# Calculating displacement?

Discussion in 'Boat Design' started by stonedpirate, Apr 4, 2010.

1. Joined: Nov 2009
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### stonedpirateSenior Member

Hello,

Is there a simple formula for finding out how much my boat can carry?

Its volume is 316cm^2.

The boat itself weighs 90 kilos.

Can i calculate roughly how much weight it will carry safely?

Thanks

2. Joined: Oct 2002
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### lewisboatsObsessed Member

first of all...volume is measured in cube not square...how much volume does it have? 316cm^2 is an area measurement not a cubic measurement. A x B x C is what you need to give us. Displacement is measured in 1000 kilos/ cubic Metre.
One cubic metre equals 1,000,000 cm^3. You have a problem with your measurement somewhere if your boat displaces 316 cm^3 let alone 316cm^2 and weighs 90 kilos. It would have to measure 17.776 cm per side and have a depth of 1 cm to match your criteria.

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### stonedpirateSenior Member

Sorry, typo. It was 3782518cm^3

I divided that by 100 then 100 then 100 and got 3.78m^3.

Lets make it simple.

Imagine a 3m * 1.5m * 1.5m rectangular box.

Thats a volume of 6.75m^3

Are you saying that i could put 1000/6.75 kilos of lead in this box?

148kilos?

Thanks

4. Joined: Aug 2009
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### Tapio PeltonenJunior Member

No.

6.75 cubic metres of water = 6.75 metric tons. So you could put 6750 kg of lead (minus the hull weight) in before the box sinks. Simple as that. Displacement in tons = submerged volume in cubic metres.

Of course this is only in perfectly flat water etc. etc., and if we want to nitpick, .001 cubic metres of water is not always exactly 1 kg, it depends on temperature and salinicity.

5. Joined: Aug 2007
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### CDKretired engineer

1.Put your boat in the water and stand next to it.
2. Measure the distance between the upper edge of the stern and the waterline.
3. Take a heavy object with known weight (your wife or girlfriend) and put that in the center of the boat.
4. Measure the distance again.

Subtract that value from the first one and you have the displacement corresponding with the weight of the object (Archimedes). With simple arithmetic you can calculate the displacement of your boat.

6. ### BostonPrevious Member

take the length width and hight at max and then start multiplying by ( some coefficient who's name I forget at the moment ) basically calculate the % of area each view takes of its max dimensions and multiply it by overall max dimension displacement
you end up with the actual displacement or at least something really close

my build design takes up about 77% of its plan view about 96% of its elevation view and about 65% of its section view at max beam

there is a lot more to not drowning than meets the eye and you might just think twice before setting out without even a radio

by the way CDK that bit about the heavy object of known weight
that better 7/8 of the relationship is going sink that thing for sure

7. Joined: May 2009
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### BertKuSenior Member

Yes, you can. If you are tired of formula's, just take a 10 liter water drum , fill it up 9 times and poor it 9 times into your boat. Make sure the boat is level. (You stated that the weight of the boat is 90 Kg)

Take a waterproof black pen and make a mark where the water line is in the boat. Now you start counting the 10 liter drums and fill the boat up, until it flows over. That will the maximum amount you can carry.

Simple, it cost you nothing other than some river water and a cloth to dry your boat at the inside.

Sorry, that I have a grin on my face.

8. Joined: Nov 2009
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### stonedpirateSenior Member

Thanks all.

Yeah, the boat isnt built yet

Just working out roughly how much different hull shapes will carry.

I am just cutting the freeboard off my hull models and taking the volume of the submerged section. As for water temp, salinity etc, i'll just be sure to load it no more than 80% of its capacity

One last question.

Does marine foam affect how much you can carry?

So if its volume minus boat weight, and you had marine foam that added twice the bouancy of the boat materials, would it be volume plus boat weight?

Cheers

9. Joined: May 2009
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### BertKuSenior Member

That is sensible, but at 80%, you cannot get too many waves rolling into your boat direction. A good spill will make your boat sink, except if you always have flat water surfaces.

All what counts is the water weight pushed out of the way , before the boat sinks by having the water rolling over your boat edge = maximim load.

Wether you use foam, steel aluminium or lead. By using foam, you reducing volume avialable. Thus: if you are using a material to make sure your boat does not sink, the weight of all, water pushed over the edge into your boat and the weight of yourself , plus the weight of everything else, must be less than the weight of the water . Thus either you build a few watertight compartments into your boat, or those compartments are filled with marine foam, or you have a hull wall filled with marine foam, it boils down to the same. The weight in total must be less than the volume of the water displaced.

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