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Fuel System Woes Revisited

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

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  1. Davidoc

    Davidoc Senior Member

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    That's great. I have never been able to figure out how much fuel I have in the tanks with those senders. I have a bunch of pictures of my gauges that I compare to the actual gallons I use to top the tanks up. Pretty prehistoric method. I have been looking at some of the new sensors available but you have to install a mini network to use them. Ralph, I will take a look at your recommendation for the wema's. I don't know if we can install it in the rear tank because many of us Post Owners have the cockpit over the rear tank with only a few inches to get to the sender. Hope all is well with you. David
  2. Berean

    Berean Senior Member

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    Great topic. I also have a 1984 '43, its an awesome boat. I tend to be pretty conservative and top off after well before I have burned 75% of my fuel. My fuel guages are of limited utility. They seem, so far, to be fairly accurate in differentiating "full" from "less than full" but indicate < 25 % remaining when in fact I am still holding much more. So from this point on the guages become useless. They fail to differentiate between "a little less than full" and "very low". Oh how I wish the fuel fills were a straight shot so that I could use a dip stick! I don't think I'll ever get to a point where I will trust my guages.

    In any event, my fuel lines are identical to yours. However, whereas your forward tank seems to remain more full than the aft because of gravity, apparently the opposite is true for me. Indeed, the aft tank is higher than the forward tank just as you described. In my case it appears that the fuel is not free to drain into the forward tank unless I use the pump.

    I figured this out when today (in the slip, fortunately) my port engine was fuel starved. It was a sloppy day yesterday offshore and I figured the Racors were full of debris. So I changed my Racor filter and thought it was odd that the starboard engine was not similarly afflicted, but did not think much of it and pressed on. After changing the element, the port engine still had no fuel. I scratched my head a for bit then decided to trace my fuel lines and discovered the configuration as you described. As my starboard motor ran great, I decided to transfer fuel forward. This was successful and the engine fired to life.

    The fuel truck is coming this week so I am very curious how much fuel I am holding in the aft tank. My prior boat had a cross feed and I always left it open, so both tanks were always level. This boat does not, and cannot because its tanks are not level with one another. As you have experienced, cross feeding would be a one way proposition. So this is new to me.

    It is also sobering that I could have had to work through this offshore. Amazingly however, the last vapors in the forward tank were consumed just as I shut down yesterday. I am curious why one engine (port) seems to have burned more fuel that the starboard motor and generator combined. I would hardly think fuel is defying gravity and going "up" into the higher aft tank from the forward tank. It might turn out that the fuel burn is very similar from both tanks and that there is but enough remaining in the aft tank to prevent fuel starvation but no more than that. If this is the case I suppose I need to re-think what I consider conservative!
  3. Davidoc

    Davidoc Senior Member

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    Just a couple of thoughts on this one.
    My starboard tank has an extra 50 gallons for the gen set. Could it just be that you still have extra fuel in the that tank because you didn't use all of the 50 reserve for the generator?
    Is your boat level, bow or stern down? Posts usually lay bow down.
    Let us know how many gallons you took on for your upcoming top off and which tank do you fill first.
    I wonder if anyone has tried the WEMA fuel gauges, that Ralph uses, on the Post configurations?
  4. Post Mayhem

    Post Mayhem Member

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    I have measured my aft tanks depth at 26.5". I contacted Wema and the recommend the sender be 2" shorter than the depth of tank. They don't have a 24.5 so I am considering a 24. The vertical clearance is one issue and distance inside tank to baffle is another. I plan to fabricate a "dummy" unit of the same dimensions to attempt installing to see if it clears prior to ordering from Wema USA.
    I know the aft tanks volume is greater but not by how much. I have been told the boats fuel capacity is 500 gal. I thought the fwd tank was 175, so aft would be 325. Can anyone verify that to be correct? I will let you know how the fit for the Wema 24"
  5. Davidoc

    Davidoc Senior Member

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    Your numbers on the fuel tank capacity do not sound logical. My 46 has 300 forward and 350 aft. 50 for the genset. If you carry 500 you should be approximately 225 and 275. Im sure you have looked for the original fuel tags on the ends of the tank. Mine are stamped with the capacity. Anxious to hear about the Wema's.
  6. K1W1

    K1W1 Senior Member

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    Put Floscan flow meters in , you will know how much you have burnt then deduct that from a full tank and you will know what you have left if it were full when you started.
  7. Capt Ralph

    Capt Ralph Senior Member

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    I plan to fabricate a "dummy" unit of the same dimensions to attempt installing to see if it clears prior to ordering from Wema USA.

