A Skipjack in Ireland

[The Boattrainman]

Hi Guys,

I'm working off the following assumptions.

The biggest requirement to sink the sub will be getting the WTC itself below the surface as its filled with air. The volume of the WTC with no equipment inside is approx 3,300ml, and taking the accepted view of a flooding cell of 10% - 15%, this puts the 450ml piston tank at 13.5% which is right on the money. Factor in a reduction of air due to the tech rack and equipment and that percentage will be even higher ,which should cover the hull and sail above the water line, so I'm not anticipating a problem.

I have room for a 500ml tank from Engel (just about, very tight fit), I'm hoping the 450ml will give me a bit of wriggle room, and I want to try to get a tank that fills to capacity for a 1 metre dive depth, as I've managed to centre the piston tank very well along the centre of gravity of the sub so far.

Rob

[He Who Shall Not Be Named]

Hi Guys,

I'm working off the following assumptions.

The biggest requirement to sink the sub will be getting the WTC itself below the surface as its filled with air. The volume of the WTC with no equipment inside is approx 3,300ml, and taking the accepted view of a flooding cell of 10% - 15%, this puts the 450ml piston tank at 13.5% which is right on the money. Factor in a reduction of air due to the tech rack and equipment and that percentage will be even higher ,which should cover the hull and sail above the water line, so I'm not anticipating a problem.

I have room for a 500ml tank from Engel (just about, very tight fit), I'm hoping the 450ml will give me a bit of wriggle room, and I want to try to get a tank that fills to capacity for a 1 metre dive depth, as I've managed to centre the piston tank very well along the centre of gravity of the sub so far.

Rob

Ain't going to cut it. And you're calculating from the wrong end to find required ballast tank weight/volume.

Its the displacement of the above waterline structures, not the weight of those structures.

And your WTC is already immersed in surface trim, so is not any part of the 'reserve buoyancy' calculation. It's not the weight. The thing could be made of depleted Uranium or duck feathers ... don't matter as all of that was countered with the floatation you put in below waterline when you established submerged trim. Its the weight of the water displaced by the structure that matters. A neutrally trimmed object displaces a weight of water equal to the weight of the object; buoyant force equals weight of object -- net vertical force, zero!

(Archimedes, call your office!)





I found it takes about 25 ounces of ballast water weight to get the model to surface to designed waterline. The 450ml piston tank ain't go'n to cut it (15 ounces of floodable volume!? .... Bzzzzzz!). That's why the video of Arkmodel's SKIPJACK shows it sitting so low in the water when in surface trim. Not enough ballast tank. His piston pump does not have the ass to push the boat up to designed waterline. (What was Feng thinking, anyway?!)



Our 1/72 SKIPJACK SD has a ballast tank that holds 25 ounces of water and will get this beast up to the designed waterline.





This is what a properly trimmed 1/72, Moebius SKIPJACK looks like on and below the surface:

https://youtu.be/7aaAVtlnbVM

David

[The Boattrainman]

Dave,

I bow to you superior knowledge on this.

So, it;s back to Engel equipment, as it happens they have a 750ml piston tank which equates to 25 Fl Ozs, but it's just not feasible. They'll custom build a tank, so I'll get the measuring stick out and see what I can get into my WTC, I'm thinking 600ml might be my absolute limit................................


Rob

[He Who Shall Not Be Named]

Dave,

I bow to you superior knowledge on this.

So, it;s back to Engel equipment, as it happens they have a 750ml piston tank which equates to 25 Fl Ozs, but it's just not feasible. They'll custom build a tank, so I'll get the measuring stick out and see what I can get into my WTC, I'm thinking 600ml might be my absolute limit................................


Rob

Sorry to rain on your parade like this, Rob. But better to give you a steer now than watch you roll off the cliff later.

The three liabilities of a horizontally sitting piston-pump: The less than ideal use of available internal WTC volume for the ballast sub-system (a problem shared with bladder type sub-systems); A significant load on the battery; and the c.g. shift as water is taken off/on. The nice feature is the ability to hover the boat (an over-rated feature in my book).

David

[The Boattrainman]

Hi Dave,

You're not raining on my parade at all, delighted to get expert feedback from you and the other guys, I very much enjoy the banter here.

If it's a compromise this time on being a partially dynamic/partially static diver, then so be it, this build is very much a proving exercise in what I can scratchbuild, as I'm already looking at the next project which will be much more expensive than the 64.99 Euro I paid for the Skipjack kit!

Anyway, here is a couple of shots of the flap that will hold the battery in place.





Keep those thoughts a coming.

The Boattrainman

[He Who Shall Not Be Named]

Very good. The nice thing about the WTC concept (wet-hull with removable system) is that if you get the ballast water amount wrong the first time, you can revisit it later with a different or modified WTC that corrects the problem.

I'm very much enjoying the high level of craftsmanship of display as I read your WIP. You're taking us all back to school with some of this stuff. Most informative, sir.

