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Traccin a punto fijo o bollard pullCapitn Eduardo Gilardoni

La traccin a punto fijo o bollard pull es la medida de la cantidad de fuerza que un

remolcador es capaz de aplicar en un trabajo de remolque en determinadas

condiciones.

El bollard pull es el resultado de la fuerza impulsora de la hlice y de los

aditamentos que la misma posea para incrementar dicho empuje.

Por lo tanto est en funcin de:

1. De su dimetro

2. De su paso

3. De su R.P.M.

4. De su calado

5. De que posea o no tobera Kort

Se debe dejar en claro que el bollard pull slo nos permite evaluar totalmente la

traccin de remolque de un buque. Tambin debemos de tener en cuenta su

desplazamiento y su maniobrabilidad.

Generalmente la prueba de bollard pull debe de ser efectuada en aguas que

exceda los 10 m. de profundidad intercalando un dinammetro cuya escala supere

ampliamente la traccin mxima que pueda ejercer el remolcador.

Se debe de utilizar una longitud de cable que permita que la popa del remolcador

este lo suficientemente alejada del muelle donde se har firme el cable parapermitir que la corriente de expulsin de la/s hlice/s de diluya antes de llegar al

paredn del muelle provocando efectos perturbadores que puedan modificar la

lectura del dinammetro

Hlices

La eleccin de una hlice para un remolcador debe de ser hecha de acuerdo con

las formas del casco del mismo.

La hlice debe de ser construida lo suficientemente robusta para que no pueda

sufrir con facilidad averas en los puertos o mares en donde encuentre desechosque la puedan daar.

En realidad en la prueba de tiro sobre punto fijo se pone a prueba la fortaleza de la

hlice y del casco.

Las hlices convencionales o de pala fija estn generalmente proyectadas para

velocidades cercanas a los 7 nudos y a esa velocidad entregan su mximo

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rendimiento pero las mismas han sido ampliamente superadas por el sistema CPP

(control pitch propeller) o hlices de paso controlable, ya que pueden adaptarse a

una variedad de empujes a distintas potencias.

Las toberas Kort y las carenas hidrocnicas terminan de optimizar el sistema

propulsor de los nuevos remolcadores ocenicosNormas del Det Norske Veritas

RECOMMENDATION FOR BOLLARD PULL TESTING

1. A proposed test programmed together with the relevant drawings and

specifications for the vessel shall be submitted to Det Norske Veritas prior to

the testing.

2. During testing of Continuous Bollard Pull, (BP cont.), the main engine(s) shall

be run at the manufacturers recommended maximum continuous rating

(MCR).

3. During testing of Overload Bollard Pull, (BP overload), the main engine(s)

shall be run at the manufacturers recommended maximum rating that can

maintained for minimum 1 hour. (the overload test may be omitted).

4. The propeller(s) fitted when performing the test shall be the propeller(s) used

when the vessel is in normal operation.

5. All auxiliary equipment such as pumps, generators and other equipment

which are driven from the main engine(s) or propeller shaft(s) in normal

operation of the vessel shaft be connected during the test.

6. The length of the tow wire shall not be less than 300 meters, measured

between the stern of the vessel and shoreline.

7. The water depth at the test location shall not be less than 20 meters within a

radius of 100 meters of the vessel.

8. The test shall be carried out with the vessels displacement corresponding to

full ballast and half fuel capacity.

9. The vessel shall be trimmed at even keel or at a trim by stern not exceeding

2% of the vessels length.

10. The vessel shall be able to maintain a fixed course for not less than 10

minutes while pulling as specified in items 2 or 3 and 6 above.

11. The test shall be performed with a wind not exceeding 5 m/seg.

12. The current at the test location shall not exceed 1 knot in any direction.

13. The load cell used for the test shall be approved by Det Norske Veritas and

be calibrated at least once a year. The accuracy of the load cell +/- 2% within

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a temperature range of10*C and +40*C and within the range of 25-200

tonnes tension.

14. An instrument giving a continuous read-out and also a recording instrument,

recording the Bollard Pull graphically as a function of the time shall be both

connected to the load cell. The instruments shall be placed and monitored

ashore.

15. The load cell shall be fitted between the wire and the Bollard.

16. The figure certified as the vessels BP cont. shall be the towing force

recorded as being maintained without any tendency to decline for a duration

of less than 10 minutes.

17. Certification of BP overload figures recorded when, running the engine(s) at

overload reduced RPM or with a reduced number of main engines or

propellers operating can be given and noted on the certificate.

18. A communication system shall be established between the vessel and the

persons monitoring the load cell and the recording instruments ashore, by

means of VHF or telephone connection, for the duration of the test.

19. The test results shall be made available to the Veritas-surveyor immediately

upon conclusion of the test program.

20. Maximum Bollard Pull, (BP max.), shall not be tested beyond 50% of the

breaking strength of the towing wire.

21. Should it not be possible to comply with one or mew of these

recommendations a notation of this fact shall be made on the certificate.