Tanks - Free Surface Area

Discussion in 'Boat Design' started by SeaJay, Oct 6, 2008.

  1. SeaJay
    Joined: Jun 2007
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    SeaJay Senior Member

    I’m specifying the water tanks for my 46’ motorsailer and am looking for input on tank size relative to free surface area. I plan on using Ronco poly tanks, which I understand do not have internal baffles. I read somewhere that in a baffled tank, you should try to limit the free surface area to around 4 square feet. The tank I would like to install is a 49 gallon B482 (20”W x 43” L x ~ 14” D). A pair of tanks would be installed opposite each other with the long dimension athwartship. (I know that it is preferred to have the long dimension running fore and aft, but this particular tank fits very nicely in the other direction.) This tank has approximately 6 sf. of free surface area.

    My questions are:

    1. Does anyone see a problem with a tank of this size, installed in this configuration?
    2. What is the “word on the street” regarding the quality of Ronco tanks?

    Regards to All
     
  2. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    ALL the plastic tanks have a different mounting systen from metal tanks.

    They have to be filled , and will grow with the fuel inside , then are foamed and strapped in place.

    The Roncos we have used have been fine.

    Biggest hassle is with the simple shape an EZ to maintain tank is not possible.

    For an offshore boat the chance of crappy fuel with water and asphalted fuel is a concern.

    While the plastic tanks will probably last the longest , the inability for the tanks to be self maintaining , or even easily cleaned IS a concern.

    Simplest method to limit free surface is smallish tanks that can transfer totally into a baffeled day tank , that Gravity feeds the engine and secondary filters.

    FF
     
  3. Sea Jay
    Joined: Aug 2006
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    Sea Jay Doug Brown

    Fred,

    Thanks for the input. In your post you say...

    "Biggest hassle is with the simple shape an EZ to maintain tank is not possible." and "While the plastic tanks will probably last the longest , the inability for the tanks to be self maintaining , or even easily cleaned IS a concern."

    What to you mean by "self maintaining"? I understand that smaller tanks are the answer to free surface area, and that is really the heart of my question. It is simpler and less expensive to purchase two 49 gal tanks rather than four 25 gal tanks but I was concerned that the 49 gal tanks may be pushing the free surface issue a bit. However, they are on the market so I expect the thanks are working satisfactorily somewhere...or maybe not.

    In any case I do intend to install a smaller day tank as well.

    Thanks again for the comments.
     
  4. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Fuel dies , less on a boat like a commercial vessel that runs 24/7 , more noticable on a pleasure boat that runs infrequently.

    To be able to clean a tank it is required to gain access and scrape the gunk from the walls and bottom.
    An access hatch to get a power wash or putty knife in is needed , hard to have holes in the SIDES of a plastic tank.

    For a tank to be self maintaining , to be able to purge water, requires at least a sump in the bottom, think of a quart jar stuck like a pimple on the bottom of the tank.

    A pump can be used to pick out the gunk but the Sparkman & Stephens folks (and the US Navy ) go further to excellence.

    To create a great fuel tank the fill must be accessible either below deck or from the deck. If accessible from a weather deck it is best if the fill is under a large deck plate , that is removed to access the fill.

    On Navy boats the fill is a 3 inch pipe with a monel (its the navy) tube that just fits inside.

    The outer 3 inch tube has rows of holes of holes that allow the fuel into the tank.

    The inner tube has 60% of its surface cut away and very fine monel screening.The inner tube can be lifted out to clear the screening as required.

    What makes the system so useful is the inner tube also has a cup that fits into the tank sump, filling it.

    The outer tube simply fits thru the bottom of the tank , and becomes the sump.

    So an operator can check the fuel condition by simply drawing the inner fill pipe and looking in the attached cup.

    No water in the lowest part of the tank, you are set to go.

    Should a load of watered fuel be taken aboard , the tank can literally be bailed. The last little bit will take a while to wait for the water to settle.

    for me, This is the system that should be on any voyaging boat , power or sail .

    FF
     
  5. Sea Jay
    Joined: Aug 2006
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    Sea Jay Doug Brown

    Fred,

    The tanks in question are water tanks, but the discussion on fuel tanks is very informative. I don't know that I'm quite up for monel fill tubes and screens. The 40gal poly fuel tanks I'm using (Moeller) do not have cleanout access, and are only tapped from the top. Fortunately I can get at them without chopping the boat apart so if they do get permanently fouled, I do have some reasonable options.

    Regards,

    SeaJay
     
  6. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    To get rid of any free surface hassles make the day tank at least 60g.

    You can run it half empty and pump a full 40g tank in ,,,, no free surface.

    If you look in scrap yards etc you can fing old MONEL tanks , they were very common in the old wooden boats.

    They can still be modified with ease (and heat) to accept the parts and configuration to create a forever grand tank.

    FF
     
  7. Sea Jay
    Joined: Aug 2006
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    Sea Jay Doug Brown

    Monel tanks in scrap yards...now there's a happy thought. I'll check it out!
     

  8. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 110, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Monel tanks in scrap yards...now there's a happy thought. I'll check it out!

    Go to big old boat yards , look behind the buildings and then ask the staff if they have any in storage.

    Not that many old wood boats are scrapped as so few are left.

    Marine Stores that sell used equippment are also a good chance, if old and big.(tanks take room to store.)

    Might be worth a trip north to Washington or Oregon , more lakes that might have more old woodies.

    FF
     
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