How does one calculate the inital trim of a boat

Discussion in 'Hydrodynamics and Aerodynamics' started by steve.1326, Jan 6, 2014.

  1. steve.1326
    Joined: Jan 2014
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    Location: tennessee

    steve.1326 Junior Member

    I am using delftship to design a flats boat. I am trying to calculate the initial trim of the boat. Then i want to add weight to the boat like a motor or some other elements and see how the boat trims. This is the information from the software I think is required to do the calculations.

    LCB 6.372'
    LCF 6.940'
    LCG 7.4'
    KML 47.941' (I believe this is from the keel)
    VCG .686'

    Can the initial trim be calculated from these numbers? If so can someone show the calculation, if not how does one calculate the initial trim?
     
  2. lewisboats
    Joined: Oct 2002
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    lewisboats Obsessed Member

    First you must assign thicknesses and weights to your panels and other furniture. You do this in the Layers dialog. If you don't have everything on separate layers... that is the first place to start. Once you have everything at it's proper weight, shift the boat so the LCB is at the 0 (zero) position and rotate your boat until the LCB and the LCG are one above the other (at the same X position). This will trim the boat to the correct angle for the given weights in the given positions. It is a tedious process and any new weight added will entail realigning the Centers. Somewhere on this forum is a small library of .part files which might help you add weights in various areas. Or you can create your own by basically building a box from an extrusion of an edge on the boat. First you make one extrusion of say .125, then you extrude again making something substantial... say 1.0. Turn on interior edges and highlight the first extrusion then delete it. Now you have a panel that is separate from the hull. Make a new layer and assign the panel to the layer. From there you can do anything you like with it. Make a box and assign the panels of the box sufficient thickness and weight to simulate a motor then move the box to the correct position. Make sure you export this as a .part so you can import it into any drawing or import multiples of it. Go into layer properties and uncheck Symmetrical, and Use for Hydrostatics so it won't affect the integrity of the hull if it touches an outer skin a bit. Remember... only the surface has weight... you can't make it a solid except by telling the layer properties how thick the face (s) are and how much each weighs. Here is a small outboard .part that you can use.
     

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  3. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    You need the Moment to Trim 1 Inch (MTI, normally in ft-tons or ft-lbs) for your displacement. Right now you show that the LCB is ~ 1 foot forward of the LCG (assuming you are measuring from the forward perpendicular), which means that the trimming moment would 1 ft* lbs displacement. However, given this seperation, it is unlikely that the MTI calculation would be valid for the excessive trim moment.

    Properly, when you design a vessel, you decide on the static trim (theta) you want. Then you calculate the displacement and LCB at that trim. Then you calculate the LCG and move weights until LCG+KG*sin theta = LCB+KB*sin theta.
     
  4. steve.1326
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    steve.1326 Junior Member

    thank you both for your responses. If you have a rectangular block of wood floating in water and you place a weight on it some where to the left of center in the longitudinal direction, the wood block will tilt. How do you calculate the angle of tilt?
     
  5. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Take a look at the bookstore on this site that has a few titles covering the basic principles, probably the best way to get a handle on things.
     
  6. TANSL
    Joined: Sep 2011
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    Location: Spain

    TANSL Senior Member

    Assuming that when you speak of boat´s "initial trim" you mean the design trim, I agree with jehardiman, you should design the boat with zero trim.
    If what you're asking is the boat acquiring additional trim when adding or traslating a weight (and this seems to be what you're really asking for), again according to jehardiman, you need to know the moment to trim 1 inch. This will give you an initial estimate, not accurate but sufficient, of the trim of the boat.
    The correct calculation should take into account that the center of buoyancy and the center of gravity of the floating change position for each trim. (Very important: keep in mind, in any case, the boat rotates around the c of g of flotation.)
     
  7. steve.1326
    Joined: Jan 2014
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    steve.1326 Junior Member

    Thanks for you replies. I am trying to design the boat with zero trim. How does one go about doing that? Do you find the CG of every piece of the structure then sum all of the moments around the LCF?
     
  8. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Steve, nothing personal, but put the mouse down and read up on the basics of yacht design. A good primer might be "The Nature of Boats" by Geer. It would bring you good insight into some of the aspects of designing a boat. Of course, you'll need to learn more, particularly if you plan on a high speed flats boat, but if you can absorb the information in this book, you can move on to "thicker" tests on the various subjects, you apparently are currently lacking in. Simply put, software will not tell you if the current set of lines you've drawn, are suitable for your needs, just the volumetrics and physical dimensions. Calculating a fairly accurate MT1, is a reasonably simple thing and you'll do it a lot, assuming you know a little more about the concepts and principles involved. Software is just a fancy combination wrench. You have to be the one that selects which end to use and the appropriate size.
     
  9. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Esteve, yes, you're on the right path. After defining some preliminary forms you calculate the CGF and LCB (do not spend time multiplying,adding and subtracting, so if you can, use a suitable software). Then, you're moving the weights so that indeed the sum of weights and buoyancy moments with respect to LCG of waterplane is zero.
    If you do not succeed, change the ship´s forms to move the CoB and CGF fore or aft, and start again. (This is a simplification of the problem, but it can work).
    If you have clear concepts, as it seems, it is best to use some software. Otherwise, I agree with PAR, comes to study the concepts in a book, and do some examples "by hand".
     

  10. frank smith
    Joined: Oct 2009
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    Location: usa

    frank smith Senior Member

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