Stranger needs advise

[cweeks]

Hello all,

I am a modeler of some experience, none of it however with subs, I have a friend who has built a model of Erich Topp's sub and wants some help. It currently dynamically dives (poorly) and he want me to help him make it dive statically.

You will have to forgive me if I miss important points in this post but I don't have a large nautical bent. (Awaiting double entendre )

The boat is around 6-7 foot long and has a stainless steel tank with a screw down lid as the pressure vessel. As there isn't room to get a ballast tank in the pressure vessel and as it's owner is very keen not to abandon the pressure vessel due to it's cost. I was thinking about creating two self contained piston tanks in the bow and stern of the boat, is this a good idea ? I gather two separate tanks can be a nightmare unless they are piston tanks. If so what material do I make the bore of the piston tanks out of ? I have tried Plexiglas but it was very eccentric to the order of around 30-60 thou, thus making a working piston fit tricky. If possible I would like them to be transparent.

Or am I better to bin the stainless pressure vessel and re-design around one central ballast tank ?

The hull is glass fibre, and was made from scratch, I have very roughly estimated the hull displacement above the water line to be 400ml.

Any help appreciated !

[anonymous]

Hi,

I take it there is

Hi,

I take it there is no space at all in the WTC? If you could fit a ballast tank in there it could use some form of pumped water system with the advantage that it could be positioned close to the centre of gravity of the boat.

However using two piston tanks would be perfectly feasible and is a commonly used commercial system coming from Germany. It could be added to the existing tank relatively easily, perhaps.

Making pistons can be tricky and another DIY variant of the piston idea would be to use flexible bellows.

Email me and I can send you more information on the various systems.

Davy

[cweeks]

Cheers Davy, message sent, and

Cheers Davy, message sent, and no there isn't much room in the WTC, I will have another look at it just to be sure

The problem with the commercial piston tanks I have seen is they are intended to go inside the WTC, there certainly isn't room for that hence considering dumping the existing WTC. Also my twin piston tank idea needs 2 small tanks that go outside the WTC, again the commercial ones would be to big especially once I had made them water proof. Hence thinking I would make my own tanks. So I would still love to know, even if it's just out of curiosity how people are making home brew piston ballast tanks.

Cam.

[Guest]

I've been experimenting with making

I've been experimenting with making piston tank tubing.



I laminated an 80mm GRP tube over a wax former (used candle wax). The tank shown is good for about 1000ml. i have another to make for my Vanguard model.

I made a mandrel from aluminium tubing which is 60mm diameter. I then melted wax over this former using a containment tube, and once this had set, I turned it down on my lathe to 80mm diameter.

I then gelcoated this, one layer of glassfibre tissue, two layers of 300 gram chopped strand mat, a further layer of tissue top coat and then filled with polyester filler to achieve a fine outside finish (not necessary but it looks nice). This was then turned up on the lathe, and the finished laminate is about 3mm thick.

I let this cure for about a week, and melted the wax out at about 90 degrees c.

The inside was very smooth, a few blemishes where my wax surface wasn't quite perfect.

I cleaned this out with various grades of wet and dry and then polished it with faraclea compound.

The finished bore appears to be fairly accurate- within 10-20 thou or so.

I'm using airzet seals to seal the piston. These have a rubber spring moulded into the seal.




I'm exceeding the dimensional design spec of these seals, but they are designed for air rather than water, plus I'm using them at a much slower speed.

We'll see how it works in practice.

BTW a lot of German modellers use seamless aluminium tubing. This is excellent stuff, and they obtain it from surplus toner catridges from large photocopiers.

I can't get it very easily, hence I grow my own. Plastics are better in the long run- zero risk of anodic corrosion.

Andy

[cweeks]

Never considered a GRP tank,

Never considered a GRP tank, certainly worth consideration. Thanks also for the tip on seals and photocopier bits, where do you obtain the airzet seals from ?

