Gauging Fuel Levels on a W32

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  • April 18, 2015 10:48 PM
    Message # 3307734

    I did a bit of searching in hopes that someone might have posted a reference guide for measuring the fuel level in the W32 fuel tanks. But, alas, I came up empty handed. Of course, I could just start with an empty tank and add fuel 5 gallons at a time,marking a stick as I went along. But that would mean completely emptying one of my tanks (which I would prefer not to do) and I would still only have a very rough estimate in 5 gallon increments. So I decided to come up with a table by means of mathematical calculations. The chart assumes the standard ~ 35 gallon tanks, as shown in Bud's Service Manual...

    http://www.westsailparts.com/ServiceManual/index.php?chapter=G#G-01

    (If you'd like to use my calculations, but aren't interest in how they were derived, simply jump to the end of this post.)

    The challenge to this method, of course, is that the W32 tanks are such weird shapes. While this makes better use of the available space, it greatly complicates the calculations.

      

    The basic formula was derived from the method for calculating the volume of a trapezoid tank...


    volume = L * (b1 + (b2 - b1) * h1 / h + b1) * h1 / 2


    There are two factors that complicate this simple method. 

    1) The two ends are different shapes.

    2) The wall facing the hull is completely skewed.

    In fact, the shape isn't really a trapezoid at all, but the formula works if you make two calculations, one volume based on the shape the forward wall and another volume based on the shape of the aft end wall, and then average the two volumes. To allow for the skewed outer wall, I made the calculations as though each tank was comprised of two individual tanks, splitting them horizontally where the hull-side wall changes dimensions at the forward end of the the tank. I'm actually not sure if I needed to do this, but it was easier for me to visualize the results in my head. (My college days of math and science are way too far behind me.)


    Since I built an automated PHP routine to perform all the calculations inside of a "loop", it made little difference how many increments were used. As such, the volumes where calculated for every inch in height. The loop could just as easily have been repeated for every millimeter in height, but that would have been really anal retentive, wouldn't it? I simply wanted something that gave me a gauge to account for the odd tank shape, without the need to manually add and mark the measuring stick at 5 gal (or even 1 gal) increments.

    Once I had the volume calculations in cubic inches, it was a simple matter to convert from from cubic inches to gallons by mutilplying Volume times the constant 0.0043290. In making the calculations, I allowed for a 1/8 inch of wall thickness and also chose to disregard the first inch of fuel in the bottom of the tank, because even if you could suck the tank that low, why would you want to? (If you chose to include the first inch, simply add 0.8 gal to all displayed values.

    The resulting chart should give a quick and dirty method to get current fuel levels using a simple dowel, marked in inches. At one point, one of tanks had a sight gauge, but it was removed for some reason. If your tank(s) do have a sight gauge, it should be a really simple matter to use these calculations to make a reference chart behind the gauge.

    I'm posting this in hopes that others find it helpful. I know I would have preferred to find the calculations ready-to-use. I no longer use my math skills as I once did. So if anyone finds discrepancy in the calculations, please let me know. 

    Jack Webb, aboard s/v Drifter



    PS: If anyone has actually done the manual measurement using 5 gallon cans, I'd love to know how your calculations compare with mine. And if anyone else is anal enough to want to see the actual calculations made, inch by inch, let me know and I'll be happy to post them.


    Last modified: April 19, 2015 5:47 AM | Anonymous member
  • April 20, 2015 5:02 AM
    Reply # 3308638 on 3307734

    Why are you reuctant to run your tank empty?  Is is because of sludge on the bottom?   If so, then maybe you could be better served by investing a few hundred dollars in having your fuel polished.   A sludgy tank is a ticking time bomb.

    After polishing, you can refill and calibrate your dip stick.

    By the way, I have boiler tubes (sight glasses) installed on my tanks.  But the glass only shows the levels from 0% to 50%.  Above 50%, I'm blind.

