View Full Version : 1/20 Koryu Type D Japanese Minisub

01-29-2010, 03:46 AM
When I first saw this Hull I wanted one, having previously seen an earlier version of the real thing lying on the seabed in Papua New Guinea. On top of that it just looks one sexy boat.

Gantu was selling these on behalf of German guy, so I think itís a relatively limited run. With limited flood vents and drains plus the fact most of the hull stays submerged, the design lends nicely to the German style of dry hull, and so thatís the route I took. Included with hull was a CD with photos showing another hull already built up including the tech rack, which was integral with the rear hull. I had concerns over how much weight was hanging on this and therefore how strong the rear bulkhead would need to be. Secondly I wanted to use the tech rack in multiple boats if necessary in much the same way that a wet hull WTC can be swapped between boats. The existing boat used a single piston tank, which is not a big issue with this model as it is not a conventional submarine, sort of pivoting around the propeller, so rising and falling at angles is not a problem.

The base kit had a main hull section, stern section, an upper hull, some moldings for the torpedo heads and part of one of the scopes, and lastly a clear vac formed cowling. The other main working parts which needed to be sourced is a 500ml piston tank by Gerhard Gasch with a proportional control board by Markus Rieger. There were some simple plans, but other than that it was up to me to make it up. No appendages!!!

I broke the build down into 4 parts

01-29-2010, 03:52 AM
This is actually the simplest part of the whole boat. The break point joint was prepped and the Engel Bayonet epoxied into place. In the bow area I drilled the main drains into the hull and fabricated the drain covers; I also cut a large v shaped hole in top of this area. This part will be free flooding with it venting into the upper part of the hull, and draining through, well the drains. I also fitted a front bulkhead to make the main hull pressure tight, this has glass reinforcing and epoxy on the pressure side. Some plasticard plates were put inside the drain holes to create the impression of an inner hull, and prevent you being able to see through to the other side. Then this was put in a cupboard out of the way.

01-29-2010, 03:54 AM
Most of this will be detailing, which I didnít want to get into until the boat has its basic functions working. I used this part as something to do while the main work was curing or some other hold up. Drilled some holes for vents in the hull and a few other areas where railings and cleats will later be fitted. Also cut out the canopy area, and drilled holes for the scopes and engine inlet etc. I am using aluminum tubing for the Ďmastsí and these are a nice tight fit into ľĒ brass tube (I know dissimilar metals but I will strip and rinse after runs). I drilled matching holes into two brass strips, one long, one short. The longer one can be attached to base of sail in the upper hull and the small piece will support the floor of the sail. Three brass tubes which terminate just inside the sail were soldered into place. The bottom, of the tubes was slightly squeezed, this should allow the masts to be removed or locked into place. A plasticard deck cut and drilled also to accept the masts, with some holes tucked into the rear of the sail floor and at the front out of sight will allow the sail to vent from the free flood upper hull.

01-29-2010, 03:56 AM
The rear bulkhead was made from two flat discs sandwiching nuts (in recesses) for the motor mount screws (avoiding through holes which would be potential leakage path). The Engel motor mount is held to the bulkhead by two screws and two studs which also serve as servo mounts. The Mount holds a simmering seal, anti friction bearing and brass shaft adaptor. The brass part also serves as the shaft thrust locator, being a nice fit between the motor bearing and the anti frication bearing. 2 x ľĒ holes fitted with brass tube will locate the bellows. A third brass tube is used to draw water into the piston tank and a flexible hose connects this to the piston tank.

01-29-2010, 03:59 AM
A hatch was cut into the lower side of the hull to allow access to the control rods and also the bellows seals. I made some overlaps for the hatch by epoxying some glass using the outline of the hull to provide the radius profile. Wax paper was used to prevent it sticking to hull (yes I borrowed that from a cabal report). These were then cut to size and epoxied into hull and hatch cover. A single screw holds it all in place.

