11. Using the appropriate curve as specified by Item 7, determine the sea state resistance (R), from
Figure G-2, using the wind and sea conditions specified by Item 8. See Note 1 at the end of this
12.Select tow speed (VTOW) in knots.
13 Select tow course.
14. Using the curve number as specified by Item 6, enter Figure G-1 to find the value for R /∆ for the tow
speed selected in Item 12. See Note 2 at the end of this section.
15. Calculate the hull resistance (RH) by multiplying 1.25 times Item 14 times Item 2. (The factor 1.25
accounts for hull roughness and other variables).
RH = (1.25)x(RH/∆x(∆)
= (1.25) x (Item 14) x (Item 2)
16 Calculate the propeller resistance (Rp) by multiplying 3.737 times Item 5 times Item 12 squared:
Rp = (3.737) x (Ap) X (Vtow)2
= (3.737) x (Item 5) x (Item 13)
See Note 3 at the end of this section.
Calculate the wind resistance (Rw) by multiplying 0.00506 times Item 4 times Item 3 times Item 10
times Item 9 squared:
Rw = (0.00506) x (Cw) x (At) x (K) x (V R)
= (0.00506) x (Item 4) x (Item 3) x (Item 10) x (Item 9)
See Note 4 at the end of this section.
18. Calculate the total steady-state tow resistance (R) by adding Item 11 plus Item 15 plus Item 16 plus
RT = Rs + RH + Rp + Rw
= (Item 11) + (Item 15) + (Item 16) + (Item 17)
19 Estimate the resistance of the tow hawser from Table 6-1 or 10% of tow resistance, Item 18.
20 Calculate horizontal tow hawser tension, Item 18 plus Item 19.
Pages G-5 through G-10 contain sample calculations.
Note 1: The method of estimating the added resistance from waves, at tow speeds, is not well developed .
The data herein are developed from stationary (anchored) theory and include no correction for tow course or
speed. However, from the shape of the curves it can be seen that there is little effect in seas up to State 4 or 5
The added resistance increases rapidly m heavier seas, which probably will require the tow to head into the
seas Furthermore, the effect of the additional speed of the tow will be small compared to the speed of the seas
m this case and can be ignored . Therefore, the amount of error introduced by assuming head seas and
neglecting tow speed is small, and, in any event, provides a conservative answer for most tow courses.
Following wind and seas will reduce the tow resistance. However, the dynamic effects of ship motions on the
tow hawser may preclude towing down-wind under strenuous sea conditions. See Appendix O. Likewise,
stability of the tug and tow may preclude towing across the wind and seas under strenuous conditions.
Note 2: These hull resistance curves are plotted on a per-ton-displacement basis. Thus they are applicable to
similar hull shapes. Hull shapes are largely influenced by speed/length ratio V√ L. Assume that it is
necessary to estimate the towing resistance of a large tanker with the following design characteristics: