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Forward fuel tank

Discussion in 'Technical Discussion' started by Capt Fred, Sep 29, 2015.

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  1. Capt Fred

    Capt Fred Senior Member

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    All: On a 2008, 47' Riviera I run has a forward fuel tank that holds approx 200 gallons. The boat will mostly be used for short fishing trips and weekend trips to Catalina Island. The need for the extra fuel is not needed. The tank is fiberglass. Would it be ok to leave this tank empty and if so what would be the process for getting this tank back in usage if the need arises for having this additional fuel reserve if it is left empty after a prolonged period of time? Thanks
  2. NYCAP123

    NYCAP123 Senior Member

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    Most (but not all) boats have a valve in your fuel line for selcting the tank you use, a shut off valve near the tank and/or a valve between them to transfer fuel. They're pretty obvious. If you haven't found it I suspect you don't and it operates on gravity or each motor feeds off a different tank (a system I don't like). When full that tank is about 1,400 lbs. Having it empty could affect balast, and leave you riding bow up, stern down. That could increase your fuel burn, slow your getting on plane, lower your speed and make your motors work harder. 200 gallons is probably only about 5 or 6 hours of running time. If I remember the song, Catalina is "26 miles across the sea" or about 2 hours each way give or take. Assuming your aft tank is of similar size that gives you total running time at 10 to 12 hours. I wouldn't mess around.
  3. Capt J

    Capt J Senior Member

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    Yes, you can transfer it to the aft and leave the tank empty. To put it back in service, just fill it back up. Most SF prefer the bow tank emptied first, however your boat may be different you just have to see how it rides.
  4. NYCAP123

    NYCAP123 Senior Member

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    Correct although some prefer to come down equally, but you don't dump that capacity. Definitely play with it and see how she rides best. Also, many places give a volume discount on fuel and that extra 200 gallons may get you there.
    BTW, if your fuel burn gives you that 10 to 12 hour running time I suspect, and seas kick up turning that 2 hours to Catalina into 3 hours, having that forward tank empty could leave you short of getting all the way home.
  5. Capt Fred

    Capt Fred Senior Member

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    The aft tank is 700 gallons so there is plenty of fuel for many round trips to Catalina. The forward tanks has a shut off valve. The planing tabs have ample authority to bring the bow down for an economical ride. Have not run the boat in a heavy head sea. My question was more on the lines of condensation collection/algae and the resulting problems. For example if the tank was empty for two years, would it be necessary to pull off the inspection plate, suck dry, clean to the best given baffle limitations and then put back into service?
  6. NYCAP123

    NYCAP123 Senior Member

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    That's 4900 lbs. sitting aft. Every boat's COG is different, so experiment, but I think you might find it better to take less in the aft tank than emptying the forward.
  7. Capt J

    Capt J Senior Member

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    No, if the tank is completely empty, you shouldn't have any issues with condensation. Condensation is usually formed by the temperature of the liquid in the tank changing expanding and contracting and drawing moist outside air inside the tank and thus condensation......But your transfer pump might not, be happy after sitting for 2 years......another option is to fill the foward tank and then just transfer it back to the aft when you fuel.
  8. Capt J

    Capt J Senior Member

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    On SF, 90% of them prefer the aft tank full and foward tank emptied first because the cockpit creates a lot of bouyancy with not a lot of weight there (besides fuel) making the LCG shift foward.....
  9. Capt Fred

    Capt Fred Senior Member

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    All: Thanks for your input. I appreciate your technical opinions.
  10. Capt Ralph

    Capt Ralph Senior Member

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    Errrrr aahhhhh?
    It may not be all the temp of the fuel, the bilge area temp vs the water temp vs outside air temp and humidity are all factors in condensation forming in your tank. Fuel temp is a factor but vented empty tanks will condense and make water.
  11. Capt J

    Capt J Senior Member

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    Not nearly as much, the fuel inside the tank (or other liquid) draws air in and out as it expands and contracts due to temperature, an empty tank with just air, not hardly at all.....if you're really worried when you want to use the tank, put 5 gallons of diesel in the tank and take the output of the transfer pump into a bucket and flush what little stuff might be in there......If you leave an empty 5 gallon gas can in your warehouse without the lid on it, you never find water in it......so why should an empty fiberglass fuel tank?
  12. Silver Lining

    Silver Lining Member

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    Uhh, I think these physics are way off. The volumetric expansion of air is almost 10 times larger than that for diesel fuel. A 40 degree F change in temperature will yield a less than 2 percent change in volume for the diesel but will yield a more than 13 percent change in air volume. The changing air volume will draw in and expel air out of the vent as the temperature changes. Of course the humidity and dew point will significantly affect the amount of moisture condensation.
  13. Capt J

    Capt J Senior Member

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    Perhaps, but I've seen completely empty tanks on yachts remain completely empty for years. Perhaps the air doesn't condensate or if it does the moisture then evaporates as it heats up. They didn't start making water. One was a holding tank on a mid 80's hatteras that was never used, as the toilets had been valved for direct overboard the vessels entire life. The people that bought the boat in 2000 took the inspection plate off to put a pickup and turned it into a diesel tank because the boat was going to central america it was dry as a bone inside and the resin looked brand new.

    I have 2 5 gallon plastic fuel tanks sitting on the shelf in my garage right now that have the pour spouts on them, open to the atmosphere and have been sitting there for almost 5 years that way, they are bone dry inside.
  14. Silver Lining

    Silver Lining Member

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    Capt J my guess is there are a few things at play, that the sticking coefficients and wetting properties are very different between fiberglass and the diesel fuel surface. And this leads to a different evaporation energy. On the dry surface, the water molecules are not strongly bound to the surface, and only a moderate thermal energy is required for evaporation. It is quite different in the diesel fuel because there is also a solubility/saturation element that effects the total amount of moisture absorbed and further since water is denser than diesel as the water is absorbed, it sinks and is trapped in the liquid and builds up over time.

    The end result is that the expanding/contracting air sucks in the moisture laden air and as this moisture condenses it is more easily evaporated from a dry surface then the water molecules that become trapped below the diesel fuel.
  15. Capt Fred

    Capt Fred Senior Member

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    Thanks guys for your insight. I'm thinking with a little inspection/cleaning/pumping 5 gallons thru the empty tank will be satisfactory to bring the tank back on line when the extra tankage in needed.