Finally finished profiling the control surfaces, and drilling all hinges. Never ever again, itís a tough material to cut and profile I should have made masters and cast them, buts its done now). The planes are located close to the screw and as such there isnít much room down there, so I decided to make this a maintenance free zone. The four hinge points were drilled 1/16Ē into the stern and once it was clear they were accurate I opened them up to 3/32 to fit the bushings. The static control surfaces also opened up to accept 3/32 tubing. I cut a 1/16 slot into the stern from the bearing end all the way to the 3/32 hole, and opened up the main shaft bearing hole to 6mm. I then took a 1/16 wheel collar and soldered a 1/16 brass rod into the grub screw hole. Also ground away some of the collar to help clear the hull (and also shaft on the one closer to the prop). The internal bore of the stern around the rudder was calculated to be 22mm and 18mm on the elevators, so I planned on 20/16mm, and bent the 1/16 brass rod to fit within this diameter with wheel collar already in place (do you like the mix of metric and imperial units). The complete assemblies could be slid down the slots and once into place the 3/32 bushing was slid over the rod and epoxied into the hull, and all allowed to set.

01-29-2010, 04:01 AM
Now to repair the damage. The stern still seemed quite strong, but firstly I wrapped some glass cloth around the prop shaft and slid it into place. Then wrapped some tape around the stern and mounted stern vertically with the bottom sitting into recess cut into a piece of wood. Previously I have drilled a 4mm hole (same as the shaft im using) to centre the stern with the shaft. The shaft was coated with silicon grease and positioned with the upper end also centrally supported within the hull. The hull to wood area was bunged up with plastecine to prevent leakage. I had drilled a 1/8Ē hole into the slots just below the linkages but above the tape. Then made a funnel and attached this to a small piece of brass tubing which I could insert into the hole. Using same concept as David M uses on his short prop shaft bearings I poured until epoxy flowed out of the slots above my tape. Bondo used to fill up the remainder of the slots, and also around the hatch cover to hide the joint.

The hull was prepped to accept the static pieces and two holes drilled into hull and control surface. I had pre bent brass staples which were inserted from inside the hull and helped to located and strengthen the control surfaces. To protect against knocks and groundings I had epoxied brass plates to the ends of the static surfaces, hope this is bullet proof enough as repairs will be a *****.

I drilled a 4.5mm hole provide some clearance on shaft then, and then a 6 mm hole drilled to accept the stern bearing.

01-29-2010, 04:03 AM
The rear bulkhead was then loosely CAíd into place using the motor mount and shaft to ensure the bulkhead was square. The motor assembly then removed and the joint made pressure tight inside with reinforcing cloth and epoxy. I havenít tried to work out what the pressure will be (due to piston tank) but I didnít want any surprises. Again copied from the merriman cabals I used bondo to tidy up the opening and hide the gaps (as you can see this still needs some work. Last bit of gluing here was to fit the Bayonet, left until last to give maximum access in to the stern, and the water inlet, close enough to break point to make removing the tubing routine but not in the way of any other workings. Finally I could re assemble, motor mount, servos and sealing Bellows. Then control arms 1/8Ē brass tubing complete with wheel collars which allow the push rod tubes to be connected to the 1/16Ē rod connected to the jumpers, permitting adjustment and removal.

Hiding the join
The OD of the bayonet ring is slightly smaller than the hull od, and so to hide the join I firstly coated the main hull ring with silicon grease, and then cleaned and prepped the stern ring, using abrasive paper. The hull was assembled and Bondo smeared over the joint. I sanded down the bondo, and after breaking the join, cleaned the face to face area. Then after cleaning and prepping the main hull ring, I coated the stern with silicon grease and repeated the process. Unfortunately some of the bondo did not adhere well despite my efforts. It is a very small contact area, and also the bondo either seems to be curing too fast, or not at all. In case I was contaminating the bondo with silicon grease I made sure only to work towards the silicon when applying the bondo but still having issues. After several attempts to repair some areas with no real adhesion, I tried an epoxy mix with micro balloons, this seems to be working better.

