Moebius Skipjack and proposed ballast system

[DanLew]

Built a Dave Welch WTC Trumpeter Seawolf back in 2006. Just got it out after long storage and it works great. Now want to try another sub, but self-built - just want to experiment.
Would like to do either the Moebius Skipjack or Revell Gato. I have 2.75" OD tube and end caps from long ago. Understand that the 'jack uses 3.5" OD x 21" tube. After looking at the ballast system proposed below, could I get away with using the 2.75" components? (Just as a matter of convenience...)
Here's the system I propose. I know there's much more sophisticated stuff out there, and many tried-and-true approaches. As I said, just want to try something here...
The proposed system uses a small PD gear pump to add/remove ballast water from a tube(s) made from a narrow bike tire. I used the bike tires in the Welch WTC rather than the BP cuff air bag, etc. The tubes are stiff enough to hold repeatably controlled geometry on air inflation/deflation. That simple Seawolf conversion holds dead level at surface and below, and can be easily held at periscope depth with the bike tire airbag system. Given that, I want to try the bike tires with water.
Here's a pic of the current Seawolf bags and a sketch of the proposed water system for the new sub. In the sketch, I show two "bag compression" trim plates - one at each end of the water tubes. But really only one-ended compression is needed if the static trim is set light at the non-compression servo end.
I do plan to get any available mechanical conversion kit and control electronics. The pitch control will, in addition to manual control, activate the bag compression servo. I may build a test WTC with the waterbag compression system even before getting the model, if that makes any sense.
Anyway, comments appreciated.
Thanks.
BTW - what's teh D of the Gato WTC?

[Kazzer]

There's blood in the water guys!

"Dave Welch and bike inner tubes" - all in the same sentence! Phew! This should be good! Where's my popcorn?

[He Who Shall Not Be Named]

Looking forward to seeing how your SKIPJACK WTC works out. And I could not make out the last sentence.

Keep us informed as you progress.

M

[alad61]

And I could not make out the last sentence.M

Being one who also on occasions spells and types so eloquently may be able to translate it as... What are the dimensions/diameter of the Sub Driver for the Gato...

[He Who Shall Not Be Named]

2.5" diameter, about 28" long

M

[DanLew]

There's blood in the water guys!

"Dave Welch and bike inner tubes" - all in the same sentence! Phew! This should be good! Where's my popcorn?

I'll supply the butter and salt....and I love to mess about....
Components to make up a WTC for testing: tube and end caps, Rx, bag compression servo, water pump servo, etc. Water pump will be fwd/rev. In final form, may be throttled.

Note caption error - tube is 2.75" OD, not 1.75".

Batts, hardware, etc available - just need a pitch control, preferably with some adjustable response parameters. Recommendation, please.

[crazygary]

Will be following this thread with great interest!! Very cool concept, here!!

Chips, salsa and jalapeno poppers on me!!!

CG

[Subculture]

The water pump into a bag is a well proven and mature system.

Like all systems it has its pro's and cons

1. Bags can and do split, especially if you over pump (geared pumps can easily make about 40psi, even the crap ones), so some sort of pressure control switch is a good idea. Either that or be careful.

2. Like the piston system, the bag is flexible for different water densities. Unlike the piston system, you can't apply feedback to the piston position, which means you have to trim your boat every time you fill and empty. If you're one of those type of skippers that dives the boat and keeps it submerged until the batteries flatten or you get bored, then this isn't a big deal.

3. The quality of geared pumps is very variable from excellent to atrocious. I don't recognise the model you have, so can't say whether it will work reliably long term. The best I know of on the market is made by a company called Kavan, they are a little chunky, but you should have sufficient room in a 1/72 Skippy for one if you find the pump you have gives up the ghost. The ones with brass gears are best.

You will need a check valve inline with the pump, because the gear pumps are a bit 'leaky' and the pressure in the cylinder will push water back through the pump. Peristaltic pumps don't need a valve, but they're a good bit slower at pumping and take up more space.

