What makes tugboats so strong

To douse fires, the Edward J. Moran , pictured here, calls on the most powerful firefighting capacity afloat. Twin hp pumps pull water through inch risers to a pair of degree nozzles that the crew controls remotely from the pilothouse. The flow rate: 11, gallons of water per minute. When we exit the river and head into the ocean, the swells pick up, and 6-foot waves, driven by winds gusting to 30 knots, crash over the pilothouse.

It's a long, rough slog out to the Princess , which finally looms into view--a British-registered, black-hulled steel monolith that left Egypt 12 days ago. We slide up against the hull in the ship's lee, and Rodney Magwood, the docking pilot, climbs the gangway and disappears inside the tanker.

We ease back feet into what's known as the in-line position and match the Princess 's speed of 9 knots. From here on, the tanker will remain tethered until she's back out at sea. It takes 2 hours for the Edward to reach the river's mouth. The tug has four crewmen: a captain, a mate, a deckhand and an engineer, and they work a week on and a week off, on standby 24 hours a day for LNG work and whatever else the port throws their way, from docking container ships to rescuing disabled vessels at sea.

Missroon is a third-generation Savannah River tug sailor. His mate, Anthony Groover, 25, is the son of a docking pilot who was trained by Missroon's father and who in turn trained Missroon. He wanted me to go to the University of Georgia, but he died in a car accident when I was a senior in high school. I changed my plans and came to the water.

He's spent lots of time on the boat, and it's in his blood, just like me. It's late afternoon when the Edward and the Princess, now under escort by a U. Coast Guard helicopter and two Coast Guard rigid-hull inflatable boats, close in on the LNG terminal, a long concrete pier parallel to the shore.

These terminals have long been controversial, but all LNG tankers are double-hulled, and during 33, voyages over the past 30 years there have been only eight leaks--none of them resulting in fires.

LNG won't burn unless it becomes a vapor and dissolves into the air at a concentration of 5 to 15 percent. The worst accident occurred in in Cleveland at the world's first commercial LNG plant, when a tank failed and spilled its entire contents into creeks and sewers. When the air-gas concentrations were right, the vapor caught fire, killing and injuring Since then there have been four accidents worldwide that resulted in fatalities, all at plants.

We're traveling at 9 knots, and it's time to slow down. From here on, Magwood, the docking pilot on the Princess 's bridge, calls the shots.

The Edward shudders violently--it feels as though we're bumping over a washboard dirt road. The meter registering the load on the Edward 's line shows 54 tons. The Edward slows to 8 knots, as the Bulldog swings round to the Princess 's bow. At 7 knots, Groover shifts to starboard.

When the Edward , straining and digging, slowly pulls the Methane Princess's stern around, 94 tons register on the line. The Edward 's bow is pushed down, its stern lifted up; it shudders as it backs against the strain. Bit by bit over the next half-hour, we slow the Princess down to 4 knots. Two more tugs join us, the Bulldog "end on"--bow forward and perpendicular to the ship--against the Princess 's bow and two older Moran tugs amidships. The berth is now about yards ahead.

As Magwood guides the behemoth in, a dance based on years of experience and intuitive knowledge between docking pilot and tugs commences. Over the next 45 minutes, the closer we get to the dock, the faster the commands come. Easy on the stern tugs, easy. With indirect towing, the tug can act as a rudder and pull the ship to one side, helping rotate the ship as it moves. Of course, tugs can also cruise right up to the side of the ship and push.

Large ships usually have designated areas where tugs can push safely. Image Credit: Workboat. What are the types of tugboats? Tugboat designs have changed over the years, the oldest being the conventional tug. These are in use all over the world and consist of a diesel engine and have one or multiple propellers.

They use a screw propeller design with nozzles and rudders that move to help them maneuver. The benefits to a conventional tug are that they are simple, low maintenance, and have a proven design. They are very reliable and still in use today. These are more versatile and agile compared to conventional designs. They can provide degree thrust, have low risk of capsizing, and can work sideways. Downsides are less pull compared to newer ASD tugs, high maintenance cost, and complexity.

They have two towing locations and use two rotating azimuth units for propulsion. Benefits of these tugs are lots of bollard pull, shallow draft, and better stability at speed. Downsides are they are a little more complex to control and limited in some maneuvers. You can see a tugboat with azimuth thrusters in the photo below.

Image Credit: Reddit. Most tugboats are still powered by diesel engines. Conventional Tug: These are the type of tugs which we are using from the time of our forefathers that are the tugs made on the oldest known principle of tug development in its early stages. The important components of conventional tugs are briefed as under: These have a classic rudder.

In the centre of the tug, there is provided with a towing hook. Along with other components of tug positioning of a hook is of huge importance. The safety and performance of the tug boat mainly depend on the towing point location. When towel hook is accompanied by gob line this point can be taken towards the aft that is its distance is reduced from its original value of 0.

