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Arkmodel piston tank question

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  • #16
    All fixes relatively straightforward for someone with the tools and know how. Many people that buy submarine kits may only have some basic handtools and maybe a dremel, which would be perfectly adequate for a well made kit.

    Sounds like Ark models kits are a bit of a curates egg- good and bad. their Type VII kit is well regarded and seems a good choice, but better to pair it with another WTC by the sounds of it, unless you are prepared to seriously tinker.
    DIVE IN! Go on, go on, go on, go on, GO ON! http://www.diveintomodelsubmarines.co.uk

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    • #17
      Digging deeper into the Arkmodel WTC, I like the overall size of the basic WTC main components. Namely the motor compartment, the 3” central compartment and the forward compartment. Plenty of room for laying out and placement of necessary operational components. Still modifying to make work properly.

      Where I’m hung up is the original manufacturer design choice of using their supplied 300ml Centrally placed piston ballast tank for a boat that seems to commonly (as far as I know) need around a 850ml tank to operate at a surfaced waterline and submerge to a slightly positive Buoyancy. If you have this Arkmodel set up and can share how well or not well it worked out for you.

      Maybe I am missing some magic voodoo here? Seems the piston tank is undersized and is only a 1/3rd of what it should be. If I’m wrong please let me know.
      Last edited by Monahan Steam Models; 01-14-2021, 10:16 AM.

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      • #18
        Bob should be able to let you know the volume of tank needed, as he has built one or two of these up. 300ml does sound small if you’re aiming for a scale waterline. I would say 500ml to 700ml would be closer to what you need. Being an injection moulded plastic hull, I’d expect the cross sectional area to be thicker than a well moulded GRP hull.

        The simplest and most accurate way to find out displacement is to place the hull in a test tank, trim it so that it is neutrally buoyant or just slightly heavy, then use blocks of polystyrene foam below the waterline to get the boat floating to the required waterline. Most hulls will need some form of simple cradle to attach the foam to, but that’s easy to make. Once you have enough, add the volume of the foam up, and that should be the tank size you need. Some will say weigh the boat, but that only works with unfilled thermoplastics which broadly have the same mass as water, plus the boat needs to be split longitudinally directly on the desired surfaced waterline.
        DIVE IN! Go on, go on, go on, go on, GO ON! http://www.diveintomodelsubmarines.co.uk

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        • #19
          Thank you Andy for sharing this very helpful information. Much appreciated!

          Bob informed me a while back that he has found that a 3” diameter ballast tank, 8” long to be ideal for this boat. I have one of David’s 250/300/250 MSD units already set up for the type 7 build.

          The Arkmodel WTC is just a curiosity and somewhat of side project. Mainly I wanted to pick one up to learn about it’s pros and cons of design. Then see if it could be modified to work better.

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          • #20
            When a boat has a lot of freeboard, and especially if the hull and upper works are moulded chunky thus displacing a lot of water, it's not a bad idea to consider a hybrid ballast system.

            For example, space permitting you could use your piston tank in conjunction with a larger aspirated ballast tank which uses a pump to move water in and out. That tank could provide the remaining buoyancy , and you don't have to concern yourself with extra pressure forming inside the module which would be a factor if you fitted either a larger or second piston tank.

            An aspirated system is very cheap to make, as you can use a small gear pump and esc with a bit of plastic pipe and some sheet plastic endcaps.

            One downside to a single piston tank is that you get a shifting centre of gravity, and if you use it to trim and account for the 1% change in water buoyancy, the boat's longitudinal trim will tend to be out by a degree or two, all depending on the boats metacentric height. Some modellers employ a shifting battery or some other weight moveable from a standard servo to adjust for this.

