Tugs Flashcards
Different types of tugs.
- Conventional Tug
- Tractor Tug
- ASD Tug
Conventional Tug.
The propulsion unit is usually a single right or left hadned with a standard rudder configuration.
Added inprovement:
- Twin screws.
- Controllable pitch propellers.
- Fittings of kort nozzles.
Towing arrangemnt situated aft or midships.
Pushing is done from the front.
Advantages of Conventional Tug.
- Simple construction and less maintenance.
- Self-sufficient and require no support system.
- Efficienct for towing from port-to-port.
- Low and economical operation cost.
Disadvantages of Conventional Tug.
- Limited Manoeuvrability.
- Slow to reposition.
- Difficult to steer when going astern.
- When working alongside they have to press heavily on the shipside to get into position.
- If the ship is making speed more than 3-5 knots, the single screw tug will sacrifice a lot of her power in maintaining position.
Tractor Tug.
2 types:
- 1. Voith Schneider
- 2. Schottel / Rotor
The tractor tug’s propulsion unit fitted forward of amidships. This enables the towing point to be placed as far astern as practicable, with the thrust being forward from the towing point, thereby created a good lever for manoeuvring.
They are best when pulling and less likely to capsize.
They work equally well going ahead or astern.
They usually have to push against the shipside with their stern to affect a push.
The fin or skeg has been removed and a third propulsion unit has been fitted dramatically increasing the tugs capability and handling characteristics.
Voith Schneider tug
- This tug has cycloidal propellers.
- The unit protrudes out under the hull, just under the bridge.
- Instead of blades they have vertical vanes that rotate in a vertical axis.
- The angles of the vanes are changed to control steering and propulsion.
Schottel/ Rotor Tug
- A set of outboard motors placed under the hull of the tug.
- These units are steerable and efficient at directing the flow of water in the required direction.
- These units can be turned through 360º thus providing an omni-directional capability without any loss of power.
Advantages of Tractor Tug.
- Full thrust over 360 degrees.
- Rapid power response.
- Outstanding manoeuvrability and able to re-position quickly.
- Simple control systems.
- Very low risk of girting.
- Can more effectively overcome interaction forces.
- Improved operational capability in a restricted area.
- Reliable propulsion units.
Disadvantages of Tractor Tug.
- High cost.
- Less bollard pull per kilowatt power.
- Repairs and maintenance of underwater units are expensive.
- Handling in open seaway poor.
- High heeling angle.
- Underwater units can get damaged or choked.
- Deeper drafts compared to conventional tug.
ASD Tug.
Azimuth Stern Drive Tug (ASD)
utilizing the benefits of both conventional and tractor tug alike.
The ASD develops its full potential when using its forward hook, and is secured alongside a ship in push-pull mode.
The reason for this is because in this case there is a large lever between the propulsion point situated aft and the towing hook situated forward.
Advantages of ASD Tug.
- Better directional stability at speed.
- More suitable hull form for open waters and working in a seaway.
- Improved bollard pull per kilowatt power.
- Azimuth units are easier to withdraw for maintenance and repair.
- Shallower draft than a tractor tug.
Disadvantages of ASD Tug.
- Sideways manoeuvring not good.
- Prone to squatting and flooding the rear when going astern and backing at full power.
- Susceptible to girting when using the after towing position.
- More risk from interaction.
- Control system complex.
- More susceptible to rubbish damage in props.
Direct/ Indirect towage
Indirect towing is performed at speeds in the range of 5 to 12 knots.
The tug uses the fin or skeg forward to generate a tow force by angling the tug to the direction of the vessel’s heading.
The attainable steering force is governed by the design of the “fin”, tug speed and the angle of “attack” (angle between the fore and aft lines of tug and vessel).
This is generally 75% to 80% greater than the tug’s bollard pull.
Advantage of the nozzle
The cross sectional area at the entrance of the nozzle is greater than at the trailing throat.
Since the water density is constant, the water must accelerate from one to the other. Hence the water is already moving faster as it reaches the propeller than it would on a conventional open propeller.
Therefore, more water is moved and more thrust created for the same input power and torque.
Bollard Pull
It is the theoretical thrust achieved at zero speed of advance and full engine RPM.
This is due to two things:
1. Propellers accelerate water as they spin so they never really are able to achieve zero speed.
2. Engines are never really able to reach full RPM under towage due to a declining torque vs RPM relationship.
Efficient bollard operations should produce about 30 lbs (130 Newton’s) of thrust per engine brake horsepower.
