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Boat Crew Seamanship Manual

COMDTINST M16114.5B

 




Section B. Forces In Towing

Overview

Introduction

Boat crews must understand the forces, or types of resistance, which act on the towed vessel and how to handle the resistance safely. They are the same forces that affect all vessels, but a distressed vessel is limited in how it can overcome them. The towing vessel must provide the means to move the towed vessel. The towline or tow rig transfers all forces from the towed vessel to the towing vessel. Learn to recognize the different forces and each of their effects individually to effectively balance and overcome them when they act together.

In this section

These items are discussed in this section:

 

Topic

See Page

 

Static Forces

17-8

 

Dynamic Forces

17-10

 

Combination of Forces and Shock-load

17-14

  

 

Static Forces

B.1. General

Static forces cause a towed vessel to resist motion. The displacement or mass of a towed vessel determines the amount of force working against the vessel. The assisting vessel must overcome these forces before the towed vessel moves. Inertia and the Moment of Inertia are two different properties of static forces which cause resistance in towing vessels.

B.2. Inertia

In this case, inertia is the tendency for a vessel at rest to stay at rest. The more mass a vessel has (the greater its displacement), the harder it is to get it moving.

B.3. Moment of inertia

The Moment of Inertia occurs when a towed vessel resists effort to turn about a vertical axis to change heading. The larger the vessel, the more resistance there will be in turning the vessel. Unless necessary in a case of immediate danger, do not attempt to tow a distressed vessel ahead and change its heading at the same time. (See figure 17-1.) Both inertia and the moment of inertia will be involved in the resistance of moving the distressed vessel, which can cause potentially dangerous situations and greater resistance for towing. Both vessels, their fittings, and the towing equipment take much less stress and strain when the two forces are conquered individually.

 

Static Forces

Figure 17-1

 
 

Overcome the effects of static forces by starting a tow slowly, both on the initial heading or when changing the towed vessel's heading. A large amount of strain is placed on both vessels, their fittings and the towing equipment when going from dead-in-the-water to moving in the desired direction and at the desired speed. Use extreme caution when towing a vessel of equal or greater mass than the assisting vessel. In such situations, the assisting vessel strains the capacity and capability of its equipment, requiring slow and gradual changes.

B.3.a. Starting the tow on the initial heading

Apply the towing force on the initial heading to gradually overcome the towed vessel's inertia. As the towed vessel gains momentum, slowly and gradually increase speed. To change the tow direction, make any change slowly and gradually after the towed vessel is moving.

B.3.b. Changing the towed vessel's heading

Apply the towing force perpendicular to the vessel's heading. Once the towed vessel starts to turn, resistance will develop. Apply turning force slowly and gradually. It is more difficult to change the initial heading of a heavy vessel (one with a high moment of inertia) than a light one. Now, begin to tow in the desired direction and gradually overcome inertia to get the towed vessel moving forward.

Once making way, the effects of static forces lessen. Until the tow achieves a steady speed and direction, apply effort to defeat any remaining inertia or to change the towed vessel's momentum gradually.

 

Dynamic Forces

B.4. General

Dynamic forces occur once the towed vessel is moving. They are based on the towed vessel's characteristics (shape, displacement, arrangement, rigging), the motion caused by the towing vessel, and the effects of waves and wind. (See figure 17-2)

 

Dynamic Forces

Figure 17-2

B.5. Momentum

Once a vessel moves in a straight line, it wants to keep moving in a straight line. The greater its displacement or the faster it is moving, the harder it is to stop or change the vessel's direction.

B.6. Angular momentum

Once the vessel's heading begins to change, it wants to keep changing in that same direction. The faster the towed vessel's heading changes, the harder it is to get the tow moving in a straight line.

The towed vessel's momentum will gradually increase with towing speed. Momentum in a straight line will resist effort to change the towed vessel's direction and will tend to keep the towed vessel moving when tension in the towing rig is decreased. If necessary to first change the direction of the tow, the towed vessel will develop angular momentum while the vessel's heading is changing. You may need to apply towing force opposite the swing, before the towed vessel achieves the desired heading. The key to dealing with momentum is to anticipate how momentum will affect the towed vessel's motion and apply an offsetting force early and gradually.

B.7. Frictional resistance

 

NOTE $

As a vessel moves, the layer of water in immediate contact with the hull move. Due to friction between water molecules, the layers of water close to the hull try to drag along. The vessel appears to move "through" the water. This attempt to drag water alongside takes energy. As speed increases, this action becomes "turbulent." This turbulence takes additional energy, and more speed requires even more power.

Frictional resistance also varies with hull shape. Greater underwater (wetted) surface area causes greater frictional resistance. Hull appendages, such as propellers, shafts, skegs, keel, and rudders contribute to wetted surface area and frictional resistance.