I don't like your idea of squeezing the bag to effect trim control. It should be left alone IMO. It's also a good idea to put the bag inside a smaller cylinder to constrict it's volume, this helps keep the volume set to a certain point and stops the bag rubbing against internal fittings. The cylinder should be vented with some small holes drilled into the walls.

If you want ballast shifting, whack the battery on a tray, fit some guide rails and push it back and forth with a servo. This is simple, inexpensive to implement and 100% dependable.

Regarding pitch controllers, they all work along similar lines, but have different software and hardware features. For instance is manual override important to you, or would you like to be able to set your leveller parameters using a torch light through the clear walls of the WTC (microgyros). Perhaps you'd prefer a leveller which has a continuously variable gain control and a part automatic override (Engel LR3), or one with specific failsafes built in (KMC).

[Kazzer]

If you want ballast shifting, whack the battery on a tray, fit some guide rails and push it back and forth with a servo. This is simple, inexpensive to implement and 100% dependable.
(KMC).

That's just as per my X Craft WTC. I hate this thing. But then, I'm biased!

[Subculture]

Shipping damages aside, that x-craft was well engineered boat that always performed very well. If it's not working right now, it's not through any fault of the design or construction.

It's not a fast boat, but neither were real x-craft. The boat benefits from pitch shifting as the 1:1 versions used to operate with their bums in the water like the early Holland boats they were based on.

[DanLew]

The water pump into a bag is a well proven and mature system.
Right. All I'm adding is a way to have it auto trim the pitch with the bag compression approach, and maybe that's also been done.

Like all systems it has its pro's and cons Thanks for all this feedback - great for my learning/thinking curve. Much appreciated!

1. Bags can and do split, especially if you over pump (geared pumps can easily make about 40psi, even the crap ones), so some sort of pressure control switch is a good idea. Either that or be careful.
Note concept drawing showing over-pressure switch and valve. (Guess I didn't post the concept drawing - will do shortly)
The bike tires hold shape well. Mechanical seals (plates and clamps with bulkhead fittings) will be used to seal tube ends, adding to integrity.

2. Like the piston system, the bag is flexible for different water densities. Unlike the piston system, you can't apply feedback to the piston position, which means you have to trim your boat every time you fill and empty. If you're one of those type of skippers that dives the boat and keeps it submerged until the batteries flatten or you get bored, then this isn't a big deal.
A trim servo will compress one end of bag to shift weight fore/aft to control pitch. Need to do more forum research about this and porpoising. Need also to learn more about availability of depth controllers and how I might have that feed back to the pump for total ballast volume control.

3. The quality of geared pumps is very variable from excellent to atrocious. I don't recognise the model you have, so can't say whether it will work reliably long term. The best I know of on the market is made by a company called Kavan, they are a little chunky, but you should have sufficient room in a 1/72 Skippy for one if you find the pump you have gives up the ghost. The ones with brass gears are best.
The P wont be that high. This pump is a freebie, from hand-held, battery-powered insecticide spray wands. "Engineering plastic". I'm betting on this working just fine. Replacements are free, anyway. Some of these toy sub components are waaay over-spec'd. (where's my fireproof suit...)

You will need a check valve inline with the pump, because the gear pumps are a bit 'leaky' and the pressure in the cylinder will push water back through the pump. Peristaltic pumps don't need a valve, but they're a good bit slower at pumping and take up more space. Good point. Might want minor leakage. Will ambient P at depth keep this backflow minimized????

