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Author Topic: Calulating bouyancy  (Read 2175 times)

Hagar

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Calulating bouyancy
« on: October 18, 2009, 08:28:42 AM »

Hi Guys!

If I know the weight og a 'cargo' that is to be transported by a boat how do you work out the required bouyancy of the vessle to carry the load.
The problem is to make a small boat that can carry a lot, but just how small can you make it. Thus the need to find the maths to work it out.
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tigertiger

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Re: Calulating bouyancy
« Reply #1 on: October 18, 2009, 10:52:32 AM »

From a purely maths point of view.

1000cc of displacement will give you 1 Kg of bouyancy.
There are several methods of calculating displacement, some accurate and some crude rules of thumb (that may still serve your purpose).
However you need to factor in the weight of the boat, and a safety margin.

If you are looking for a specific answer to a specific problem, then some more information may help.

What is your planned purpose for the boat?
- is there a functional use or is it just modelling?

How heavy and bulky is your planned cargo?
- 30kg of conrete is one thing, 30kg of popcorn is another.

Do you have a style of boat you wish to construct?
- e.g. are you scaling up a barge or coaster and wish to know how far to scale it up?
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derekwarner

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Re: Calulating bouyancy
« Reply #2 on: October 18, 2009, 11:38:25 AM »

....Ian......the model vessel & the load [mass carried]  are at the same scale ...both are volumetric...& hence considered as displacement...or 3D as different to a 2D pictorial representation

So if you build @ 1:10 or 1:100...the ratios remain constant to the same scale ratio  O0

1000cc is actually a unit of volume [consider .....length x breadth x height] & hence the displacement of that volume/mass is relative to the specific gravity of the product & yes fresh water is derived as 1kg/litre

As tt suggests....the more information or detail you can supply will assist......Derek
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Derek Warner

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tonyH

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Re: Calulating bouyancy
« Reply #3 on: October 18, 2009, 12:48:45 PM »

Hi Ian,

Simply put, the volume of the hull below the waterline has to carry the cargo plus the weight of the boat. So if the volume below the waterline is, say, 5 litres, then the cargo and boat can weigh 5 kilos, 10 litres can support 10 kilos etc. This assumes, as TT says, that you can fit the cargo in!

If you're not worried about how the boat looks under the water, you can cheat and increase the volume by adding depth to the hull shape below the line.

Tony
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Hagar

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Re: Calulating bouyancy
« Reply #4 on: October 18, 2009, 05:01:24 PM »

Need to work out two thing really.
one project bashing about in my head is a bait boat, which I know some of you are familiar with. So the scale thing is out off the window there, I am thinking of a loading capacity of around 1.5 - 2kg max. that being ontop of the weight of the boat and running gear and batterys etc.

The second brain wave is a bit more serious, concequent wise. I am comsidering the idea of building a trailer top that can be used as a boat which whould be able to carry about 300kg without sinking of breaking in half!!!

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BarryM

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Re: Calulating bouyancy
« Reply #5 on: October 18, 2009, 06:12:24 PM »

Er 300kg, there isn't a certain Great White Whale coming into this equation somewhere is there?  :o

Barry M
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Hagar

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Re: Calulating bouyancy
« Reply #6 on: October 18, 2009, 08:31:20 PM »

Er 300kg, there isn't a certain Great White Whale coming into this equation somewhere is there?  :o

Barry M

 <*<
well lest see me 104kg and just as much fishing gear or a mate who is also the wronge side of 90kg.......most of the time it will just be me and a big bukket of bait, on the odd trip there will be more or less lull load so 300kg displacement would be kind of nice.

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tonyH

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Re: Calulating bouyancy
« Reply #7 on: October 18, 2009, 08:32:11 PM »

Since 1litre is basically a cube 10cmx10cmx10cm of fluid, in this case water, which will weigh 1Kg and you know the size of the roof of the car/van/stagecoach etc. you can work out the draft of the basic box hull and then you can decide how deep the sides above the water need to be for safety.
You'll need to add a percentage for the shape of the hull, curved corners etc. but that depends on how 'boaty' you want it to be.

As far as the bait boat is concerned, you'd probably need to allow 2 - 3Kg for the hull, batteries etc so the loaded weight could be 6kg so the wet volume would be 6litres or a box 30cmx20cmx10cm.