            Twin piston tanks fore and aft bypass this problem, but it adds cost and complexity.
            DIVE IN! Go on, go on, go on, go on, GO ON! http://www.diveintomodelsubmarines.co.uk

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            • #21
              Andy,

              Again great information! I would like to build a twin Engel piston tank boat one of these days. Your suggestion of using the piston tank in conjunction with a larger aspirated tank is spot on. Yes it add complexity but I’m fine with that. Most likely if I end up using anything from the Arkmodel WTC Kit, it will only be the end caps, center tube and some other bits. The piston tank will be put on a shelf and used for a future project.

              With that said, more than likely the Arkmodel WTC will get a single aspirated central ballast tank and system like David developed for his SubDrivers. Will also most certainly use a gas emergency backup. The Arkmodel WTC center tube measures 90mm in diameter with a 3mm wall thickness. If 850ml capacity for the ballast still holds true for the model using this WTC, then a 6” long tank would be required. Shorter and maybe less sloshing around in the tank? Baffles inside the tank could also be added.

              During my work experimenting with the gas and electric torpedoes, along with the launch mechanisms, I began considering various ways to make fine ballast trim adjustments to the fore and aft of the boat while using just as you mentioned one large main ballast tank.

              One of the options I posted about a few months ago was using peristaltic pumps and syringes as trim tanks. One syringe place fore and one placed aft. Similar idea to twin piston tanks.

              Click image for larger version

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              Here’s a link to a video from today demonstrating the concept.

              https://youtu.be/g8hO1cLHi30


              You make a good point about the use of a single piston tank’s effect on trim during the fill and empty cycles. This was another reason I had already had concerns. The shifting of a weight to adjust trim is a very clever method Manfred introduced to me recently. Has anyone similarly explored the idea of sliding a single piston tank to offset the emptying effects on trim? Another option for a single piston tank would be to use twin opposing pistons in a single cylinder. Having individual control over each piston would allow trim adjustments fore and aft.


              Best Regards,

              Nick



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              • #22
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                As I mentioned earlier I noticed the end caps for the WTC kit suffered from misaligned inner and outer forms during the casting process. This messed up the servo pushrod penetrations as well as the motor shaft penetrations. The way the in Inner and outer manufacturing casting forms are designed, each creates a stepped bore to hold specific components in each of the penetrations which require concentric alignment to have the installed parts function and seal properly with minimal resistance for the parts.

                For example. In regards to the pushrod penetrations, the inner form creates a 5mm diameter bore in the aft bulkhead that is 6mm deep (meant to hold an oil-lite bushing) before reducing to a 2mm thick wall with a 3mm diameter hole in the center for the bushing to stop against. The outer casting form for this same pushrod penetration creates a 6.75mm diameter bore that is roughly 8mm deep before it meets the wall with the 3mm center hole created by inner form. This is meant to hold the u-cup pushrod seal. Long story short. These bores were way off and the inner form was shifted to port or to the left for the land lovers.

                Wow that was a pain in the rear to explain. Really need to take more before photos. The same alignment issues are seen with the inner and outer bores created from both forms for the motor penetrations.

                Not only were the casting forms misaligned to each other left to right but also deck to keel or top to bottom. When assembling the kit per the instructions. (Inserting the oil lite bearings and u-cup seals into the pushrod penetrations and inserting the ball bearings and u-cup seals into the motor penetrations) both were locked up frozen with no ability to spin or travel.

                The fix for the pushrod penetrations. There’s a lot of different ways to do this but it starts with mounting the end cap casting vertically on a squared up milling machine or drill press. Align the center of the 6.75mm cup seal bore with the center of the machines chuck. That’s the trickiest part. Easiest way is to create a stepped drill bushing on a lathe. The bushing will have an OD of 6.75mm with an inner bore diameter of whatever you chose. In this case 5mm.

                Mount a 5mm bit in the machines chuck and with the bushing installed in the casting, take the first step to straighten that misaligned mess out. The following steps use fluted reamers mounted in the machine’s Chuck to gradually open up the bore from 5mm to 6.75mm while it is mounted on the square machine table.

