Answer:
1) First up draw BODY PLAN for your tug boat with reasonable number of stations..
2) Find the sectional areas (of each station) from the body plan till the required draft...
3) Then using simpson's rule,simpsonize the areas and find the volume...
4) Then find the summation of volume.. this gives the volume of displacement..
5) We can hence find bollard pull using the relation
Bollard pull = ((Displacement * 60)/1,00,000)+ 40