anyone for maths !!!! Buoyancy question

Depends what your buoyancy is made of - and what extra weight in excess of the hull that you are trying to cater for (e.g. fittings, crew etc.).

IIRC 1 gallon of water weighs 10 lb. therefore for your 3 cwt you need 34 gallons of air buoyancy

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Simple - 3 cwt else it sinks

or 3 cwt= 3X112lbs = 336lbs 1 cuft of water (fresh) = 64 lbs
1 IMP gallon fresh water weighs 10lb
therefore buoyancy bag to keep the hull afloat needs to hold 33.6 gallons MINIMUM
or have a volume of 336/64 cuft = 5.25cuft. MINIMUM

Salt water is a little more dense but safer to work in fresh water figures.

Think I got the numbers right!

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er - about 3cwt if it is to float - less if you would like it to sink.

3 hundred weight (cwt) is about 152.5 kg (seems like a small boat!), which in fresh water would require a displacement of 152.5 litres or 0.1525 m^3 and in salt water would require a slightly smaller displacement of about 148 litres or 0.148 m^3

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Thank you al for your prompt responses.

I need to go out at lunch time and get some buoyancy bags for the grand launch!!! of the 30 yr old wooden sailing dinghy I have just spent 7 months restoring.I've never sailed before, so wanted advice on striking the correct balance between safety, and getting taken for a ride by a zealous chandler.

so if i get a bag to go in the bow that has 150 L capacity i should be ok.

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Thank you al for your prompt responses.

I need to go out at lunch time and get some buoyancy bags for the grand launch!!! of the 30 yr old wooden sailing dinghy I have just spent 7 months restoring.I've never sailed before, so wanted advice on striking the correct balance between safety, and getting taken for a ride by a zealous chandler.

so if i get a bag to go in the bow that has 150 L capacity i should be ok.

<hr width=100% size=1>
 
Thank you al for your prompt responses.

I need to go out at lunch time and get some buoyancy bags for the grand launch!!! of the 30 yr old wooden sailing dinghy I have just spent 7 months restoring.I've never sailed before, so wanted advice on striking the correct balance between safety, and getting taken for a ride by a zealous chandler.

so if i get a bag to go in the bow that has 150 L capacity i should be ok.

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NO, NO, NO, 150L is the minimum. You still need to consider the buoyany required for outboard, additional kit, bodies, beer, etc - say at least an extra 50Litre although wood generally floats anyway so even if swamped you should stay afloat so you might get away with 150L

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what's all this horrible imperial mathematics. Surely it's much easier to work in metric where 1 Litre of fresh water weighs a magical 1kg.

150 Litres of air = 150Kg of buoyancy

Simple

Of course it would have been even simpler if you'd weighed your boat in Kg

Sorry if I sound a bit terse but this is a pet rant!!

Steve

<hr width=100% size=1><A target="_blank" HREF=http://"www.nwcc.info">North Wales Cruising Club</A>
 
What's wrong with 'a pint of water weighs a pound and a quarter'?

<hr width=100% size=1>my opinion is complete rubbish, probably.
 
As an ex dinghy sailor, ignore the fact that wood floats. You need to be able to be in the boat to bail it out. In the good old days of wooden Enterprises, I had to help with the rescue of one with a smaller than normal bow bag, and slightly smaller than normal side bags. It was just about awash at best. You couldn't get in it to bail because it sank three feet until you were buoyant enough to stop sinking yourself...

Before anybody then draws theinference that you can't have too much buoyancy, it is really difficult to get back into a very buoyant dinghy, or in the case of a capsize reach teh centreboard to get it back upright (505 and Mirror spring to mind at opposite ends of the scale

<hr width=100% size=1>Larry Botheras

Anderson 26 "Amber"
 
Ok, now i'm confused,

too little = bad
too much =bad

think i'll start with 150 litres, and play it by ear.



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Re: imperial maths

When I went to skool we were taught in imperial and I still find it easier to do many calculations in imperial measure and if necessary convert the answer to this decimalised system you young uns use today. There is nowt wrong with feet and inches and pounds and ounces anf rods and chains and CABLES and Nautical Miles and Guineas and crowns and half crowns and florins and shillings and sixpenny bits and thrupenny bits and pennies and ha' pennies and farthings. Not to mention horse power.

Funny how people still use the old measures "I got 2 X 350 horse Power engines and can do 35 knots"



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Go for 150L in the bow and a couple of smaller buoyancy bags to go under any thwarts, or tied on either side of the dinghy at the stern, then you will float level.

If you have too much buoyancy then if you capsize then the dinghy floats too high in the water and it is difficult to get back upright,

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in the simplest terms, the buoyancy of a dinghy needs to be enough to float:

the dinghy itself (150kg)
the crew (2x75kg)
and enough extra to keep the gunwales above water while you bail it out (50kg)

total 350kg or 350 litres.

QED



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Yes, it's easier to work in metric, but easier still if you remember the relationships between the systems.

The metric tonne is almost identical to the imperial ton (1 tonne = 0.984 ton). Taking the two as identical, 3 cwt is 0.15 tonnes, or 150 litres of fresh water.

On a slightly different point, 150 litres of buoyancy in the bow should keep the bow up nicely, but with the stern hanging down there's no way that you can bail the boat out. I'd suggest another 100 litres in the stern. And it wouldn't be a bad idea to try filling the boat with water before you set out, just to see how she would lay.

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The problem you have is that what appears to be a simple question with a simple answer isn't simple at all.

You need to arrange to have enough bouyancy that the dinghy will float not too high to make righting difficult (can't get on the centre board/dagger board) and yet enough so that when you do get the thing back upright again you can bail it out whilst retaining some sort of stability and sufficient freeboard with all of you on board. I recall some dinghys that you used to have to stuff a towel down the centreboard case to enable you to bail them out...

The maths are not easy, and you might be better to try a little trial and error. 150Kg of bouyancy will not be enough. Double that might be right - or too much - or too little. It depends on the shape of the boat and the weight of the crew and gear as others have pointed out.

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