FINDING A SHIP’S DRAUGHTS
Knowing the weight and cg position for any given ship condition, the draughts can be found by:
• using the hydrostatics, the draught amidships at design trim corresponding to the weight can be read off together with the LCB position.
• the weight multiplied by the distance between the LCG and LCB gives a moment taking the ship away from its design trim. The change of trim will be by the bow or by the stern depending upon whether the LCG is forward or aft of the LCB respectively. It is best to produce a small sketch of the ship to show which way moments act and hence the way the ship trims.
• the trim change between perpendiculars in metres is found by dividing the trimming moment by the MCT. This trim will be about the LCFL1 the position of which must be noted. The draughts at the perpendiculars or at the draught marks can be calculated.
If the overall change in draughts between perpendiculars, L apart, is t, say, the change of draught per unit length along the ship is t/L. If a set of draught marks is at a distance k from the CF, the change of draught at those marks will be tk/L. Whether this is an increase or decrease will depend upon the way the trim is changing and whether the marks are forward or aft of the CF. A simple diagram will assist the student in determining which.
It should be noted that the draughts obtained from the above will only be reasonably accurate if any change from the design trim is relatively small. If the change of trim is significant, a new amidships draught must be found and the process repeated.
FORE AND AFT MOVEMENT OF WEIGHT If a weight already in the ship is moved forward or aft the effect is to move the LCG and the draught will increase at the end of the ship towards which the weight is moved. If the weight is w and it is moved a distance d forward the trimming moment created will be wd, that is the product of the two. The ship must trim to compensate, that is the new LCB must move so as to be in line with the new LCG. The change in trim is found by dividing the moment by the MCT.
The displacement of the ship is constant so the trim will be about the CF position and the new draughts can be found.
ADDING WEIGHT If a weight is added, or taken off, there are two effects:
• The ship will sink (or rise) a little so that the increased (decreased) buoyancy equals the change in weight.
• The ship will trim due to the fore and aft movement of the overall ship LCG. It is convenient to calculate the changes in draught in two stages:
• Assume the weight is initially added in the same vertical line as the CF of the waterplane at which the ship is floating. The ship will sink by an amount equal to the weight divided by the TPC. At this stage there will be no change in trim.
• Move the weight to the position it is to occupy and calculate the changes in draught as above.
• The final draughts are obtained by summing the two effects above. Again, it is useful to draw a thumbnail sketch to show how the draughts change forward and aft.