Question: What is the requirement for a towline on an Auxiliary facility 18 ft long.
Answer: There is no "standard" length. This is something that should be taught in Coxswain training and is different for the type of waters the facilities would be patrolling. Small lakes are different from waters like the Gulf of Mexico or the Coastal waters where longer lines would be required. Most local DIRAUX's will have standard size requirements for facilities in their AOR.
What is important is that tow lines should be of the length that may be necessary to minimize the shock forces by keeping the vessels "in step" with one another. It is important to adjust the length of the tow line to minimize the shock forces caused by wind, waves, and/or current.
Of course as explained above, that varies in different waters where 100 feet may be fine in one area but 200 feet would be required in rougher waters.