Ship Speeds

Factual Questions
1

A question came up at lunch recently - I don’t think this has been covered previously.

Assumption: A ship is designed for optimum fuel efficiency at a certain cargo load. Say that with a load of 10,000 tons, the ship sits at say, load line of “10” on the hull, and the hull has been designed to consume 5 tons of fuel an hour at constant speed of 20 knots at that load (all numbers totally imaginary).

Will adding or subtracting weight reduce and or improve the fuel efficiency?

Obviously, the intuitive answer, which seems very likely, is that more weight = more power needed to move forward = less fuel efficiency, but an alternative theory is on the table.

It is asserted that adding or subtracting weight will change the measurement of the load line relative to the water surface - thereby REDUCING fuel efficiency either way. IE: Adding weight to the ships cargo will drive the load line lower (say to 8) making the hull less efficient. And conversely, that removing weight would drive the load line higher, say to 12, and still make the hull less efficient. There are obvious theoretical variations, but in the real world, even a zero cargo load leaves the hull partially submerged. And given routine stability requirements, we might leave out scenarios where the total cargo load exceeds the displacement weight of the vessel ( I think this would cause it to sink!)

Given that the ship’s engines must push the hull through the water, and that the hull’s need for power is impacted by need to break water tension, how do changes in weight affect a moving ship?

Any thoughts?

2

IANANA, but two thoughts immediately come to mind:

1.) The higher the cargo weight, the lower the hull sits in the water, increasing the hull drag. The ship’s engines must overcome this drag, thus reducing efficiency.

2.) The higher the cargo weight, the more mass that needs to be displaced, also reducing efficiency.

Thus, it looks like a 1-2 punch. Removing weight to make the hull ride higher in the water will decrease the hull drag, but I don’t see how this will result in an decrease in efficiency. Generally, the less hull in the water, the better. This is the driving principle behind catamaran hulls.

3

I understand and tend to agree. But in modern shipping, we must account for bulbous bows (the torpedo shaped thing forward of the hull). Sorry, I should have mentioned this earlier.

The hypothesis is, if you remove weight enough, the bulbous bow RISES, possibly even breaking the water, therefore reducing efficiency. Lose/lose.

4

I think the bulbous bow issue is being revisted by shipping. As a matter of fact, I spent today aboard an LPG carrier (of relatively new construction), and noted from the ships drawings that it had no bulbous bow. The last LPG carrier I was on a few months ago had the same flat, straight up and down stem with no bulbous bow. When I asked someone about it, they basically said that bulbous bows don’t offer that much efficiency, and can actually be detrimental to efficiency under certain load conditions. Therefore, some ships are now being constructed without them.

Also, when ships are in ballast (no cargo), the bulbous bow is almost always partially out of the water. Commercial tankers and freighters spend about half their time in ballast, loading in one port, discharging at the next. Container ships are rarely without cargo, so perhaps they would benefit more from a bulbous bow.

5

Enormous ships are unstable with no cargo, so tankers usually pump in seawater for ballast when returning to the port where they load up.

In the early days of the American colonies and the young USA, ships carried bricks for ballast on the “dead head” run to the Americas, and that’s why most of the surviving colonial houses were made of brick.

6

The bulbous bow goes back to the Greek triremes, where it was used as a battering ram to break the other ships. This is now done with topedoes and cannon.

7

Ships in ballast use less fuel. Not as much less as you might think, but less. Many charters will give a daily consumption rate for loaded/in ballast and the latter is always in my experience less.

P.S.

and

Many of my shipowner clients would be horrified (horrified, I tell you!) by these statements. If their ships were spending half their time in ballast they would be sacking their brokers, pronto. There are exceptions, because ballast legs are unavoidable on some routes (which causes the hire/freight rates on those routes to be appropriately loaded), but generally savvy shipowners know that the way to make money in shipping (indeed in any transport industry) is to have a customer paying for all movement of your transport assets.

8

The bow wave, the water pushed up at the stem of the ship is the most critical concern with respect to drag, and drag impacts directly on fuel on fuel economy.

Next concern is the bow wave impact on the stern where turbulence can create energy dissipation.

For this reason all displacement hulls have an optimum hull speed where the bow wave completes one or more complete wave cycles exactly at the stern. The captain can determine his hull speed by watching his tachometers and speedometer.

Theoretically, the introduction of the bulbous bow is meant to create a wave that immediately gets cancells some of the wave caused by the stem and thus lowering the height of the bow wave. It seems obvious to me that any variation in cargo weight will affect the designed location of the bulbous bow relative to the surface of the water and negatively influence the height of the wave with respect to design.

Thus I can see the possibility that a lower cargo weight can result in loss of fuel economy.

Waterline length of a displacement hull can also influence drag. The longer the hull in the water the less drag. An unloaded ship will have less water line length but given the length of these large cargo ships. a few feet is not going to make much of a difference.

9

Thank you Dutchman… and everyone else :). It looks like your comment is becoming the consensus here… that there is an optimum load and that LESS load can adversely affect fuel efficiency as can MORE load.

The ballast point may also be relevant - except that ballast was cited as an improvement to stability without commenting on the fuel efficiency discussion.

FWIW - the ships that started my original conversation are Cruise ships - floating cities ranging between 100,000 and 150,000 gross ton displacements. They NEVER run empty - but consume ;large amounts of fuel. The load question came up while discussing various ways to reduce fuel consumption.

BTW - All of those ships have bulbous bows.

