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This may sound crazy but bear with me. I was thinking about why cargo ships are built from such heavy material as steel, while smaller boats are often made of plastics. I know cargo ships carry a lot of weight and the sea can be rough. Those ships of course need to maintain structural integrity and such.

But wouldn't it be possible to build a structure from steel, supported by the strength of the containers that fill the ship. And then build a thick hull around it made from a combination of plastics and compartments of some kind of foam (polyurethaan?).

The advantages are that plastics are flexible (a bump doesn't result in damage as easily), easy to work with, easy to repair, cheap, and really light compared to steel.

The idea is kinda like how tall buildings are built from a strong structure, but have an outside "hull" of glass (see image below).

I didn't do any calculations on it (I'm a software engineer, unfortunately not a hardware engineer).

Does anybody know the reasons why big (cargo) ships are not made from plastics?

enter image description here

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  • $\begingroup$ A light bump will cause more damage to plastic that may require extensive and difficult repair... some metal hulled ships have deep scratches running for metres that just have a coat of paint... $\endgroup$ – Solar Mike Mar 30 '19 at 7:16
  • $\begingroup$ Containers don't have much strength. They have pillars at each corner so they can be stacked without crushing the ones at the bottom, of the pile but that's about it. Also, on container ships there is usually 3 or 4 times as much volume of cargo above the water line as below it, so most of the containers wouldn't "strengthen" the ship hull at all. $\endgroup$ – alephzero Mar 30 '19 at 13:47
  • $\begingroup$ Also what happens when the ship is empty? Important strength members missing! Do not underestimate the forces generated by slamming & pounding in head seas. These are non-trivial. I have seen a 50 tonne crane reduced to scrap by a single wave strike over the fore-deck. $\endgroup$ – pHred Mar 31 '19 at 5:27
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In short, because steel is less expensive on a strength per dollar basis of anything else, and for large structures, it's less expensive to work, again on a strength per dollar basis. That's why steel construction predominates in cars, too.

For small boats, the cost equation leans more toward the ease of manufacture of that size of structure out of fiberglass (which isn't just plastic) or aluminum compared, plus a consideration for the corrosion resistance of fiberglass.

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  • $\begingroup$ Alright, thanks. So does the cost argument also hold when you take into account that plastic is lighter, and the ship will therefor use less petrol to move it? I guess so, but I've sailed a metal boat before and that just moves so much slower so I figured it takes a lot more energy to move it. $\endgroup$ – kramer65 Mar 30 '19 at 7:04
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    $\begingroup$ Apart from the very small fraction of the voyage when the mass of ship and cargo are being accelerated to sea speed the majority of the propulsion energy is expended overcoming skin friction and wave making resistance. Skin friction is a function of wetted surface area and wave making resistance is a function of waterplane area (and its shape). As a vessel's displacement is directly related to the mass of cargo carried (Archimedies) the change of structural material will have little or no impact on the additional energy required to propel the cargo proportion of the ship + cargo total mass. $\endgroup$ – pHred Mar 31 '19 at 5:14
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Anything is possible and lowering the lightship weight (LS) of ships will save fuel, but the key issue would be any change must not reduce the seaworthiness of ships.

This question is very broad.

Regulations have been established for commercial steel ships (DNV GL, Lloyds, ABS, individual countries). These regulations take into account the harsh marine environment. In many cases, they represent the hard learned lessons from accidents like the Titanic. Regulatory agencies, would have to be overcome. It is on thing to do it for small craft, another for commercial ships.

Safety (which probably has more to do with regulations).

Fire: Steel melts at around 1370°C. Plastics melt at temperatures below 300°C. A candle burns at 1000°C. A typical fire in a steel compartment can be extinguished by closing off the air supply. On a plastic hulled vessel, the hull would burn (which would probably extinguish the fire, but cause stability problems).

enter image description here

Strength: A ship is built from the keel up. The hull steel plates and stiffeners must displace the displacement weight of the ship. The hull must be able to withstand hurricane force wind and waves. Any plastic would not have the strength of steel, so more structure would have to be used, so any loss in hull weight would probably be offset in structure weight. A steel ship already flexes in heavy seas. The longer the ship, the greater the flex.

enter image description here

Impact: There are a number of estimations for steel weight estimation. They are summarized in Preliminary parametric estimation of steel weight for new ships. They report a 6.25% Error for a 8500TEU container ship when using the following formula:

$$ Steel Weight = 0.034 \times L^{1.7}\times B^{0.7}\times D^{0.4}\times C_B^{1.7} $$ $$ Steel Weight = 0.034 \times {317m}^{1.7}\times {45.8m}^{0.7}\times {25}^{0.4}\times {0.6}^{0.5} = 24, 780mt$$

Assume half is structural and half hull or 12,390mt.

From Electrical Systems and Engine Room: Displacement, LWT and DWT

$$Displacement = C_B \times L \times B\times D\times \rho_{SW}$$
$$Displacement = 0.6 \times 317m \times 45.8m\times 25\times 1.025 tonnes/m^3 = 223,224mt$$

These are just estimates, but at best with no increase in structure and if plastic had no weight, you'd save < 5.5%.

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  • $\begingroup$ Thank you for the insights. I'm currently planning to build a solar powered robot boat and thanks to your post I completely changed my designs.. :-) $\endgroup$ – kramer65 Apr 3 '19 at 13:34

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