[PSUBS-MAILIST] Water jets
Marc de Piolenc via Personal_Submersibles
personal_submersibles at psubs.org
Tue Oct 28 05:44:04 EDT 2014
I'll say it again: eyeball engineering doesn't work with ducted
propulsors. The Navy's river patrol boats used in Vietnam had water jet
units built, so I am told, by the Jacuzzi firm, makers of whirlpool
baths! Somebody there must have understood propulsion, though, because
the units worked well.
A ducted unit optimized for low speed and static operation will achieve
a mass flow equivalent to a (much) larger open propeller, and will get
better static thrust. Lower bollard pull for the same hp just means the
ducted unit was not correctly designed for the application.
Best,
Marc
Ducted fans: http://massflow.archivale.com/
On 10/27/2014 1:35 PM, Phil Nuytten via Personal_Submersibles wrote:
> A quick weigh-n on water jets. Years ago (many) I talked to the pilots
> of the Cousteau “Puce de Mer” (Sea Fleas) And the famous ‘SP 350, Diving
> Saucer’ about the efficiency of their water jet propulsion units. They
> confirmed that they were very ‘weak’ in a comedic way . . .they had a
> phrase in French which made the two Sea Flea pilots break out laughing –
> my trusty translator also laughed and then tried to keep a straight face
> as he said “they are saying that the things that you ask about are not
> strong enough to “ pull a penis out of a bucket of lard”!
> Some years later we tried a high volume water pump coupled with a duct
> engineered to work most efficiently with the pump. We hooked it up to a
> fixed pad eye inside our large test tank and used a spring scale to
> determine the bollard pull. Our froggie friends were quite right: using
> a conventional prop powered with exactly the same horsepower gave a
> large increase in bollard pull.
> *From:* Alan James via Personal_Submersibles
> <mailto:personal_submersibles at psubs.org>
> *Sent:* Sunday, October 26, 2014 9:30 PM
> *To:* Personal Submersibles General Discussion
> <mailto:personal_submersibles at psubs.org>
> *Subject:* Re: [PSUBS-MAILIST] Water jets
> Marc,
> didn't want to have to Google again.
> Wiki.....Classic prop-drives are generally more efficient and economical
> at low speeds, up to about 20 knots (37 km/h; 23 mph), but as boat speed
> increases beyond this, the extra hull
> <http://en.wikipedia.org/wiki/Hull_(watercraft)>resistance generated by
> struts, rudders <http://en.wikipedia.org/wiki/Rudder>, shafts (etc.)
> means waterjets are more efficient in the 20-50 knot range (up to 90
> km/h; 60 mph).
> Alan
> ------------------------------------------------------------------------
> *From:* Marc de Piolenc via Personal_Submersibles
> <personal_submersibles at psubs.org>
> *To:* Personal Submersibles General Discussion
> <personal_submersibles at psubs.org>
> *Sent:* Monday, October 27, 2014 1:24 PM
> *Subject:* Re: [PSUBS-MAILIST] Water jets
>
> A blythe generalization that is essentially meaningless.
>
> As Pazmany used to say: poot some nombers to it!
>
> Marc
>
> On 10/27/2014 3:50 AM, Alan James via Personal_Submersibles wrote:
>
> > Also I did a bit of research on jet propulsion a couple of weeks back
> > & what I read said it was inefficient compared to conventional
> propulsion.
> > Alan
> >
> > ------------------------------------------------------------------------
> > *From:* Alec Smyth via Personal_Submersibles
> > <personal_submersibles at psubs.org
> <mailto:personal_submersibles at psubs.org>>
> > *To:* Personal Submersibles General Discussion
> > <personal_submersibles at psubs.org
> <mailto:personal_submersibles at psubs.org>>
> > *Sent:* Monday, October 27, 2014 8:11 AM
> > *Subject:* Re: [PSUBS-MAILIST] Water jets
> >
> > I don't think water jets are necessarily inefficient, but if there is
> > considerable piping around of the water before it exits through the
> > jets, that's what I was referring to. However, even that was not meant
> > as criticism, it is just a normal trade-off. It is surely less efficient
> > than a conventional direct-coupled prop, but you also get
> > entanglement-resistance and maneuverability.
> >
> >
> >
> > On Sun, Oct 26, 2014 at 10:11 AM, Marc de Piolenc via
> > Personal_Submersibles <personal_submersibles at psubs.org
> <mailto:personal_submersibles at psubs.org>
> > <mailto:personal_submersibles at psubs.org
> <mailto:personal_submersibles at psubs.org>>> wrote:
> >
> > There is nothing inherently inefficient about waterjets. There are
> > commercial fast ferries using them and getting very good thrust per
> > horsepower - better than any supercavitating propeller could do at
> > the same speed, certainly.
> >
> > Lousy design will of course produce poor results, and jets are much
> > less amenable to rule-of-thumb construction than open propellers.
> > But whether it is "well known" or not, good design will produce good
> > results.
> >
> > Marc de Piolenc
> >
> > On 10/26/2014 8:41 PM, hank pronk via Personal_Submersibles wrote:
> >
> >
> > Graham,
> > In the world of marine jet drive it is well known that jet drive
> > takes two times the horse power to do the same job. Jet drive
> > is well suited to a craft that needs a shallow draft. I think
> > you will find it very complicated to control and it will be very
> > inefficient. I have been down this road, I love the concept but
> > abandoned it. If you go forward may I suggest you start with a
> > jet drive. It is not a simple part to replicate. The impeller
> > tolerances are critical. I have a jet unit on the shelf I could
> > donate to your project. The jet is from a jet ski. I would
> > consider a single rear motor on a full gimbal.
> > Hank ------------------------------__--------------
> > On Sun, 10/26/14, Graham Bayliss via Personal_Submersibles
> > <personal_submersibles at psubs.__org
> <mailto:personal_submersibles at psubs.__org>
> > <mailto:personal_submersibles at psubs.org
> <mailto:personal_submersibles at psubs.org>>> wrote:
> >
> > Subject: [PSUBS-MAILIST] (no subject)
> > To: "'Personal Submersibles General Discussion'"
> > <personal_submersibles at psubs.__org
> <mailto:personal_submersibles at psubs.__org>
> > <mailto:personal_submersibles at psubs.org
> <mailto:personal_submersibles at psubs.org>>>
> > Received: Sunday, October 26, 2014, 7:39 AM
> >
> > #yiv9417249292
> > #yiv9417249292 --
> >
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> > #yiv9417249292 Hi I am near the end of my build
> > of my k350 and have started to look at new designs I am
> > interested in building a flyer sub and have come up with a
> > design which will include a vectored motor drive this will
> > enable my flyer to hover and stop where ever I want. I have
> > designed a motor unit which will drive a fan type propeller
> > as an intake this will force water to the rear of the unit
> > where it is compressed then it is forced into ducting
> > which will take the water to four nozzles on the side of
> > the craft. The nozzles are able to turn three hundred and
> > sixty degrees both side are independent of one another so
> > rolls will be achievable. The reason for a vectored motor is
> > you only need one motor source and one power pack this will
> > ease the maintenance and increase the enjoyment of using
> > your sub. I am interested in the clubs thought of this kind
> > of design. Graham
> >
--
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Polymath weblog: http://www.archivale.com/weblog
Translations (ProZ profile): http://www.proz.com/profile/639380
Translations (BeWords profile): http://www.bewords.com/Marc-dePiolenc
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