Its stern consists of the power plant complex. These tugboats are designated on the basis of a number of propellers in them as: Single screw tug implies one propeller conventional tug, Twin-screw tug implies two-propeller conventional tug and Triple screw tug implies three propeller conventional tug. Advantages: The main advantages or features of conventional tugboats are as follows: The construction of conventional tugs is quite simple which requires less maintenance, so the maintenance and construction cost if these tugs are very low as compared two other two types of tugs.

These are self-sufficient thus requires no support system. Smaller is the propeller larger be the open water. These tugs have an upper hand and maximum efficiency in towing carried for port-to-port.

These provide complete assistance with pushing and tugging operation while carrying towing on a line. The operation cost is quite low and seems economical while working with moderate-sized vessels. There may be certain disadvantages of conventional tugboats and these are briefed as follows: The designing principle is very old thus it degrades the performance standards.

They are unidirectional and cannot work in the reverse direction until provided with a reduction gear to facilitate reverse flow. There are more chances of cavitation in these tugs.

The stability and strength of these tugs is less as compared to the other two. Their usage is limited to small and medium boats due to obsolete design principles so azimuthal stern drive tugs and tractor tugs are employed to assist the mega boats. The repositioning of the tugs is quite slow due to restricted manoeuvring capacity. There are more chances of capsizing or overturning of the vessel which results in fatal accidents.

To overpower this issue of capsizing and halting the development of girting, a release hook is required. Conventional tugs have low values of astern power 2. Tractor Tug: The key to the true tractor lies in the use of 2-multidirectional propulsion unit, of which some are rather like large rotating outboard motors with other consisting of rotating vertical blades.

Advantages: It being the most important type of tugs, tractor tug has many positive points. Some of its advantages are briefed as below: These are the only tugs which provide full thrust over degrees. They have rapid power-on response time and are well known for their outstanding manoeuvrability. In contrast to the conventional tug, tractor tugs are adaptable to repositioning swiftly on the will of the pilot or on demand of the towing job in which they are employed.

So these are readily available tugs with high-efficiency values. Their control systems are very simple and they pose a very low risk of girthing or capsizing. These have an extraordinary performance with fast speed.

They can effectively overcome the interaction forces when interacting with ships at some close distances. They can work efficiently in sideways movement due to the close location of the propulsion unit to the turning point. They have improved operational capability in a restricted area such as a lock flow channel.

In these types of tugs, a rudder is not required that is the use of rudder can be eliminated. They have reliable and robust propulsion units. Disadvantages: The tractor tugs are the most effective and most powerful sort of tugs.

As everything has certain cons along with pros these too have certain loopholes mainly their complexity and cost and other drawbacks of these tugs are as follows: These kinds of tugs have less bollard pull as compared to ASD tugs.

They are considered to have very high capital investment costs it implies a lot of money is required to buy these tugs and maintain these tugs. In short, it confers that initial and maintenance cost of tractor tugs is very high.

The repair and maintenance of complex under-water units of these tugs are quite expensive Handling in an open seaway might be poor with the short distance between the pivot point and the thrust creating a short turning lever Heeling angle with full side thrusts may be up to 21 degrees with some tugs. Azimuthal Stern Drive ASD Tugs : These tugs are midway between conventional tugs and tractor tugs as these utilize some of the benefits of both conventional and tractor tugs.

Advantages: These are considered more efficient than conventional tugs but less efficient than tractor tugs. Major advantages of using ASD tugs are as follows: Better directional stability at speed They are more suitable hull form for open waters and working in a seaway They have improved bollard pull The azimuth units are easy to withdraw for maintenance and repair The maximum heel with side thrust is less than 15 degree which is in contrast with tractor tugs having the value of twenty-one degrees.

These tugs have a shallower average draft of 3m Disadvantages: These tugs are difficult to work with as compared to tractor tugs and other drawbacks working with these tugs are as follows: The side-stepping ability of ASD tugs is not so good. The squat at the stern and flooding of the aft deck has been acquainted with occurrence with certain types when backing with full power They are still susceptible to girting or capsizing when using aft towing position.

Tractor Tugs With Cycloidal Propellers: Before Second World War tugboats developed were of high power rating and large tonnage capacity, the propeller used were cycloidal to serve the manoeuvrability purpose; these were renowned as Cycloidal Tugboats. Reverse Tractor Tugs: The tractor tugs are termed as reverse tractor tugs if these have Z-drive aft-mounted propulsion units. Z-PELLER: It can have two towing locations, one forward and one amidships and main propulsion is from two rotating azimuth units which are placed rather like a traditional twin screw tug.

Giano Tug: The Giano Tug is highly stable and can serve the purpose of both support as well as escort tug. Eco-Tugs: Hybrid technology tugs or tugs with LNG as a running fuel are categorized in an eco-tugs category. Ice Tugs: The tug boats escort ferries or barges and serve as ice breakers or salvage boats to make the ferries and barges to speed up to their destination, are termed as ice tugs.

The important features of this tugboat are as follows: It is flexible in operation and can switch swiftly. It emits less fuel by virtue of its hybrid conformations and its maintenance is quite economical and easier.