Marine jet vs conventional diesel

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Marine jet vs conventional diesel

Postby HiCaptEd » Mon Feb 13, 2012 5:43 pm


Newbie here. Active harbor pilot writing a book on a High Speed Ferry. I desire to learn more about "efficiency" of high speed vessels (HSV) when compared to conventional propulsion systems/ships. I would tremendously appreciate the bountiful depth of information available here (and life experiences) and hope you all might be able to help.

I once read (long ago and far away) that all these new HSV jet powered cargo/pax ferries were the 'most efficient' method of transport of people and cargo...up to a certain distance. Beyond that theoretical distance, their fuel consumption versus their payload carried, quickly started to diminish. My recollection was that distance was between 700 and 1000 miles, depending upon weather (sea state) and traffic encountered and other variables that may slow them down. I recall reading this in a professional magazine and it seemed a broad sweeping generalization. But if anyone out there can lend some truth, data, support, or example of this; I would be eternally grateful.

My desire is to put FACT in place where many laymen have made the allegation that running a ferry between the Hawaiian islands where the longest geographic distance run was 140 miles (on a specific proposed route) between two ports was an incredible, wasteful, inefficient use of diesel fuel. A conventional tug (4000 hp twin screw) towing a flat deck barge (11,000 ton capacity) does this typically in 18 hours. This is difficult for me to explain to anyone in "non-engineering" terms. But I get the idea this is an apples to oranges comparison. Could anyone offer something to support the "concept" of moving people/cargo quickly between two points as being efficient? Further can any tug engineers provide some accurate fuel consumption data? I know the existing tugs that make this run are equipped with CAT engines, with a total horsepower of 4000 Hp. Boat makes about 8 knots towing with a moderate load on the barge. 18 hours dock to dock. Can't give you much beyond that.

Thanks in advance for any/all replies! :D

Capt. Ed
"The Captain might have permission to take the ship out for a spin, but the Chief Engineer has the keys."

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Big Pete
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Re: Marine jet vs conventional diesel

Postby Big Pete » Tue Feb 14, 2012 4:08 am

Hi Captain Ed, Welcome to the Diesel Duck Forum.
There are several different linked issues here.
Obviously, as we all know, increasing the speed of a ship means using more power, therefore bigger and heavier engines, more fuel consumption therefore bigger and heavier fuel tanks, also more expensive spare parts and Lubricants.
So simply making the same Hull faster will increase the initial cost and the running cost as well as reducing the payload, against this the higher speed will enable more voyages to be made in a given time. If the extra Freight (English usage) earned on the extra voyages is greater than the extra costs and the lost earnings from the reduced payload per voyage, the ship will make a greater profit and therefore be more efficient financially.

If the ship is specially designed for high speed the Hull will be finer lined than a slow speed hull, reducing the wetted surface area and displacement, this will enable higher speeds to be reached with smaller, lighter engines and relatively small additional costs, except for a much bigger loss in Payload/ deadweight.

If you look at the fastest ferries, which are very specialist, Hovercraft, Hydrofoils, planing Hulls, High speed Aluminium cats etc they usually have lightweight high speed diesel engines or Gas Turbine engines, inherently less fuel efficient than Medium or Low speed engines, but for them, the saving in engine weight and volume is crucial to give a useful payload. On long voyages the extra fuel carried on board would start to outweigh the weight savings on the engines. This type of ferry is almost exclusively used for shorthaul passenger ferries with very limited freight capacity.

