Air Tank for reserve of buoyancy

[Mr Efficiency]

Polystyrene is superior to PU foam in many ways, but of course can't be poured, and is vulnerable to contact with fuel. It is probably wise to paint it with acrylic house paint, a couple of coats. Cheap mis-tinted paint from a paint shop will do.

 

[Rumars]

In production you would have the foam CNC cut to shape, smear with glue, put it in the cavity, glue on the lid. If one or more sides remain exposed, either infuse a light layer of fiberglass beforehand on it, or just paint with the rest of the interior.
PET foam is the other alternative, but it's more expensive then XPS.

 

[fredrosse]

The distribution of flotation elements within a boat generally requires that, as the boat floods (due to piercing of hull, virtually anywhere) an even keel/upright boat orientation is maintained. This distribution can be attained with expanding foam, expanding foam and encased plastic bottles, or just plastic bottles, as long as they are captive in a compartment or encased in un-used space within the hull.

I do not understand the need for "goretex vents or rubber diaphragm" for pressure equalization?

 

[Rumars]

With temperature and outside air pressure variations the (sealed) tank walls can experience significant forces so it's better to either have the same pressure both sides (goretex vents, only air passes), or use a rubber element (diaphragm, lenght of capped hose) to accommodate the inside air expansion and contraction. Big steel boats probably don't need it, but it's standard for small ply and composite ones.

 

[fredrosse]

OK Rumars, I understand. I had assumed that the compartments housing the flotation material would not be built "air tight", and it is true that if they are actually "air tight" then they would need to be strong enough to withstand the typical variations in atmospheric pressures as well as the pressure variations coming with temperature changes. Alternately, a flexible diaphragm to equalize pressure would often be needed.

 

[Mr Efficiency]

Boats seem to sink despite buoyancy chambers, but then sometimes the term can be a bit flexible. I recall a new GRP power cat sinking, that allegedly had watertight compartments, (advertised as such)but while the compartments were all sealed to above the waterline, and under a sealed sole, there was a little gap between the sole and the compartments, and when water leaked in and filled the rear compartment due to an improperly sealed engine mount bolt hole, overnight, it was enough to drop the stern enough so that the water now spilled into the next compartment, and so on. Such so called sealed compartments remind of the Titanic !

 

[fredrosse]

Yes, virtually the same story as the Titanic, with one very notable exception! The Titanic was designed to withstand the hole in one watertight compartment, but several compartments were holed at once, and this led to a condition exceeding design criteria, the flooding came over bulkhead height, and she sank. Any boat that is designed with watertight compartments should be designed to withstand at least the hole in one compartment, evidently not properly designed in the "new GRP power cat" you mentioned.

Ordinary commercial ships are designed to allow flooding of one compartment, separated by bulkheads. US Army seagoing dredges look like typical merchant ships (sort of), and are designed to allow flooding of any two watertight compartments. Military ships of course, can typically withstand flooding of several compartments.

 

[Rumars]

One of the methods used was a lenght of surgical tubing capped but with a small (1mm or under) hole at the far end. This allowed pressure equalisation while beeing waterproof enough to maintain a seal if the boat capsized. Nowadays, for a few dollars, you can get a small screw in cap (it looks like a normal machine screw), metal or plastic, wich has a small piece of goretex inside, and performs he same job. There are also glue on goretex patches, the surfboards and some waterproof cases use them.
If the chambers are not airtight you have a problem, either you locate the vent at the top, and have problems if you capsize, or you have it somewhere in the middle, but then you need double size tanks to insure the hole is always above water.
Of course everything is simpler with foam, and as I said today there is no problem with using a rigid one, it just gets cut to shape and installed before the compartment is permanently closed.

 

[Mr Efficiency]

Yes, virtually the same story as the Titanic, with one very notable exception! The Titanic was designed to withstand the hole in one watertight compartment, but several compartments were holed at once, and this led to a condition exceeding design criteria, the flooding came over bulkhead height, and she sank. Any boat that is designed with watertight compartments should be designed to withstand at least the hole in one compartment, evidently not properly designed in the "new GRP power cat" you mentioned.

Ordinary commercial ships are designed to allow flooding of one compartment, separated by bulkheads. US Army seagoing dredges look like typical merchant ships (sort of), and are designed to allow flooding of any two watertight compartments. Military ships of course, can typically withstand flooding of several compartments.

The boat I referenced was super expensive, but the methodology was totally defective, unless the compartments are completely sealed, not worth much. I guess it was easy to hide underfoot, which is why when I buy a boat like that, out comes the hole saw, I want to know what is hidden from view.

 

[fredrosse]

"I want to know what is hidden from view."
If I could count the times? A vendor, of pumps, or turbines, of heat transfer equipment, or whatever, have given false representations of the capabilities of their product. Learning to play "20 questions", leading to the truth of a claim, has been around for longer than I can remember. Another game: "I'm from Missouri, show me" also works.

 

[Cacciatore]

Thanks for the several replies. My catamaran is an high level boat so i can't use ping pong balls or bottles for floating anyway my dubts are regarding the method of ISO for this air tank and pressure valve that seems noone have used.The last choice will be XPS milling with CNC .

 

[Rumars]

For the air pressure valve I would do following: presuming that your air chambers have screw in inspection ports, take an exta cover, mount an air hose connector to it, and use it to test each tank. If you don't have inspection ports, drill a hole and use a rubber compression plug with a air hose fitting for testing. After testing you can plug the holes, but I really recommend having inspection ports in all air chambers.