calculating psi at depth

Discussion in 'Boat Design' started by jstepp590, Jul 26, 2004.

  1. jstepp590
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    jstepp590 New Member

    Here's an off idea that is a casual fancy of mine that I need some help with. I came to the conclusion a long time ago that, except for human error, the next most likely cause of ocean fatalities in small craft is weather (just a belief, not studied fact). If this is the case or close to it why not design boats with the ability to sink to calm waters until the storm blows past? Any scuba diver or submariner will tell you that no matter how rough the weather is on top it's serene below.

    It wouldn't need to go anywhere underwater so a propulsion unit is unneccesary. A simple CO2 scrubber and minimal gear could keep the air clean and breathable, old 1950's submarine technology. It would stay down to depths of up to 100ft for up to a week. At that depth an ocean liner could probably go right over it with no danger. It could be an aid for security as well in dangerous waters, pirates can't board it can they?! This especially makes sense for cruisers where comfort and safety are more important than racing.

    What I don't know how to do is calculate psi per foot of depth on an enclosed hull of normal air pressure. Until I know that I have no idea what materials, hulls and techniques are feasible. I know, the idea's a little out there but it would mean there will be no size limits on small craft on the open ocean any more due to weather. A boat the size of a Flicka would be safer on the open ocean in a Cat5 hurricane that at the dock!

  2. John David
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    John David Junior Member

    Approx. 0.5 Psi Per Foot. To Avoid Most Wave Turbulence, 100 Feet Is Good. Don't Forget Depth Control Means. Otherwise You Will Go Deeper And Implode. Very,very Tough Problem. Good Luck.
  3. Arrowmarine
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    Arrowmarine Senior Member

    LOL. It took me an hour to realize that john david is correct. (Sorry John, didnt have the right math glasses on.). Yes, I think I recall my begining scuba class several years ago. Pressure at sea level is somewhere around 15psi or 1 atmosphere. Every 33 feet down that figure multiplies. 66 feet=2 atmospheres or approx. 30psi. 99 feet=3 atmospheres or approx. 45psi. Etc....Etc. Which of course is, low and behold, somwhere near 0.5 psi per foot. I hope my memory serves correctly. (Someone verify this for me)
    Anyway...... just my two cents. Hope it helps.

    Great Idea and good luck, Joey
    Last edited: Jul 27, 2004
  4. carlg
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    carlg Junior Member

    One of the constants I definitely remember from my years of Physics: 0.433 lbs per square inch per foot of depth in water.

    (For fresh water at 62.4 lbs/cu ft)
  5. Come on guys, seawater is 64 pound per cubic foot and there are 144 inches on the bottom of a cubic foot so 64 dived by 144 is .44 pounds per square inch per foot. Remember displacement and all that good stuff. <BG>
  6. tom28571
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    tom28571 Senior Member

    Salt water weighs approximately 64.5 lbs per cu ft, depending on salinity. At a depth of one ft the pressure is 64.5lbs/144 sq in or .445 PSI. One atmosphere is 14.7 PSI at sea level.

    Therefore water pressure is one atmosphere, or neutral pressure on a closed container, at (14.7/.445) ft, or 33 ft as was said above. At 99 ft, the pressure on the container is 99/33 = 3 atmospheres or 44.1 PSI. The net pressure the skin must handle is 44.1 - 14.7 = 29.4 PSI.

    If you go back to fundamentals, you don't have to remember all the
    derivative stuff.

    Lots of people died, and still do, in developing moderately reliable submarines. I expect many more would die in this one than would die if it stayed on the surface. Think about it, but don't do it.

