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Fuel System Woes Revisited

Discussion in 'Post Yacht' started by Post Mayhem, Jun 23, 2015.

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  1. Post Mayhem

    Post Mayhem Member

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    hello again everyone.first let me thank you guys for your previous assistance with my new to me 1984 Post 43. She is up and running and I am using her for charters on Ches. Bay in VA. My issue/problem is this: after a trip on the bay I always top off fuel. I still need to replace my sending units so never do more than a trip without a top off. My aft tank will need about 48 gal and the fwd tank only 2-4 gal. I have traced the systems. Stbd eng and genset runs from and both return to aft tank. Port eng runs from and returns to fwd tank. I do have a transfer pump and plumbing which contains no valves. The aft tank is situated higher in the boat than the fwd tank. I have a valve setting next to me that I plan to install in the transfer system to determine if the aft tank is allowing fuel to run "down" to the fwd tank. I am interested in your input on whether this sounds like a plausible explanation before I start breaking open fuel lines and change design set up. Oh yeah, the tanks ARE both full. I got a miniature geyser of fuel when I removed a screw from the sending unit flanges. Thanks for insight, you have been spot on for me so far!
    John
  2. mfan46

    mfan46 Member

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  3. NYCAP123

    NYCAP123 Senior Member

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    Those numbers sound like your port motor is running off both tanks or the aft tank. Also, watch that geyser. Leave 10% for expansion, especially in the heat of summer.
  4. mfan46

    mfan46 Member

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    I had the same problem and installed a shut off valve at the transfer pump,worked fine.Hope it will help.
  5. Post Mayhem

    Post Mayhem Member

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    Next few trips will get no top off so I can get new sending units in and to sound like my friends in Boston, "It's wicked hot here today"
  6. Post Mayhem

    Post Mayhem Member

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    Good enough, when I get the fuel level down I will install the valve and give a reply as to the results, thanks
  7. mwwhit1

    mwwhit1 Senior Member

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    Your theory is right on. Aft tank is syphoning into the front tank. I've had this happen if the impeller in the transfer pump breaks a blade(s) or isn't sealing for some reason. Shut off valve will stop it. I wonder if there is a 12v valve that could be installed that ties into the pump power so it will open automatically when you use the pump. Probably not worth the bother for the few times this pump gets used.
  8. chesapeake46

    chesapeake46 Senior Member

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    I have purchase stainless steel, 12 Volt DC solenoid valves in the past that you could do what wht1 suggested. Mcmaster Carr catalogue is,where i got them.
  9. Post Mayhem

    Post Mayhem Member

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    Thanks for the confirmation. The valve our dock master gave me to use for my test is too small so will have to get a different one. Might look into the solenoid valve mentioned by Chesapeake 46. Tho you are right, I don't transfer fuel very often, in fact so far never. Would install aft end fwd tank so easily accessible from engine room.
  10. Davidoc

    Davidoc Senior Member

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    You are spot on relative to the location. You don't need a solenoid, just put the shut off at the rear of the forward tank and tie wrap it closed. You really only need it when one engine is down and you need the fuel in the other tank to get you home. Been there done that. The impeller on the transfer pump leaks. The fuel does flow forward. It is very slow but over some time it will keep the forward tank topped off. That one drove me nuts!
  11. Post Mayhem

    Post Mayhem Member

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    As mentioned earlier this keeps the fwd tank full each trip. I think the impeller in the pump is shot so it is a direct pipe from tank to tank. I burned some fuel today so hopefully tomorrow I can put a valve in. Thanks
  12. Davidoc

    Davidoc Senior Member

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    Right. The impeller in mine is metal. The pump cannot hold any gravity feed back. This is per the manufacturer of the pump. The shut off should nail it. Put it in before you top off for sure. Posts lean forward at rest. I fill my forward tank first because the fill goes to the aft of the forward tank.
  13. P46-Curaçao

    P46-Curaçao Senior Member

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  14. mwwhit1

    mwwhit1 Senior Member

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    If you were only putting in a few gallons after each trip, the syphon seems pretty strong. More than likely your front tank is full again after only one 45 gal trip. Back tank needs to be around 1/2? to stop the flow? You probably need to open the line going to the pump and break the syphon by getting some air in and draining that pickup line.
  15. mwwhit1

    mwwhit1 Senior Member

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    By filling the front tank first, bow comes down farther and the air bubble trapped in the front tank has a better chance to come toward the rear of the tank and vent. Fill the back tank first and it traps the air in the front tank. How much of a difference? Hard to know. 5-10 gallons?
  16. Davidoc

    Davidoc Senior Member

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    That's correct. If I fill the aft tank first it will start to bubble back at me much sooner. Both the fill and vent on the forward tank are at the aft end of the tank. In addition my aft tank is 350 gallons and she does sit higher than the forward tank. The forward tank is 300 gallons and does not feed the genset. Always fill the forward tank first. In my case I loose over 20 - 25 gallons.
  17. Post Mayhem

    Post Mayhem Member

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    Today was a washout. Got the aft bilge pump float switch replaced and spent the rest of the day at dentist. Correct about fwd tank will still be full. Planned to break line by pump as mentioned and cap it or something so I can effectively lower fwd tank. Thanks again guys for sharing your knowledge. Davidoc, I am jealous! I can't wait for the day I can cruise my Post to the islands.
  18. Davidoc

    Davidoc Senior Member

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    Be sure that you have transferred some fuel to the rear tank so the bubble is at the aft part of the forward tank. Then you should be able to break the line at the fuel transfer fitting on the tank and not dump fuel all over the place. Then simply install a shutoff valve right at the tank. The is a good thing to do anyway so when you have to do maintenance at the transfer pump, which is low in my boat, you will have a shut off from the forward tank. I have one on the aft tank as well.
    Let me know when you are ready to go to the Islands. We can talk about it more.
    David
    OUTPOST 46-225
  19. Post Mayhem

    Post Mayhem Member

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    Thanks everyone. Installed the valve and that ended the automatic fuel transfer. Got the new sending unit in the fwd tank and it is working great. I have installed the new unit in the aft tank but it appears to be hanging up in baffles no matter which of the 3 available directions I point it ( fwd, port or aft). I wonder if there is a gap it is supposed to snake through to have full range of movement. I am going to borrow or buy one of those flexible camera devices to put in the tank and look. Made an offshore fishing trip a couple weeks ago and needed to use the transfer pump. I was amazed how quickly the fwd tank filled. Thanks again. Trip was 273 NM. Burned 406 gal.
  20. Capt Ralph

    Capt Ralph Senior Member

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