Shiphanding with Tugs Slesinger Flashcards
What are the three general categories of tugs used in shipwork?
- Conventional
- Tractor
- Azimuthing Stern Drive (ASD)
What are the five key elements in tug design?
- Towing Point
- Propulsion Point
- Propulsion and Steering
- Hull Shape
- Superstructure and Fendering
Note: TP and PP determine Maneuvering Lever (ML)
What is the Towing Point?
Last physical Point on the tug that fair leads it’s line to the ship. When pushing , Point of contact between the tug and the object being pushed.
What is the Propulsion Point?
The physical focal point of the tug’s application of horsepower underwater.
What is the Maneuvering Lever?
The distance between the Towing Point and Propulsion Point. It is what gives the tug leverage to swing its bow or stern around the Towing Point.
Describe the length and orientation of the maneuvering lever for:
- conventional tugs
- tractor tugs
- ASD tugs
Conventional: Pushing is best lever with TP at bow and PP at stern. Towing with TP midships of aft of midships is shorter lever.
Tractor: TP aft and PP just forward of midships is best lever. TP on bow is shorter lever.
ASD: TP on bow and PP aft makes for lever the length of the boat. TP aft of midships is shorter lever.
What causes a tug to capsize while on a towline?
Short Maneuvering Lever and the towline pull falling off to the side. If the tug does not have enough power and leverage to push its stern around and Towing Point back in line with the heading of its tow, it will be tripped.
What advantage does a single screw tug have over a twin screw tug?
Single screw is 20% more efficient than twin screw. Also, helping submarines to/from berth proffered over twin screw due to less likelihood of damage from propeller.
What advantage does twin screw tug have?
Maneuverability (compared to single screw)
What are the disadvantages of a twin screw tug?
Less efficient in converting HP to bollard pull (compared to single screw)
Much less power when backing (compared to tractor and ASD)
What advantages does tractor tug have over conventional?
The orientation of Propulsion Point to Towing Point. The omnidirectional propulsion units are forward of amidships and allow the Towing Point to be moved further aft for a longer and more effective Maneuvering Lever.
What advantages does ASD have over a tractor tug?
Shallower draft than a tractor, most efficient propulsion units to convert HP to thrust, and initial capital outlay is less than cycloidal units.
What are the four components of propulsion and steering systems?
- Power source
- Power transfer and control system
- Hydrodynamic driver (thrust)
- Thrust directional control mechanism
What is the purpose of the power transfer and control system?
To transfer rotative force to the tugs hydrodynamic driver.
What effect has the Diesel engine had on the design of tugs?
They are powerful and efficient, making it possible to increase both the power and range of tugs. Compact powerful assist designs could not have been built without Diesel engine technology that places more HP in a smaller footprint.
Describe the DR system of engine control.
The engine is connected directly to the propeller. To reverse the propeller, the engine is stopped and restarted in the desired direction.
What made the DR system obsolete?
Modern machinery which allows tugs better maneuverability has made DR obsolete.
Describe the DE system of engine control.
A diesel electric (DE) system consists of main engines turning generators supplying power to electric propulsion motors.
What are the advantages of DE drive?
Delivers shaft speed from dead slow to full ahead (astern) and responds without delay to changes in speed and direction of rotation. Engine maintenance is minimized, with multiple engines only the engines/generators required need to be run, more flexibility in engine room layout.
What are the principal disadvantages of DE drive?
Cost and marine electrical vulnerability to dampness in salt in atmosphere.
How does an SCR system differ from the conventional DE drive?
Main generators produce AC suitable for ship supply and other applications. Current is delivered through rectifiers which convert it to DC for driving the propulsion motors.
Are there any particular advantages to the SCR system?
AC generators are cheaper than DC and compatible with AC vessel electrical systems.
Describe the CPP system.
Three components. A reduction gear to reduce to a suitable shaft speed for the propeller, a hydraulic system to control pitch, and a uniquely designed propeller hub to house the mechanism. The CP propeller is always turning and thrust (ahead and astern) is generated by changing pitch of propeller blades instead of changing rotation of propeller shaft. Shaft speed may be constant or combinator (automatic matching of propeller RPM with pitch) control
What Maneuvering function is housed in CPP?
Pitch and therefore thrust. Also, reversing.
What are the drawbacks of a CPP system?
Propeller always turning, neutral pitch disrupts flow to rudders reducing or negating rudder effectiveness, repairs can be expensive and typically require dry-docking.