  • April 20, 2015 6:06 AM
    Reply # 3308675 on 3307734
    Deleted user

    Dick.....When I installed new fuel tanks on Tamzin I had Bud include the sight glasses and then I marked the glass at every five gallons when I filled it the first time.  The next time I'm down I'll take your chart and see how close they are.

  • April 20, 2015 7:16 AM
    Reply # 3308771 on 3308638
    Dick Mills wrote:

    Why are you reuctant to run your tank empty?  Is is because of sludge on the bottom?   If so, then maybe you could be better served by investing a few hundred dollars in having your fuel polished.   A sludgy tank is a ticking time bomb.

    After polishing, you can refill and calibrate your dip stick.

    By the way, I have boiler tubes (sight glasses) installed on my tanks.  But the glass only shows the levels from 0% to 50%.  Above 50%, I'm blind.

    There are two main reason I don't like to run the tank(s) empty...

    1. It can be a pain to bleed the injectors.

    2. Bio growth is less less likely in a full tank.


    Nonetheless, my fuel has been polished. But the tanks have not been emptied and cleaned. I keep them as full as practical in order to reduce growth. But I have little doubt that there's at least some crud in bottom of the tanks, nonetheless.

    One of my tanks did have a sight glass, but it was removed for some reason. It would be nice to have them on both tanks!


  • April 20, 2015 7:20 AM
    Reply # 3308774 on 3308675
    Larry Smith wrote:

    Dick.....When I installed new fuel tanks on Tamzin I had Bud include the sight glasses and then I marked the glass at every five gallons when I filled it the first time.  The next time I'm down I'll take your chart and see how close they are.

    Larry,

    Based on the content of your post, I'm assuming this was intended for me and not Dick, as indicated. Had my tanks been empty, I too, would have just done it with the 5-gallon container.

  • April 23, 2015 6:32 PM
    Reply # 3314760 on 3307734

    Jack,

    I did the empty tank, five gallon jerry tank method with a fuel stick marked accordingly.

     Factory standard tanks from 1976.

     5   Gallons     5 3/8 inches

    10  Gallons    9 1/8  inches

    15 Gallons    12 3/8 inches

    20 Gallons    15 1/4 inches

    25 Gallons    17 1/2 inches

    30 Gallons    20      inches

    35 Gallons    22      inches


    Looks as if your calculations and the manual measurements are very close.


    Regards,

    Werner


    Last modified: April 23, 2015 6:42 PM | Anonymous member
  • April 24, 2015 1:45 PM
    Reply # 3315869 on 3307734

    Thank you, Werner... that was the feedback I was hoping to receive.

  • May 04, 2015 6:17 AM
    Reply # 3326397 on 3307734

    Jack & Werner,

    I'm glad you both posted your conclusions.  I had wondered about the levels in gallons for years.  Jack, your mathematical mind is remarkable - you make it seem like a simple exercise for fun, but for me it would have been a nightmare!

    Tom Koehl

  • May 05, 2015 9:49 AM
    Reply # 3328291 on 3307734

    Had I been been smart, Tom, I would have just called Werner to begin with. I should have known he's already done this. 

    The problem with posting a query for an answer on the forum is that so few members (and fewer owners overall) actually monitor the site on a regular basis.  And when I searched the archives, I couldn't find that anyone had previously posted these measurements. Thankfully, at least Werner had an answer!

    When I get my boat back in the yard this fall, I'm going to empty and clean them well, then refill them. For now, this at least gives me a good reference. I'm thinking about installing an electric fuel pump as a means to simplify bleeding the system. Do you know anyone who has done this?

    Jack

  • May 07, 2015 5:43 AM
    Reply # 3335027 on 3307734

    Awh shucks Jack, you give me too much credit but thank you. It was 20 years ago but since I had only one 5 gallon Jerry can I do remember the 7 trips to the Shell station and the long walk down the dock very well.

    BTW, got the hard dodger built and installed. I'll post the pics on Picassa one of these days.

    Werner


    Last modified: May 07, 2015 5:48 AM | Anonymous member
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