Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[fatcat123]

Hi guys,

Bit of advice needed here please. This may be a simple calculation but i'd rather make sure i get it right.

After just completing my tug, I need a towing barge. I plan to make the barge from ply with internal framing to support some 5 litre containers (car washer fluid type).

Now i know 1 litre of water weights 1kg and i plan to use 4 or 5 of these containers so we're looking at 20-25kg of ballast in the barge. I'm thinking if i make the barge too small then it will simply sink so my question is, how big does the barge have to be to sit right in the water with the ballast on board?

I guess my question is, how much cubic air does it take to keep 1kg afloat...  The barge will be around 13" wide, around 8" deep and i guess the length would be determined by the cubic air needed..

Any help welcome :)
Dan

[dreadnought72]

Turn the question around!

A 1 litre plastic container, full of water, will be just about neutrally bouyant. If you add any bouyancy to it, it'll float. So...

If you're aiming for 20-25kg of water ballast in the barge, then you'll need a barge of "just larger than" 20-25kg displacement in order to float. The more bouyancy, the more it'll float - and the better the freeboard.

Think about a basic box. Let's assume the surface area of the box base is 80*25 cm. That means for every 1cm of depth in the box, you can hold ~2000cc of ballast, which, if water, will be 2kg.

To just float 25 kg will need 12.5cm of hull in the water. Add maybe 5cm of freeboard, and you get 10kg of positive bouyancy.

Andy

[fatcat123]

hey Andy,

Cheers. Yeah i thought it may be something like that but would of rather asked. So we're basically saying 1 cubic meter of air will make 1kg neutrally bouyant so make the barge a little bigger than the bottles and it will sit right..

Dan

[dreadnought72]

Cheers. Yeah i thought it may be something like that but would of rather asked. So we're basically saying 1 cubic meter of air will make 1kg neutrally bouyant

1kg of water is neutrally bouyant. Add any "air" to the hull volume that that ballast is put in, will provide positive bouyancy, though it might be marginal until you have enough.

...so make the barge a little bigger than the bottles and it will sit right..

Yes - and try to keep the bottles as low as possible, else she'll roll like anything. Or, maybe, "just roll 180 degrees the once"!

You could try a practical experiment - float your washing-up bowl and ballast it with water bottles. You'll soon find how wallowy it can be with only water ballast.

Andy

[fatcat123]

Thanks Andy,

I'll post the build when i come round to it in a few weeks.

Dan

[Bunkerbarge]

Just to turn this on it's head and offer an alternative point of view, why would you decide on an amount of water you want them make a barge to suit?  Wouldn't it be better to make the barge you need to look right and to scale with the tug, then decide on how much ballast to out in?

[fatcat123]

Yes thats true.

Ive decided to make something in the region of 40" by around 17" by 8" deep.

I may also convert the barge to a motorised version in the future to make a working crane barge so the size will come in handy.

Dan

[Umi_Ryuzuki]

I built several self ballasting barges 30"(762mm) x 18"(457mm).. Essentially they sink till they hit the foam inside the barge.
The result is a 42 lb (19kg) barge sitting at the water line. These were relatively small barges.
A small air vent at the corner allows it to settle quickly, and makes it easy to lift out of the water.

The next one is 48"(1219mm) x 24"(609mm)  at 6" deep Needs a bit of foam taken out, and an air vent added.
It should weigh 156 lbs (70.76kg) when ballasted down.  The crane on top ought to add a bit of weight as well.