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Author Topic: How big a ballast tank do I need  (Read 3669 times)

Ian Robins

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How big a ballast tank do I need
« on: January 02, 2007, 10:56:42 pm »

Hi all,
I am attempting to build a 1941 japanese mini-sub.
I want it to be static diving with piston tanks.
Is there a way of working out the capacity needed in the ballast tanks.

IE. could it be 5% or 10% of internal volume??

I am about to measure the models volume and will post this volume later

Thanks
ro88o
NDMBC
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Mankster

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Re: How big a ballast tank do I need
« Reply #1 on: January 03, 2007, 01:35:14 pm »

The minimum volume for your ballastst tank for a static dive = the volume (not weight!) of all the hull and other internal structures (such as the top of your WTC possibly) that will be above the waterline when the sub is surfaced. So building your top hull out of thinner material and keeping the top of the WTC below the waterline will allow you to get away with a smaller piston tank. keep all foam you may use below the waterline, or you'll end up needing a bigger ballast tank. I normally aim for 30% extra volume in the ballast tank for reserve buoyancy to pull away from weeds/ overcoming a small leak etc. Easiest way to work it out is to mark the water line on the sub, turn it upside down and slowly imerse it in a brimmed tub of water down to the waterline. The amount of water displaced will be the minimum size of the ballst tank.

dreadnought72

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Re: How big a ballast tank do I need
« Reply #2 on: January 03, 2007, 04:45:31 pm »

Just for completeness (Mankster's answer is excellent) if you know the surface displacement and the submerged displacement you can work it out. Take an Ohio-class sub for example (can't find much online about minisubs):

Surface displacement = 17033 tonnes
Submerged displacement = 19000 tonnes
Difference = 1967 tonnes

At, say, 1/72nd scale:

1967 tonnes = 1967000/(72^3) kg = 5.27 kg = 5.27 litres of fresh water.

Andy
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gantu

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Re: How big a ballast tank do I need
« Reply #3 on: January 03, 2007, 05:50:27 pm »

Hi all,
I am attempting to build a 1941 japanese mini-sub.
I want it to be static diving with piston tanks.
Is there a way of working out the capacity needed in the ballast tanks.

IE. could it be 5% or 10% of internal volume??

I am about to measure the models volume and will post this volume later

Thanks
ro88o
NDMBC

Hi ro88o,

wich kind of the 1941 japanese mini-sub you want to build? May you post a pic?

Gantu
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Subculture

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Re: How big a ballast tank do I need
« Reply #4 on: January 03, 2007, 06:55:39 pm »

Good luck comparing 1:1 with a scale model regarding displacement. It can give a VERY rough idea, but only if the boat completely apes the originals pressure hull and doesn't feature many free flooding areas.

Andy

Just for completeness (ro88o's answer is excellent) if you know the surface displacement and the submerged displacement you can work it out. Take an Ohio-class sub for example (can't find much online about minisubs):

Surface displacement = 17033 tonnes
Submerged displacement = 19000 tonnes
Difference = 1967 tonnes

At, say, 1/72nd scale:

1967 tonnes = 1967000/(72^3) kg = 5.27 kg = 5.27 litres of fresh water.

Andy
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sheerline

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Re: How big a ballast tank do I need
« Reply #5 on: January 03, 2007, 11:52:56 pm »

Try this as a rough guide: If you can roughly work out the weight of the upperworks of the hull which you intend to be above the surface when fully blown, this will be the weight of water your tanks will have to take on to make the boat sink (almost) or at least be at neutral buoyancy. You can now do a calculation to find the tank volume based on a water figure of 10lbs per gallon. The next bit sounds daft but bear with me!
Once you have the theoretical tank volume, increase it by something like 5%. this will make the boat too buoyant so then kill off the extra buoyancy with additional lead ballast to bring the boat back to the correct waterline. What you will achieve by this is to create tanks with a bigger range which will allow the boat to actually sink and surface to your intended waterline.
I hope this does'nt come across as too long winded.. It's late and I am Knackered!!
Regards....... Chris
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Subculture

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Re: How big a ballast tank do I need
« Reply #6 on: January 04, 2007, 01:01:55 pm »

I don't go by weight, I go by displacement, volume is all that matters when it comes to calculating the size of your ballast tanks.

The weight of your materials affects the metacentric height of the boat, the volume/thickness affects the size of the tank.

