I was just reading the ABS rules and it says that if you have a cylinder of air or oxy inside that if it is totally released the internal pressure should not be greater than 15 psi. Also if you had an air leak then the internal pressure could rise so that when you hit the surface the hatch could pop before you vent the MBTs. You also have to have a relief valve with an isolating valve wired open. What do you set the relief valve at. Also the name plate on the vessel must show maximum external pressure and maximum internal pressure. This would also be required to determine the number of bolts holding the viewports in. Hugh
Minimum internal pressure? Hmm. I'm not sure it matters as even a hard vacuum would be marginally less than your 15 pounds. Maybe I'm missing the point. For the seal test, an inch of vacuum would be more than enough. Half a pound or so, I think.
Thanks Vance, that’s good to hear. Another question. What is the minimum internal pressure allowable in a PVHO?? I have seen reference to 15 psi but the catches etc need to be looked at.
No Perry that I ever heard of completely flooded a battery pod. My two experiences had partial flooding, taking on water steadily while we surfaced and managed the recovery. Typical to those events was that the pod went heavy on the back upon surfacing due to the forward mount of the MBTs. Complete flooding would shift your CG to the heavy side and move your CB. But certainly not enough to drag the sub all the way onto its side. The metacentric heigh would be rotated that way, but not overcome. Worst case, the wet pod would become your keel, more or less.
A 20"X120" pod takes on about half a ton of water in the worst case scenario. This may be glaringly obvious, but don't forget the battery volume displacement when you are doing the figures. Batteries aren't quite as heavy underwater as they are in air, natually, and whatever volume they take in seawater is subtracted from the flood quantity. Plus the acid is lighter than seawater. It isn't a calculation for weight gain. It's strictly a negative displacement puzzle.
How big are your ballast tanks and how are they distributed (side to side or fore and aft)? The Perrys are shoulder tanks, of course, and that was done for a couple of reasons, mostly to clear the bow for viewing, but secondarily to offset the heavy pod in case of flooding. You could vary the lift if needed.
One of the standard "hanging-in-the-paint-locker" items was a pair of big teardrop floats with short pennants and snap shackles attached. These lines could then be wrapped around the guard rail wherever necessary and snapped back to themselves for lift in case of a flooded or partially flooded pod. The swimmer does that, and you should be able to figure how many buoys and what size to compensate for the unplanned loss of buoyancy. The idea is to spot the buoys where needed for lift to bring the sub level and steady on the surface before opening the hatch if that turns out to be necessary.
Bailing out would be an absolute last-ditch measure offshore, and never done to my knowledge, even with an extinguished fire on board and the boat full of smoke, which I did once. That's what the BIBS and scuba masks are for, to keep you breathing and clear eyed until the topside crew can save you and the million dollar underwater pickup truck. We surely didn't want to lose one of those subs, and didn't. It was never even close. Planning is the key.
Phil and his crew do vacuum checks on the void spaces (pods and hull) before every dive. We didn't do that, but I'm betting we would today. It's just too simple and elegant a solution to ignore. A couple of minutes while buttoning up will check for leaks and set your O-rings so that this wouldn't or at least shouldn't ever be a problem.
Vance, I am interested in your comment that “if flooded it would stand the sub on its tail.”
Is that a must to avoid? How many subs are able to have a compartment flooded and not come up on some obscure angle? I.e. If a battery pod is flooded would it not make a sub come up on its side so that the hatch if opened would flood? I am not being facetious but concerned about mine’s ability to come up with a compartment flooded. I haven’t done the calcs yet but what designs would qualify as same. My philosophy or belief was as long as you can get to the surface you are rescueable. Interested in your comments on this one. Hugh
You can't dfepend on neutral buoyany for a module as you describe. It will need to have a variable ballast tank and fairwater tanks to hold the unit up on the surface. Along with the controls for that system, you will need intake and exhaust and cooling valves interconnected to the pressure hull (or controlled in the module by reach rods), PLUS your electrical distribution to the control panel for charging and/or hotel loads, including propulsion. AND you will need an additional dropweight to offset the volume of your propulsion/charging center, as it will be way aft of your CG/CB and might stand the sub on its tail or thereabouts if flooded. Nothing to it.
Hi Jens. That's what I was thinking. We were sailing last weekend on San Francisco Bay with a friend and his 26 foot boat has a little two cylinder diesel motor. My thought was to use a pressure vessel with a bolt-on end cap dome, and install a small diesel motor, generator, and fuel tank in it. Make the "unit" completely separate from the cabin hull with a "disk type" thru-hull for cable and controls. The unit could provide power for running electric motors OR have a propeller shaft coming out the back end ( OR both.)
It would need to be neutrally buoyant as a unit so the sub could dive even if it was removed. This "power module" could easily be bolted to the sub and removed as a unit if it needed repair or maintenance. Cooling would be an issue but if the motor was water cooled, just a water pump and plumbing would be needed.....no radiator.
The small two cylinder diesel on my friend's boat pushed his boat at about 5 knots at 3/4 speed and he said it will run for days on a tank of fuel. I think the motor brand was a "Universal". Probably made in China but a quick search on Google would show what's available in a compact motor that was water cooled.
I designed my sub to accept a small 20 horse outboard on the rear transom plate for long distance surface transit. The cables and wires will pass through a small electrical thru-hull, and for steering the motor has a link to the electric thrusters/rudders linkage. The outboard is small enough to be put inside the sub if needed but most likely will be loaded onto the surface boat when diving.
A permanent diesel electric "module" seems like a fairly simple thing to fabricate, and the sub could still be used with or without it.
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