When to go for a more extensive and dense vessel baselines
Vessel baselines are key for performance management as they allow to compare actual measured values against the expected ones, represented in the baseline. Vessel typically come with two baselines from the yard, that describe the needed power and / or fuel consumption over the sailed speed at certain drafts (and good weather):
1. The Seatrial, where the yard needs to prove speed / consumption performance under ideal conditions to the owner of the vessel.
2. The Model Tank Tests, where the vessel design is towed and analysed to come to speed / consumption predictions.
The advantage of the model tank test is, that it has already more measured values (is more dense) as it typically covers more speeds and also laden, ballast and sometimes even mid draft conditions.
3. Computational fluid dynamics (CFD) is used to assess vessel performance in a virtual model tank in the computer
Again the reason lies in the achieved density of the conditions for the baselines. Once the vessel is modelled in the computer hundreds of conditions can be ran in the virtual model tank at little additional costs (as opposed the physical model tank where each run is a costly item). For a hydrodynamically more complex vessel like a container vessel, we typically assess 7 speeds, 7 drafts and 7 trims. Model tank test results look rather thin in comparision. In the below graph you see 3 dimensions, which are draft, trim and speed.
The problem with the model tank test now comes, when you are sailing away from the tested conditons, like in slower speeds or mid-drafts, which is very likely due to slow steaming regimes and cargo on board. What you need to do then is to interpolate the draft and extrapolate to a lower speed, which can be highly inaccurate depending on how hydrodynamically complex the vessel is. Lets take an example of a semi-submerged bow, so a vessel with a bulbous bow sailing half laden. While the laden and ballast vessel condition does not have significant waves, the semi-submerged bow does. That is nothing more than additional energy loss for propulsion. So higher power demand and fuel consumption for a given speed.
Interpolation of power demand at mid-draft between laden and ballast draft is most likely wrong. So the needed power that the baseline is assuming at this condition is far too low in this real example. As a rule of thumb the more hydrodynamically complex the hull shape is (e.g. bulbous bow, transom) and the more days you sail away from your model tank tested conditons the bigger the error. Container vessels are more prone to this error than bulkers, chemical and product tankers more than crude oil tankers. The issue is displayed below in two examples:
The left graph is a post panamax container vessel, where the classical model tank test baseline would underestimate power demand by 4.000 KW at 16kn and even 6.000 KW at 14 kn. The problem is smaller for the 76.000 DWT bulker on the right graph, here the mistake is only 150 KW at 11 and 10 kn speed, which is below 5% inaccuruacy at 11 kn.
The consequence of this mistake might be costly decisions taken wrongly. Lets take a big fuel saving lever as an example, which is hull degradation. Here you want to know how much more power do I need to achieve a certain speed, so you keep comparing measured power values with the expected ones at comparable speeds, drafts and trim (and corrected for weather of course). If your baseline is wrong your assessment is wrong. In the below example of the same post panamax container vessel you would have organized a hull cleaning asap if you had a model tank test as baseline, while with the better, more dense baseline (from CFD) the vessel is still clean. This was actually proven by visual inspection from the responsible superintendent.