Need help with simple trim calculation

Discussion in 'Boat Design' started by schuh, Apr 9, 2020.

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  1. schuh
    Joined: Oct 2013
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    Location: Panama City, Fl

    schuh Junior Member

    I am an 80 yr old boat builder who’s having a little trouble calculating how the trim on a boat I am designing will change with an added load. Any help would be greatly appreciated! Here’s the problem:
    Assume the boat is a simple box
    Beam = 4’
    Length = 10’
    Height = 1’
    Displacement = 200 lbs
    Calculated draft with no load = 0.95” (assumes 63 lbs/ cu ft brackish water)

    The problem: Calculate the draft at the bow and the stern if a 250 lb load is placed 7.5 ‘
    back from the bow.

    These are my calculations (these assume that the change in trim is very small)
    Displacement = 450 lbs
    Draft if load was at center = 2.14”
    Increased draft = 2.14” - 0.95” = 1.19”
    Since additional load is 75% towards the stern:
    0.75 x 1.19 = 0.89’ additional draft at stern
    0.25 x 1.19 = 0.30 less draft at bow
    Total draft: Bow = 2.14 - 0.30 = 1.84” Stern: = 2.14 + 0.89 = 3.03”

    Are these calculations correct? Do they provide a good approximation?
    Thanks in advance ....
     
  2. Heimfried
    Joined: Apr 2015
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    Location: Berlin, Germany

    Heimfried Senior Member

    Hi schuh,

    unfortunately the calculations are not correct and if they provide a good approximation can not be said, because you didn't give the vertical distance of the center of gravity of the load. The latter is very important. If it is low, there will be "little" trim, if it is high enough, there will be an immediate knockdown.
    In general, a correct calculation of the trim of a box will help not much to calculate the trim of a normally shaped boat hull.
     
  3. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    No, it's a lot more complicated than that. To basically sum it up, the CB and CG must be vertically aligned. The quick calculation I did assuming the vessel is a rectangular prism and that KG = 0 in both cases (a bad assumption) was drafts fwd 0.357, aft 3.928. Correcting for the proper heights (and possible motions) of KG will most likely result is even deeper drafts aft, and a complete loss of stability if KG is high (or moves) enough.

    Edit: was working on the solution, didn't see Heimfried's comment.
     
  4. schuh
    Joined: Oct 2013
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    Location: Panama City, Fl

    schuh Junior Member

    Gentlemen
    Thank you for your help. I suspect in this case the box model will give me a close approximation because the design is of a flat bottom skiff wth a 90 deg transom.
    Does it help any if the CG is estimated as amidships and 4" above the bottom? Showing me your calculations or directing me to a program or spreadsheet which does the calculations would be most helpful.
     
  5. Heimfried
    Joined: Apr 2015
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    Location: Berlin, Germany

    Heimfried Senior Member

    This is your floating box in metric units. (The calculations are simple in a mathematical sense, but very tedious.) So I use a calculation tool. (Calculated is fresh water 1000 kg/m³, but your brackisch water is 1009kg/m³, not very different.)
    floating_box.jpg
     
  6. schuh
    Joined: Oct 2013
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    Location: Panama City, Fl

    schuh Junior Member

    Dear Heimfried

    Vielen dank for taking the time to figure out my problem. It really was much appreciated and gave me the impetus to continue my studies in hydro-statics.

    If I may, I think I have found a slight error in the input data to your model. The model shows an initial displacement mass of 204 kg. The initial displacement was actually 200 lbs or 91kg. The added load was 250 lbs or 113kg .... so the 113 kg was added in twice resulting in somewhat larger fore and aft displacements.

    I was able to use Delftship to compute a metastatic height of 110 ft. With this datum and the MCT1in formula I was able to compute a total change in trim of 3.57 in or 91 mm. I believe this translates to 2.69 in (or 68 mm) down by the stern and about 0.89 in (or 23 mm) down by the bow.

    I have put this all into a spreadsheet which calculates change in trim when loading a box or barge. I may post it somewhere on this forum (with the appropriate cautions!). If you see it, your comments would be most welcome!

    Again, thanks for taking the time to help a complete stranger !
     

  7. Heimfried
    Joined: Apr 2015
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    Location: Berlin, Germany

    Heimfried Senior Member

    Hi schuh,
    to explain my steps, this is the initial state, deriving from the data of the box, I located the CoG in the center of the box. The weight is 91 kg, the calculated draft is 24 mm.
    box_ini.jpg

    After this i loaded the 113 kg and set its position in relation to old CoG. Click button load results in adding the weights calculating the new draft and the new CoG.

    box_loaded.jpg

    Next step is alter trim until the CoB is vertically under CoG. This step is done (partly) automatically. The result is given in #5 above.
     
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