive economics (a raggie asks)

[TallBuoy]

Are jet drives more or less economic on:
- fuel consumption
- routine maintenance
- replacement parts

than shaft and props?

If they are better, why are there not more of them ?

 

[Whitelighter]

I think to your three areas its three strikes.

The benefits are I think more performance at higher speeds

 

[Greg2]

I reckon around 30+% more fuel consumption than shafts.
Not sure one maintenance and parts but there isn't a lot there when compared to outdrives but perhaps more than props. Basically a belt and an impeller, which I understand can be expensive. Buckets are hydraulic . They are an expensive drivetrain in term of initial cost when compared with shafts/outdrives. That and fuel burn are the main reasons why you don't see many in leisure use IMHO.

Main benefit is that they are in a league of their own when it comes to manoeuvring.......once you get the hang of them!
Have to forget everything you know and start again but to be fair it doesn't take long. Within half an hour I was close quarters manoeuvring with confidence i.e. moving sideways, spinning in own length, etc. etc

 

[bigwow]

And shallower draft, no props to damage.

 

[BruceK]

And shallower draft, no props to damage.

Couldn't resist that could you?!

 

[bigwow]

Couldn't resist that could you?!

Just saying :encouragement:

 

[benjenbav]

Great fun once you get the hang of the buckets, or so I'm told...?

 

[Greg2]

Great fun once you get the hang of the buckets, or so I'm told...��

Yes, they are!
Thing is to set the engine revs where you want them and then use the buckets to manoeuvre......with great precision!

 

[jrudge]

This summer in Sardinia there was a 100 odd feet azimut with jet drives next to us. He told me the fuel burn which was frightening. I think it was iro 1000 lph !

 

[Bandit]

My experience is with large commercial jet drives at speed circa 35 knots.

Economy they are slightly better than props & Shafts

Manoeuvrability especially with a joystick and computer is excellent , the ability to go sideways etc.

They don't hit underwater objects in the same way as they have a lesser draft and no hanging appendages under the hull but they do act like a great big vacuum cleaner sucking up objects off the seabed in shallow water ie ropes, tarps, etc I have even seem a metre wide rock sucked up ( into a big jet intake).

At higher speeds they avoid the problems of propeller cavitation, Hydrofoil props had a life of about 600 hours water jets would run almost indefinitely with the same impeller.

A lot less vibration than shaft & Props.

 

[Latestarter1]

My experience is with large commercial jet drives at speed circa 35 knots.

Economy they are slightly better than props & Shafts.

Your headline number says it all............

The power absorption efficiency of the jet pump goes up by the cube of the RPM which is very reason why jets are so effective at higher speeds.

A jet driven boat needs more engine rpm than a propeller driven boat for the same speed until vessel approaches the 25 knot + threshold. Until you get above 25 knots fuel consumption is significantly more greedy than shafts.

A serious plus is that a properly sized water jet can never overload an engine regardless of the displacement or hull resistance due to say weeding, as power absorption remains constant, characteristics totally different from conventional propeller demand curve.

A surface drive which does not use a supercavitating propeller is an odd concept and I can understand why they would suffer from cavitation damage. Rolla supercavitating props on Arneson drives should certainly last longer than 600, although I agree Arnesons are heavy on maintenance in commercial environment.

 

[Bandit]

Hydrofoil props were not surface drive or super cavitating they were traditional shafts and props but when at cruising speed perhaps 1.5 to 2m below the surface with no hull above so they could easily cavitate in rough conditions. In my mind the loading of props and long shafts, full load and then props sucking in air lead to problems with engines.

I understand that surface drives are very poor for manoeuvring .

A waterjet has the engine continuously engaged for manoeuvring , it uses a bucket to redirect the flow either to balance forward and aft thrust for manoeuvring for the equivalent of out of gear or fully reverses the thrust for astern . you can with the engines at full power put full reverse bucket and full reverse thrust at full speed with no overloading of the engines for an emergency stop which goes completely against a master or engineers instinct and provide a crash stop of 1.5 boat lengths which is extraordinary to see and feel at 35 knots.

 

[PilotWolf]

A waterjet has the engine continuously engaged for manoeuvring , it uses a bucket to redirect the flow either to balance forward and aft thrust for manoeuvring for the equivalent of out of gear or fully reverses the thrust for astern . you can with the engines at full power put full reverse bucket and full reverse thrust at full speed with no overloading of the engines for an emergency stop which goes completely against a master or engineers instinct and provide a crash stop of 1.5 boat lengths which is extraordinary to see and feel at 35 knots.

My experience (now) is also with big jets around 30kts cruising speed.

Have done a crash stop in a small 9m jet boat I wouldn't want to try in a loaded passenger vessel. Personnel have their own momentum!

W.