Jet Propulsion

MarkM · 19 October 2003, 19:41

Read on Internet much concerning water jet propulsion and the advantages of it. But nowhere I can find something concerning the fuell consumption in proportion to propeller propulsion at equal speed. I want to purchase a rib boat, with aluminium keel, of 6.30 m. which has an inboard Steyr diesel engine of 212 hp with such a jet propulsion. Is the fuell consumption much more and what are the possible other disadvantages? Who has experience on this subject?

tue · 19 October 2003, 20:40

In general jet pumps tend to be a less efficient means of propulsion compared to a propeller drive system. This would mean that in order to get your boat to, at the same speed as the same craft with the same engine with propeller drive you need to use more power (higher revs) on your engine thus using more fuel.

thewavehumper · 19 October 2003, 22:26

This is true, but they are a lot of fum with a 500 hp diesel pushing. Before you all say anything it will be fixed soon!

This steyr boat has it got a costoldi drive?

MarkM · 19 October 2003, 22:43

No it's a Twin disc waterjet.

chrisallse · 19 October 2003, 23:15

they get more efficent the faster you go, flat out they are as good if not better than a prop.

Dirk Diggler · 20 October 2003, 19:09

Quote:

Originally posted by chrisallse
they get more efficent the faster you go, flat out they are as good if not better than a prop.

And where did you get that from?

chrisallse · 20 October 2003, 19:33

read a few articles on them i think hull design has a bearing as well.....dont you agree then?

UruG · 20 October 2003, 20:00

Hasn't anybody got some scientific calculation methods to compare jet to propellor propulsion? It must be possible to calculate a hp to propulsion ratio...

chrisallse · 20 October 2003, 20:46

Hamilton's web page is interesting.

A quote from it,

"High efficiency

Propulsive coefficients as good or higher than the best propeller systems achievable at medium to high planing speeds"

Dirk Diggler · 20 October 2003, 21:25

Quote:

Originally posted by chrisallse
read a few articles on them i think hull design has a bearing as well.....dont you agree then?

No I don't, and you haven't answered my question. As for Hamiltons web page, you don't suspect that they are a little biased then do you?

chrisallse · 20 October 2003, 21:44

What part of "read a few articles" dont you understand? and i am sure they are biased but i am also sure they are experts in their field....or dont you agree with that..?

qsiv · 20 October 2003, 21:56

Quote:

Originally posted by Dirk Diggler
No I don't, and you haven't answered my question. As for Hamiltons web page, you don't suspect that they are a little biased then do you?

Maybe - but true. Do you think that most of the fast ferries in the world would use them if they weren't efficient? It was also the propulsion of choice for the transatlantic record run (Il Destriero). Propellers are great, but they are only at peak efficiency at one speed, and one loading condition - at all others they become markedly inefficient

Dirk Diggler · 20 October 2003, 22:15

Well, that answered that one then. A bit like buses really, you wait for ages, then a load (of experts) turn up at the same time!

thewavehumper · 20 October 2003, 22:31

To get this back on thread a little bit, My son used to drive an Ali boat with a Steyer and a jet drive (about 6.5 metres ) when he was working for an oil exploration company in Poole harbour a few years back.

And they were excellent boats so I think a rib with a Steyer and a jet drive would be a very useful craft. The other thing to remember is that Diesel is V cheap so any loss of eficioecy doesn't really reflect in the running costs.

We have a Jet drive boat but havent used it enough yet to offer an informed opinion but my gut feel is that if the engine had a prop attached the boat would be faster, but not as much fun

MarkM · 21 October 2003, 13:27

Thanks Wavehumper that's what I wanted: a clear answer.