The Docking as a Performance Event: What a Yard Visit Resets, and What It Ratchets

Every other course in this specialisation manages a hull that is getting worse. Trim is pulled on a hull whose resistance is rising; a shallow-water correction keeps a false speed loss out of a ledger whose true entries are all one sign; a cleaning recovers part of a slope and leaves the rest. The docking is the one event in the five-year cycle that reverses the slope, and it is the most expensive fortnight in it. The specification that decides what the hull will cost for the next five years is usually written by the superintendent and the paint maker's representative, priced by the yard per square metre, and never read by the analyst who will spend those five years explaining the trend it produces. This course puts that analyst in the room, and this lesson says why the room matters.

What the fleet statistics say a docking does

The best public measurement of a docking's effect is not a study of one ship but a regression across hundreds. Gundermann and Dirksen took the CASPER performance-monitoring database, several hundred vessels and 1,391 events, and analysed 237 dry dockings, 127 hull cleanings, 445 propeller polishes and 394 combined hull-and-propeller cleanings from it (HullPIC 2016 proceedings, free PDF). Three findings from it frame everything that follows.

First, the size of the problem: "most vessels have added resistances in the range between 10 and 40 percent, with development rates between 0.3 and 1.5 percent per month." Second, what the docking recovers: "dry dockings in average reduce the level of added resistance by 2/3 of the pre-dock level," and the fit is Fb after = 0.018 + 0.37 Fb before, so a ship that enters with 30 per cent leaves with about 13. A hull cleaning took off about a quarter of the pre-cleaning figure and a propeller polish a little more than a fifth. Third, what stays: after docking, added resistance grows with the ship's age at about 0.64 percentage points a year on their regression, "consistent with the hypothesis that ships develop a permanent resistance over time." That is the plating, the set-in panels, the appendage damage, the things no blast removes.

One more line from the same paper is the seed of this whole course. "The result of the second and the fourth dry-docking after delivery is on average better than the first and the third, probably due to the fact that often only a partial hull blast is applied at the first and the third dry-docking while a full blast is more often carried out at the second and fourth." The scope decision, the subject of lesson 4, is visible in the fleet statistics from the outside.

The ratchet

A hull's roughness is measured as average hull roughness, the AHR of lesson 2: the mean, over about a hundred stations, of the highest peak-to-lowest-valley height found in each 50 mm of a gauge traverse. Its history over a ship's life is a ratchet, and the teeth are the dockings.

The starting point is the newbuilding. Townsin's group put a new ship "usually between 95 and 125 µm," with 70 about the best practically possible, as Candries reports in his Newcastle thesis (Candries, 2001, free PDF); International Paint's brochure Hull Roughness Penalty Calculator (AkzoNobel, 2004) takes 120 as "the approximate roughness value for a typical newbuilding"; and Jotun's HullPIC 2021 paper puts "newly applied coatings commonly found on ship hulls" at "around 100-150 µm" (HullPIC 2021). Take 120 as the round number.

Then it grows. The figures International Paint published: about 20 micrometres a year for a self-polishing coating and 5 for a foul-release silicone, both of which Hempel's research paper quotes (Hempel R&D whitepaper, free PDF), and 30 for the old hybrids and 40 for the controlled depletion polymers, as International's own calculator brochure and the AAST review give them. A fleet-wide fit to International's 2003 to 2014 surveys found a gentler 10 a year across the raw data, and the 1980s figure was 25 (Yeginbayeva and Atlar, Biofouling 2018, open access). Quote the spread, and use the coating family's number for your own ship.

