Boyancy

thornbackflound · 27 April 2010, 18:02

Hi all this may be a silly question but when i go out on my small sib i feel relatively safe with the wooden deck if it filled up with water,but what would my 4m with a heavy 5 peice alloy floor do if flooded? would it float?

PeterM · 27 April 2010, 18:04

Quote:

Originally Posted by thornbackflound

Hi all this may be a silly question but when i go out on my small sib i feel relatively safe with the wooden deck if it filled up with water,but what would my 4m with a heavy 5 peice alloy floor do if flooded? would it float?

Yes it would - think of the force trying to hold a football under water in a swimming pool. I weigh 'several' stone and it keeps me up .Now think of how many footballs worth of air are in the tubes.............

If however you mean the floor - no

thornbackflound · 27 April 2010, 19:41

Quote:

Originally Posted by PeterM

Yes it would - think of the force trying to hold a football under water in a swimming pool. I weigh 'several' stone and it keeps me up .Now think of how many footballs worth of air are in the tubes.............

If however you mean the floor - no

thats what i mean ,will the floor be so heavy with the weight of the water drag the sib down?

Poly · 27 April 2010, 19:52

Quote:

Originally Posted by thornbackflound

thats what i mean ,will the floor be so heavy with the weight of the water drag the sib down?

No!

How much does the floor weigh? I'm guessing probably less than a person. So its just the same as throwing the floor overboard and having one more person on board.

When building a cardboard boat a few years ago we worked out that about 2 cubic feet of air will hold one adult completely out the water. So your tubes which are probably 20 cubic feet at least, will keep a lot afloat.

m chappelow · 27 April 2010, 19:56

most sibs/ ribs will stay afloat fully swamped and with the engine on and crew inside ,all you need to do is drop down the drain trunks and its inherant bouyancy should start to drain its self at least up to before it was swamped ,in theory though with waves filling the boat back up i doubt it in lumpy conditions ,even better though if the engines running and you can keep moving ,if you also have an inflatable keel that will give a fair amount of bouyancy and take a certain amount of volume space under the floor .just to give you some idea ,,rough guide is a 50 gallon oil drum when empty will support about a 1/4 ton ,,250 kgs so if you compare that with the tubes on you boat it will give you some idea just how much bouyancy there is in your boat , or think of even a 25 ltre drum and how much that takes to push under water ,,,

thornbackflound · 27 April 2010, 20:11

Arh many thanks gents,yes i feel a lot safer now ,mind you my boat as you know is a lifegauard made boat,with inflatable keel,and crikey the tubes do take some air,she is nearly done so photos to follow real soon,trailer should be with me next wk and i have a 15hp marinar coming soon,,,,,,,,,,,,,,,, staying with original alloy flloor ,comes with it so is ment to be used sod making me own,lol whoopeeeeeeeeee ,deep joy

m chappelow · 27 April 2010, 21:38

looking at some of the coding regs cat c and d your particular boat should ,, when fully swamped be capable of supporting its full outfit of equipment,the total number of persons it is allowed to carry and support enough mass to include the engine and a full fuel tank,

prairie tuber · 27 April 2010, 22:25

Quote:

Originally Posted by m chappelow

looking at some of the coding regs cat c and d your particular boat should ,, when fully swamped be capable of supporting its full outfit of equipment,the total number of persons it is allowed to carry and support enough mass to include the engine and a full fuel tank,

...and still be quite stable! (unlike a hard boat)

biffer · 28 April 2010, 06:19

from my diving days if i can remember, 1 litre of air equals 1 kilo of lift, it changes at depth but you're not going there, the big foot pumps we use are 10 litres, it don't take much working out you ain't going to sink

doggypaddle · 28 April 2010, 06:32

Quote:

Originally Posted by biffer

from my diving days if i can remember, 1 litre of air equals 1 kilo of lift, it changes at depth but you're not going there, the big foot pumps we use are 10 litres, it don't take much working out you ain't going to sink

Thats quite right Biff, but the relative density of seawater is greater than 1 so 1 litre of air will give you a kilo in fresh water, but is well over a kilo in seawater

so even less likely to sink