Unsinkable Monohull - Why Not ?

[mitiempo]

The wood is buoyant but fiberglass isn't. Neither would most equipment or the engine be buoyant. And 14,350 lbs of lead. And anything above the intended awash waterline would be dead weight.

There is unsinkable and then there is being able to sail after holing - tougher to achieve.

[Matt sachs]

[QUOTE=mitiempo;825398]I wonder how much foam is required to keep a 36,500 lb FD12 afloat? You would lose a lot of storage space.

Sea water weighs about 64 pounds per cubic foot. The foam would have to displace about 570 cubic feet of water for the boat to be neutrally buoyant when filled with water. Thats a lot of storage space

[mitiempo]

Thanks for the math. Too much lost space to foam.

But that is probably one reason few builders have tried it. If you are looking at 2 boats the same size and one has a lot less space and less storage that is usable guess which will sell better.

If it was such a great idea buyers would have lined up to buy Sadlers or Etaps at the expense of other builders - but they didn't.

[HopCar]

Permanent flotation just takes up too much space. Inflatable bags are expensive and take up space. You are pretty unlikely to need either despite what the multi hull guys say. I think your time and money is better spent on damage control preperation, a life raft with epirb and insurance. Just my opinion.

[psneeld]

remember...it's not the weight that needs to be floated it's the DIFFERENCE between objects and their weight underwater (specific gravity)...so things like fiberglass and a LOT of interior things don't have a specific gravty much above water so only a few cubic feet of foam floats a lot. Look at how much foam in a life jacket keeps a 200 pound man above water.

[Matt sachs]

Thats very true. I think a good insurance policy and a life boat is the best bet.

[nwdiver]

Put your life raft inside and pull the cord, if the boat goes down you go down with the ship you are the Captain.

A friend with a big power boat carries a 19’ Whaler as a dingy, he likes it because it’s unsinkable.

[Matt sachs]

Why would you inflate the life raft inside? Maybe I missed something.....

[s/v Jedi]

Calculations are wrong... You only need that foam volume calculated minus the volume of the materials of the boat (they displace water just like the foam)

cheers,
Nick.

[mitiempo]

You would still need 204 cubic feet of foam just to support the 14,350 lb keel, which has a volume of 20 cubic feet. 204 cubic feet is a bit more than 4' x 5' x 10'. A lot of foam.

Makes a good case for a light displacement boat - less foam.

[psneeld]

Originally Posted by mitiempo

You would still need 204 cubic feet of foam just to support the 14,350 lb keel, which has a volume of 20 cubic feet. 204 cubic feet is a bit more than 4' x 5' x 10'. A lot of foam.

Makes a good case for a light displacement boat - less foam.

Do you know what the specific gravity of the keel material is? If not...then your new calculation isn't accurate either...sorry but you can't simplify the calculations THAT much....

[mitiempo]

You need enough flotation to support the weight of the keel minus the cubic area the lead itself displaces.

Cast lead weighs 708 lbs/cubic foot. A keel weighing 14,350lbs is just over 20 cubic feet. To support that weight would require 224 cubic feet of foam. I deducted the cubic area of the lead itself to arrive at the 204 cubic feet of foam.
The specific gravity of cast lead is 11.35.

[s/v Breakaway]

James Baldwin has a good article about adding water tight compartments to boats.

Atom Voyages | In Search of the Unsinkable Boat by James Baldwin

[Lancerbye]

Originally Posted by mitiempo

I wonder how much foam is required to keep a 36,500 lb FD12 afloat? You would lose a lot of storage space.

I think the better solution is a water tight bulkhead forward and another aft. As an example the Dashew designed Sundeers had this, and only 2 through hulls, none in the center section between the bulkheads. Paine's Apogee 50 has a crash bulkhead forward and a watertight door to that section.

It will require 584 cu. ft of foam to totally float that weigt , however you should have some sealed bulkheads and such which will reduce that considerably.

[mitiempo]

And if you are not able to sail it when the foam is doing its job all you end up with is a larger liferaft. And in any kind of swell or waves below decks is not habitable.

As I recall the Etap boats are able to be sailed at a slow speed - at least one was in a Yachting Monthly test years ago - in calm water.