I don't like your idea of squeezing the bag to effect trim control. It should be left alone IMO. It's also a good idea to put the bag inside a smaller cylinder to constrict it's volume, this helps keep the volume set to a certain point and stops the bag rubbing against internal fittings. The cylinder should be vented with some small holes drilled into the walls. The bag(s) will lay at the bottom of the cylinder with constraint above, preventing intrusion into upper part of cylinder.
Total air volume impacted, whether overall tube venting or a sealed, smaller bag chamber will depend on total bag volume change required, and having cylinder air P adequate for an emergency bag blow thru a failsafe valve that would open on power failure.
If you want ballast shifting, whack the battery on a tray, fit some guide rails and push it back and forth with a servo. This is simple, inexpensive to implement and 100% dependable. Great idea. Now you have me reconsidering the whole approach...but the bag thing is sooo enticing.

Regarding pitch controllers, they all work along similar lines, but have different software and hardware features. For instance is manual override important to you, or would you like to be able to set your leveler parameters using a torch light through the clear walls of the WTC (microgyros). Perhaps you'd prefer a leveller which has a continuously variable gain control and a part automatic override (Engel LR3), or one with specific failsafes built in (KMC).
Wow - these have come a long way since my last look in about 2005 (or likely I don't remember) Thanks - I'll research these and try to get up to speed.

Again, Thank you for the help.
DanL - old guy in Minnesota USA, where there's still 2ft of ice on our lake!

[Kazzer]

Shipping damages aside, that x-craft was well engineered boat that always performed very well. If it's not working right now, it's not through any fault of the design or construction.

It's not a fast boat, but neither were real x-craft. The boat benefits from pitch shifting as the 1:1 versions used to operate with their bums in the water like the early Holland boats they were based on.

Agreed, but I don't like the upside down arrangement of the access to the WTC. I still haven't found the leak yet (too busy with house renovations) so it hasn't had the problem resolved. Maybe this summer!

[Subculture]

Bicycle inner tubes will be strong, so splitting shouldn't be an issue.

You will absolutely need a check valve on the hose, trust me on that one. Without it the boat will just start to pop up as soon as you shut off the pump.

The pump you have sounds pretty good. It's not pressure that does the gears in usually but heat when the pumps run dry after emptying the tank. The driven gear loses its keyway, which is usually just a moulded in 'D' shape. Most of the production of these pumps has been moved from Europe, where the quality was decent, to China where it's lousy. They've taken good designs and completely buggered them by using cheap and crappy materials. So if you've found a decent cheap pump let people know about it, because it will help the wider community.

Leveller tech has only marginally improved since 2005. The big leap forward was the introduction of chip based accelerometers in the early noughties, since then it's basically been a case of lowering costs, and further miniaturisation, to the extent that even the very smallest boats can benefit from an automatic planesman.

[Subculture]

Agreed, but I don't like the upside down arrangement of the access to the WTC. I still haven't found the leak yet (too busy with house renovations) so it hasn't had the problem resolved. Maybe this summer!

As you have a semi tamed master modelmaker in your employ, why don't you crate it up, and let the expert loose on it. I expect he'll have it sorted out in two shakes, and you can crack on with the honey do list!

Re the underside access hatch, the idea was to hide the gap when the boat was in the water. it's not that big a deal to acces to be honest. perhaps you could get a stand made which enable you to revolve the boat. Think of the way Austin Mini shells used to be made.

[Kazzer]

As you have a semi tamed master modelmaker in your employ, why don't you crate it up, and let the expert loose on it. I expect he'll have it sorted out in two shakes, and you can crack on with the honey do list!

Re the underside access hatch, the idea was to hide the gap when the boat was in the water. it's not that big a deal to acces to be honest. perhaps you could get a stand made which enable you to revolve the boat. Think of the way Austin Mini shells used to be made.

If I sent it to The Cave, it would linger there for years. The monster that lurks within is busy doing orders and also has a pile of projects.:-(

I am almost tempted to cut the hull and re-glue it so the top comes off, rather than the bottom. I have a Japanese saw and I've earmarked the places to cut. It's just a case of plucking up the courage.

I've got my Skipjack to run if the mood takes me. But first, I'd better finish this kitchen-


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