[PSUBS-MAILIST] Water jets
Hugh Fulton via Personal_Submersibles
personal_submersibles at psubs.org
Mon Oct 27 17:05:14 EDT 2014
Hi Cliff,
Have you looked at the Rice nozzle as opposed to the Kort. Supposed to have
advantages.
Interested in your seal experience. I am using 2 mechanical seals with Oil
between them at 5 psi above ambient pressure but using a spring loading
rather than air over oil/water.
The torque per seal is very high comparatively. Using carbon/silicon caride
faces. What were the seals you used? And what diameter.
Regards, Hugh
From: Personal_Submersibles [mailto:personal_submersibles-bounces at psubs.org]
On Behalf Of Cliff Redus via Personal_Submersibles
Sent: Monday, 27 October 2014 3:08 p.m.
To: Personal Submersibles General Discussion
Subject: Re: [PSUBS-MAILIST] Water jets
On a thread on jet pumps for psubs, I will have to put by two cents in as I
have one. I have been using a jet pump from a jet ski on my boat for about
five years. Like any propulsion choice there are pros and cons. Like any
major component on a psub, it is nice to have that component accomplish more
than its primary task to save weight. I chose a jet pump for a number of
reasons. As I have divers and swimmers around the surfaced boat all the
time, I felt that the enclosed ducted impellor was much safer than an open
prop and by utilizing an articulated jet housing, I get yaw and pitch
control along with propulsion. With forward speed developed by the jet
pump, ailerons give me roll control. One of the boat objectives was to be
fast and maneuverable at higher speeds than is normally associated with
psubs, i.e., I wanted to be able to fly underwater. I addressed some of the
issues that Marc pointed out in the design process by converting the
impellor to fit my application. The first was to re-pitch the impellor
(more power, less speed), the second was to use a high speed double
mechanical seal with glycol/water fluid as a barrier fluid crossing a heat
exchanger to dissipate heat for this high speed shaft. Cavitation, has to
date, not been a issue due to re-pitching the prop and also due to the fact
that water depth suppresses the tendency to cavitate due to added ambient
pressure (local pressure has to drop below the vapor pressure of water or
less than 0.3 psia to cavitate at 70F).
Having said all this, I am in the middle of replacing the jet pump
propulsion unit with two Minn Kota 101 thrusters with custom Wageningen
Propeller series Kort Nozzle 37 for the following reasons. The first is
poor low speed maneuverability. While I was happy with the jet pump at high
speeds, I found that for the inland lakes I frequent, I just can not take
advantage of the speed and end up running as slower speeds were control
surfaces are not all that effective. The second reason is that the drive
train is noisy. While the double mechanical seal arrangement was fine for
sealing the drive shaft, it was very noisy both internal and external. Also
the main propulsion motor and controller were noisy as they were inside the
pressure hull. I power up the boat in the water column and fish scatter.
The third reason I am replacing the drive train has to do with the
mechanical seal. I use and air over liquid reservoir that uses regulated
air pressure that floats about 20 psi above ambient water pressure to
pressurize the seal cooling system. I found that the seal would
occasionally leak the barrier fluid into the interior of the boat (not
ambient water). As the air/liquid reservoir only holds one quart of
coolant, the small leak over several hours would occasionally drain the tank
and then regulated air would bleed through the seal and into the boat
causing the pressure into the boat to increase slowly. Because the drive
motor, seal and reservoir were behind the pilot towards the aft the boat,
maintenance on the drive train was difficult.
So in the sprit of KISS, I am simplifying the propulsion and maneuvering
system by going to fixed thrusters with Kort nozzles and Minn Kota motors.
While I do drop the boat speed dramatically, I get a simple propulsion and
maneuvering system that is quite and easy to maintain. In a few months, I
will let you know if I was successful.
From: Marc de Piolenc via Personal_Submersibles
<personal_submersibles at psubs.org>
To: personal_submersibles at psubs.org
Sent: Sunday, October 26, 2014 7:22 PM
Subject: Re: [PSUBS-MAILIST] Water jets
Surely isn't sure at all. And as speed through the water increases, the
advantage, which might have rested with the open prop at lower speeds,
definitely shifts to the jet. The jet's single biggest advantage, in
fact, is being able to control inflow to the rotor to prevent cavitation.
Marc
On 10/27/2014 3:11 AM, Alec Smyth via Personal_Submersibles wrote:
> 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>> 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>> wrote:
>
> Subject: [PSUBS-MAILIST] (no subject)
> To: "'Personal Submersibles General Discussion'"
> <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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