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Old 10-10-2006, 10:09   #1
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adequate protected reserve bouyancy

I've seen this discussed before in general, but would like to focus on what would be adequate protected reserve bouyancy. What I mean by protected reserve bouyancy are the watertight chambers in catamarans. I know that a cu ft of air has a bouyancy of 63 lbs in saltwater, so when calculating the amount of cu ft that we have in the hulls in water tight chambers 4 cu ft of air would lift 252 lbs. The heavier the catamaran, the greater the cu ft of air in these water tight chambers would be needed to provide adequate bouyancy. But how this translates into "my boat wont sink" if I had a hole in the central part of the hull I'm unsure. Is it as simple as "if my protected forward and aft water tight chambers in each hull provide 6000 lbs of lift and if my boat is 20000 lbs, then half of that is 10000 lbs and I've got a deficit of 4000 lbs and therefore a hole in the center of the hull would eventually sink the boat?"

I know that it also depends on which chamber is compromised, obviously the forward section is bouyed by the large central chamber and small transom chamber, so that's not a big deal, and the aft section being flooded would also not be a huge issue. But what of something like a failure of the seal on the engine driveleg or some other through hull which is in the main compartment.
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Old 10-10-2006, 12:16   #2
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Quote:
Originally Posted by schoonerdog
"if my protected forward and aft water tight chambers in each hull provide 6000 lbs of lift and if my boat is 20000 lbs, then half of that is 10000 lbs and I've got a deficit of 4000 lbs and therefore a hole in the center of the hull would eventually sink the boat?"
Othes that are better than me at mathematics will explain why better than I can but...You are in much better shape than you think. Your boat may well weigh 20000 when sitting on the land but when you put it in salt water what matters is whether the specific gravity of the whole structure is less or more than the specific gravity of salt water. In my case with foam filled hulls, deck and even furniture, the complete structure will have a specific gravity quite a bit less than salt water. So, even if you fill the boat with water it will still float even with no watertight compartments.

That's correct isn't it you mathematicians?
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Old 10-10-2006, 15:42   #3
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In reality you only need to provide 'floatation' for the difference in weight between the materials your boat is made of and the weight of the water that those materials displace to keep her from going to the bottom (when the hull is in free communication with the sea).

A cubic foot of wood displaces the same 64lbs of seawater that a cubic foot of lead does, but generally woods are lighter than 64lbs/cuft and they float while a cuft of lead is around 700lbs and would require almost 11 cuft of sealed airspace to keep it afloat.

The method for determining how much 'floatation' a boat requires to keep it from completely sinking, as well as the method for keeping level floatation is on the Coast Guards web site. The complete method is found in the regulations and a quick method is described in a publication they put out for home-built boats.

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Old 11-10-2006, 07:29   #4
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I talked with our boat designer which was very useful. He mentioned that basically with our boat having around 100 cu ft of air space forward in a water tight foreward locker (8 ft long, 6 ft high) for around 6300 lbs of lift and probably around 10 cu ft for 630 lbs of lift in the aft transom in another water tight compartment. Should we get a breach amidship, our boat would float with the bows above the water and the transom awash IF we actually make the forward compartment completely water tight by replacing the access door with a water tight door to prevent it flooding. So obviously thats what I'm going to do! If we kept the forward compartment with a non water tight access hatch or had just a big opening up to the bow with a birth in it, then any breach would have no interior bulkheads to stop the entire hull from flooding and sinking. One thing he mentioned as well, which we never thought of and are very guilty of, is not being diligent about making sure the forward escape hatches are dogged and locked before we go out. If we were to flip with those hatches open, then the otherwise trapped air flows out the escape hatch and the boat sinks pretty far down to the level of the opening of the hatch.

Basically the boat needs trapped air to float and the mast, the galley stove the engines, heavy provisions all weigh the boat down. A criticism that boat designers have with boat builders is that often the boat builder will compromise all of the water tight compartments that they designed to try to make the floor plan in the hulls open and maximize the useable space in the bows with things like forward births that can't be sealed off.
Of course the heavier the boat, the more trapped air one would need in water tight compartments to remain floating.
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Old 11-10-2006, 18:22   #5
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It's heaps easier with metrics. One litre of (fresh) water weighs one kilogramme. So for each tonne of weight you need to occupy more than 1000 litres. (1 cubic metre) Preferably a lot more.
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Old 12-10-2006, 06:19   #6
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Cruisingcat,

Although you are right that the calculations using metric measure are easier, they don't really work well when it comes to boats. Displacement of a vessel is figured in long tons (2240 lbs). That 'ton' works out better when doing displacement calcs and propeller calcs than either the standard ton (2000 lbs) or the metric ton.

Then there's the ubiquitous 'gross tons' and 'net tons' documented boats have carved into a main beam. It is a volume measurement (100 cuft). And there is also 'Thames tonnage' which is a cargo carrying capacity that I don't recollect the value of at the moment.

I wrote in last month's "Motorboating" magazine about how when they quoted the diplacement of a cruise ship they used the wrong 'tons'. It's very easy to confuse as with everything boating related. Let's not even get into 'plimsol marks'.

Richard
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