Ship New Drafts
1. **State the problem:**
A ship 55 m long has drafts 3.65 m aft and 2.75 m forward. Two loads (27 t and 70 t) are added at 18 m aft and 19 m forward of amidships respectively. We need to find the new drafts forward and aft after adding these loads.
2. **Calculate initial mean draft:**
Mean draft $T_m = \frac{3.65 + 2.75}{2} = 3.20$ m
3. **Calculate initial trim:**
Trim by stern (difference in drafts): $\text{Trim} = 3.65 - 2.75 = 0.90$ m
4. **Calculate moment of inertia arm from amidships:**
Ship length $L = 55$ m
Distance of center of flotation (C.F.) from amidships $F = 1.46$ m forward (positive forward)
5. **Calculate moments introduced by initial draft and loads:**
The position of C.F. from amidships implies initial moments balance around a point 1.46 m forward.
6. **Calculate initial displacement ($\Delta$) and TPC:**
$\Delta = 2351$ t at mean draft 4.65 m
TPC (tons per cm immersion) $= 6.12$ t/cm
7. **Calculate change in displacement ($\Delta W$):**
Total load added $= 27 + 70 = 97$ t
8. **Calculate new mean draft ($T_m^{new}$):**
Change in draft in cm $= \frac{\Delta W}{TPC} = \frac{97}{6.12} \approx 15.85$ cm
New mean draft $= 3.20 + 0.1585 = 3.3585$ m
9. **Calculate moments due to added weights:**
Position of 27 t load is 18 m aft of amidships (negative position) $= -18$ m
Position of 70 t load is 19 m forward of amidships (positive position) $= +19$ m
Calculate moment of added weights about amidships:
$$ M = (27)(-18) + (70)(19) = -486 + 1330 = 844 \text{ t m}\ $$
10. **Calculate change in trim:**
Moment to change trim relates to TPCM (tons per cm moment):
$$ TPCM = \frac{TPC \times L^2}{100} = \frac{6.12 \times 55^2}{100} = \frac{6.12 \times 3025}{100} = 185.73 \text{ t m/cm}\ $$
Trim change in cm:
$$ \Delta \text{Trim} = \frac{M}{TPCM} = \frac{844}{185.73} \approx 4.54 \text{ cm} $$
11. **Calculate new trim:**
Initial trim $= 0.90$ m $= 90$ cm
New trim $= 90 + 4.54 = 94.54$ cm (trim by stern)
12. **Calculate new drafts forward and aft:**
Let forward draft $T_f$ and aft draft $T_a$ satisfy:
$$ T_m^{new} = \frac{T_f + T_a}{2} = 3.3585 $$
$$ \text{Trim} = T_a - T_f = 0.9454 $$
Solving:
$$ T_a = T_f + 0.9454 $$
$$ \frac{T_f + T_f + 0.9454}{2} =3.3585 \Rightarrow 2T_f + 0.9454 = 6.717 $$
$$ 2T_f = 6.717 - 0.9454 = 5.7716 \Rightarrow T_f = 2.8858 \text{ m} $$
$$ T_a = 2.8858 + 0.9454 = 3.8312 \text{ m} $$
**Final answer:**
New forward draft $= 2.89$ m (approx.)
New aft draft $= 3.83$ m (approx.)