Required Bollard Pull for a Harbour Tug

[Rob Moody]

Can anyone provide me with info on calculating the required bollard pull of a harbour tug.

Also any info referencing justificaton in the choice of using either a Voith-Schneider or Azimuth propeller. Note the tug in consideration will probably be used >95% of time for berthing/unberthing ships. On the odd occasion these movements could be with a 'cold' ship.

Largest ship under consideration is about 23,000 tonne deadweight in a 20 knot wind.

At present there is a range of tugs (Voith and Z-Drive) being used, the max with a bollard pull of 35 tonne. However, max size that is presently being handled is about 14,000 tonne deadweight.

Any input would be appreciated

[Kay9]

I wouldnt go with anything less then 10,000 LBS bollard pull. If your intrested I have listings for many ship assist tugs here in the US that are for sale in the 60K to 260K range that would do this job.

Also either drive you mentioned will work nicely. Most of the time the assist vessel (tug) is positioned prior to the move for a specific direction of pull. The assist vessel is then re-positioned for the next pull. So even an old fashion single screw tug will do the job nicely and might save you millions in cost over a new Z-drive.

K9

[Guillermo]

After inspecting a 130+ tugs fleet in 6 countries I've found Voith propulsion being presently the favourite for most harbour tug masters. Then azipods.

The attached empirical formula (see downwards) is widely used for determining the required bollard pull. A record of safe towing in Canadian waters has been established using the following K values at a towing speed of six knots.

(a) for exposed coastal tows K = 1.0 to 3.0
(b) for sheltered coastal tows K = 0.75 to 2.0
(c) for protected water tows K = 0.50 to 1.5

A quick and rough estimative for engine's power in BHP is 100 to 125 times the bollard pull in tonnes.


Also the following formula allows a direct rough calculation of BHP (Break Horse Power) but taking no account for external factors such as wind and waves:

BHP = 2D/3*v^2 /120
where:
D = Displacement of the tow (t)
v = towing speed in knots

To pass from BHP to bollard pull in tons you may use:

Fixed pitch propeller: (freewheeling) BHP / 100 = (t)
Fixed pitch propeller and kort-nozzle: BHP * 1.08 / 100 = (t)
Controllable pitch propeller: (freewheeling) BHP * 1.125 / 100 = (t)
Controllable pitch propeller and kort-nozzle : BHP * 1.26 / 100 = (t)


Another formula to roughly determine the requested Bollard Pull under consideration of aerodynamic resistance and Seas state:

Bollard pull (tons) = ((D^2/3 * v^3)/7200 + Cmv*B*D1)*K

where:
D = Displacement of the tow (t)
v = Towing speed in knots
Cmw = coefficient for the mean wind speed
B = Width of the tow (m) (transverse to movement)
D1 = Height of the wind facing area above water level, incl. Deck cargo (m)
K = Factor 3 - 8, depending to the circumstances

This formula should only be used during following two situations:
• Ordinary towing conditions (BFT. 4)
V = 6 knots
Cmw = 0,0025
K = > 3
• Keep on station during heavy weather (BFT. 10-11)
V = 3 knots
Cmw = 0,015
K = 8


I hope this can be of use.
Cheers.

[Ad Hoc]

I've always used the rough rule of thumb, 75kg pull per 100HP.

[mhdarya]

is any good source for calculation of Bollard Pull?

Originally Posted by mhdarya

is any good source for calculation of Bollard Pull?

Go two (now three) posts up!