Ballast issues - Uboat MKV11c

[mark v11c]

Hi all.
I am brand new to this so please excuse me!
I've picked up an old Darnell MKV11c from ebay and I love it. She was listed as a static diver, but I don't think this is true as she has to be surfaced to take on air to pump out the tanks. Motors, pumps and all running gear is intact and working - if a little rusty in places.
As you'd expect, it is a narrow vessel and the stern is cluttered with push rods for the rudder and elevator - as well as the dual props. The owner had custom built three coffin-shaped ballast tanks that squeeze in around the various rods, but when I tested her in the paddling pool one of them leaked and she listed heavily to port as a result.
My question is - if I dispensed with the air intakes secreted in the conning tower (and which obviously take on the air - but only when partially surfaced) and tried to just pump water into the tanks thereby compressing the air inside and hopefully sinking her - would this work? I think I would need non-return pumps (if there is such a thing) or maybe a servo-controlled return valve. On top of this I would need ballast tanks that don't mind being pumped up and will not explode.
Any thoughts? Or am I best just to stick with the snorkels and mend the stern ballast tank? I have to admit that I warm to the cool of the static diver!
Many thanks for any thoughts.
Mark

[Guest]

Re: Ballast issues - Uboat MKV11c

That's not the original Darnell system, the previous owner has done their own thing. The original Darnell system used a small steam pump, which moved air from flexible bags to a plastic bottle/reservoir mounted in the forward compartment.

A lot of people modified the basic system by getting rid of the steam pump, and fitting a small air compressor, the type used to inflate car tyres from the cigarette lighter socket. This improved dive times.

Darnell models have also been adpated to work with pumpe water ballast systems piston tanks etc.

There is no one 'perfect' ballast system, they all have pros and cons. Some are less expensive than others, some need special tools to build etc. etc.

The system you are proposing is the same as that used in the Sheerline range of model submarines. Water is pumped into a sealed tank, and the air compresses above it. A pinch valve prevents the water shooting back out of the pump when you stop pumping. This system is simple and inexpensive, but there are a few things you need to consider. First of all the tank must be completely airtight and of sturdy construction to take the pressure. Some people fit an auxialliary reservoir to help keep the pressure inside the main tank within reasonable limits. Aerosol tins are good for this, as they're built to take pressures much higher than we will require. If you retain a single tank, then a piece of PVC or acrylic pipe is very suitable with plastic endcaps bonded at each end, and held together with three or four threaded rods.

As the tank is only partially filled, it must be baffled to avoid the water sloshing about.

The pump you choose must be capable of the reaching at least 40psi if you're to achieve a reasonable fill of the ballast tank. Geared pumps work very well, although their flow rate tends to be on the slow side. Kavan produce a good quality pump suitable for your size of boat. They cost about £25-30 (try ebay) new.

[mark v11c]

Re: Ballast issues - Uboat MKV11c

Thank you for your prompt and full-some response. Most helpful.
As you will be aware, I am very new to this and am beginning to realise a long-held ambition. I have been visiting the U-Boat pens in Lorient and St Nazaire frequently for the past few years and am absorbed by this part of history so naturally wanted my own detailed model. When an actual working one came up on Ebay I was delighted and have set about trying to understand its mechanics. It's nicely made and painted up as returning to base.
I only think it's a Darnell because the previous owner said so - however he didn't have it for long as his wife told him to get rid of it to make space, so he could be wrong. (I have an image of this boat being passed around from bloke to bloke across the UK as their wives get fed up with it cluttering up the place!)
There is a central, rectangular waterproof control centre about 12" long and the width and depth of the hull that contains all electrics, powered by a 6V motorbike battery - so plenty of juice. There are two water pumps - one for'd and one aft - which pump water in and out of the ballast tanks. The forward tank looks like a 900ml pvc standard model fuel tank and the rear ones were hand made to fit around the steering, prop and hydroplane gear (all of which works nicely). However the 3 aft ballast tanks are bits of sheet plastic cut and glued together (the side ones look like coffins) and will not withstand pressure. I know cos I already tried! There is a safety cut-off system in case you lose contact underwater, but I do not see how this can possibly work unless you have an air supply on board - either compressed in the ballast tanks as you describe (and as I hope to achieve) or from a separate reservoir.
I wondered about looking at an r/c aircraft undercarriage piston which is basically a pressurised cylinder which pushes landing gear up and down. The beauty would be that you recharge it with a bicycle pump and it holds lots of compressed air - so cheap to maintain and plenty of dives per charge. The drawback that I can see is actually fitting the cylinder into the waterproof control box as the hull is already quite cramped. A nice idea though.
Sorry to ramble, but one last question. Can you get non-return 6V water pumps so that once water is pumped into the ballast tank it doesn't just seep back out again? Or would I - as you seem to suggest - need a separate valve for that?
Your advice is very gratefully received.
Mark

[Guest]

Re: Ballast issues - Uboat MKV11c

To address your questions.

Can you get non-return 6V water pumps

You can make any pump non return by putting a one way check valve in line. Problem is, how do you pump the water out again?

Peristaltic pumps are non return, and pump bi-directional. The snag is they're very slow, so really not well suited to a boat like a Type VII with large ballast tanks.

An r/c aircraft undercarriage piston which is basically a pressurised cylinder, recharge it with a bicycle pump and it holds lots of compressed air.

