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Old 02-06-2009, 11:25   #1
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Challenge: Add How Much Flotation?

We have a Cal 25 (4500 lbs displacement empty) that we'd like to have float when filled with water. Our initial thought was to cram 2 liter plastic drink bottles throughout. Question is how many would that take?

Figures I've heard: sea water weighs about 64 pounds per cubic foot. Each bottle would displace about 4.5 pounds of sea water. So would a thousand bottles do it?

A related question: if the bottles were free in the cabin and she started to sink, clearly the 2,000 pounds of lead in the keel will be leading the way to Davy Jones locker. What are the chances of the force of the bottles pushing against the overhead separating the deck from the keel?

We don't care if there's only one inch of freeboard if she gets holed and fills with water.
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Old 02-06-2009, 12:17   #2
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1000 bottles will lift 4500lbs, so yep, 1000 bottles will do it. The boat itself will have some buoyancy so you could probably do with less, but 1000 will do it.

The second depends on how big the overhead is(I don't have a boat yet so don't have any point of reference). Basically you've got a downwards force of 4500lbs(the weight of the boat) and an upwards force of 4500lbs(the buoyancy of the bottles) so whatever the bottles are holding her up with(the overhead/deck) will be experiencing 4500lbs of force pressing against it. Divide this by the area of the overhead to get the pressure that will be applied upwards - a totally uninformed guess would say that if she can survive a big wave coming at her deck then she can survive being lifted from underneath, but it may be built so it can survive a lot of pressure from the outside but not so much from inside - very hard to say.
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Old 02-06-2009, 12:27   #3
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I dont see the deck coming off due to the force of the bottles. In rough terms you are talking about 55 feet of deck attachment (Deck perimeter 25+25+5). About 80 lbs force per foot or 6.5-7lbs per inch. Also, the more the boat goes down, the less the submerged part weighs in the water right? Also, figure some bouancy from your empty portion of the fuel tank and water tank. Also, the diesel weighs less than water, but that's negligible!
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Old 02-06-2009, 12:27   #4
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I think Ben got it right, except perhaps, for the boat having some inherent buoyancy.

Since a boat’s structure is almost always heavier than water, it’s "buoyancy" is derived from it’s interior air space (excepting structural displacement, per Cheechako). When flooded, there is no buoyant air space (displacement) - hence no buoyancy.
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Old 02-06-2009, 12:34   #5
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Gord: Everything has some buoyancy... equivalent to how much water they displace - things that sink have less buoyancy than their weight, but the buoyancy is still there and can be added when calculating how much air to add to increase the buoyancy. At least that makes sense to me, but It's possible I could be wrong.
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Old 02-06-2009, 12:43   #6
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Another way to look at it - how much air inside her is under the waterline when she's floating? That's how much air you need to bottle up.
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Old 02-06-2009, 13:54   #7
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Yea, lifting a "one man rock" under water takes less force than lifting it if it's out of the water.... not that it matters for this exercise.... :>)
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Old 02-06-2009, 14:19   #8
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hmm is there an edit on this forum? I just noticed that Gord did actually cover what I meant ("excepting structural displacement"), but didn't realise that's what I was talking about when I said buoyancy of the boat itself. Sorry to insult your intelligence by re-explaining that Gord
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Old 02-06-2009, 15:08   #9
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Greetings, and welcome aboard captainmark.
I hadn't noticed that you were a new contributer.

Of course Cheechako & bencoder are right about the displacement of the boat structure, reducing the amount of addiditional lift required to float the boat.

Note that, once you've provided positive floatation with air-filled drink bottles, you'll have a suirvivable capsule, but probably not a liveable cruiser. Accordingly, whilst an intriguing idea; probably not a practical "safety" solution.
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Old 02-06-2009, 15:25   #10
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hmmm... is alcohol lighter or heavier than water? :>)
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Old 02-06-2009, 15:27   #11
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Hey, as long as we're on the subject, figure this one out: if water weighs nothing in water, does water ballast only work once it is lifted above the waterline by heeling?
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Old 02-06-2009, 15:43   #12
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Quote:
Originally Posted by Cheechako View Post
Hey, as long as we're on the subject, figure this one out: if water weighs nothing in water, does water ballast only work once it is lifted above the waterline by heeling?
No, it works all the time. You have added extra weight within the hull, which changes the displacement by X. However that weight X is still off center and will have a moment arm which acts on the center of gravity.


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Old 02-06-2009, 15:53   #13
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The key is that water weighs more than air...
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Old 02-06-2009, 16:51   #14
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So I'm trying to logic this out in the real world... If i put a 5 gallon bucket of water with a lid on it in the water, put on my diving gear and take it to the bottom, your saying it will still weigh about 40 lbs...?
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Old 02-06-2009, 17:16   #15
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I'm not totally sure on this one... my guess is that since you actually have to move that mass(of the water) with the bucket, as you move it around, it'll offer greater resistance to movement as opposed to just displacing the water around it. It's not to do with buoyancy or weight, but just the mass that has to be moved. In a keel that will help stabilise the boat, because more energy will be required to roll the boat, with the added advantage that it'll shift the centre of buoyancy when the keel leaves the water, helping the boat to stabilise itself.

There's also the advantage that you can empty it when you're out of the water, making it easier to trailer on land, which, as far as I can see, is the only advantage to having a water ballast as opposed to a heavy metal(also cost).
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