CAUTION !

Frictional resistance will constantly affect the tow, normally keeping some steady tension in the towing rig. Since the shape and wetted surface area of the towed vessel will not change, frictional resistance is managed with towing speed. Higher towing speed causes higher frictional resistance and more strain on the towing rig.

B.8. Form drag

Form drag plays a large role in the ability to control changes in the towed vessel's movement. Different hull shapes react to motion through the water in different ways. The shape and size of the towed vessel's hull can either help or hinder effort to move in a straight line, when changing heading, and motion changes in response to waves due to buoyancy. The less water a hull shape has to push out of its way, the easier it will move through the water. A deep-draft, full-hulled vessel takes more effort to move than one with a fine, shallow hull. A large amount of lateral resistance, spread evenly over the length of the hull, will hinder effort to change a towed vessel's direction, but will help offset angular momentum in steadying up on a desired heading. A towed vessel may be able to help offset form drag by using its rudder.

B.9. Wave making resistance

 

 

 

CAUTION !

 

NOTE $

A surface wave forms at the bow while the hull moves through the water. The size of the bow wave increases as speed increases, causing the wave to create resistance for the bow to be pulled or propelled through the water.

Keep in mind the different hull types of maritime craft, including the towing vessel. In any towing evolution, the boat crew must be able to recognize a vessel's hull type, as well as its critical capabilities and limitations. Dependent on the type of hull, towing vessels must be careful not to tow a vessel faster than the design speed of its hull. Refer to Chapter 8, Boat Characteristics, for discussion of different hull types.

It is not always safe to tow a planing hull type of vessel above planing speed. Going from displacement speed to planing speed, or back, can decrease the towed vessel's stability and cause it to capsize. Also, wave drag (even one large wake) could slow the hull down to displacement speed and cause a severe "shock-load" as the towed vessel tries to get back on plane.

"Shock-load" or "shock-loading" is the rapid, extreme increase in tension on the towline, which transfers through the tow rig and fittings to both vessels.

 

B.10. Wave drag, spray drag, and wind drag

These frictional forces act on the hull, topsides, superstructure, and rigging. They all have a major effect on the motion of the towed vessel, and the transfer of forces to and through the towing rig. These constantly changing forces all vary with the towed vessel's motion relative to the environmental elements and are directly related to the towed vessel's amount of exposure to them. These forces can add up and cause shock loading. Wind and wave drag also cause a distressed drifting vessel to make leeway, that is motion in a downwind direction.

B.10.a. Wave drag

Wave drag depends on the "normal" wetted surface area of the hull and the amount of freeboard exposed to wave action. Wave drag has a large effect on the strain of the tow rig.

  • In large seas, be aware of:
  • Combination of wave drag and form drag could overcome the towed vessel's forward momentum and cause the towed vessel to stop and transfer a large amount of strain to the tow rig.

 

  • Shock-load could damage vessel fittings, part the towline, and endanger both vessel crews.
  • In head seas, be aware of:
  • Towing vessel can only control the effect of wave drag by the speed and angle that the towed vessel encounters the waves.
  • Limiting speed and towing at an angle to the seas to prevent them from breaking over the bow of the towed vessel.
  • In following seas, be aware of:
  • Wave drag causing the towed vessel to speed up as the crest approaches, increasing speed to keep tension in the towing rig, and reducing speed as the crest passes.

B.10.b. Spray drag

Spray drag also provides resistance to the tow. The spray from a wave could slow the towed vessel and increase the amount of shock loading. Spray drag could also adversely affect the towed vessel's motion by imparting a momentary heel, pooling on deck or in the vessel cockpit, and in cold weather form ice, and thus decrease stability.

B.10.c. Wind drag

Wind drag can cause shock loading and have a bad effect on the towed vessel's motions and stability. A steady beam wind can cause list and leeway, while a severe gust can cause a threatening heel. List, heel, and leeway may cause the towed vessel to yaw. A headwind increases tow rig loading in a direct line with the towed vessel while the towed vessel crests a wave, causing shock-loading.

B.11. Buoyancy response and gravity effects

WARNING I

Once making way, a vessel's buoyancy response or the effect of gravity in a seaway may cause severe shock-loading.

Develop a feel for the towed vessel's initial and reserve buoyancy characteristics, overall stability, sea keeping, response to the prevailing environmental conditions, and the response to being towed. Though a distressed vessel may seem stable and sound at rest, its response in tow could be a capsize. A towed vessel's bow may react to an oncoming wave by pitching skyward, or by "submarining." Buoyancy response to following seas could cause the towed vessel to yaw excessively or gravity may cause it to gain speed and "surf" down the face of a wave.