Then the docking, which is where the ratchet either clicks forward or resets. Overcoat the old scheme and the new coating inherits the roughness underneath it. Hempel measured exactly this: on a smooth substrate a top-tier self-polishing antifouling came out at 64 micrometres and a silicone at 41; on a substrate of 211 micrometres, which they describe as "a ship that have been through 3-4 dry dockings," the same two coatings came out at 161 and 134; on a substrate of 322, "a ship that is at the end of its lifetime without any intermediate full abrasive blasting," at 267 and 209. Their rule: for a substrate roughness below approximately 40 to 60 micrometres, "the final macroroughness will be determined by the coating roughness. Above this threshold, the substrate will be the predominating contributor to the final roughness." Townsin found the consequence in the fleet forty years earlier: "68% of the hulls increased in roughness during dry-docking when re-blasting was omitted. Re-blasting, however, can render the surface to 'nearly new' condition if corrosion has not occurred." And the paint makers write the ratchet into their own guides. Jotun's application guide for a self-polishing antifouling: "if the coating exhibits weak adhesion or has been spot repaired for more than 3 dockings or 15 years, the general recommendation is to blast the surface to Sa 2 as per ISO 8501-1" (SeaQuantum Classic S application guide, free PDF).

What a docking can reset, and what it cannot

It helps to be precise about the list, because the specification is written against it.

Reset: the coating's accumulated roughness, but only if the old scheme is removed rather than overcoated, which is the scope decision; the fouling, everywhere the wash and the divers reach, including the niches that our Biofouling Management course owns; the propeller's surface, from whatever Rubert grade a year of cathodic chalk and slime left it at back to a polished blade, lesson 6's cheapest kilowatts on the ship; and the performance record itself, because the sensors get serviced, replaced and re-zeroed while the ship is dry, which is either the best thing that happens to the record all cycle or the worst, depending on whether anybody wrote it down.

Not reset: the plating. Indents, set-in panels between frames, the welds and the appendage damage are the permanent resistance in the CASPER data, and they accumulate. Nor the coating choice, which was made at the last docking and will be made again at this one on the operating profile the ship actually has; that decision belongs to the biofouling course and is cited here, not re-made. Nor the ship's trade, draught mix or speed, which set how much any of this is worth.

Two things follow. A docking is a surface event, and the surface is where friction lives: our Ship Resistance Fundamentals course puts friction at about 70 to 80 per cent of a slow ship's calm-water resistance (this course's lesson 2 explains why its own arithmetic uses a smaller share, the flat-plate friction alone), so a percentage recovered here is worth more than the same percentage anywhere else on the ship. And a docking is a dated event with a before and an after, which is precisely the shape of measurement our ISO 19030 course is built to make. The two facts together are why the analyst belongs in the room.

The analyst at the specification meeting

The yard file usually holds the last docking's paint report and a photograph of the propeller. Four numbers make it a performance document, and the analyst is the only person who can bring them.

1. The speed-loss trend of the closing cycle, from the in-service performance indicator: what the hull has cost, per month, since the last out-docking. This is the number that prices every scope option in lesson 4, because it says what a rough hull costs this ship in her trade.
2. The dry-docking performance of the last docking, if the fleet measured it: what the previous scope delivered against the one before it. A spot repair that scored badly is the strongest argument for a full blast that a superintendent will ever hold.
3. The in-docking roughness survey, ordered for the first day in dock, after the wash and before any blasting: the AHR the ship is carrying, station by station, so that the scope can be chosen on the hull's condition rather than on habit, and so that the out-docking survey has something to be compared with.
4. The propeller's Rubert grade from the last diver's report, because the polish is the item most likely to be dropped when the dock schedule slips, and it is the item with the shortest payback in the whole specification.

The rest of the course follows the ship through the dock in the order the work is actually done. Lesson 2 makes the roughness number mean kilowatts. Lesson 3 is the wash, which comes before any decision and decides more than it is given credit for. Lesson 4 is the blast grade and the scope. Lesson 5 is the coating job and the weather it is done in. Lesson 6 is the propeller and the other metal, including the sensors. Lesson 7 is the device question, and the confound that comes with fitting one at a docking. Lesson 8 is the report card: the two indicators that say, a year later, whether the money was well spent, and the docking report that carries the answer into the next specification.

One habit is worth forming before lesson 2. Whenever a docking is described to you as "full paint job" or "spot and sweep," ask for the number that describes it: the out-docking AHR. A docking that was not measured on the way out cannot be judged on the way back in, and the fleet statistics say the odds are two to one that an unmeasured overcoating made the hull rougher than it found it.