Sea Trial Speed Shortfall on a 32,000 t Bulk Carrier
A 32,000 t deadweight bulk carrier (Lpp 180 m, design draft 11.0 m, TPC 46 t/cm) is on sea trials. The contract guarantees 14.0 kn at 75% MCR in calm, deep water with a clean hull; MCR is 8,500 kW at 105 rev/min. On the measured mile the ship averaged 13.55 kn at 6,630 kW (78% MCR) and 94 rev/min. Conditions recorded at the time: trial displacement 2% below the design loaded condition, water depth 25 m, a 12 kn wind on the starboard bow, and a small residual current not fully cancelled by the reciprocal runs. The propeller is a 6.0 m fixed-pitch design. The yard's trial report states the result is 'within measurement tolerance'.
1. List the corrections that must be applied to the measured speed and power, and state the direction of each.
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Depth. h/T = 25/11.0 = 2.27, which is shallow water for a full-form hull. Squat and the return-flow field raise resistance, so the measured speed at a given power is low; correcting to deep water increases the speed. An allowance of the order of 0.2-0.3 kn is plausible at this h/T - it must be justified by a documented method, not assumed.
Wind. 12 kn on the bow adds aerodynamic resistance; the trial should be corrected to calm air, which increases the speed (order 0.05-0.15 kn).
Current. Reciprocal runs cancel a steady current only if the runs are symmetric in time and the tide does not change; a residual here means the mean is biased and the scatter between runs is the honest measure of uncertainty.
Displacement. The ship is 2% light, so it is faster than at the contract displacement; correcting reduces the speed (order 0.1 kn).
Water temperature and density affect both resistance and propulsor thrust and should be normalised to the contract reference.
The net correction is positive, but it is built from terms of the same order as the claimed 'tolerance', which is precisely why the correction method has to be agreed before the trial, not after.
2. Apply the corrections and decide whether the vessel meets the contract.
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Taking defensible mid-range values:
V (corrected) ≈ 13.55 + 0.25 (depth) + 0.10 (wind) - 0.10 (displacement) = 13.80 kn
Now compare power at the contract speed, using the propeller law P ∝ V^3 about the measured point:
P at 14.0 kn = 6,630 x (14.0/13.8)^3 = 6,630 x 1.044 = 6,924 kW = 81.5% MCR
The contract requires 14.0 kn at 6,375 kW. The vessel is short by roughly 550 kW (about 8%) at the contract speed, or equivalently about 0.2 kn. That is outside any reasonable trial tolerance and is a genuine performance shortfall, not a measurement artefact - but note that the conclusion is sensitive to the depth correction, so that term must be defended.
A secondary check on the engine: at 78% MCR the nominal propeller curve would give n = 105 x 0.78^(1/3) = 96.7 rev/min; the measured 94 rev/min is slightly below that, i.e. the propeller is running marginally heavy, which is consistent with the ship needing more thrust than predicted at that speed.
3. Assuming the shortfall is real, how do you separate a hull problem from a propulsor problem?
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Plot the trial points on the design P-n plane. The engine and propeller are coupled through one curve; hull resistance moves the ship along that curve, while a propeller defect (pitch error, blade roughness, wrong blade area) moves the curve itself.
- Hull fault (rough coating, fouling, poor fairing, a bulbous bow running wrong): at the measured speed the propeller is heavily loaded, so rev/min drop and torque rises. Trial points lie on the design curve but displaced to lower speed for a given power. Check ITTC/BMT hull roughness against the value used in the prediction, and inspect the boot-top and the flat of bottom.
- Propulsor fault: the points sit off the design curve - e.g. high rev/min for the power absorbed (light propeller, under-pitch) or low rev/min with excess torque (heavy, over-pitch, damaged blades). Verify the measured mean pitch, blade surface finish and the bossing/clearance geometry against the drawing.
- Measurement fault: confirm the power from the shaft torque and rev/min independently (strain gauge vs. torsionmeter), and confirm the speed from DGPS over the measured mile rather than from the log.
Only after this split can the yard be told what to fix; re-running the trial before the diagnosis is a waste of a docking.