 

Combination of Forces and Shock-load

B.12. General

A boat crew rarely deals with only one force acting on the tow. The crew usually faces a combination of all the forces, each making the situation more complex. Some individual forces are very large and relatively constant. Crews can usually deal with these safely, provided all towing force changes are made slowly and gradually. When forces are changed in an irregular manner, tension on the tow rig starts to vary instead of remaining steady.

Example:

CAUTION !

Shock-loading may cause severe damage to both towing and towed vessels and overload a tow rig to the point of towline or bridle failure. Shock-loading could also cause momentary loss of directional control by either vessel and could capsize small vessels.

In calm winds and seas, a towing vessel encounters a steady, large amount of frictional resistance, form drag and wave making resistance when towing a large fishing vessel with trawl lines fouling its propeller and net still down. The tow rig and vessel fittings will be under heavy strain, and the tow vessel engine loads will be rather high, but the tow proceeds relatively safely. If suddenly the net tangled and caught on an unseen obstacle, this new "force" acting through the tow rig could immediately increase stress to a dangerous level. This shock-load could part the towline or destroy fittings.

(In the example above, the prudent solution would be to make a "safe" tow by recovering the net or marking it and letting it loose before starting the tow.)

Though this example began as a safe and steady tow, a single unexpected incident could have caused a very dangerous situation. Always keep in mind that some degree of shock-loading can occur during any tow evolution.

B.13. Shock-loading prevention or counteraction

Because of the potential dangers, the tow vessel must use various techniques to prevent or counteract shock-loading, or reduce its effect.

 

 
  

Action

Effect

Reduce Towing Speed

Slowing down lowers frictional resistance, form drag, and wave-making resistance. Reducing these forces will lower the total tow rig tension. In head seas, reducing speed also reduces wave drag, spray drag, and wind drag, lowering the irregular tow rig loads. The total reduction in forces on the tow could be rather substantial. When encountering vessel wake in relatively calm conditions, decrease speed early enough so the towed vessel loses momentum before hitting the wake. A small towed vessel slamming into a large wake will shock-load the tow rig, and may even swamp.

Get the Vessels "In Step"

 

 

 

 

NOTE $

Extreme stress is put on the tow rig in heavy weather when the tow vessel and the towing vessel do not climb, crest or descend waves together. Vessels in step will gain and lose momentum at the same time, allowing the towing force to gradually overcome the towed vessel's loss of momentum, minimizing shock-loading. To get the vessels in step, lengthen rather than shorten the towline if possible.

 

When operating near bars and inlets, getting the vessels in step may be impractical due to rapidly changing water depth and bottom contours.

Lengthen the towline

A longer towline reduces the effect of shock-loading in two ways. The weight of the line causes a dip in the line called a catenary. The more line out, the greater the catenary. When tension increases, energy from shock loading is spent on "flattening out" the catenary before it is transferred through the rest of the rig and fittings. The second benefit of a longer towline is more stretch length. Depending on the type of towline, another 50' of towline length will give 5'-20' more stretch to act as a shock-load absorber. Remember to lengthen the towline enough to keep the vessels in step and minimize the shock-load source.

  

 
  

Action

Effect

Set a Course to Lessen the Effect of the Seas

Do not try to tow a vessel either directly into or directly down large seas. Tow on a course to keep the seas 30-45 degrees either side of dead ahead or dead astern. This may require "tacking" to either side of the actual desired course.

Deploy a Drogue from the Towed Vessel

This device (covered under Equipment) may help to prevent the towed vessel from rapidly accelerating down the face of a wave. The drogue does add form drag to the tow, but could prevent shock-load.

Constantly Adjust Towing Vessel Speed to Match that of the Towed Vessel

 

NOTE $

 

 

 

 

 

CAUTION !

In large seas, constant "finesse" techniques may reduce shock-loading. This requires the coxswain to constantly observe the towed vessel, and increasing or decreasing towing vessel speed to compensate for the effects of approaching or receding seas on the towed vessel. This takes much practice and experience.

 

Safety demands emphasis on preventing shock-load and reducing its effects. Shock-loading presents a definite possibility of vessel fitting or tow rig failure. One of the more feared possibilities is towline snap-back. Think of this as a greatly magnified version of stretching a rubber-band until it breaks. Remember, some nylon cordage can stretch up to an additional 40% of its length before parting.

 

Shock-load can also capsize or swamp the towed vessel. The additional towing force from a shock-loaded towline could cause a smaller vessel to climb its bow wave and become unstable or it could pull the bow through a cresting wave.

  

 

Boat Crew Seamanship Manual

COMDTINST M16114.5B

 




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