David

[Albion]

Sorry to rain on your parade like this, Rob. But better to give you a steer now than watch you roll off the cliff later.

The three liabilities of a horizontally sitting piston-pump: The less than ideal use of available internal WTC volume for the ballast sub-system (a problem shared with bladder type sub-systems); A significant load on the battery; and the c.g. shift as water is taken off/on. The nice feature is the ability to hover the boat (an over-rated feature in my book).

David

During transition from surface to submerge with a single tank you will get some weird angles. I have a single tank in my koryu but due to the fact it sits at an angle in surface trim the descent doesn't look too funky. I did put a ballast shifter on mine so that i can hover the boat ;), and trim it to suit the hover point. All a bit irrellevent as i need to secure that module inside my hull, if i raise to quickly the module moves and i end up with a permanently porpoised Sub :(

https://forum.sub-driver.com/forum/b...ighlight=koryu

This is a very interesting build and waiting to see how it goes next. i also suffer from not having toys like lathes available all the time and having to improvise

[The Boattrainman]

Hi Albion,

I'm hoping that the Pitch Controller will help with keeping the sub on the level when diving (when under way), as I was forced to mount the piston tank with the cell flooding from the aft on dive.

Just saw your thread on the ballast shifter, very good, Norbert Bruggen would be impressed!

Rob

[The Boattrainman]

A main power switch with extension arm (1.5mm brass rod) is made from an old model railway switch and Plasticard mount, a 12V mini LED will act as Power On indicator. I always have an indicator beside the main switch as this is helpful to know the electrics are 'live' when working on them, especially as the model won't be fused, I don't think fuses and subs are a good mix.

I'm hoping this will fit at the top rear of the tech rack.






The Boattrainman

[The Boattrainman]

The caps are filed with a round file where the inner O Ring will sit, to create a U-shaped groove for the ring to sit in. This job was a pain in the butt, as just a few microns too heavy and the gap between the inner surface of the WTC and the O ring could be ruined. A job not possible when the cap parts are all separate as you can't file a U-shape on an edge.

During this process I thought a compression rate for the 3mm O ring of 33% (i.e, 1mm for 3mm depth), it's more light 5-10%, hence a lot of filing of the inner space, took around 3 hours to carefully file the excess.




The inner and outer O Rings on the forward cap, there is a compression gap at this end between the end cap and the tech rack, to give room for the outer rings to be squeezed tight.




The forward cap gets a fitting to hold it to the correct axis as per the bow cap (threaded rods at 12, 3, 6 and 9 o'clock), lining up with the horizontal of the tech rack. It just a scrap piece of 3.2mm styrene but with all rounded ends so the fitting slips easily into place on the rack. It also holds the battery holding flap down.





So confession time, I made a stupid mistake with the size of the inner O ring. As sized to fit on the 80mm inner disc, at 80mm cross section it is perfect, but when you sand the inner disc as above it reduces in diameter and the 80mm O ring is now a sloppy fit and too big.

Lesson, when ordering O rings, buy them 2/3mm under sized in inner diameter to be a tight fit and allow for a bit of fettling.

The Boattrainman

[trout]

use teflon tape, wrap the channel 5 or 6 times (or as many times as needed). You could also take some marine epoxy and run it in the channel. Keep the endcap on a motor or drill (slow) until the epoxy firms up. That prevents sagging or thick areas. Either of these will take up that slop.

[HardRock]

Trout, I get the impression that the Boattrainman might be a tad more anal than that. I suspect that if it isn't perfect, he'll start again and make it so.

[The Boattrainman]

Hi guys,

Prob didn't explain that right, it's the O ring diameter that's now wrong, by dropping it down to 75mm cross section it will be a tight fit again. I over-estimated the amount you can compress an O ring, I though by leaving 1mm excess all round the join the inner face of the WTC would squeeze the O-ring flat, but not so.

The compression rate of the O ring appears to be very small, hence the filing of the U channel to reduce the excess to around 0.2/0.3mm. As I said very tricky the only job I didn't enjoy on this build, as over-file the channel and the there would be a gap between the O ring and the inner wall of the WTC (then I would need the teflon tape) . I estimate the channel gives room of 2.7/2.8mm, so a 3mm thick O ring is still perfect, just a different diameter.

Sounds more complex than it is!!!
.

[trout]

Sounds like you have a plan already and have it worked out.
Not to counter you, but you can get a very good compression of o-rings. They will need to have room width wise in a channel (for me, a flat bottom or square channel works easier) to spread when it it getting smooshed (not a real word). I see you are using a o-ring at the end and the tube is compressing that, correct? Then my next suggestion will not work, but you need a bevel on the inner edge of the tube to help the compression process.

[The Boattrainman]

Hi Trout,

It's a learning curve for me, I get your bevelled rim idea, the next stage will be testing the integrity of my inner and outer O rings when the revised diameter ones arrive.

Rob