In a message dated 11/20/2010 10:23:38 P.M. Eastern Standard Time,
JimToddPsub@aol.com writes:
I'm expecting to
have a single, fully articulated Kort thruster at the
The articulating Kort nozzle seems like a good idea, and shouldn't be too
complex to fabricate.
This assumes the motor/prop is "fixed" with maybe the motor inside the
sub.
I think the big advantage here is the ability to use a really big motor
as it will be inside the sub and no size restriction except for power
available.
If an external motor is to be used, like a Minnkota, it might be easier
and more efficient to articulate the whole assembly. This would simplify the
nozzle geometry and keep the nozzle and prop "lined up" so a closer tolerance
of prop tip to nozzle could be used.
When we visited the Johnson Sealink 1 at Harbor Branch Oceanographic
Institute I saw that the clearance on their props inside the Kort nozzles was
very small. Less than 1/8 inch.
One thing to consider may be the " crash factor ".
This has always been a big consideration with designing and building
Git Kraken.
Anything with VERY tight tolerances has the potential to jam, and a
device like main propulsion out in the flow is likely to receive an impact at
some point.
Typically the " high dollar " small subs I've seen have
vertically mounted thrusters for elevation control and often horizontal
thrusters in the bow for turning in tight quarters. This allows very precise
positioning at low speed or static position holding. Very useful when
trying to snuggle up close to an object for observation and seems useful also
if a water current is present.
The fully articulating nozzle, with or without the motor, seems most
useful while the sub is moving about at some speed in open water.
On Git Kraken, I'm using vectored thrust by rotating the two rear
thrusters left and right but have no means of moving the assembly up and down.
My thinking here was to add stern dive planes if sea trials show that
they are needed.
The design I'm using has a lot of the "crash factor" in there.
My prop guard rings will give a little of the "Kort effect" but the
clearance from prop to ring is about 1/2 inch and if ( ----when---- ) an
impact occurs to the ring/rudder assembly it would take a pretty big "hit"
to bend the thing enough to jam the prop.
Again, my thoughts here were to keep the mechanism simple enough so not
much can go wrong and the whole assembly can take a beating without
stopping.
The two thrusters are each mounted on solid stainless bar stock at 1
inch diameter. It's going to take a pretty big "hit" to bend those mounts. The
swivel mechanism is protected from impact and/or entanglement and requires
only a simple two wire 12 volt rocker switch to steer the sub. Not much to go
wrong back there and it's built pretty beefy.
The one draw back I'm expecting is that currently both props are "right
handed".
I've included "vanes" in the prop guard rings to help some with
that but actual testing in the water will show how much of a problem that
turns out to be.
Because the Minnkotas are reversing, it should be a simple fix by
replacing one prop for a "left hand" one which is something I will consider
after the boat's fully functioning. I'd like to replace the stock
Minnkota "Weed Wedge 2" two bladed props with 4 blade props
designed to make the most of the Kort effect of the prop rings.
Unfortunately, the side thrusters have no prop guards so the props are
exposed. In the interest of diver safety I will probably add guards to them as
well, but for the time being, the weed chopping ability of the stock
props is needed in case of kelp or something brushing past them.
Getting back to the articulating nozzle thing......
The two side thruster/dive planes can rotate from straight vertical for
precise positioning up and down to a 30 degree down angle for diving.
This means a total of 120 degrees arc swing for each side thruster. The rear
thrusters have the same geometry so a good tight turning radius will result
with just the rear thrusters alone. Once coupled with the side thrusters
forward/reverse the sub should be able to spin on it's own axis
while remaining in a fixed position.
My little models showed during flow testing that by setting one dive
plane to the up position and one in the "dive" position, I was able to make
the sub "lean" to one side a little bit. It appeared to be about 10
degrees but I didn't measure it accurately. The dive planes on my
model weren't quite as large ( in scale ) as the ones on the actual
sub so that "banking" effect should be even more on the real thing.
I have left enough room at the rear of the sub to add another set of
dive planes at the very back end. This would be controlled by simply adding
one more actuator and two fiberglass dive planes on a simple swivel, again
operated with just a single two wire rocker switch for up and down.
There is also extra capacity in the through hulls to accept those two
wires.
My main focus is always to keep the stuff as simple as possible, with
ease of maintenance near the top of the list.
I just wonder if the mechanism to have a fully articulating nozzle could
be a little overly complex, meaning prone to problems if the assembly were to
suffer an impact.
A lot depends on what the sub is used for and where it will be
driven.
Because of where I'll be operating, I expect Git Kraken to suffer LOTS of
impacts, with some likely to cause damage. I designed the parts most likely to
suffer the damage to be easily and quickly replaceable with spares, and as
many off-the-shelf parts as possible so replacing those parts in far
off locations should be possible.
You mentioned Cliff's R300 and the articulating nozzle he has. That's an
amazingly beautiful craft but understand it is made for relatively fast travel
and not so much for static positioning. On the other hand, the Kittredge type
boats are made for precise positioning but won't go very fast. It's pretty
difficult ( but not impossible) to have both.
Frank D.