10

Addendum

Regarding the bow waves - it is my understanding that the hull forms on these enormous cruise ships are designed with self cancelling bow wave configurations. That is to say, the hull is specially shaped to create a second wave form, after the “naturally” occurring one - the second being of opposite phase to the first. This is done to prevent wakes - which would be disastrous when these enormous vessels enter ports.

FYI

11

All ships have wakes, bulbous bow or no. I have never heard of wake reduction being the purpose of bulbous bows, as such.

And by the way, cruise ships aren’t very big. VLCC’s and ULCC’s are big. Cruise ships just get all the glory by being high profile.

12

I wasn’t clear enough in my earlier post. The bulbous bow has nothing to do with the wake cancelling I discussed. The hull itself is reshaped to provide that effect. There is a very good documentary out on the design and construction of the Voyager of the Seas, which goes into detail about this (Discovery Channel or Travel channel originally). I will look for a cite for you.

13

I’m stuck on one point in the OP:

I don’t know jack about boats but would add that more weight means more power needed to accelerate forward at the same rate, but does not automatically mean that more power is needed to maintain the same speed in steady state.

The weight per se doesn’t affect fuel economy except to the extent of how it affects the other performance characteristics like displacement and stability. In contrast, adding weight in a land vehicle adds load to bearings, increasing friction, but in an idealized frictionless situation, weight doesn’t matter except when accelerating. The hull and the water bear the weight of a boat so the only thing that matters is how the weight affects other stuff that does matter.

Assume a boat with a nominal load vs. a boat with an extra ton (assume that a ton matters) of cargo plus helium balloons with a ton of buoyancy. The boat still has the same weight and therefore draught (right word?) and same center of gravity, but now the inertial mass has increased significantly, so more energy is needed to accelerate. But once you’re up to speed, no net effect on fuel economy.

14

A couple of terms need to be redefined, efficiency which the op asked about vs economy. Certainly most things have less economy when load is added but typically effiiency is defined as cargo load / fuel economy. An Airbus 380 crammed full of coach class passengers will get poorer econmy than a Cessna 150 but may in fact have better efficiency.

A ship with extra cargo will certainly use more fuel due to more wetted hull area being drug through the water but is that increase in fuel useage proportional to the inrease in cargo or is greater or less?

15

I can see how this would work for container ships, but how can an oil tanker ship oil back from a country that uses more than it produces? Oil tankers can’t just fill up with wheat, for example.

The practice of filling tankers with enough seawater for stability was once a big pollution problem for many ports, because tankers would dump out oil-laden seawater at the dock before taking on new oil. Now, there’s an oil-eating bacteria that’s added to the tank when the seawater is pumped in, and by the time the ship gets to the other end of the run, there’s no oil left to pump out. Most ports now demand that tankers pump the ballast water out of their tanks before entering the harbor.

16

IME, that’s correct, and tankers end up spending much of their time in ballast. What I typically see in my port is tankers that load up overseas, and discharge here, load up overseas, discharge here, rinse, repeat ad infinitum. Same for some bulk freighters. On occasion, a tanker will discharge here, then load up somewhere in the US to go back overseas. This would minimize the time in ballast, but it does not happen very often in this part of the world. YMMV. My previous post should have been more specific to my experiences; Princhester is correct that shipowners will do anything (anything!) to minimize costs.

As far as ballast water is concerned, all new tankers these days have segregated ballast tanks. The ballast water never sees the cargo tanks.

17

Tankers and bulk carriers (particularly the former) will find it harder to avoid ballast legs than, say, container and general cargo vessels. However, they will try to avoid the problem by finding round or triangular routes if they can.

It’s hard in the bulk trades if you are carrying dirty products (coal, for example) because it can be hard to then get clean enough to carry something else.

But you can carry say fertiliser/coal/fertiliser, for example, without the need to clean much.

Cite for your second and third sentences? I’d be amazed if you find one though, because I am pretty much certain that this is complete twaddle.

All modern tonnage has, and is required by international treaty (MARPOL, which treaty is made law pretty much everywhere) to have Segregated Ballast Tanks (SBT’s). That is, they have seperate tanks for ballast precisely so that they don’t get their ballast dirty with oil cargo.

There were grandfather clauses in MARPOL under which old tankers could avoid this, but I think that even those clauses have expired.

This is the reason dirty oil ballast is no longer a problem.

I have never heard of oil-eating bacteria being used to clean dirty ballast. If this is or ever has been done, it is certainly not done widely.

Dirty ballast, if any is created, is discharged via an oil-water separator which reduces the oil content below 5 ppm. Even then, the cleaned ballast must be discharged at certain distances from land. The oil separated out by the separator goes to a waste oil tank and is burned in the boiler or discharged ashore.

Finally, if there are any ports at which your last sentence is true, I’d be surprised. A tanker with no ballast tends to be basically uncontrollable, because it has so much freeboard that it has very high windage and because the prop and rudder are significantly out of the water.

What tankers do is discharge as much ballast as possible before they arrive off a port, leaving enough on board to allow safe passage into port. The last ballast will be discharged as cargo is loaded. However, even this is not to avoid discharge of ballast into a port (it wil be clean because it will come from SBT’s anyway) but to avoid pumping delays.

18

Here is a link to the definitions of Grey Water - Black Water etc. used by Royal Carribean Cruises. Some types of water can be dumped off shore, and others can’t. BILGE WATER CHART

This might help advance the conversation a bit.