For instance, if you have a 0.5mm etched brass deck on a Type VII u boat, the deck will weigh approximately four times more than a Type VII fitted with a 1mm ABS plastic deck. This is because the molecular weight of brass is roughly 8 times that of water or ABS.

Now which will require the bigger ballast tank assuming they both adopt the same surfaced waterline?

Answer is- the plastic deck boat will require a bigger tank, because that deck will displace twice volume of water of the brass deck.

However be careful with top weight, as it will reduce the metacentric height, which can lead to an unstable boat underwater.

Andy
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Turbulent

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Re: How big a ballast tank do I need
« Reply #7 on: January 04, 2007, 02:29:32 pm »

I tend to weigh the Top half of the hull & allow a % for fittings, say, 1Kg, this equates to 1litre of water.

Not sure how scientific it is, but it works for me!

Subculture

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Re: How big a ballast tank do I need
« Reply #8 on: January 04, 2007, 04:20:32 pm »

The weight method tends to work because many plastics (notably ABS and styrene) tend to be around the same density as water, however it is inaccurate and misleading to refer to weight as a factor when deciding ballast tank volume. You could quickly run into problems if you build yourself a boat from sheet metal, or wood. Even GRP can give surprises.

In actuality, GRP laminates tend to be considerably denser than water- upto 1.6 or more owing to the glass content of the reinforcement.

There is only so much you can do with a paper and pencil. Ultimately as most subs are built to a wet-hull principle, trying to calculate the required volume is a bit of a nightmare, and likely to require a significant fudge factor, especially if the boat has an elaborate shape and/or lots of fiddly fittings.

Ultimately, it's best to make a few rough calculations, then build the boat and the dive module/wtc/pressure hull get it in a test tank and find out the volume of the boat at surface trim and submerged trim and subtract the difference, that will tell you very accurately what size tank you need, and it isn't difficult to do either.

Andy
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Mankster

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Re: How big a ballast tank do I need
« Reply #9 on: January 04, 2007, 09:18:01 pm »

Yes its the volume/displacement thats important, and its not just the hull; often you will find that some of your WTC will be above the waterline, and you'll need to measure that too. Thats why dry hull subs need a huge ballast tank compare to wet hulls despite the hull weighing the same.
Another misconception is that when you fill your ballst tank to dive you are weighing you sub down by the amount of water you take on board. In fact you are not changing the mass of the sub at all, what you are doing is reducing its buoyancy by reducing the volume of air in the ballast tank. (the mass of the sub on the pond side is the same as when it is submergered and all the free flooding area are full of water). To get to a point of neutral buoyancy from the surface, you need to reduce the buoyancy of the sub (reduce the volume of air in the ballast tank) by the same amount of positive buoyancy exerted by the top hull as it enters the water (the volume of the top hull). And buoyancy of the hull is independent of its mass. 500cm3 of lead has the same buoyancy as 500cm3 of foam. So it doesn't mater if you build you sub out of lead or foam, as long as the volume is the same the size of the ballast tank will be the same (obviously the lead sub will need more buoyancy to get it to float but the size of the ballast tank to get it to dive will be the same).


magpie

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Re: How big a ballast tank do I need
« Reply #10 on: January 06, 2007, 08:18:32 am »

Yes its the volume/displacement thats important, and its not just the hull; often you will find that some of your WTC will be above the waterline, and you'll need to measure that too. Thats why dry hull subs need a huge ballast tank compare to wet hulls despite the hull weighing the same.
Another misconception is that when you fill your ballst tank to dive you are weighing you sub down by the amount of water you take on board. In fact you are not changing the mass of the sub at all, what you are doing is reducing its buoyancy by reducing the volume of air in the ballast tank. (the mass of the sub on the pond side is the same as when it is submergered and all the free flooding area are full of water). To get to a point of neutral buoyancy from the surface, you need to reduce the buoyancy of the sub (reduce the volume of air in the ballast tank) by the same amount of positive buoyancy exerted by the top hull as it enters the water (the volume of the top hull). And buoyancy of the hull is independent of its mass. 500cm3 of lead has the same buoyancy as 500cm3 of foam. So it doesn't mater if you build you sub out of lead or foam, as long as the volume is the same the size of the ballast tank will be the same (obviously the lead sub will need more buoyancy to get it to float but the size of the ballast tank to get it to dive will be the same).



An excellent description mankster!!!
Well done!!!
Jason
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