Warped Gato main hull halves

[mermaid]

I noticed when trial fitting the !/72 Gato main hull halves that there is quite a bow to the parts resulting in a 1/4" gap amidships on both top and bottom. The gap can be closed by pressing the parts together. Is it just my particular kit or is this a common problem with the Gato kits in general? I plan to RC the model and will need access to the hull interior. Problem is, if I cut out the top section, the upper part of the hull will spring outwards and the deck won't fit properly. Any suggestions?

[aeroengineer1]

See David Merriman's cabal report

See David Merriman's cabal report on the subject. He addressed this in one of his last reports.

Adam

[mermaid]

http://www.subcommittee.com/forum/icon_redface.gif Where is Dave's cabal

Where is Dave's cabal report located? Would that be Subpirates site?

[crazy ivan]

Yes, Subpirates. But if you

Yes, Subpirates. But if you cut your access similar to the way I show it in my build thread, rather than splitting it at the waterline as David did, then it will not be a problem. How wide an access will you need?

[rokket]

I'm not going RC, but

I'm not going RC, but I have the same problem. I actually thought for a moment that there was a strip piece that filled the gap! But pothers (static fellers) have successfully closed with huuge clamps. I imagine for Mermaid some extra internal supports would be the answer, circle ribs that would hold most of the hull no matter what was cut for access.

[mermaid]

http://www.subcommittee.com/forum/icon_question.gif I'm not quite sure

I'm not quite sure what access I will require as this will be determined by my final WTC & ballast tank designs and placements. I'm considering building two WTCs into the hull fore and aft, the layout being very similar to Jim Butt's Batfish. I believe that the hull wall thickness is sufficient to make this feasible. Ideally, I would like to have the entire deck removable from the turtle back to mid fore plane just as Jim did. The WTC bulkheads should reinforce and support the hull internally. The two internal bulkheads provided in the kit should do the same. By the way, I notice that you mentioned a reverse RCABS ballast system. I have a vague idea about the principle. Would someone please explain in greater detail. I have been considering a gas system since I am familiar with the design.

[crazy ivan]

What you are describe is

What you are describe is commonly called the dry hull method, or in this case, semi-dry hull. I don't know... it's one thing to seal off portions of a monolithic fiberglass hull to create dry spaces, something else to do it with a glued together styrene boat and maintain watertight integrety at depth. While I agree that the Revell's hull is plenty thick enough, my concern would be the bottom seam cracking open after a time. Just my 2 cents. Maybe it could be reinforced somehow. Your call.

RCABS: ReCirculatingAirBallastSystem. As originally conceived by our own Dr. Art Broder, it is based on a small air pump (of the type commonly found in home blood pressure measuring systems) and an inflatable bladder (most often used as the cuff for these same machines). The bladder is located in the wet space outside the WTC, and it is inflated by the pump with air from inside the WTC. In this condition, the sub floats on the surface and the WTC holds a partial vacuum. The amount of vacuum pulled depends on the volume of the WTC relative to the bladder. To dive, a valve is opened and the vacuum in the WTC draws the air back out of the bladder, which deflates the bladder, submerges the sub (if it is properly trimmed) and restores the WTC to atmospheric pressure. Then to surface, the pump refills the bladder from the WTC and the cycle repeats. No external air or gas is ever required. There is nothing to recharge, except your main battery which won't even notice the 200 - 250 ma draw of the pump. You can dive and surface all day long without even thinking about it. It is a very simple and reliable system.

Reverse RCABS, or RCABS-R, works the opposite way. The bladder starts out inflated under very low pressure and the sub floats. The pump pulls the air out of the bladder and stores it under pressure in a sealed container (pressure vessel). With the bladder deflated, the sub submerges. To surface, a valve is opened and the pressurized air in the pressure vessel inflates the bladder. The sub rises. None of the air is used from the WTC, which remains at atmospheric pressure at all times. The pressure vessel may be located inside or outside the WTC as space permits. Since these pumps develop around 11-15 psi, the volume of the pressure vessel must be equal to the volume of the inflated bladder. That volume is driven by how much material you need to raise above the surfaced waterline. With this Gato, that can be quite a bit. The main down side of the reverse system is the additional space required to accomodate the pressure vessel.

