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Old 02-06-2009, 17:19   #16
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Yea... it's confusing. If I have a bucket full of water (open top) in the water, and I'm kneeling on a dock, when I try to pull that bucket out of the water, it gets much heavier as it exits the water...... "Bob Perry... where are you?"
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Old 02-06-2009, 17:27   #17
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Picture a hollow cylinder. It doesn't care which way is up as you rotate it. Strap a container of water to the inside of the cylinder. The cylinder will now rotate until the water is at the lowest point. It doesn't matter whether the container is above or below the waterline.

If you've designed your water ballast poorly, such that it is allowed to move in the cylinder, again the cylinder doesn't care which way is up.

As another said, the water is in air, even though it is below the waterline in the cylinder.

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Old 02-06-2009, 17:31   #18
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hmmm... that helps!
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Old 03-06-2009, 00:11   #19
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I'm going with about 600 bottles. Lead density about 11, for each cc of lead in fresh water 11-1 = 10 grams in water, 2000 * 10/11 = 1818 lbs in water. Also going to ignore the 3% difference for salt because of all the rough estimates.

Going to assume rest is fiberglass, roughly guessing the wood floating versus other metal objects sinking besides those objects being a small percentage of the weight of the glass will more or less balance each other out.

This doesn't seem to be a bad guess for the density of glass.
Composite Materials: fiberglass, cubic centimeters, grams to lbs

4500-2000= 2500 lbs of glass, 1.5 gm/cc density of glass, 1.5-1= 0.5,

2500* 0.5/1.5 = 833 lbs in water.

1818+ 833 = 2650 lbs in water, or 600 bottles.

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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 03-06-2009, 05:00   #20
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cal40jon,

Come on, man, you've gotta tell us what's behind this crazy scheme! Are you really serious about filling up your boat with empty plastic bottles? Do you have any credible reason to justify your fear that the boat will develop a major leak and sink?

Hmmm. Maybe you simply have "multihull envy", and want to make your monohull unsinkable? That's it, I bet. You're tired of hearing the multi guys brag about their unsinkable boats, and you've decided to do something about it!
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Old 03-06-2009, 05:40   #21
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cal40jon,

Come on, man, you've gotta tell us what's behind this crazy scheme! Are you really serious about filling up your boat with empty plastic bottles? Do you have any credible reason to justify your fear that the boat will develop a major leak and sink? ...
... and that the boat will develop a major leak and the plastic bottles will remain uncompromised?
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Old 03-06-2009, 08:00   #22
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Make the keelbolts explosive, strategically add a few airbags, and wire them all to a panic button in the cockpit.
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Old 03-06-2009, 08:43   #23
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cal40jon,

Come on, man, you've gotta tell us what's behind this crazy scheme! Are you really serious about filling up your boat with empty plastic bottles? Do you have any credible reason to justify your fear that the boat will develop a major leak and sink?

Hmmm. Maybe you simply have "multihull envy", and want to make your monohull unsinkable? That's it, I bet. You're tired of hearing the multi guys brag about their unsinkable boats, and you've decided to do something about it!
This is in the challenge catagory, right? The challenge was to determine how many bottles to float the OPs Cal 25. I think using the displacement of the vessel was not right, so I took a stab at it and got about 400 bottles less than he did.


I don't have multihull envy, at least not in the way you make it out. My first choice in a boat would have been a multihull. I used to own and race a Hobie SX18, I currently own a Hobie Miracle 20, I have chartered a Privilege 39 and Kennex 445 in the Caribbean. My personal requirements for a sailing vessel versus money constraints to achieve early retirement to go cruising resulted in after a 2 year search acquiring a 40 year old Cal 40 with new sails and new engine at an extremely good price. A multihull at minimum would have been at least 4 times the price. I'm very familar with the Cal as I have spent years maintaining and sailing (even to Hawaii) a friends Cal 34, so I am happy with my choice within my money constraints.

John
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Old 03-06-2009, 09:04   #24
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Nice one cal40, I didn't think the buoyancy of the boat materials would have such a large effect as that, but with FG density of 1.5 that reduces it a lot.

