Large draft barge - Problem

[philgib]

I need to build the following barge, and I need the draft to be MINIMUM one meter when empty, that is where the red line is.


So I guess I need to design a ballast layer at the bottom which I can fill from the top through an electric pump.



Question is : how much water should contain the ballast to make the boat "sink" by 1 meter, almost by half, when empty ?

The barge is 5.80 meter long at the top and 5.2 meter long at the bottom,
and is 2.3 meter large and 2.2 meters high

The barge volume is therefore of about 5.5.x 2.3 x 2.2.= 28 m3
The barge will be made of heavy steel.

Is the answer really basic like since I want half of the height below water, I just need to put 28m3 / 2 of water, minus the barge net weight ?

Thanks a lot

Philippe

[marshmat]

Hi Phillipe,

You can start by calculating the displacement at the design draught of 1.0 m. For a simple box of the dimensions you give, this would be (5.2 x 2.3 x 1.0) = 11.96 cubic metres. In fresh water, that's 11960 kg, in saltwater it's approx. 12260 kg. You should calculate this based on the actual shape of your barge, accounting for the slope at the ends, etc.

To find how much ballast is needed to get it to draw 1.0 m, subtract the weight of the barge from the displacement you just calculated.

You may also be concerned with where that ballast goes, in order to maintain level trim. You can do this by "build and guess" (I'd advise against this), or by calculating moments about some convenient reference point and comparing to the location of the centre of buoyancy (the volume centroid of the below-water part of the barge).

Can you elaborate on what the barge is for, and why you need it to draw 1 m when empty?

[philgib]

Thank you Matt. That is clear.

I want to put acrylic "windows" at the bottom half of the barge to make a kind of "underwater observatory". The barge is 2 meters high so that an adult can be straight up (ok, no basketball guys), and is 1 meter underwater so that I can have decently sized underwater "windows".

The water ballast idea would be great because I could easily tow the barge with ballast empty and "go deep" when needed.

Let's say the barge weighs 2000 kilos. That is 2 cubic meters of water. That means ballast volume should be roughly 10 cubic meters. That means another 90cm of draft, to add to the 100 cm of height I need underwater.

That means total draft would be almost 2 meters. Ouch...

Maybe I should forget about water ballast and think about lead ballast to reduce total draft. That would change the ballast layer thickness from 90 cm to 9 cm ! Problem is it would be much heavier to tow. But total draft would be 109 cm, which is just fine.

Do you think the barge would be stable enough with 11 tons of lead at the bottom ?

[marshmat]

You'd have to do the stability calculation to find out. A square-bottomed shape with very low CG is generally a pretty stable form, though.

Keep in mind, when considering water ballast, that ballast weight is, effectively, relative to what it is displacing. Water ballast works because it is internal to the boat, thus displacing air. External water ballast would be displacing water, and so would not add weight to the barge.

(I should note here that liferafts do have external water ballast bags. While these have absolutely zero effect in calm water, they do have a significant effect when the raft is being tossed around by waves that would otherwise toss it airborne. I don't think your barge is likely to go flying anytime soon.)

Of more concern from my perspective is the acrylic windows issue. Engineering an acrylic window to withstand water pressure is a somewhat more difficult task than determining how deep a box will float. Do you know how to do these types of calculations, or are you looking for some reference books? (Engineering is rather hard to teach in a few sentences on a forum.)

[philgib]

Indeed I think I will forget about the water ballast and use a definitive keel made of steel and cement on a small layer right below the barge. It will be heavier but once the barge arrives at its destination from inside a container, it should be pretty fixed, so ciment could be poured at destination.

Acrylic windows : I will use a specialised company which makes acrylic windows for submarines (Google semi-submarine)

Now I have to decide wether I would have the barges built locally to save on shipping or built and shipped.

I have to build my first one, use it for a while, I guess I will have hands-on improvements to make anyway...

Someone told me I should look at ferro-ciment building as it would be pretty heavy and I would have my 1 meter draft quite easily without adding extra-draft for cement-ballasting. I have to look around as I have zero background on that one. The steel price is so expensive it maybe worth looking at it.

Thx again for your help

[philgib]

Hey Marshmat,

Here is a better pic of the project

[Stumble]

I don't know why, but I was envisioning the portholes on the floor of the barge. Given this configuration I would think that useing lead balast would be your best option. While concreat or some other material will work, the density of lead is much higher, and would allow you to build a much shallower bottom than other materials would.

Since lead is about twice as dense as concrete this would allow you to build a shallower draft barge that would be easier to tow. However lead is likely more expensive and would be harder to pour in place once the barge is where you want it.