Both systems will use about the same amount of battery power. Both can be used proportionally to a degree, but because water pressure increasing with depth will begin to compress a partly filled bladder, it is generally used either completely full of air or completely empty. The same holds true for the basic gas system you are familiar with.

[mermaid]

http://www.subcommittee.com/forum/icon_idea.gif Thanks for the RCABS

Thanks for the RCABS primer George. I'm re-evaluating my original idea for going with a gas system. I'd hate to put a partial vacuum on my WTCs when diving however, reverse RCABS sounds ideal!! What about using the WTCs as an air reservoir? That way, you put positive air pressure into the WTCs as you dive. This would help keep the water out. Where could I obtain the hardware required to set up such a system?

[crazy ivan]

Well, I forsee three potential

Well, I forsee three potential problems with pressurizing the WTC. First is designing an enclosure that can hold up to one atmosphere without coming apart and still be readily accessable inside. If you were to use the hull as part of it as you suggested above, you would really be stressing the seams and they would not hold up for long. The reverse RCABS system tolerates no air leaks. Even with a small one, you would find yourself unable to surface before long. If you went with the more commonly used cylinder approach, it might be more feasable if you can keep the endcaps from blowing off.

Then you would have to deal with the second problem: seal leaks around the penetration areas. Any place a control rod or prop shaft exits the WTC would be potential problem. Actually, the low friction commercial seals we normally use in this hobby are designed to hold back external water pressure and will not seal against internal air pressure. You could have an isolated section without any penetrations to hold air pressure. Big Dave did this in one of his prototype WTCs. But he glued the end caps on and reinforced them in place with a tierod. No components were located inside, just air. If you do that, you might just as well go to a seperate metal tank.

The third problem is that the volume of of the WTC would need to equal the inflated volume of the bladder, since the pump develops a maximum pressure of one atmosphere. Normally, however, the size of the WTC is dictated by the parts it must contain. Too large, and it would not store air at high enough pressure to surface at depth. Too small, and the pump will not be able to force in enough air to fully deflate the bladder. If you are familiar with the Ideal Gas Law, you can plug in some volume and pressure numbers and see why this is true. Also, you won't truly know how much air you will need for the bladder until you fit everything in place, including the WTC, and can do some float tests.

If all this sounds too complex, don't be put off. In practice the reverse RCABS system is quite simple: a pump, a bladder, an electric valve, and a tank. The pump and bladder can be purchased from Big Dave's Designs located at:

http://www.frontiernet.net/~bwelch/

A Clippard electric solenoid valve can be obtained, along with an electronic switch to activate it (and the pump) from MIKE'S SUBWORKS]http://www.mikessubworks.com/[/url]

A pressure tank can be made from a suitably sized hollow metal sphere. Copper and stainless steel floats of various sizes can be had from Naugatuck Manufacturing Company. They also sell them as flagpole ornimental balls. I used a 3 inch copper float in my Akula and it works great. Find them at]http://www.naugatuckmfg.com/default.html[/url]

McMaster Carr carries them as well. If you can find the room, (I couldn't, Big Dave could) a Robart aluminum air tank (as used for model airplane pneumatic retract systems) is ideal. Most hobby shops should have them. Failing that, an empty aerosol spray can of the appropriate size (my current solution) will fill the bill.

[mermaid]

http://www.subcommittee.com/forum/icon_question.gif Thanks for the info.

Thanks for the info. Yes I am familiar with the ideal gas law. I figure that if I use miniature servos, ESC and receiver, I should be able to manage to incorporate a pressure vessel in one of the WTCs. or possibly, one along the keel below the bladder. I realize that until I get the running gear into the hull, I won't be able to finalise the required bladder size. I'm thinking that a children's BP cuff bladder might work. I'll try to minimise the weight above the WL.