Why A Jet Drive?

I very recently posted asking for input on the merits of a jet drive for a

small, high-performance submarine I am designing to build. My thread was

poisoned and derailed by a jaded, pessimistic, paranoid, close-minded,

know-it-all who apparently has no knowledge of jet-drives.

I have abandon the thread.

Here, http://www.psubs.org/projects/1237684922/r300submersible/

a fluid mechanics engineer has built an impressive, expensive sub with a

jet drive. He did it for safety around the grand-kids he says. His vehicle

compells me to ask those in the know, why a jet drive? If you are pro jet

drives, please let me know why.

My sub is low drag, requires 500 Newtons thrust to achieve 15 knots with

an 8 kw motor and enough batteries and compressed air to fly around for

an hour with an additional hour reserve. That's with ~90% efficiency.

What kind of efficiency can I expect from a jet-drive? Can counter-rotating

forces of torque be cancelled by stator vanes? Are there other benefits to

jet-drives, especially for my application?

Please note: if John (AKA "jehardiman") responds on this new thread I will

abandon it too and would ask you to personal message me your input.

Thanking you, once again, in advance,

Tom

 

No Merits

That's kind of what I thought too, no good points to a jet drive.

Inefficient sums it up in a word.

Silly that the R300, with all it's costly construction, used one.

What a waste. He claims 25 knots at the top end. He's dreaming.

Props it is for my sub!

Cheers, Tom

 

Jets versus props is a bit like asking which is best, steel or aluminium or GRP.

Everyone has their own views, however there are as with steel, ally etc each has their own pro's and con's and must be viewed objectively and in the context of the design.

Generalising here, however, jets are usually best at higher froude numbers, simply because the efficiency is better and they are more suited to low length displacement ratio designs. BUT, how many people care that much about the efficiency if all the other 'desirables' of the design have been met and she performs as 'desired'?

Almost everyone here who contributes fails to appreciate that the final design is the desire of the client. If the client wanted the boat made from Switz cheese and engines run with little mice running around a treat mill, its crazy, yes. But and this is the point. If the client wanted that, then it matters not what the efficiency is or if aluminium is better than the cheese. It is all about what the client wants. So, how does one design the boat to perform made from the cheese, that is the interesting part.

As such looking at the pro's and con's of both jets and prop's, which, for your boat, has more pro's than con's...it is a simple at that.

Design is all about finding the solution to the problem, (or SOR)...there is no real right or wrong answer, only different solutions.

It becomes derailed, threads etc, by those who have never designed a boat before and/or only have computer programs to back up their very weak, if any, technical knowledge (or very myopic) and the fact they are only looking at one aspect, because that is all the program can do, and not the "whole design", with is the end product.

When i get into my car, there are some bits i don't like..but does it make the whole car crap..of course not. It is all about the holistic design....not whether the a/c switch is where i want it, or the accelerator peddle being slightly bigger etc.

Same is true in designing boats, well, those that actually are trained and qualified naval architects and design for real. It is all about the holistic product working as one to achieve the desired goal, by whatever means.

 

The guy asked about a jet for his sub, he got your resume and your dislike for your accelerator pedal in your car.

Your opinion of us "almost everyone here" is eye opening, did you study for your superiority complex or is it a gift.

If the guy wants to design a sub he can do wether you agree or not.

It makes me wonder if you have any designing to do as your here most of the time.

Im retired,-- all the time in the world,-- retired at 38 you wont be able to do that.
Now if you would read his question again he explains adequately and gave lots of numbers for you to play with.

Try not to mention how good you think you are. "Most of us here" already know.

 

"..Your opinion of us "almost everyone here" is eye opening, did you study for your superiority complex or is it a gift..."

Nope, just by reading others replies..formed my own opinion, you can take it or leave, as I'm sure you have your own too. But i wonder how many fully qualified naval architects there are, compared to wannabes and amateurs, care to guess?.

Going back to his question:
"..I very recently posted asking for input on the merits of a jet drive for a
small, high-performance submarine I am designing to build..."

There are no numbers, just opinions required, which is given above. Without any hard data, that is all one can give, opinions, unless you are in receipt of more information than anyone else, please share?

"..It makes me wonder if you have any designing to do as your here most of the time..."..time differences and the magic of pop up emails is quite amazing..either i choose to reply or not...and this 'time issue' is important to the posting because..???

 

Most subs I've seen look like a cucumber with a shrouded prop at the end. There were lots of them in '40-'45 and they kept on building them that way. The torpedo's had the same shape, just scaled down, so that probably proved to be the best way to propel a submerged vessel.

The only refinement that comes to mind is a duoprop system because it eliminates the rotational forces so there is no need to compensate these with fins. But on the other hand, fins or rudders are necessary anyway....

Whether or not a shrouded prop should be called a prop or a jet drive is a matter of personal taste: both use a rotating object within a confined space.

 

You guys crack me up.

I found lots of info on related threads.

I should have looked there in the first place.

Tom

 

This site is the "Odd Duck" of boat forums! You would think with the word "design" in the name of the website that most of the members would be somewhat OPEN to new and/or creative ideas or questions.

Hmmmmm...

 

Jet Drive vs Prop

Hi Tom,
If this subject still interest you, you might have a look over at this subject thread on another forum. The webmaster of this forum, Carl, has lots of experience with this subject. Posting a question there just might draw him out into the discussion.

http://www.yachtforums.com/forums/technical-discussion/4470-jet-drive-vs-prop.html

 

Thank you Brian, I'll have a look.

-Tom

 

Brian,

Thank you. Really interesting reading. Just what I was looking for.

Too bad the thread came to a halt 1 1/2 years ago.

Any further reading you'd recommend?

Cheers, Tom

 

PM Baeckmo,

thats all you need.

Regards
Richard

 

I wish it were that simple Richard.

I've emailed and PM'ed him, no response.

So, I thought I'd try a post.

Thanks though,

Tom

P.S. Wait a minute, who's Baeckmo, Brian Eiland?

 

Ok Tom, got your message and have checked the link you referred to earlier. I will write a short tech explanation to you. Very briefly one can say that the Cliff-something guy is loosing a substantial amount of operating radius because an enclosed propulsor (~waterjet) has very low total propulsion efficiency in this application. I would say that the selected inlet arrangement is making things even worse. Unfortunately I find no technical info on that page, so there is nothing to base a calculation upon, except for pics on the jet nozzle diameter. The job is admirable, but he is lacking basic understanding of propulsion mechanisms.

If you have any (realistic) figures on expected hull drag, cruising speed, motor power and rpm's for your rig, please send so they can be incorporated as example.

 

I thought I gave them to you, but here they are:

Max Drag: ~500 Newtons at 16 knots

Mass: ~300 kg

Cruising speed: 5 - 10 knots

Max shaft power: ~8kw

RPM: as needed

Dive time: One hour max

No surface operation, buoyancy is +/- 1 kg

Prime objective: FUN!

-Tom