This seems like a short-sighted solution to the problem (I guess these all are). Wouldn't the answer be to tune the engines so that they run more efficiently at the their normal crusing speed. If they spend 90% of their time at this speed, we could live with a little inefficiency at the low and high ends. Slowing down may help with overall efficiency with boats, but that means more suppliers will ship a portion of goods by air and by sea in order to get products to market quickly. So, the net result may be negative.

Also while I agree with their stats as far as total GHG contribution, water shipping is much more efficient than shipping by air. So, I agree it needs to be tackled as an issue, but its like complaining that a Prius's mileage isn't good enough while surrounded by SUV's.

"Wouldn't the answer be to tune the engines so that they run more efficiently at the their normal crusing speed."

That's not really how it works. Going faster means more drag, more drag means more fuel used. There are ways to make ships less draggy and more efficient, but most cargo ships are constrained in their geometry by having to fit through the Panama canal, which precludes really radical design changes.

I expect there will be big resistance among shippers to this. Adding 10% to the time required to ship something almost certainly costs more than the fuel it would save. It pretty much eliminates one ocean crossing per year.

PRP's nailed it. Engines need to be optimised for peak performance.

Let's compare the volume of goods sent by ocean going freight, and compare that to the tonnage of goods sent by air. That would likely show that ocean shipping is perhaps 1,000 times less polluting than air cargo.

But ships should be efficient and pollution minimizing too. Maybe exhaust could be scrubbed, or particulate traps installed

Lots of ship are already too wide to fit through the Panama Canal. Their efficiencies in loading make up the costs of additional travel time and fuel needed to get around the Cape.

But some ships may be able to adjust their travel speed so that they arrive at port at favorable tides and when the port is ready to dock them. Ships often must moor outside ports until conditions are right, losing some of that travel speed advantage.

Don't forget that they are also testing out using giant computer controlled kites to help pull the ships along. First test was a success (Something like a 2% fuel savings), and if I remember right they said they only needed to make 5 trips to pay for the system.

Sails have huge disadvantages, but modern controls could overcome the large crews needed in the past, and better communication and radar/satellite could relay the best conditions to avoid dead calm or heavy weather to an extent.

Maybe a motor sailer is what we need?

A ships speed is proportional to length, so with modern steel structure, a sailing ship could be made much larger than in the past, so possibly those old speeds could be exceeded.

Maybe the time has come to resurrect the concept of nuclear powered cargo vessels. The US navy currently uses nuclear power because of enhanced strategic performance (ie keep a submarine underwater for months at a time or aircraft carriers that can continuously support air launches without stopping to refuel). The US navy is now considering expanding its nuclear fleet to reduce its fuel costs. The Russians operate a fleet of nuclear icebreakers to keep their northern ports open.

The US built a nuclear cargo ship several years ago (NSS Savannah). The performance was excellent but it was more a showboat than a working vessel.

Since fuel is inexpensive, nuclear power would permit a ship to travel at hull speed, about twice what is used by most commerical shipping. That would tend to mitigate the extra cost of the reactor.

from the article
"said that dropping average speeds of 18-19 knots by just two knots could save 5 per cent of fuel use and emissions alone."

So slowing by 9.5% could have 5% on your fuel bill? Hmmm

Speed provides some important commercial advantages. Slowing down to save on fuel may seem like an easy choice but that choice also has a negative effect on productivity. If all ships on the ocean slowed down by 10%, the overall effect would be a new requirement for 10% more ships to carry the same quantity of goods.

When ships are carrying high value cargos, every additional day at sea adds to the finance charges that the cargo owners have to pay, since goods in transit are not goods in use.

The people that decided to replace sailing ships with primitive coal fired steam ships were not stupid, they understood and saw with their own eyes that a ship that could move a bit faster, with greater predictability and could carry more cargo per trip could return a much greater return on investment. The advantage of combustion power over sail increased as the engines became more refined and required fewer people to operate.

As Bill Young points out, there is a readily available, proven alternative to combustion today. Nuclear fission powered ships have proven their mettle over the past 50 years in the world's most challenging environments as propulsion plants for submarines, aircraft carriers, and ice breakers. Fission provides the opportunity to eliminate emissions while at the same time reducing fuel costs by an enormous margin. (Commercial nuclear fuel costs about 50 cents per million BTU, oil costs about $19-30 per million BTU depending on location and grade.)

N. S. Savannah, like the S. S. Savannah that first crossed the Atlantic using a steam engine, was a financial failure, but the conditions that made that happen in 1972 - with oil prices at $3 per barrel - have changed rather dramatically.