I did consider using my mill to vertically bore the Plexiglas tube to make it concentric but stopped on the thought that there had to be an easier way, hence the post. Granted I could also do it in the lathe but I would need to manufacture a running steady, again more complication.

[Guest]

Yes I understand the problem.

The

Yes I understand the problem.

The GRP tube is a bit of a fiddle. Tufnol supply filament wound tubing for compressed air application with a very accurate and smooth bore (designed for moving seals). It is rather pricey though. 50mm I/D tubing worked out at about £50 a metre, and you had to order a minimum of 2 metres-

http://www.tufnol.com/tufnol/default.asp?id=85

I got the airzet seals from a fella in Germany. You can purchase them over here too, but they're not cheap by any means.

An o-ring works very well too, slightly higher in friction.

The other alternative to making big piston tanks is to build a hybrid system. Make a small piston trim tank and have you main ballast tank using either a water pump or compressed air.

A small tank can be made very easily. Sources for good quality small bore tubing can be the following-


1) Caulking or silicone rubber gun tubes
2) Toothpaste tubes (the pump action type)
3) Old toner cartridges from laser printers or fax machines contain small bore aluminium tubing, high quality.

Andy

[cweeks]

Thanks Andy, valuable information.

I did

Thanks Andy, valuable information.

I did consider a hybrid system but (without upheaval) the water/air tanks would again need to be a pair and outside the WTC, wouldn't this cause problems ? I could imagine it would in theory, but I have no experience of the practice.

[Guest]

If you can only fit

If you can only fit twin tanks front and rear, then piston tanks are the only thing practicable.

I've seen other systems with twin tanks and they are a headache to trim.

Making these tanks proportional is a good idea. Premade boards are available for this, based on magnetic encoder principle. They cost about £50 per board, and you will need two.

The boards include MOSFET control of the motor, a failsafe and water detector. They're very good value.

If you're interested in these I'll pass on details on how to obtain some.

With twin tanks under proportional control, you not only have control over the vessels bouyancy, but control of the longitudinal trim.

This is very valuable in a model sub, as it enables you to adjust the trim of the boat to suit varying water conditions from the lakeside, without having to engage in messing about with bits of lead and foam etc.

You need to design the tanks to be 80% flooded when at neutral bouyancy. i.e. for 600ml of lift you want 750ml of displacement in your tanks.

This gives bit of downthrust if required, and also allows for a shift of longitudinal trim, fore and aft.

Andy

[cweeks]

Well I think that's all

Well I think that's all my questions answered, at least for now Further details on the boards you mention Andy would be greatly appreciated.


Thanks again, Cam.

[Guest]

Please PM me your email

Please PM me your email address and I'll forward you some details in PDF format.

Cheers,

Andy

[cweeks]

PM sent.

PM sent.

[scott t]

Since you are building a

Since you are building a piston tank here is a Rube Goldberg design tank.

http://subcommittee.com/SubComm/images/photos/piston%20tank.JPG

The idea is to remove the jack screw on a typical piston tank which takes up
valuable space as it extends.

[Guest]

That system was adopted by

That system was adopted by Ron perrot many moons ago.

Increases the friction of the tank (which is already quite high) by quite a margin.

Plus the potential for another seal to fail........

Andy

[cweeks]

I did rack my brains

I did rack my brains to try and find a simple way to eliminate the extending screw jack and thus save space, but the mechanical genes in me feeling are little queezy after seeing that design, one word springs to mind unreliable.

Cheers for the email Andy, that controller looks like a very nice bit of kit.

I will now have a word with my friend, and explain the options.

Cheers, All.

[Guest]

Excuse the crude paintbrush drawing

Excuse the crude paintbrush drawing (I'm not much of a draughtsman I'm afraid).

Here is an idea-



The thread remains fixed with the piston running up and down the threaded shaft like before. However instead of sealing the thread, you cap it off with a watertight tube that is attached to the piston and reciprocates backwards and forwards with the piston.

Only downside is that you gave to fit a microswitch in the wet area, but that needn't cause too many problems.

Andy