cliff
Well-Known Member
cwt => hundredweight = 112lbs = 8 stone.
1 stone = 14lbs
1 lb = 16 oz (ounces)
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1 stone = 14lbs
1 lb = 16 oz (ounces)
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anyone for maths !!!! Buoyancy question
- Thread starter griff1n
- Start date
peterb
Well-Known Member
See, that's the problem with 'modern maths'. Doesn't teach you anything about real units.
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William_H
Well-Known Member
The question raises more questions. The dinghy will probably float if it is of wood without added buoyancy however that won't help you much it just means you wo'nt lose it to the bottom. You most probable scenario is to capsize or more specifically dip the gunwhale under water ansd take in lots of water. I reckon the best thing there is to have side buoyancy tanks up to deck lervel. It gives you something to sit on on gunwhale when leaning out to balance the boat and when it does go over too far the water can't come in until it reaches beyond the tanks. When it is capsized on it's side then the buoyancy is in the right place to aid righting ie lifts the gunwhale up. Plus of course when upright and full of water it won't sink with you sitting or standing inside.
If you put tanks or bags in the bow and sternusually under seats or thwarts then it will hold a lot of water and settle low before the bags take effect. You will find with a lot of watwer in the hull that it loses it's righting ability so you have to bail out before starting to sail again. I have spent many years teaching kids to sail and the first lesson always involved gouing out and de;iberately capsizing the dinghy then righting it. The prefered method although this varies with boat type is to have one person lay in the water inside the dinghy. Avoid trying to lift yourself out of the water is this will pull the boat over on top of yourself to fully inverted. (This can be fun and you can surface inside the turtled dinghy in beautiful blue light.) The other person leans on the centreboard to right the boat. The person in the boat bails the water as the one in the water can not usually board until most of the water is out becuase of loss of righting ability. If the boat goes right over (turtle) then 2 people pulling on the centreboard, toes on the gunwhale can slowly bring it to where it is on it's side. So a big heavy dinghy may never capsize in which case disregard all above. On the other hand try the exercise. You will be surprised how hard it is to capsize an empty dinghy and how easy with half full of water. Make sure the centre board and the rudder are locked down and in place and will stay there fully inverted. Do your ptractices close to help and where the wind will help you drift ashore. It is often easier to drive ashore to empty the boat than to bail it and carry a bucket tied in. in fact everything tied in. Obviously you may need wet suits if water is cold.
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If you put tanks or bags in the bow and sternusually under seats or thwarts then it will hold a lot of water and settle low before the bags take effect. You will find with a lot of watwer in the hull that it loses it's righting ability so you have to bail out before starting to sail again. I have spent many years teaching kids to sail and the first lesson always involved gouing out and de;iberately capsizing the dinghy then righting it. The prefered method although this varies with boat type is to have one person lay in the water inside the dinghy. Avoid trying to lift yourself out of the water is this will pull the boat over on top of yourself to fully inverted. (This can be fun and you can surface inside the turtled dinghy in beautiful blue light.) The other person leans on the centreboard to right the boat. The person in the boat bails the water as the one in the water can not usually board until most of the water is out becuase of loss of righting ability. If the boat goes right over (turtle) then 2 people pulling on the centreboard, toes on the gunwhale can slowly bring it to where it is on it's side. So a big heavy dinghy may never capsize in which case disregard all above. On the other hand try the exercise. You will be surprised how hard it is to capsize an empty dinghy and how easy with half full of water. Make sure the centre board and the rudder are locked down and in place and will stay there fully inverted. Do your ptractices close to help and where the wind will help you drift ashore. It is often easier to drive ashore to empty the boat than to bail it and carry a bucket tied in. in fact everything tied in. Obviously you may need wet suits if water is cold.
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mirabriani
Well-Known Member
William H gives a good answer. Unless I have missed it no one has mentioned an important point which is to securely fix the buoyancy bags in place. We nearly lost a Firefly because of this when a bag came adrift and she could not be righted.
Briani
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Briani
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Bodach na mara
Well-Known Member
No you won't. The boat (if capsised) will JUST float bow up in the water and you will be able to do nothing to get it up again except yell for help.
In my dinghy days in Enterprises, GPs and YW Dayboats, the usual configuration was a combination of a 300 lb bow bag, 150 lb bag at each side and another 300 lb bag at the stern (although the Enterprise used two more side bags. All need to be securely fixed in with webbing going through the latches on the bags and OVER the bag. The side bags had two latches each and that was not enough. A third strap was needed roung the middle or the centre of the bag floated up and did no work in lifting the boat.
Hope this helps. I capsised in an Enterprise and the GP once each and recovery is possible, but difficult. For the Dayboat (with a steel plate) the bow and stern bags were increased to 600 lb but I never tested recorery in it.
<hr width=100% size=1>Ken Johnson
In my dinghy days in Enterprises, GPs and YW Dayboats, the usual configuration was a combination of a 300 lb bow bag, 150 lb bag at each side and another 300 lb bag at the stern (although the Enterprise used two more side bags. All need to be securely fixed in with webbing going through the latches on the bags and OVER the bag. The side bags had two latches each and that was not enough. A third strap was needed roung the middle or the centre of the bag floated up and did no work in lifting the boat.
Hope this helps. I capsised in an Enterprise and the GP once each and recovery is possible, but difficult. For the Dayboat (with a steel plate) the bow and stern bags were increased to 600 lb but I never tested recorery in it.
<hr width=100% size=1>Ken Johnson
aitchw
Well-Known Member
What class of dinghy is she?
Whilst it is an excellent idea to understand the principles behind a decision on buoyancy it is much easier and more sensible to use the collective experience of thousands of sailors who have helped arrive at the buoyancy required by the class rules for your dinghy.
Go to your class association and find out what they insist on. It will be a safe and reliable level which has taken account of the issues.
Howard
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Whilst it is an excellent idea to understand the principles behind a decision on buoyancy it is much easier and more sensible to use the collective experience of thousands of sailors who have helped arrive at the buoyancy required by the class rules for your dinghy.
Go to your class association and find out what they insist on. It will be a safe and reliable level which has taken account of the issues.
Howard
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steve_cronin
N/A
This is basic Archimedes principle stuff....
So you need to know the "upthrust" You only get to know this if you can submerse the hull first and observe what capacity of water it displaces.
Why is this important? Well ask the alternative question to the original:-
How much buoyancy would I need for a closed cell foam dinghy weighing 3cwt? Answer - NIL
Or "How much buoyancy would I need for a lead hull weighing 3cwt? Answer 3cwt less the weight of water (very little) displaced by 3cwt of lead in water (which will provide upthrust).
Steve Cronin
Am I right David Teal, Past Head of Battle Abbey School?
Apologies if anyone else has already mentioned this - I've just got back from Greece - where I met the President and I'm telling everyone!
<hr width=100% size=1>The above is, like any other post here, only a personal opinion
So you need to know the "upthrust" You only get to know this if you can submerse the hull first and observe what capacity of water it displaces.
Why is this important? Well ask the alternative question to the original:-
How much buoyancy would I need for a closed cell foam dinghy weighing 3cwt? Answer - NIL
Or "How much buoyancy would I need for a lead hull weighing 3cwt? Answer 3cwt less the weight of water (very little) displaced by 3cwt of lead in water (which will provide upthrust).
Steve Cronin
Am I right David Teal, Past Head of Battle Abbey School?
Apologies if anyone else has already mentioned this - I've just got back from Greece - where I met the President and I'm telling everyone!
<hr width=100% size=1>The above is, like any other post here, only a personal opinion
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