[philgib]

Thanks Stumble. I was skipping lead for contamination issue. Is this an issue or is there common eco-friendly protection possibilities ?

[marshmat]

Lead has a density of 10 660 kg/m^3. Concrete, depending on what aggregates you make it with, has a density of somewhere between 2500 to 3000 kg/m^3. Tungsten, used on big-dollar race yachts to make smaller (ie, lower drag) keel bulbs than possible with lead, is 19 250 kg/m^3.

One practice sometimes used for heavy boats with internal ballast, where the volume of the ballast is not such a big concern, is to embed an assortment of scrap steel in concrete. This is cheap and effective, with a density of 3000 to 6000 kg/m^3 depending on the ratio of iron to concrete.

I see no point in using lead for internal ballast on a barge. It is used on external keels so that the keel can be smaller and cause less drag. It is toxic, even in trace amounts. If you can do it with concrete and/or iron, it'll be much cheaper, safer and easier than smelting lead. Perfectly suitable 20-MPa concrete can be bought, delivered right to your moulds, for forty or fifty bucks a tonne- less if you buy a lot of it. As of yesterday, lead on the open market was around $1500 a tonne. For eleven tons of ballast, I'd think you'll save something like $16,000 by using concrete instead of lead- enough to buy the electronics suite, or a kick-ass sonar, or.....

[Stumble]

Lead is toxic, but there are a lot of safe ways to work with it. Just think about the thousands of sailboats out there with lead keels. But it is more expensive, which may be problematic.

From what Marshmat said I am now thinking that a scrap steel/concrete mix may be a good mix of density and cost depending on your design criteria.

[rwatson]

make your ballast in stackable "ingots" so you can make adjustments for other gear, and if its lead, paint it with something inert.

With "stackable" ballast, you can remove it for road transport etc

[TeddyDiver]

Originally Posted by rwatson

make your ballast in stackable "ingots" so you can make adjustments for other gear, and if its lead, paint it with something inert.

Something like "gold" paint.. Think how nice to lay in you uw bed two stacks of "gold bars" beside of the bed

[philgib]

Thank you guys. You actually make me realise that I could use the bed volume (1.12 m3) to be part of the keel. That is 8 cm saved from the bottom layer and 2.8 tons of weight not needed inside the bottom cement layer...

Stairs could also be made of cement but then I would have to balance the weight on the left side, not so easy if I want a flat layer. I will forget it and have steel stairs instead.

I do need the slimmest keel layer as possible as max height for a 20' container would be 230 cm. The highest the bedroom, the more comfortable.

I am now thinking that 50 cm high windows should do it if they are large enough. Then draft would only need to be 70 cm (windows located 10 cm above the keel layer bottom and 10 cm below the draft line). That is about 9 tons of displacement, less 3 tons of steel, that would make the ballast weigh 6 tons. Not bad. Minus the 1.4m large bed ballast, then I need 4.6 tons of cement. Taking your number of 2.5 tons per cubic meter for the cement, that is 1.84 cubic meters of cement, divided by 13.8 square meters of the bottom area, including the front slope : 13 cm high-layer. If you guys think I am not mistaking, than this is absolutely great. My young kids would kill me to use lead as they are dangerous eco-warriors :-) Money saved is absolutely amazing too.

3 questions for the specialists here :

1- what structure should I use for the bottom ballast layer so that it accepts so much ballast without bending ? Do I need like 10mm steel, or a special structure ? I think 10 kilos ingots is a great idea but then the bottom structure should accomodate exactly the ingots so that it does not move once fixed. 10 kilos ingots size would be like 30 cm long by 10 cm large by 13 cm high, and about 460 of them to reach 4.6 tons. I guess a simple day of work with a chain of friends from the truck into the barge...and looooots of beer :-)

2- I have written on Internet to a few barge builders but they do not seem to take me seriously, or maybe not enough money to make from that mini-barge. What is the other way ? Should I hire an engineer to make the plans than simply hire any marine welder to build the steel barge ? Or send the engineer plans to the barge builder to look more serious ?

3- How thick should be the top steel deck ? Of course I will cover it with teak to make it nice to walk on, but how thick should it be so that it does not bend when people walk on it ? Let's say maximum 500 kilos per square meter or about 100 pounds per square inch since some of you are Americans ?

Thank you so much again

[webbwash]

hire a naval architect to do the design and stop guessing - better yet get yourself a degree in naval architecture so you can do the design and be SAFE about it for your purpose.

[RAraujo]

I don't know how do you intend to use it or the applicable regulations there but have you considered what happens if you damage one of those windows? I would consider to have that thing subdivided internally...