Hull structural designers know one very important rule: steel is cheap. No one cares about reducing the structure weight just to save on some steel costs, right? WRONG! And when you see the next bunkering bill, you will care too.
In the field of cost cutting, people often decide to eliminate any significant engineering optimization in the structural design. After all, why bother paying the engineer to agonize over a balance of minimum structure weight and production costs? Just use thick plate. Increase your scantlings all around and eliminate any concern. Certainly safe and cheap, but you will pay for it all the rest of your days. Or at least for the rest of that ship’s life.
Extra structural weight means extra displacement; your ship sits lower in the water. And you don’t get that for free. It takes extra fuel to push that extra structural weight through the water. Extra fuel that you must pay for, again, and again, and again. You will continue to write checks for this extra weight every time you bunker, for the entire life of the ship. I grind my teeth in annoyance when I remember this extra weight is completely unnecessary, does nothing for the ship, and there is no way to remove it. Today we talk about the hidden cost of steel: extra fuel costs.
I talk about steel because it is the most common marine building material, but this also applies to aluminum, fiberglass, or any material used to create the ship’s hull structure. All those tonnes of girders and thick plates that do the very important job of keeping the ocean on the outside. But they don’t start life looking like a ship.
Your steel was born in a steel mill, looking like a very boring stack of flat plates and long skinny beams. This juvenile steel arrived at the shipyard where it got cut, bent, welded, and tortured into the adult role of a ship. All of that labor is the problem. In most Western countries, shipyard costs are dominated by worker labor rates. Engineers face great pressure to find new inventive ways for minimizing these labor costs, even to the point of sacrificing the structural design.
That sacrifice never endangers the safety of the ship, it merely reduces the efficiency of the structure. We never question the long term effects because few question if a structure could accomplish the same job with less weight.
You pay for every metric tonne of weight that you drag through the water, including the tonnes that do absolutely nothing to help you. Lousy loafer tonnes. Fast forward to a year after your new ship leaves the shipyard. You gasp in horror at the next bill for fuel bunkering. Fuel is one of the largest operating costs that we face. And you will continue to pay those fuel bills for the rest of that ship’s life. But it’s just a few extra tonnes. How much can it really cost?
Quite a lot. A simple a rule of economics: any number multiplied by the life of your ship becomes something to worry about. Take the example of the Maersk Triple E class. As a large container vessel, they provide a great extreme example. For sake of argument, assume that the vessel could possibly 6 mm (1/4”) off their steel plating on the underwater portion of their hull.
That minor increase in plate thickness added 1,307 MT in steel weight. Merely 2.4%. But watch how it grows. Due to the extra weight, the engines work harder and burn more fuel. Using a rough calculation, the main engines burn an extra 2.82 MT/day. (Table 2‑1)
Table 2‑1: Change in Fuel Consumption
That may not seem like much, but remember that there are 365 days in a year, and many compounding years to a ship’s life. Taking the current cost of heavy bunker fuel, that little increase in structural weight will set you back almost $4 Million Dollars (USD) over the life of the ship! (Table 2‑2)
Table 2‑2: Fuel Cost of Extra Weight
When examined over the life of the ship, structural optimization is a sound investment. The best part, the engineering effort would be fairly minimal to achieve these savings. For a vessel the size of the Maersk Triple E, the cost of additional optimization is probably between $50,000-$100,000 USD. That extra engineering cost yields a 40x to 78x return on investment (ROI). This is one case where planning ahead pays major dividends.
Results may vary, depending on your ship and fuel consumption. You can download the spreadsheet and try it for yourself. The included download link includes a copy of the example calculation I did for the Maersk Triple E and a spreadsheet to explore cost savings with your own vessel.
A lifetime savings of almost four million dollars. That is amazing. You could build another ship with those savings. A small one. (Personally, I’m picturing an elegant silver yacht reflecting the warm crimson sunset in the background.) Imagine the competitive advantage achieved if every ship in your fleet saved you $4 million dollars. Any increase in fuel consumption is still important when applied over the entire life of your ship. So forget justifying structural optimization by focusing on steel material costs. The true cost of steel is a lifetime of fuel bills.