Does a reverse reduction gear change propeller shaft rotation?
Yes
What factors determine a propellers efficiency?
Quality and velocity of water flow on the intake or discharge side of the propeller, size and speed of rotating propeller blades, and the number, pitch, and shape of the blades.
How do nozzles increase propulsion efficiency?
They improve propeller thrust.
What are the disadvantages of nozzles?
Lateral resistance underwater, hindering maneuverability. 50% of ahead thrust when backing (similar to unducted). Subsequent nozzle designs have addressed these deficiencies and can produce 70% of ahead thrust when backing.
What three factors determine a rudder’s efficiency?
Velocity and quality of water flow and rudder shape.
What is the difference between an unbalanced and balanced rudder?
Unbalanced: all the rudder’s length is on one side of the rudder post.
Balanced or semi-balanced: leading edge of the rudder extends forward of the rudder post to use propeller flow efficiently and produce mechanical advantage to steering gear.
What is a spade rudder?
A balanced rudder hung independently of any skegs.
What is the difference between a steerable nozzle and a steerable propeller?
Nozzle turns 25-30 deg with fixed propeller vs. nozzle and propeller turn together through 360 deg.
Why are flanking rudders effective when operating astern?
The flanking rudder is then operating on discharge side of the propeller.
Describe the Voith-Schneider cycloidal propeller.
A controllable pitch propeller rotating around a vertical axis. A rotor casing ends flush with the ship’s bottom. The rotor casing is in a constant circular motion on a horizontal plane. Attached to the rotor casing are vertical blades where each can rotate about its own vertical axis, performing an oscillating motion about their own axis while following the uniform motion of the rotor casing. Gears and mechanical links control the pitch angle of the vertical blades at any desired point along the circumference of the rotor casing’s circular motion. As a result, the same amount of thrust can be generated in any direction. Typically two units mounted forward in tractor configuration.
What are the disadvantages of VSP?
HP to bollard pull ratio is lower than conventional or steerable propeller tugs, vulnerability to underwater debris and grounding, high initial cost, high repair cost.
Describe a steerable propeller system.
Engine power is transmitted to a drive unit consisting of a propeller (and sometimes nozzle) that can be rotated through 360 degrees by hydraulic motors. Most SPS have the propeller running in only one direction requiring rotation of drive 180 deg to reverse thrust.
What are the disadvantages of a SPS?
Vulnerable to underwater debris and grounding, high initial cost (less than VSP), high repair cost
What is bollard pull?
Thrust in pounds or kilograms delivered by the engine under static conditions ( pulling against a dock of fixed structure).
Is bollard pull a fair criterion for judging a tug’s serviceability for shipwork?
Yes, however other criteria must be considered such as: maneuverability, stability, and backing power - deficiency in these may limit a tugs ability to effectively apply bollard pull.
Describe the important aspects of the hull of a tug used for shipwork.
Must be able to withstand heavy contacts without sustaining damage, reinforced in critical areas where propelling power is brought to bear, and have a high amount of stability (stiff) to avoid capsize from heeling forces (line leading off side of vessel, etc.)
Is the hull reinforced?
Yes
Why are deck structures set so far inboard on Harbor tugs?
To prevent damage to the tug and ship when the tug works under the flare of the bow or overhang of the ship’s side near the stern. Also, to minimize likelihood of damage from rolling of both vessels.
What is a bullnose?
A large closed chock
What is a staple?
Functions as the Towing Point on tractor and ASD tugs and is subject to extraordinary strain. As such, it is heavily constructed and mounted in a manner that solidly connects it to the tug’s frames, bulkheads, and plating creating a web of strength and structural integrity.
Where are the bits located on conventional American harbor tugs?
Shoulder, quarter, and H bitt aft of the deckhouse (and on bow if no tow winch)
What are the structural requirements for the bits and deck fittings?
Stout enough to withstand heavy line strain when the tug is backing or coming ahead with full engine RPM.
What type of mast should be installed on a harbor tug?
Set up in a tabernacle for easy lowering.
With respect to visibility, how should the wheelhouse be arranged?
Operator’s field of view as unobstructed as possible. Horizontal view as close as possible to 360 deg. For working alongside large vessels with high freeboard-additional small windows that face upwards.
Why are model hull single screw tugs proffered for assisting submarines?