    WEMAs go straight down. The tube is just under 1/2" in diameter and the float about 1 1/4".
    Length measured from the tank top to the bottom of the tube.
    Never understood that 2" spec unless to keep away from trash or water. Maybe a reserve. We went to the longer model when partial inches were involved in our water and glass fuel tanks. Assuming access at an angle may be a problem; I guess a broom stick with a T on the end would be a good tool/dummy to help determine if you could get one in there
  8. Post Mayhem

    Post Mayhem Member

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    I will try the broom stick idea. The clearance above the tank is the first obstacle to clear so it must go in at a very steep angle which will bring baffle location into play.
  9. Davidoc

    Davidoc Senior Member

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    I would love to have them but when is the last time you were quoted a price for them? One for the fuel line, one for the return fuel to the tank and times 2 for the second engine. Very expensive and a fair amount of plumbing.
    I'm still looking for an electronic solution to measure fuel consumption.
  10. Berean

    Berean Senior Member

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    You certainly could be right. I did try to read the one plate facing forward on the aft tank and could only see the numeral "2" followed by the remaining two digits that could not be read. Your proposal would make sense. This did not occur to me before.

    in any event, The fuel man came by today and I put 190 into the foward tank and 175 in the aft. Assuming you are right I would have been left with 35 gal in the forward tank. Given that it was snotty my last trip, I think it was the filters afterall. That was my very first suspicion and I changed them out before any thing else. I suspect that the fuel starvation symptoms on the port engine that did persist for a bit after swapping filters was a function of a tired and exacperated and late for dinner Capt that did not mind the air introduced durnig the filter swap as cafefull as I could. The fact that the engine started after I finally transfered some fuel from the aft tank is probably coincidental and it was only at that point that I had sufficiently flushed the air out. So had I tried to start the motor instead of transferring fuel it would have started anyway. Just my thought anyway.

    Floscan would be awesome! I hope to eventually benefit from the info it would provide.

    I never met Wema so I posess no opinion. I heard Fred was nice though.


    Thanks everyone for your amazing collective knowlege base!
  11. Capt Ralph

    Capt Ralph Senior Member

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    Some intresting FlowScan kits on e-bay now.
  12. Capt Ralph

    Capt Ralph Senior Member

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    Experience and record keeping could prove more accurate.
    I'm a fan of FlowScans, Not really for their attempt of fuel use recording, but you can detect problems early when the needles and pointers are not where they should be while running. Early detection of problems can save money and headache.
  13. Berean

    Berean Senior Member

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    I just read your post more carefully, and if in fact your tank size numbers are accurate then my forward tank would have been empty.

    A vacuum in fact. A black hole from which not even light could escape...

    I did get 190 gal in it yesterday (?) I don't know if possibly some of that went to the aft tank before the forward tank appeared to be at capacity? There is no fuel in the bilge which is always a wonderful thing. So I would have to adjust my previously stated theory. At the dock MG II does seem to rest a wee bit 'bow down' by "eye" typical of her type, but only very slightly. I would and will put a level on top my tanks to quantify her attitude. Underway and certainly planning we always have coolers full of monster fish :) in the pit so she is not running bow down, but I do tab aggressively when cruising except in following seas.

    I really need to get my tankage down. I am going to try to read the aft tank plate again, maybe I'll recruit younger eyes to take a look. I could not make out the number stamped on the plate for the life of me.
  14. Berean

    Berean Senior Member

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    I do agree. And every time I fuel up I calculate my fuel burn, which is of course a cumulative average for that interval between fuel appointments. Sometimes we are cruising, sometimes idyling and then everything in between. But that calculated fuel burn tends to be fairly consistant each time, around 15-16 gph. So I make my fuel appointments based more on my time indicated and much less on guages.

    Knowing what I actually burn at cruising is difficult to know because we are not always cruising. The best I can do is calculate my cumulative use, and here the Floscan would be very useful.
  15. K1W1

    K1W1 Senior Member

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    I am not sure we are talking about the same thing. It is my understanding that the Floscan Units are installed in the supply and return lines. This means only one cut into each line.

    The cabling is a bit more complicated of you want to broadcast the signal.

    I have not made enquiries for a set for at least 10 yrs so can't say what they were worth back then.
  16. bobhorn

    bobhorn Member

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    On our FLs there are three return lines, so they have to be combined to measure total return flow. Last I looked they had no broadcast or networking capability, although they will accept a GPS signal from the chartplotter to compute and display nm/gal.
  17. K1W1

    K1W1 Senior Member

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  18. bobhorn

    bobhorn Member

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    Beats me, I didn't design the engine. There is return from the engine mounted fuel filters, a return from the injection pump and a return from the head which is a return from the injectors.

    The flow meters are in the engine room but the displays, one/engine, are on the console on the flybridge. One of the displays has a switchable window for either fuel flow or nm/gal.
  19. Capt Ralph

    Capt Ralph Senior Member

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    What engine are we talking about here? Never heard of such.
  20. bobhorn

    bobhorn Member

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    Probably not one found in real yachts, Ford Lehman SP225.