01-29-2010, 04:05 AM
which comprises Piston tank, Tank Controller, Battery, RX, BEC, APC and ESC. Unlike the usual German designs mine is separate from the motor drive, servoís and stern piece, I hope that it will be transferable between boats. The diameter of the piston tank and the inner diameter of the Engel locking ring only gives a small radial clearance, so I was forced to use 3mm stud to form the tech rack, not as rigid as 4 or 5mm that I had originally planned on. Now I better understand the workings I plan to make new bulkheads to improve the finish and will probably use 5 or 6 studs next time for added rigidity. I go to UK once a year to see family and save up my machining requirements for the father in laws lathe, so this job will be added to the list.

The initial wiring was connected and fired up, The BEC LED lit up and all looked good. Connected servoís they responded nicely to the TX. Powered down and connected the piston tank and controller. Switched on, tank controller LEDís blinked nicely, then without my operating it, the piston tank started a fill, arrgh. I moved the stick and it responded, but when I let go, it moved back in another direction arrgh. Spent the next hour or so checking my wiring, switching connections on the rx to see if that channel was faulty. It had been a long day, things werenít making sense, my piston tank had a life of its own. Science fiction movies about man-eating plants, cars and manikins all coming to life, yeah, Piston tanks nope. So put my toy away and sulked, Bombay Sapphire helping to erase the memory of machine overcoming man. The following morning I decided I should read the instructions, seeing as how I bought this well over a year ago, I had a distant memory of a pdf document being more like sales blurb than actual practical information. Doofus, these were Germans, who stick to zee facts. The Engel tank controller I have on my other piston, responds to left stick or right stick by either filling or emptying, let go of the stick it stops. I had forgotten this was not a Volkswagen but an S Class Mercedes Controller (no offence Engel), with proportional control. With the stick centered as default the Piston was heading for half full. Push stick all the way left, it fills, all the way right it empties, anywhere in between it will reflect that location. There was also a slight delay in responding to my controls, not a big issue but this was causing me more confusion when I hadnít understood the problem. I will need a new TX with a dial type knob for one channel. Otherwise it works like a dream and the lost signal trip works fine. Iíve been having a lot of issues lately with various electronic not doing what they say they will on the packet, and saw this was another problem I didnít need.

Also I realize the original battery has to go. The 12V gel cell seemed a good idea, Iím gonna need a lot of ballast in here and the gel cell wouldnít hurt on that account. However it fouls on the piston screw if too high and wont go through the hull hole if mounted too low. I found two 6V which mount really nicely, but with low AH. So will try these for now and investigate a better solution.

At the moment the tech rack is not fixed in the hull but can just be slid in so I can find the best location for trimming. The Koryu does not raise perpendicular but kind of pivots from the stern, due to its MBT being in the bow, so I hope to be able to achieve similar effect by shifting the tech rack around until I reach the optimum place.

The tech rack connects to the stern using a tamiya plug for power to the motor and the two servosís using extension cables. Lastly the water inlet/outlet, a rubber tube from the stern bulkhead is attached directly to piston tank.

01-29-2010, 04:08 AM
I now have four pieces, stern complete with servos and motor, main hull which is just a pressure cylinder, the upper hull which is free flood area, and Tech rack

The main pressure hull was assembled with tech rack placed inside as far forward as possible. Firstly I cycled the Piston tank to listen for hisses and or deformation of the hull due to pressure. Couldnít hear any, but then the piston makes some noise, I placed the hull into the test tank (bath, it just fits) and immediately saw bubbles on the seams. I placed some white tape along the hull next to the seams and used a marker to indicate the leak locations, just 3 points not too bad. I also added some lead into my hole at the bow and in a bag over the stern to get a rough idea of how much lead is required. That done there wasnít much more I could do so dried the hull and stripped it down, not much water had got in and the tech rack was dry  Wasnít a great test, but could have been a lot worse, phew

Weekend spent getting resin and cloth into the far end of a 5Ē tube, not easy, but hopefully found all the holes. Tried a quick test and no I hadnít, another hole appeared on the seam, so I ground out the crack and filled with epoxy, as the inside was already covered in cloth, so some how water was creeping past the cloth.