Hope that (a) my sums are right and (b) it helps.

Tony
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Bryan Young

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Re: Calulating bouyancy
« Reply #8 on: October 18, 2009, 11:02:13 PM »

It isn't "buoyancy" you should be looking at. Buoyancy is a residual. The real "problem" has been touched upon in some of the earlier postings. It's about "displacement". Put your hull into some water and load it up until it sinks to your desired waterline. That is now the total weight of the model. Easy. BY.
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tigertiger

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Re: Calulating bouyancy
« Reply #9 on: October 19, 2009, 12:58:47 AM »

<*<
well lest see me 104kg and just as much fishing gear or a mate who is also the wronge side of 90kg.......most of the time it will just be me and a big bukket of bait, on the odd trip there will be more or less lull load so 300kg displacement would be kind of nice.



Hi Ian
A car-topable/trailerable boat for 4 people should be adequate.
One for 5-6 people is more than adequate, and give you plenty of room to use fishing rods.

Look at the small boat plans listed on the Duckworks site http://www.duckworksmagazine.com/r/plansindex/dories.htm
The description will tell you if it is car topable or trailerable. Some of the small boats show a sail, this is optional.

As the wieght is people and tackle this is not dense and can easly be spread out. So little danger of the boat snapping in half.
A 300kg lead bar would be a different matter.


Several of these plans are easy-build

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Hagar

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Re: Calulating bouyancy
« Reply #10 on: October 19, 2009, 07:19:14 PM »

Tanks for the replies thus far. keep em comming.

@BY; thats too easy! Problem with that solution is having a boat to load...

@tiger; Thanks for the link, I'll have a look at that in a moment. I spent most of last night storming the www for boat palns and have seen a few I can tell you! Didnt find the site you linked to though!

So I can deduce that to safly carry a max load of 300 kg I need a design that will have a dispalcement of 300 liter.

Being a total newbee at the design side of things, could someone clarify a few things.
Which hull type is most stable and less likly to flip. A flat bottomed boat is pretty solid platform, but I understand that they can suddenly flip. A more rounded hull form wll roll more but will by more predictable with regard to a capsize.

Am I totally wronge or.....
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Bryan Young

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Re: Calulating bouyancy
« Reply #11 on: October 19, 2009, 07:52:12 PM »

As far as "static" stability is concerned it makes very little difference if the hull is round or flat bottomed. I would go for the flat bottom out of sheer practicality. My 1930s era trawler has a "semi-round" hull and it's a real pain having to keep putting it into a cradle to keep it upright. A flat bottom job means you can just plonk it down anywhere.
The big difference could occur when a deck edge is submerged. Then the basic rules of ship stability go out of the window. Depending on how far the hull goes over, the hull shape changes drastically, and then so do all the previously set parameters like C of G, metacentric height..and thus depleting the length and strength of the "righting lever". Very important on full size ships, but although less on models as most models are bottom heavy anyway (hence why so many....including some of mine...bob around a bit unrealistically).
A good example that was always used to teach us pre-sea cadets was to show the impossibility of getting a square sectioned log of wood to float in a square way. Natural tendency is for the log to float with the widest possible "beam". Submerge one side of the "beam" and the thing will always flip over. However, take a good slice off the "bottom", and with any luck the log will return to the upright. (Obviously the same amount or more will have to be removed from the top).BY.
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malcolmfrary

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Re: Calulating bouyancy
« Reply #12 on: October 20, 2009, 01:16:30 PM »

Getting the underwater volume in units of 1000cc will give you the total weight of the boat in Kg.
A fairly accurate way is to work out the cross-sectional area at suitable distances along the length, then work out the section volumes and add 'em all together.  A scale drawing on graph paper and count the squares can get you near enough the area of the sections, or break the section down into rectangles and triangles, work out these areas and add them together.  For the volume of the sections, add the area at each end, divide by 2 to get the average area, multiply by the length of that section and there you are.  Then add the volumes together. 
For a modest size, Bryan's empirical method is best, but it does need a hull.
I can't remember the name of the series (Salvager or something like that) but there was a guy on TV who made scrap furniture out of railway sleepers, coach bolts and bits of battleship.  He did a later series terrorising the French, when he made a "boat" from the fibreglass roof of a Transit van.  This could be either warning or inspiration.
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