In the Auxiliary Operations course AUXSAR (Auxiliary Search and Rescue); a sizeable portion of this course is about towing. This course provides all the theoretical information about towing. The course material states "Almost everything done during a tow is potentially hazardous; a successful tow is one during which no damage is done to the engine(s) of the towing vessel, no damage is sustained by either vessel, and no one sustains an injury."
Essentially, there are four factors that impact a towing situation: the hull characteristics of boat doing the towing, the hull characteristics of the boat being towed, the construction and diameter of the line used to tow the disabled vessel, and the sea state (waves, wind, and current). With all the different makes and models of vessels, as well as different line types, you can see that every tow is unique, making towing as much an art as it is a science.
I mentioned stress before, as one of the many reasons why you really need to learn how to tow a vessel before you actually just do it. There are three types of forces that a tow boat, the towed boat and the lines that connect them, undergo. These are: acceleration forces, steady forces and shock forces.
A brief definition will help you understand the dangers involved.
Acceleration Forces is the stress placed on the vessels and the towline during the time the towed and towing vessels are dead-in-the-water, to the time they reach their maximum (constant) towing speed.
Steady Forces is the stress placed on the vessels and the towline during the phase after maximum (constant) speed is reached. These forces are just involved in pulling the towed vessel through smooth water at a constant speed.
Shock Forces occur because of the sea state. Towing in calm, smooth water would produce little or no shock forces. Towing a vessel where there are five foot waves, at 30 second intervals would produce considerable shock forces. Just picture your boat slowing down and speeding up as it goes up and down waves. The towed boat is doing the exact same thing. But, they probably are not in synch, so the towline is being stretched and then goes slack, and then get pulled tightly again and stretches.
An average size vessel towing a vessel of equal size will, at a minimum, incur several hundred pounds of force, depending on the type of line used, sea state, etc. While many lines may contain ratings for several thousand pounds of force, those statistics are for brand new line. Lines that are well used, or that are weathered, are probably capable of sustaining loads much smaller that what they are rated for.
Deal Breakers - What Can Go Wrong, Usually Will
So great, now you know the factors that influence a tow and some basics on what the forces are - so what? Without doing all the math and physics involved, all you need to understand is this: Recreational vessels are often ill equipped to handle the stresses of towing for a variety of reasons:
While every piece of equipment has different breaking characteristics, given enough stress, any part of this towing system could break, and often with catastrophic results. If you must tow another vessel, examine its hardware (cleats, bits, etc) as well as your own to make sure it is bolted through.. Never attempt to tow another vessel using a "ski rope" or other lightweight line incapable of sustaining the stresses outlined above. Under no circumstance should anyone stand directly in line with the tow line, because if it were to break, it would "snap back" like a rubber band, wreaking havoc with everything in its path.
The cleats and deck fittings on most boats can only accommodate smaller lines; which limits the amount of force they can take, and thus the size of the vessel you an tow. How fast you tow another vessel can impact the forces exerted.
The pitch of most propellers on your average recreational vessel is geared towards maximizing speed of the vessel, not torque. Using the average propeller with a pitch of 19" or 21" results in a great deal of slip (inefficient movement of water through the propeller), making towing inefficient and stressful on an engine.
The amount of power it takes to tow a vessel, if done improperly could cause serious damage to one of the most expensive pieces of equipment in your vessel - your engine(s)
The average recreational vessel does not carry lines 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.
If the boat doing the towing is an outboard or an inboard/outboard, you have another potential disaster - getting the towline caught in the prop of the tow vessel. At the very least, this usually means cutting the tow line free from the prop, to totally disabling the tow boat, resulting in the need for another potential tow.
Given the information above, I hope you can see that there are a myriad number of things that can go wrong when towing another vessel. In any case, if I was a professional gambler who was asked to bet on whether the average recreational boater could tow another boat without incident, I would pass, as the odds favor the house. The "house" here is the fact that you'll likely experience damage to either the towed boat, the towing boat, or that someone on either vessel would sustain an injury.
