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Re: [PSUBS-MAILIST] Articulating kort nozzle



Hi, Frank,

 

With the main motor inside the sub (i.e., outside the main pressure hull but inside the rear fairing), I can conceive of the prop in a rigid tunnel with an articulated nozzle aft of that, however that’s different than what I have in mind.

 

For a sub as small as mine (cigar shaped, bubble canopy) I intend for the [external] motor, prop, and Kort to move as a unit which will allow greater range of motion than having the motor fixed.  Whether I end up giving it the capability to move about one axis or two, the mounts/push-pull rods will incorporate some type of in-line shock absorbers or springs so the propulsion unit can sustain a bump, deflect, and return to its original position without damage or at least as little as possible.  Also I want to avoid the impact being transferred to the sub itself.  I need to spend some time researching other’s designs to see how many degrees of articulation they have.  I expect to use electric actuators mounted inside the rear fairing to control the rods.  This is a little more complex than direct mechanical linkage to the cockpit, but it avoids mechanical penetration of the pressure hull.  It’s just a prejudice of mine to have as few mechanical penetrations as possible (or none).  I want all penetrations to be for electric, COM, fluid, or air.  We’ll see if I have to bend on that.  There are some advantages to mechanical linkage.

 

One of the advantages of full articulation is that when you are stopped, you can still maneuver the rear end around.  The bow thrusters might be fairly small.  I’ll have to see if I’m satisfied after experience in different situations or decide to upgrade.

 

I won’t have a lot of ability for a powered, level, straight-up ascent (i.e., without forward or reverse travel).  I’ll have to ascend either by pointing the nose up or adjusting ballast.  Yes, I could go rear end first, but generally try to avoid that in all areas of life.

 

There are lots to decisions to make, and most of them can’t be made in isolation.  Everything I consider seems to affect at least two or three other decisions.

 

Thanks much for your comments.  They always cause me to think things through again and draw more pictures in my head.

 

Best regards,

Jim

 
In a message dated 11/21/2010 9:21:18 A.M. Central Standard Time, ShellyDalg@aol.com writes:
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.