Also I noted that it was stern heavy in dived state, so I worked out the CP point for the hull and compared that to the CL of the piston tank. I found that I could not get the piston tank far enough up the hull and so the CL of piston tank was sitting about 3cm further back than the Sub theoretical CP. I re looked at the layout of the front of the tech rack, I had space in the battery area and also by relocating the piston tank controller I was able to reduce the tech rack by 5cm (2Ē), woohoo.

Re Assembled with two 1kg weights taped to the outside of the hull fore and aft of the tech rack. Placed it in the bath, I mean test tank, I then commanded a dive and yet again watched bubbles appear where bubbles werenít supposed to be, this time they were coming out between the stern plate and the hull arrgh. I had just added some vent holes in the stern free flood area, so maybe this had been leaking all along I just hadnít seen them before. After a few dive cycles I found I needed a total of 1.27kg in the bow and 1.68kg in the stern to achieve waterline. I believe it should be more than this as some water was taken on, so all up weight will be around 7 Ė 7.5Kg (16lb), not too bad for the length. Aside from this new leak it was a success with boat keeping an even keel in dived state. Now need to strip down the stern to reinforce the seal, and clean up my re wiring of the Tech rack in its shortened state.

01-29-2010, 04:10 AM
Spent some of Xmas in UK machining bits and pieces for my various boats, after returning home I got back to work on the Koryu, the Minister for finance and fun was still in UK giving me unlimited free time and use of work areas not normally allowed, kitchen, living room, plus being able to leave my tools out all weekend, leaving just enough space on the kitchen work top for a single food bowl.

I had been struggling to get CG further forward, hampered in part by a large lump of lead which had moved while the epoxy was setting and stopped the tech rack being fully inserted. The day after I had chopped up bits of an old dive weight and epoxied them in I had found a source for nice little flat sheets, but the damage was done already. It was still far enough down the hull to make reaching it with a screw driver to pry it out near impossible. However whilst in the UK, I had found my old super long snap-on screwdriver hiding in the shed, so first task was to attack the aforementioned piece of lead which didnít stand a chance, problem one solved.

Secondly I had been considering the fact that the original vessel had forward mounted ballast tanks which would make it rise at the bow. However, the current lay out of my Tech rack meant the piston tank drew in water to the rear of the tank, making the stern part dive first, not good.

So rebuilt the Tech rack using my newly machined bulkheads. I secured a cut off from a sheet of 5mm Lexan from a sign maker for a very reasonable fee. Cut into 4 90mm squares, then using a bolt through the centre turned them into nice round blanks. Secondly I found a source for SS studding to hold it all together, and lastly made up new trays for the electronics, the original battery tray looks a bit the worse for wear but is still functional.

The piston tank now has water inlet at the front, meaning I need to draw water in through the length of the tech rack, which I accomplished by using a Brass tube which IMHO looks quite neat. Also much less wiring is traversing the piston tank, only the main power leads and my RX aerial, altogether much neater, just need to see if I get interference. The revised layout means the tech rack is slightly longer, but the CG of piston tank moves forward 3cm.

A bit of final cutting was required on the third bulkhead to get the piston to pass through the bayonet. The hole in the bayonet is 87mm, the bulkheads are 86mm, not a problem, however the piston drive motor creates an offset of 3mm! That means that the tech rack needs to be offset as the motor part passes through the bayonet, hard to explain in words easier to look at photos.