    Robert sneaked the answer in ahead of my post.
  7. jstepp590
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    jstepp590 New Member


    Wow, thanks for the responses! I really do appreciate it. I find this idea to be intriguing and feel I may follow up on it when I can connect a submarine engineer, a sailboat designer and the proper capital. At the moment I am looking to see if it is even feasable while keeping the boat light enough (and inexpensive enough) to sale. It will definitely be applicable to monohulls as I feel the engineering required for a multihull will be much more. Again, thanks for all your help.
  8. One of the larger problems that you will face is that the ideal shape for a submarine is a round pressure hull and that’s not a very good shape for a boat. If you look at a picture of a sub you will notice no ports, no deck and a shape that is not very conducive to deck parties. This is done to give it a shape that will survive the pressure. It’s a nice project to think about but it will not be very practical as a boat.
  9. Willallison
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    Willallison Senior Member

  10. Karsten
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    Karsten Senior Member

    From my experience subs are great in rough water. Underwater it's relatively quiet and on the surface you just close the hatches and use the periskope. Only problem are seas from the side. Because the hull is usually round rolling is an issue.

    The problem with your idea is that if you want to make the boat withstand the pressure at 30ft you don't have to dive to survive any storm! Diving has it's own problems. It's an instable condition. If something like the turbulence of a wave makes the boat go deeper the pressure on the hull increases. This reduces the volume of the hull. If the weight stays the same you just keep sinking! Therefore you need some sort of active control (depth rudders). If you need depth rudders you need speed to make them work. If you need speed you need an engine and a power supply for the engine. If you have speed you want to make sure not to hit things. Since looking out the window doesn't work you need sonar which sounds expansive. Also lead batteries generate hydrogen and hydrogen combined with oxygen is explosive. Your boat wouldn't be the first one to blow up!
    Outside you can't use any standard equipment because it's not made to be underwater. One advantage is that you don't have to worry about washing the deck. Just go for a dive.

    Good luck with the project. I think you'll need it. Talk to somebody who has been on a sub!

  11. redcoopers
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    redcoopers Member

    You're looking to sell this idea?

    I think you'll have at least one more problem besides implosion: marketing. Who will really want to "buy the boat that sinks?"

    Anyway, most fatalities in sailing involve 1) drinking and 2) not wearing a pfd. The other component of fatalities which accounts for about 2% is racing (i.e. Sydney-Hobart and Fastnet). About 80% of these fatalities involve human-error (inexperience, poor navigation, poor meteorology, not deploying warps, not heaving-to when you should, etc).

    Overall, boating safety is mostly related to the human at the helm, not the boat. Inexperience kills. If you want to make a safer boat design, what I would do first is ensure that a capsize cannot kill people belowdecks. By this I mean that I would make sure floorboards cannot fly out and hit someone on the head (this happens more than you would think!). Secondly, I would make sure the ports have sufficient strength in wave impacts. Third, I would make sure the boom height is above the head height of anyone standing in the cockpit. I can go on...

    Finally, a racer would not buy a 30' boat which is about 20k lbs or more...

    In the end, your idea sounds like a very cool technical project and would be a lot of fun to work on, but if you're trying to focus on safety, I think that crew training should be your primary area to work on.

  12. When I saw the submarine that Will designed, this thread, at least for me went from humorous to serious. I see that SNAME has published “Submersible Vehicle Systems Design” and I under stand another volume of standards for passenger carrying submersibles. Are there many under water boats in private hands. Are ther any underwater yachts out there. Is this a real market?
  13. grob
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    Great site,

    On the incentives page it says "If you know someone who can, and does, purchase one of our luxury diesel electric submarines, we'll pay a minimum of $50,000 in a referral fee."

    Try Dr Evil, he has a submarine lair see and can be contacted on can I have my $50,000 now.

  14. John David
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    John David Junior Member

    Margin of Safety

    Tom Lathrop,(aka,Tom28571)

    You are correct in that the density In pounds per cubic divided by 144 yields the pressure in psi per foot. However, in the submersible and underwater equipment field, engineers like to use 1/2 psi per foot. 1) It is easy to remember 2) It provides a 10% margin of safety which is comforting in the face of so many uncertainties.

    Interesting but not the reason I am posting this reply. You suggest that because the pressure in the boat is atmospheric one is entittled to subtract 15 psi from the working pressure on the hull. This would result in a dangerous understatement of pressure. The absolute pressure on the hull is actually the water depth pressure plus atmospheric(15 psi) This in turn cancels the atmospheric pressure within the boat. Therefore it is correct to use the calculated water depth pressure as the design pressure.

    So much for remembering fundamentals.
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