One problem with your question is what efficiency are you talking about?
In one respect the most efficient Hull shape would be a perfect cube, It would take the minimum amount of material and labour to construct, bending moments would be minimised because of the reduced length, so scantlings could be much lighter in relation to payload, it would not require any plate bending so it would be very cheap to build and there would be no wasted spaces where it was difficult to load cargo. In terms of Hydrodynamics/ propulsion efficiency seaworthiness etc it would
be a total disaster!!
The next thing up from this are Lighters, Barges, large Tankers and Bulk Carriers, they are usually very rectangular in shape, with a high Block and Prismatic Co efficient and a minimal amount of streamlining fore and aft. These are designed to carry heavy low value cargo at low speed, they operate well below the speeds at which wave making resistance becomes a problem. In the case of the Large Tankers and Bulk carriers they are nearly always fitted with slow speed Diesel Engines which are the most fuel efficient propulsion, but are heavier and take more volume than medium speed engines. These types are the most fuel efficient in terms of the fuel consumption per ton of cargo carried per mile and also the cheapest.
These are designed to carry large masses and volumes of low value commodities as cheaply as possible
Where you are carrying higher value cargo, such as containers and cars, the shipper is willing to pay a higher freight rate for his cargo to arrive quickly. The manufacturer has bought raw materials paid wages and run his factory to make a product and he will not get paid until after it reaches the buyer. Send a ship full of Porsche cars from Germany to Australia and you can imagine how much interest goes on Mr Porsche's overdraft during the voyage, every day saved on the voyage is a day's interest saved, so he is willing to pay a higher freight rate for a faster voyage which pays for the extra fuel burnt. At present with Bank Rates at record lows, the incentive to pay for faster voyage times is greatly reduced which is one reason for the large number of ships slow steaming rather than being laid up. When interest rates are higher there is a much bigger incentive for the shipper to pay premium rates for fast delivery.
Ships carrying this type of cargo will have much more streamlined Hulls below the waterline to make the hull efficient at higher speed and also because these types of ships rarely load to their maximum Deadweight/Payload, because the density of the cargo is less than Bulk cargoes, so they do not require such a large immersed volume to provide the buoyancy to support their cargo. Some of both types have medium speed engines to enable cargo to be loaded over the top of the Engine Room with small casings for stairwells and uptakes going through the Car decks to the Accommodation. Medium speed engines also have a lower centre of Gravity than Slow speed engines so improve stability. The additional fuel and maintenance costs of Medium speed engines being outweighed by the extra freight earned because of the saving in weight and space.
Most Passenger freight Ro/Ro ferries have Multiple Medium speed engines to save weight and space and allow the freight decks to run through the entire length of the ship without any interruption, for the reasons above.
Once you start carrying passengers speed becomes more important and they and their cars pay a high freight rate for very little weight, so it is possible to have a very streamlined underwater Hull to support a rectangular box for parking the cars and passengers. However this has led to structural problems with a lot of feries and car Carriers, the combination of a Destroyer like Bow underwater transforming into a rectangular box one deck above has caused severe structural damage in head seas.
Refrigerated cargo Ships are also very streamlined and designed for High Speeds, again they often carry light and bulky cargoes such as Bananas, that have a relatively high value and are perishable which generate a financial incentive for fast delivery. Similarly they have narrow streamlined Hulls below the water and then a large volume above water. They usually have Slow speed Main Engines for economy, stability is not an issue for them, and they dont require an uninterrupted cargo Deck from forward to Aft.
With the High Speed Ferries, great efforts are made to reduce weight by using Aluminium Alloys, composite materials such as carbon Fibre etc. These give the required strength for much less weight. However, they require much more energy to produce than Steel so some people regard them as being environmentally unfriendly, however their use saves fuel when the Ferry is in service.... a bit like the arguements over the Hybrid cars like theToyota Prius and batterry cars.
They also use Aircraft Derived gas turbines or High Speed Diesels which have very low power to weight ratios, but worse fuel consumptions than Medium or Slow speed diesels.
Many years ago I was told that if you took the cost of Lubricating Oil and spares for a slow speed engine as one, then the cost for a medium speed engine, running at the same power for the same hours, would be two, and for a high speed engine four. I am not sure about the relative costs for Gas Turbines.

It sounds like you are writing this book in support of a plan to use a High Speed Ferry. But I think I have managed to give a general outline of the Economic factors above, and I am sure other people will contribute more detailed specifics.

I would think thaat the claim that HSVs were the most efficient form of Transport over short distances is based on profitability for the operator more than anything else, nothing wrong with that. All the factors I mention above are costed out by Naval Architects, Shipyards, and Shipowners before they agrre to build any ship. In a Capitalist Society Money is how we measure the input and output of any operation and therefore its efficiency.