                There are other ways to do it. Like single point machine boring for harder materials but this is plastic or Tupperware so this method is fine.

                Here is the results afterwards. The pushrod bores are straight and true to the centerline of the casting and are now a consistent 6.75mm without bore steps or walls. The concentric alignment between the bushing and the seal is perfect. The pushrods now align perfectly into the servo tray’s pass through holes. The resistance for pushrod travel is much smoother now.

                The same modifications were made to correct the forward end cap with the same results. Correction to the motor penetrations and seals will be next and will be slightly different.

                My original post was not intended to go into this detail of a review, built and correction of the Arkmodel WTC piston tank kit.

                Maybe I should start a new topic or continue here?

                Nick

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                • #23
                  Originally posted by Monahan Steam Models View Post

                  One of the options I posted about a few months ago was using peristaltic pumps and syringes as trim tanks. One syringe place fore and one placed aft. Similar idea to twin piston tanks.
                  Same principle. Those pumps tend to be a bit slow, but are fine for trim function. Some modellers speed them up by fitting hotter motors, but they're still slow and it's no hardship to make a screw jack. I wonder how long the tubing lasts with increased RPM?

                  Originally posted by Monahan Steam Models View Post

                  The shifting of a weight to adjust trim is a very clever method Manfred introduced to me recently. Has anyone similarly explored the idea of sliding a single piston tank to offset the emptying effects on trim? Another option for a single piston tank would be to use twin opposing pistons in a single cylinder. Having individual control over each piston would allow trim adjustments fore and aft.
                  I've not seen that variation, although I'm sure someone has tried it- most things have. One potential issue is all the extra wiring plus the filling hose would need to be flexible to allow the movement without snagging or coming adrift. Moving a battery or some other heavy lump with a servo is a lot easier IMO.

                  DIVE IN! Go on, go on, go on, go on, GO ON! http://www.diveintomodelsubmarines.co.uk

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                  • #24
                    For balance why not pump air between a fore and aft syringe?
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                    • #25
                      Air isn't so controllable, and you tend to get hysteresis with the movement of the pistons. Air is great for moving large quantities of ballast, but not easy to control small volumes in a precise way.
                      DIVE IN! Go on, go on, go on, go on, GO ON! http://www.diveintomodelsubmarines.co.uk

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                      • #26


                        The reason I am interested in exploring options for making fine trim adjustments both surfaced an submerged in addition to the main vented LPB ballast tank is due to this boat’s weight and buoyancy may be changing dynamically during a patrol and I would like to adjust for that.

                        The boat has 4 torpedo tubes in the bow and will hopefully have one aft when I get the chance to develop that tube and mechanism. Although the tubes and equipment are below water level while surfaced. The electric torpedoes will add a slight amount of buoyancy until launched. And to over think the need for this a bit more, the boat is also capable of launching both gas weapons. Each of these types of weapons have a different stored weight while in the tubes. The electric are slightly buoyant. The gas slightly negative buoyant. The boat also has in addition to the emergency gas backup tank, a separate gas tank for operating the launch tube solenoids and potentially other features. Unclear what the second tanks use may do to change final trim? I’m expecting the use of the liquid gas overtime will make the boat lighter. This is all new territory and learning for me as I have not built a sub with this many features before. I don’t have the answers but I can say it has been a fun experience going down this challenging design path so far.

                        The boat is maybe what some would consider a small to medium size (compared to 32nd scale) and to be perfectly honest, I have no clue what amount of effect these small weight changes will have on overall trim. Significant or subtle.

                        Other reasons I would like having the ability to make fine adjustments would be the ability to hover the boat at a depth statically or adjust for different water conditions. Most likely I will size the main ballast tank so when fully flooded the boat will be positively buoyant with the conning tower mid height above the surface or maybe decks awash with both trim tanks empty.