I think Hud3 got confused between you and the original poster, his question would be better aimed at the OP.
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Old 03-06-2009, 09:42   #25
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You're right, bencoder. I made a mistake by not looking up the original poster. I was just joking with him, but I actually am curious why the question was asked. Sorry, Cal40Jon!

Captainmark, are you really planning to stuff your boat with bottles???
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Old 03-06-2009, 11:36   #26
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all sailors ain't so dumb

I'm impressed with the quality of analysis/discussion in this forum. Also, I am feeling somewhat less stupid myself as I see that the question I posed is not so simple that I have to cringe for not knowing the answer straight out.

Thank you to all who posted.

I'm intrigued with Bencoder's observation about observing the interior volume of the boat below the waterline. Does that define the displacement? It's how much seawater (at about 64 pounds per cubic foot) the hull pushes out of the way to support the entire weight/density of the boat.

If so, that's about 70 cubic feet of flotation. That's a volume seven feet by ten feet by one foot high. A lot to fit into a 25 foot sailboat.

Hmmm, how much volume within a 25' boat? Maybe it's not a lot different from a 22' long 7' diameter soda bottle. She is 8 feet in the beam but less than 6 feet of headroom, so 7' as a first approximation is reasonable.

Lemme see, pi R squared = 3.14 times 12.25 times 22 (LWL) = about 846 cubic feet.

So, in theory (emphasis on theory) less than 10% of interior space devoted to floation would do it. That is, maybe "unsinkability" does not have to be achieved at total sacrifice of cruising livability.

The McGregor 25 floats when totally swamped and has plenty of room below. Of course, this design has sea water ballast instead of 2 tons of lead hanging off her bottom.

If you're still with me, a related question: how hard is the 2,000 pounds of lead pulling her down? The specific gravity of lead is 11.34. That makes it about 11 times heavier than sea water. But doesn't that also make that lead in effect weigh about ten percent less when submerged?

Elsewhere in this forum, I want to get feedback on the idea of dumping a perfectly good Atomic 4 for an electric drive.

Thanks again, skippers. I feel lucky to have stumbled into this place. Someone buy me a beer.

md
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Old 03-06-2009, 11:56   #27
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An other old sailor and I are planning to sail the Cal25 to Europe summer of 2010. We are not planning on taking a lifeboat, but make the boat be her own lifeboat.

We would be glad to sacrifice considerable creature comfort for increased tendency of the craft to stay on top of the water.

Hey, someone joked about airbags and jettisoning the keel. I've toyed with the idea. There are no doubt some dead sailors out there who would have preferred to be wallowing around post storm without a keel. I don't see any practical way to dump the keel. (I have this image of trying to back off rusty bolts in the bilge while the water level rises in the cabin.)

What I would really like to do with the keel is figure out how to replace the lead with lead acid batteries running a 90 pound electric drive instead of relying a 500 pound stinky diesel.

md
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Old 03-06-2009, 13:34   #28
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Water ballast can be some nasty **** if you cant control it and trim the boat as you like. But if you can, its realy practical. You just need to make watertight holes aft and fore, (like tanks) that you can empty and fill opp as you like. The most imoprtant thing is keeping the water from moving side to side, this problem is solved by making some "compartments" inn the tank. just to keep the water from pendling.

This gives you the possebility to have tanks full off air for flotation, or, for trimming the boat for more comfort crusing, making it more stabil or manuvreble (bad spelling, i know) in f.eks strong curentes.

Have fun
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Old 03-06-2009, 13:42   #29
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70 cubic feet under the water line? Probably a pretty good estimate - that's just under 2000 litres (or 1000 bottles of air)
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Old 03-06-2009, 14:09   #30
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Any reason you picked bottles and not foam sprayed in whatever nooks you can find? Or just blast the overhead then scrape it when you arrive? You could go so far as to cover your overhead or other parts of the boat with velcro-backed cling wrap, blast on a good layer of foam, then simply remove the foam and clean up the glue from the velcro when you arrive. Get crazy--paint the foam so it looks like it belongs there. Make foam couch cushions. If you sink, they will float to the overhead. What about carrying an outdated salt water activated life raft in the cabin?

And don't even think of changing to electric propulsion on that size boat if you are trying to cross the pond on the cheap.

Brett
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