To avoid sub side shell plating damage from propellers on twin screw tug or chine of v-bottom tug.
What structural characteristics of a ship can make it difficult or dangerous for a tug assisting it?
Subs with no ship side for above water tug fenders to lay against, aircraft carriers with overhanging structures and many projections
What are pocket or recessed chocks and why are they useful?
Chocks/bits recessed into the ship’s side to which a tug can make fast. Useful on vessels with high freeboard or limited chock locations (car carriers, cruise ships).
What are the three functions of fendering?
- Absorb and dissipate energy
- Enhance or reduce traction
- Create space
What are the components of a towline system?
Working lines (material and construction), fastening/release mechanisms(bits/cleats, winches, tow hooks), best practices (operational procedures, handling and care)
What types of synthetic lines are used in shipwork?
Polyester (Dacron), polypropylene, blends (nylon, polyester, and polypropylene), Aramid, and HMPE (high modulus polyethylene)
Are 100% nylon lines used for shipwork?
No
What are Dacron lines used for primarily?
Slings, mooring lines, and working lines on tugs.
What are polypropylene and polyethylene lines used for?
Polypropylene used in light work applications or as component in blend. Polyethylene is used in blends or alone as working line if HMPE.
What are blended lines used for?
Bonds use two complementary types of fiber that compensate for each other’s respective deficiencies. Most have qualities well suited for working lines in shipwork.
What are the two common trade names for HMPE fibers?
Spectra and Dyneema
What purpose do wire rope pendants serve?
They protect the soft line from chafe and abrasion where the lines pass through the ship’s chocks.
What are the five common rope construction patterns?
- Three strand
- Six strand
- Eight strand
- Twelve strand
- Double braid
What types of wire rope are suitable for pendants and working lines?
6x19 or 6x37 IWRC with fiber core.
What are the purposes of a composition line?
Use abrasion resistant components at high friction points (ship’s bitts/chocks), create an easily replaced, disposable component in high areas of wear, accommodate dynamic loading, create an inherent and purposeful weak link in the line, and provide flexibility and softness to working ends handled manually.
What are the four important characteristics of a winch?
- Braking capacity
- Maximum line-pull capacity
- Slack line speed
- Remote release
What are the two main operational best practices regarding rigging for shipwork?
Choose the right rope for the job and use 5he rope properly.
List the common items of auxiliary equipment.
Heaving lines, tag lines, gob lines
Describe the heaving lines commonly used on tugs.
15 fathoms long, 3/8-7/16 dia, made of Manila or other non slippery material.
Describe and explain the purpose of a tag line.
Lengths of line secured to the outboard end of the tug’s working lines. 30 to 40 feet long, typically 3/4 to 1 inch dia and made of Manila or other non slippery material. They are attached to the bitter end of a heaving line which is passed to the vessel first. They enable the ship’s crew to pull in 5e heaving line and get a hold on the tag line before the weight of the hawser is felt.
Can tag lines be made of Manila?
Yes
What is a gob line and what is its purpose?
They act as a preventer to keep the towline from leading over the side and capsizing or tripping the tug.
What is a quick-release strap and how is it used?
Used as a substitute for a towing hook. It is a short line (6-8 fathoms) with an eye at one end that is rove through the eye of a ship’s hawser and secured to the after tow bitts, permitting the hawser to be cast off in an emergency.
What is a critical decision point?
A point in the maneuvering sequence that requires critical action at a specific time.
Describe the effect of propeller torque on conventional single-screw tugs with a right hand propeller.
A “right-hand propeller” will have a tendency to move the stern to starboard and the bow to port when engaged ahead. When reversed, it will move the stern to port and the bow will fall off to starboard.
How is a single-screw tug maneuvered when it is necessary to back for some distance?
It is steered when moving astern by giving an occasional “kick” ahead on the engine to correct the heading. This is done briefly enough so that it does not destroy sternway.
How is a single-screw tug turned in a short distance?
It is turned by “backing and filling”, i.e. alternately going ahead and astern on the engine with the helm turned toward the direction of the turn.
Will a single screw tug turn more readily in one direction than the other?
Usually it will since the stern will be cast to one side or the other when the tug backs its engine. This will be to port with the standard right-hand propeller. Tugs with nozzles may not react the same way.
What is the principle advantage of flanking rudders or a steerable nozzle rudder on a single-screw tug?
They enable the single-screw tug to steer astern and maintain position when backing during shipwork.