Displacement vs. positive ballast

I have a small keel boat, a Gloucester 16, that all up weighs 900 lbs. I think we all agree that little boats due to thinness of hull and low freeboard are more prone to sink than large boats. So how much positive flotation do I need to make it unsinkable? 900 divided by 64 equals 14 cuft, but once, say, three quarters of the boat is submerged, do you really need that many cubic feet equivalent of air to keep her afloat? Put another way, if she sank, would I need 900 lbs of lifting crane to get her back up off the bottom to the surface in, say, 20 feet of water?

 

Taking your questions in reverse order, you would need in excess of 900lb. lift to get her off the bottom if she sank; if lift = weight = displacement, then you have neutral buoyancy.

You will need about 14 cf of positive flotation to compensate for that 900 lb., or more if the flotation compartments are filled with foam instead of air, since the weight of the foam would have to be compensated for. But you may already have some positive flotation as a part of your boat already, which could offset that number above.

 

If she sank you would have the 900 lbs of the boat plus two or three tons of water.

 

What you need to make your boat float even if it is flooded is anything better than neutral buoyancy. Weight in itself is meaningless. What counts is density. Anything less dense than water will float regardless of the weight and anything more dense than water will sink. If the materials of your boat are only slightly more dense than water however, (as might be the case with fiberglass and wood construction), then you might only need a little bit of help to keep it afloat. If you have lots of heavy ballast on the other hand it would take much more floatation.

So if your boat weighs 900 lbs, then the total unflooded volume of your boat needs to be a bit over 14 cubic feet. Imagine if you were to put your boat into a car crusher and smash it down to just the dimensions of its materials plus the sealed compartments. Well, if the volume of that equaled 10 cubic feet, then you'd have to add at least four cubic feet of flotation to make it positively buoyant. If your boat were largely made of plywood, balsa and Styrofoam, then you might have over 14 cubic feet of material and you would need no additional flotation.

 

Haile,

Did you mean "Displacement vs. positive floatation"?

Here are a few points for you to ponder:

A 100 pound (~40 kg) outboard engine requires ~70 pounds (~30 kg) of

floatation to be neutrally buoyant and a couple more to send it to the

surface.

How much of your hull would you like exposed once it fills with water and/

or capsizes while you await rescue?

A 6000 pound Zodiac 810 has 12 000 pounds of buoyancy in it's air tubes.

Your questions seem simple but the answers are trickier than one may

think and certainly more than you have thus far been led to believe, IMHO.

Good luck,

Tom

 

Adding up all the cubic footage of material a boat is made from including wood, lead ballast, fiberglass skin, and so forth, you might arrive at say, 3 cubic feet. If the boat weighs 300 lbs it will displace about 5 cubic feet of water. Subtract the 3 cubic feet of material volume and you get 2 cubic feet.
A flotation volume (foam or an air cavity, e.g.) of a mere 2 cubic feet will provide a positive flotation of 124 lbs in fresh water. Even if two or three adults are clinging to the boat the boat will remain unsinkable (though the crew will no be able to raise themselves clear of the water because when they attempt to do so, they push the boat down rather than raising themselves up.
Therefore, some additional bouyancy is meeded. I would think at least enough extra bouyancy to allow a crew of four to gain control of the boat by boarding quickly and bailing, since it's possible that the water is cold enough to be dangerous at times.
An additional 200 lbs per person might be about right, so 800 lbs (about 13 cubic feet) of additional flotation.
Based on this you'd need 15 cubic feet of foam and/or air chambers. I don't know how this compares with standards, it's just my own reaoning.

 

I think White and Submarine are on to it. Once the boat sinks, it weighs 900 lbs LESS the weight of the volume of water the submerged materials displace (Fudd), in this case iron keel and fiberglass hull, now that they are UNDER water. This would be the weight Charles Atlas (or the Governor of California for you West Coast fans) would be holding aloft (while he held his breath) directly beneath the sinking hull once it was an inch below the surface with nothing above and no trapped air. Iron is alot heavier than water, but there's only 200# of it in this little boat, so about 80# lift required, per Sub Tom. Glass is not much densier than water. So I would expect one would only be lifting maybe 250# to stay neutral. Thus I might only need say 400# positive flotation to bring enough up to start bailing. Or what? I imagine a removable deck plate in the cabin top to insert a hefty bilge pump, with drop boards in place, that is.

 

My mistake. Iron is nearly eight times denser than water, as Submarine Tom indicates, so the calculation is not 80# but more like 150#.

 

I think you are on your way Haile.

Water is ~800 times more dense than air.

A telephone booth of air (~75 cubic feet) weighs about 6 pounds (~2.5 kg).

You may want to talk with your insurance agent to be sure you're within their

requirements for positive floatation status as you may receive a discount.

Remember: better to avoid ever needing that floatation in the first place

but nice for you to know it's there just in case you do need it.

Good luck,

Tom