With regard to the "break even" point of HSV being 700 to 1000 miles you would have to calculate that for a particulatr vessel on a particular trade, as it depends on the additional cost of fuel per mile versus the additional freight earned per mile, when compared with a conventional ferry.

A Few thoughts/questions below:-

With regard to transport between the two Ports you are thinking about, I would try to analyse the existing traffic, for instance you are saying it is loaded on a Flat deck Barge, so I assume that there are no passengers, refrigerated cargo or anything that could be damaged by weather, or does this Barge carry containers as well? is it equiped with its own generators to power reefer Boxes??

Would a proper ferrry attract additional passengers, car traffic, RO Ro Freight, Container cargo, and thus generate more income for both Ports and the areas around them?
A Proper passenger Ro Ro ferry would certainly employ a larger crew than a tug and be an advantage to the local economy.
A tug and Ferry arrangement is probably not very efficient on a Sea Passage like this, a tugs propellers are too small in relation to the power transmitted to be efficient, the lower the Prop speed the more efficient, but that means a larger Blade area and Diameter. An 11,000 ton Ferry would have more draft and therefore more space for a big propeller than a tug. Propelling a single Hull should also take less power than two.
Are there any draught restrictions in either Port that would restrict operation of a Ferry? A Ferry of 11,000 ton capacity would almost cetainly have a deeper draft than the existing barge
A Ferry would probably have higher capital costs than a tug and barge, (Depending on age) would it be able to run nearly fully loaded all the year round, to pay the finance costs, or is demand Seasonal?
Does the existing barge carry large loads that could not be put on a ferry?
Could you charter a small high speed passenger/ car ferry with limited space for premium cargo as well as the Barge and Tug operation? Then see how demand shapes up, if it is good try chartering a bigger one.

Good Luck. BP
It is always better to ask a stupid question than to do a stupid thing.

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Big Pete
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Re: Marine jet vs conventional diesel

Postby Big Pete » Tue Feb 21, 2012 4:55 am

Hi Captain Ed,

I hope you have read the Post. Was it helpful?

One thing I forgot to mention is thet Steam and Gas turbines are inherently less thermally efficient than reciprocating engines because thermal efficiency is related to the maximum gas temperature in the engine cycle. The materials of the piston and cylinder head of a Diesel Engine do not reach the maximum gas temperatures because for most of the cycle they are in contact with relatively cold gas. In a turbine the gas temperatures are constant at each stage, so the heat soaks right through the metal bringing the core of the metal up to the same temperature as the Gas and weakening it. So they have to be designed to run with lower gas temperatures. The latest gas turbines try to partially overcome this by blowing compressed air through bores in the blades to cool them.

With reference to your ferry, how many round trips a week does the existing barge do? You estimate 36 hours at sea for a round trip, if you have good facilities for cargo handling you should be able to turn the barge round in an 8 hour shift,
making about 52 hours for a round trip. Three round trips a week. Say 33,000 tons capacity each way per week.

A high speed ferry might make 40 knots and do the round trip in 7 hours at Sea and with much less cargo capacity, should be able to turn around in 3 hours at each end, making 13 hours for a round trip. Making 13 round trips per week.
This would give you the same freight capacity as the Barge if the High Speed Ferry can load 2,500 tons.

However, most High Speed Ferries are Ro Ros to speed up loading and discharge. This works much better if you have regular scheduled sailings at fixed times, so the trucks turn up at the right times!
In the example above every sailing would be at a different time.
So you either have to reduce the turn around time, and /or have a faster ferry to complete a round trip in 12 hours. Ferries running over 40 knots are going to really burn some fuel and are not likely to be viable.

Alternatively, a Ferry running at 15.5 kts could do the voyage in 9 hours, making a round trip take 24 hours, so you could have one sailing each day in each direction and always at the same times.
You would require a top speed slightly in excess of 15.5 kts to allow for weather and to be able to catch up on any Port delays.
In this case you would be making 7 voyages a week, you would require 33,000/7 or 4,700 ton capacity.
It is always better to ask a stupid question than to do a stupid thing.

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