Despite still using 4 pieces of 3mm stud for the tech rack, the shorter length means the whole structure is much more rigid

The forward racks now hold all the electronics, quite tight on space and a mass of wiring, however once I am happy itís all working ok I can shorten some of the wires, and reduce the spaghetti. The RX is very close to the piston tank motor, will this be a problem???
Re wired and on the bench it all still works, not a bad weekend Next step is to re assess the ballast requirements.

01-29-2010, 04:12 AM
Ballast Testing Stage 2 Ė With Chinese New Year Vacation fast approaching (4 days off), and still no management in the house, I wanted to try and get her seaworthy and ready for trials. Picture 1 as Ballasted so far, with empty tanks, photo 2, tank at 80% filled. The piston tank controller is set up so that at 20% of dial, I get 80% fill. The 20% - 100% of dial proportionally controls the tank over the range 80-100%. I was aiming for about 20-30mm of sail visible at 80% fill, this theoretically gives me scope for diving deeper or offsetting different gravity Ďseasí. Photo 3 the final lead locations.

01-29-2010, 04:14 AM
Well with the hull now pressure tight, I taped the upper hull on and finally took it out for a spin. Also its sporting a nice big Yellow duct tape flag.

01-29-2010, 04:19 AM
Ground back the glass on upper hull, and prepped main hull. Then epoxied the two together, left for a week to harden whilst on business trip. Then applied glass matting to
cover the joint, then filler over the matting and sanded down. I had cut back part of the upper hull to form a horizontal slot where the ballast tank would drain, unfortunately I should have cut it back further :(, might need a bit of time spent with rat files to make the slot more defined

Oh those cleats on the front, you can lift it by them!!!!

01-29-2010, 04:21 AM
Lost a lot of the weld lines during work, so I masked up troughs where the welds are, and then dabbed thick paint into the trough and allowed to dry, rinse and repeat until it built up. Then sprayed a thin coat over and lightly sanded so they werenít quite so angular.
Also whilst leaving the hull drying one of the cats knocked against it. Wouldnít have been too bad except it was upside down at the time, and with all that lead in the lower part of the hull it flipped right way up and mashed up part of the rudder. So small repair was in order

01-29-2010, 04:24 AM
This is where we are at today. Made my own flag transfer, and have a small amount of rust starting to show. Not sure how far to go with weathering on this. Did these things get beat up at all??? Buying myself an airbrush and compressor for birthday next month  so maybe I can beat it up a bit with that.

Still need to finish the canopy area, and have plans for tech rack Mk2A featuring a trim shifting device.

Next job tho is a bit of sailing

He Who Shall Not Be Named
01-29-2010, 08:07 AM
Now, THAT's the way to present an entry into this contest. Well done sir!

I love that little boat ... must have .... one .... NOW!

Could use a bit more weathering, but who really knows -- as far as I know none of those boats made it to the Fleet. Below are some shots of a Bob Wallace 1/16 KAIRYU I assembled and weathered a number of years ago:


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01-29-2010, 08:05 PM
Thank you, praise indeed. I remeber seeing your Kairyu build and the weathering. I would like one of those too. the white/grey streaking looks very effective. I must dig out the Cabal and have a read

08-05-2010, 09:11 PM
As with Thresher, need some natural sunlight shots to make this look , like it really does

He Who Shall Not Be Named
08-13-2010, 08:38 AM
Very, very good job there -- a vast improvement over your initial THRESHER weathering effort.


08-14-2010, 11:14 AM
Dave, Your KAIRYU is your camera ship isnt it?

He Who Shall Not Be Named
08-14-2010, 03:14 PM
Dave, Your KAIRYU is your camera ship isnt it?

Yup! Here are some shots of the camera set-up:


10-01-2010, 07:34 AM
Where did you get your Ballast tank Albion? I am facing broblems on building my submarine since, i want to use a similar structured ballast tank as you are using.
Any help is appreciated!

10-01-2010, 09:21 AM
Where did you get your Ballast tank Albion? I am facing broblems on building my submarine since, i want to use a similar structured ballast tank as you are using.
Any help is appreciated!
will pm you with gerhard email address.