Sub hull thickness and fibrglass layup - help

[oracle_9]

I will be scratch building a sub of some sorts. It will be about 36" long, circular cross section of 4" diameter. Imagine a tube running straight for about 24" of the length, and then the last 12" it will taper down to about 0.5". The front end is basically a half a sphere.

It will be a static diving sub or what others call it an "assisted dynamic diver". I am guessing it will dive to a depth of somewhere between 1 ft outside, and maybe 4 ft inside in a pool.

I dont know exactly how strong to make the hull for this model to withstand the water pressure, or maybe sinces its only a maximum of 4 feet, it's not much pressure? Anyway, I am planning to use Fibreglass and hollow inside. I would like to ask what wall thickness does my sub needs to have? And if possible any layup recommendations, like X number of layers of Y oz of fibrglass, etc.

Another question, same scenario, but lets say its not hollow, but rather it has foam inside.

[adriaticsea]

Hi Oracle 9,

considering, as you

Hi Oracle 9,

considering, as you outlined it, the design of your submarine (long straight central section, tapered stern and hemispherical bow), my advice is to use the "wet hull" principle and go with two different materials for the submarine's hull. Wet hull means that you'll make watertight and pressure resistant ONLY the central (i.e cylindrical) part if the submarine, while the bow and the stern will be free-flooding areas where no pressure differential acts on the hull's materials.
For the central section, UPVC pipe is a very good and cheap material: it will withstand 10 ft of water with no problems and will machine easily.
For the bow&stern fiberglass is tha way to go but I'm no expert on the layup.
Hope this helps,
BEST REGARDS MAURO

[himszy]

Isn't a wet hull where

Isn't a wet hull where there is a independent watertight chamber inside the hull - thereby making the hull 'wet' on both sides?

As opposed to a dry hull where the hull itself acts as the pressure vessel?

Michael

[adriaticsea]

Hi everyone,

you're right Michael, I

Hi everyone,

you're right Michael, I should have said dry hull in this specific case...Oracle 9, the concept I was talking about is indeed called dry (or single) hull, but the above mentioned explanation and materials are correct. Sorry for the mistake, I was typing in a hurry when I wrote the message.
BEST REGARDS MAURO

[oracle_9]

Thanks for the suggestions so

Thanks for the suggestions so far.

But, I would really prefer the fibreglass hull methods, and I guess the dry hull method.

[Wheelerdealer]

It would be easier to

It would be easier to make a wet hull boat with WTC. You can still make a fibreglass hull. It won't matter about the thickness/ strenght of the hull if you do it this way as the hull won't be holding back any water pressure. Also you sub will be a lot lighter.

[adriaticsea]

Hi everyone,

Oracle 9, if you

Hi everyone,

Oracle 9, if you want to go with fiberglass, then Wheelerdealer's advice is a very good one: first, you won't have to do structural calculations in order to avoid deformation of the fiberglass hull under pressure and you will be able to use the same UPVC pipe I mentioned earlier to build a reliable cylindrical pressure proof compartment.
BEST REGARDS MAURO

[Guest]

Structual calculations on a sub

Structual calculations on a sub hull?

Awww, come on. I use a very scientific way of judging pressure hull integrity- I sit on the blighter. If it takes 168 pounds of my fat backside it's good enough for any depths I wish to go.

Personally, I'd make the boat a dry hull. Use 4" PVC pipe (or as close as you can get 110 mm is common here in the Uk, which is colser to 41/2") for the main centre section. Cap it off with a moulded hemisphere, or mould one in GRP, and make the cone like tail from GRP.

I would cap of the rear cone with flat plastic sheet and vent the rear to free flood. Seal off the main pressure hull with a single o-ring. Unless you have access to a lathe, use axial compression for the o-ring.

Andy

[oracle_9]

Typically what is the wall

Typically what is the wall thickness of these PVC pipes?

[Guest]

The stuff I have is

The stuff I have is about 3.2mm thick, which is adequate. If the pipe was GRP you could probably halve that, as it's much more rigid- PVC is relatively soft, which is why it machines nicely.

Andy