Not as much as you'd think. Maybe one or two blows of the tank at most, and a lot of hard work pumping it back up again.

The ballast system you have fitted into the boat sounds somewhat Heath Robinson (Ruby Goldberg for the American readers) to me. If you want a reliable dependable sub, it sounds like you need to remove this, and build a new one based on a single central ballast tank. This needn't be expensive, as you can reuse most of the components you have, like the pump(s).

As you are a member you have access to the back catalogue of Sub Committee reports. I realise wading through all the columns looking for the information you need maybe difficult. To make life easier I have collated a few of the articles relevant to ballast systems into a single PDF, which can be viewed on the computer or printed off. I can send you this file if it's likely to be of benefit.

[mark v11c]

Re: Ballast issues - Uboat MKV11c

Thanks again. All very useful. And yes - Heath Robinson is exactly the right description!
If you could send me the file I'd be hugely grateful.
KInd regards
Mark

[Guest]

Re: Ballast issues - Uboat MKV11c

Okay, I've sent a link to the PDF to your email address. Hope you find it useful.

[chuck chesney]

Re: Ballast issues - Uboat MKV11c

'Mornin', Mark11c.
Just a comment regarding what Sub culture said....you will be hard pressed to find a more experienced or helpful guy than Sub culture. His posts are always well thought out, and based on many years of "hands on", actual working knowledge. Both of you are Brits, so you have a first rate resource right there at home. Sub culture can save you vast amounts of time and money while helping you to cut through many of the questions that you will have as you enter this arcane world of model submarines. Good luck, and smooth sailing !

[mark v11c]

Re: Ballast issues - Uboat MKV11c

'Evening' Chuck. And cheers - your post is much appreciated. He has already been immensely helpful and made me realise what a botched job my MkVIIc actually is! I think I may have to gut her and start from scratch - albeit reusing existing motors, servos, linkages etc. So much for taking her out on her maiden voyage on New Year's Day!!
I feel a project coming on! Hey ho.
All best wishes to SubCommittee colleagues over the pond.
M
ps But I still love her!!

[JWLaRue]

Re: Ballast issues - Uboat MKV11c

You can make any pump non return by putting a one way check valve in line. Problem is, how do you pump the water out again?

The way I have solved that problem is to use a solenoid-actuated (reversing) valve. The one I use is by MAC. I have both the pump and the valve connected to a SubTech SES switch....which is then controlled by a 3-way switch on my transmitter. You'll need to add a diode to one leg of the SES connections to the valve so that it is only actuated for one of the two operating/switch positions.

An r/c aircraft undercarriage piston which is basically a pressurised cylinder, recharge it with a bicycle pump and it holds lots of compressed air.

Not as much as you'd think. Maybe one or two blows of the tank at most, and a lot of hard work pumping it back up again.

The assumes that the air from the pressurized cylinder is vented to atmosphere when the ballast tank is filled with water. The air could be cycled back into the cylinder to create a closed ballast cycle.

-Jeff

[mark v11c]

Re: Ballast issues - Uboat MKV11c

Thanks Jeff. So you have an enclosed ballast system, just pumping water in and out under pressure? That's what I want mine to do rather than use its existing snorkel arrangement.
Your solenoid solution looks promising. I'll look into it further.
What do you use for ballast tanks?
All best wishes
M

[mark v11c]

Re: Ballast issues - Uboat MKV11c

Sub Culture - many thanks for the PDF file. Extremely useful.
Cheers
M

[JWLaRue]

Re: Ballast issues - Uboat MKV11c

Hi Mark,

For the ballast tank I use the same blood pressure cuff 'bags' that are commonly used for the RCABS systems. I use Robart tanks as my pressure reservoirs so that I do not need to build the dry space (dive module) to hold the over pressure. The plumbing for the system does, of course, run through the dive module.

-Jeff

[Guest]

Re: Ballast issues - Uboat MKV11c

Where are you based in UK, Mark?

Also do you have workshop equipment, or access to such things as a pillar drill and a lathe?

You can get by without these, but it makes life easier if you do have access to them, and know how to use them.

[mark v11c]

Re: Ballast issues - Uboat MKV11c

Thanks Jeff. Duly noted. I am looking at various options now, having browsed further into the site. The sealed tube seems the most popular - but expensive. I have to say the wealth of pictures and debates in here is astounding. I am going to have to be very careful and take guidance from you more experienced hands.
All the best
M

[mark v11c]

Re: Ballast issues - Uboat MKV11c

Sub - I am based in Dorset - near to Ringwood (Hants). I have a pillar drill (well a bench one anyway) and could access a lathe if I needed to.
I am pretty handy with tools but am entering a whole new and exciting world here. At least I have a decent hull, motors, pumps, electrics, radio etc and I know she sails as I've taken her on my kids' old paddling pool. Submerging, however, is a different matter and kind of what it's all about! But I love a challenge and I kind of need a project to allay the pressures of work - so this is ideal.
My aim is to join you UK guys later in the year at a meet! (St Albans is not too far. Also, my family all live in Kettering so Wicksteed's is really easy.)
Lots to do before then though. My first challenge will be to remove the old WTC - which is resined in - without damaging das boot.
I will definitely be back in here for advice along the way. A tenner well spent in my view!
Happy sailing all.
Mark