Comerciante

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 (seedownwards) 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 sixknots.

(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'spower 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 factorssuch 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 pitchpropeller: (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 pitchpropeller 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:

Bollardpull (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 duringfollowing 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