                        I would like to hear input from anyone who has experience with what I just outlined. Pros Cons

                        There was a very interesting thread started by Redboat219 on RC Groups in 2009 about non gas (propel) ballast systems that I ran across. The first number of pages of posts in the thread had some very relevant information regarding hybrid systems like this but unfortunately many of the following pages and overall topic discussion was badly derailed early on by a forum member who was eventually banned from that site. Would’ve like to have seen the relevant information exchange continue in that thread.

                        Link to the thread for those curious:

                        https://www.rcgroups.com/forums/show...allast-Systems


                        Nick


                        Last edited by Monahan Steam Models; 01-15-2021, 10:14 PM.

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                        • #27
                          Originally posted by Scott T View Post
                          For balance why not pump air between a fore and aft syringe?
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                          Scott,

                          Are you referring to the buoyancy angle effects of using a single piston tank? If so that could be an interesting way to compensate angle keeping level during the filling and emptying of the tank statically. Or keeping a nose down during dive and nose up during surfacing dynamically. Like Andy mentioned air is not as smooth in transition as a non compressible media but your proposed example is worth exploring nonetheless. I have a few more trim variables to account for in the build I am currently working on that will be outside of the operating conditions of the main ballast tank trim. I may need to make the boat heavier as well as more buoyant both fore and aft and a mix of both during operation. I appreciate your input, please keep it coming! I’m experienced and well educated in designing as well as in different ways of making things but I’m always learning too. That is the point of these experiences and sharing of knowledge. The willingness to share, or go down deep rabbit holes of research as well as listen and learn from others who have already experienced first hand the knowledge you are seeking is all part of this experience I enjoy as much as build. For example I’ve always been a builder of things since I was a kid. Learned that from my dad. I just enjoyed the experience of building different things and using them. I’ve never cared for attending shows, showing them off and getting reactions. Wasn’t what I was looking to gain from the experience.

                          Nick

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                          • #28
                            Originally posted by Monahan Steam Models View Post
                            . I’ve never cared for attending shows, showing them off and getting reactions. Wasn’t what I was looking to gain from the experience.
                            That's not the reason for attending shows and events. The idea is to share information, get to know others, and have a bit of fun.

                            DIVE IN! Go on, go on, go on, go on, GO ON! http://www.diveintomodelsubmarines.co.uk

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                            • #29
                              Originally posted by Subculture View Post

                              That's not the reason for attending shows and events. The idea is to share information, get to know others, and have a bit of fun.
                              Yes I completely agree. That’s how I first got into radio control model ship building. I was part of the San Francisco Model Yacht club at Spreckels Lake in Golden Gate Park for decades. I helped organize events there. We had a great group of guys at that lake for many years and still do but no where near the number we had in the late eighties and nineties.

                              One of the reasons I stopped participating in the shows was due to a couple newer personalities who started hanging around that were good builders but completely full of themselves. The arrogance was off putting and at times inescapable.

                              Nothing like the old guard. These guys and really one in particular only wanted to talk at you about themselves and their build. Seemed the only words they could ever hear in a conversation was compliments.

                              Also the lake was becoming over run by the go fast boats and the RTR crowd. I’m more into scale and scratch builds. So I stopped attending and stayed in touch with the guys I consider friends to this day.

                              I find the forums to be a better platform for sharing and exchanging ideas much like it was in past with the guys at the local pond. If you’re fortunate enough to still have a good pond and group of modelers there, then consider that lucky. San Francisco and the surrounding area has changed dramatically over the last decade mainly due to the tech boom and it has not changed for the better.

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                              • #30
                                Only too well aware of that., and it only takes one bad apple to spoil the barrel.

                                Used to be fortunate to have access to five model boating ponds within a five mile radius of where I live in North London, but they've gradually fallen into disrepair and then either drained then left empty, filled in or been repurposed into wildlife ponds full of aquatic plants.
                                DIVE IN! Go on, go on, go on, go on, GO ON! http://www.diveintomodelsubmarines.co.uk

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