Towing Flashcards
Forces are also considered ___
Types of resistance
___ forces cause a towed vessel to resist motion
Static
The ___ of a towed vessel determines the amount of force working against the vessel
Displacement or mass
Inertia is ___
The tendency for a vessel at rest to stay at rest
The moment of inertia occurs when ___
A towed vessel resists effort to turn about a vertical axis to change heading
An attempt to tow a distressed vessel ahead and ___ at the same time should not be used
Change its heading
Overcome the effects of static forces by ___
Starting a tow slowly, both on initial heading and changing heading
Dynamic forces occur once ___
The towed vessel is moving
Dynamic forces are based on ___
The towed vessels characteristics, the motion caused by the towing vessel, and the effects of waves and wind
Once a vessel moves in a straight line, it wants to keep moving in a straight line
Momentum
The greater its displacement or the faster it is moving, the harder it is to ___
Stop or change the vessel’s direction
Once a vessel’s heading begins to change, it wants to keep changing in that same direction
Angular momentum
The faster the towed vessel’s heading changes, the harder it is to ___
Get the tow moving in a straight line
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
As a vessel moves, the layer of water in immediate contact with the hull moves. Due to friction between water molecules, the layers of water close to the hull try to drag along
Frictional resistance
As speed increases, frictional resistance becomes ___
Turbulent
Turbulence takes additional energy, and ___
More speed requires even more power
Frictional resistance will constantly affect the tow, normally ___
Keeping some steady tension in the towing rig
Frictional resistance is managed with ___
Towing speed
Frictional resistance also varies with ___
Hull shape
Different hull shapes react to motion through the water in different ways
Form drag
A large amount of lateral resistance, spread evenly over the length of the hull, will hinder ___, but will help ___
- Effort to change a towed vessel’s direction
- Offset angular momentum in steading up on a desired heading
A towed vessel may be able to help offset form drag by ___
Using its rudder
Going from displacement speed to planing speed, or back, can ___
Decrease the towed vessel’s stability and cause it to capsize
Wave drag could slow the hull down to displacement speed and cause a severe ___
Shock-load
A ___ forms at the bow while the hull moves through the water
Surface wave
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
Wave-making resistance
___ is the rapid, extreme increase in tension on the towline, transferring through the tow rig and fittings to both vessels
Shock-load
The frictional forces of wave drag, spray drag, and wind drag act on ___
The hull, topsides, superstructure, and rigging
Wind and wave drag also cause a distressed vessel to make ___
Leeway
Motion in a downwind direction
Leeway
Wave drag depends on the ___
Normal wetted surface area of the hull and the amount of freeboard exposed to wave action
In large seas, 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
In head seas the towing vessel can only control the effect of wave drag by ___
The speed and angle that the towed vessel encounters the waves
In following seas wave drag causes the towed vessel to ___
Speed up as the crest approaches, increasing speed to keep tension in the towing rig, and reducing spped as the crest passes
Spray drag adversely affects the towed vessel’s motion by ___
Imparting a momentary heel, pooling on deck or in the cockpit, and possibly forming ice
A steady beam wind can cause ___, while a severe gust can cause a ___
- List and leeway
- Threatening heel
List, heel, and leeway may cause a 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
Reducing towing speed will ___
Lower frictional resistance, form drag, and wave making resistance; lower tow-rig tension. In head seas reduce wave drag, spray drag, and wind drag
Getting the vessels in-step will ___
Cause the vessels to gain and lose momentum at the same time, 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 depth and contours. ___ may be used
Counteracting shock-load
Lengthening the towline will ___
Reduce the effect of shock loading via catenary and stretch length
The weight of the line causes a dip in the line called a ___
Catenary
Catenary causes ___
Energy from shock loading is spent on flattening out the catenary before being transferred through the rig and fittings
Depending on the type of towline, another 50’ of towline length will give ___
5’-20’ more stretch to act a a shock absorber
Tow on a course to keep the seas ___
30 - 45 degrees either side of dead ahead or dead astern
Setting a course to lessen the effect of the seas may require ___
Tacking to either side of the actual desired course
Deploying a drogue may help to prevent the towed vessel from ___
Rapidly accelerating down the face of a wave
A drogue does add ___ to the tow, but could prevent ___
- Form drag
- Shock-load
___ instead of other synthetic fiber cordage should be used for a good combo of strength and stretch
Nylon
Towlines are usually ___
2-in-1 DBN, 2-4” in circumference, up to 900’
Towline is typically secured to the reel with ___
Small stuff
Unless slipping the towline in an emergency, keep at least ___ turns of towline on the reel
4
Inspect towlines for ___
- Cuts
- Chafing
- Flattening
- Fusing
- Snags
- Hardening
Fusing of towline is caused by ___
Overheating or over-stretching
Hardening of towline is caused by ___
Heavy use
Towing pendants and bridles are made from ___
DBN or Kevlar
Pendants and bridles should have a breaking strength ___ than the towline
Equal to or greater than
A pendant is used to ___
Reduce wear and chafing at the towline end
A pendant must be long enough so the ___
Towline is clear of obstructions on the towed vessel
A bridle is used when ___
Both legs can be rigged to exert an equal pull on the hull of the distressed vessel
A bridle is preferred for ___
Heavy weather towing
Use a long bridle when ___
The best attachment points for the towed vessel are well aft, but maintain a fair lead forward to reduce chafe
The amount of tension on each bridle leg increases with the ___
Size of the angle between the bridle legs
Keep the legs of the bridle long enough so the angle of the legs stays less than ___
30 degrees
The bridle legs must be long enough to reduce ___
Towed vessel yaw
When deploying the drogue ___
Pay out all 200 feet
When recovering the drogue ___
Bring the towed boat to a stop
Vessels with a large superstructure forward or outboard engines will tend to lay ___
Stern-to the wind
Vessels with deep draft and low superstructure will tend to lie ___
Broadside to the seas
Before starting the approach, an ___ is established
Imaginary danger zone
The size of the danger zone depends upon ___
The conditions and the arrangement of the distressed vessel
Safe distance keeps you out of the ___
Danger zone
The position taken by the rescue vessel which allows the crew the safest opportunity to pass the towing equipment
Optimum position
Optimum position provides ___
Better vessel control, while keeping appropriate distance, and maintaining a safe escape route
A 90 degree arc; 45 degrees forward and aft of your tow bitt
Maneuvering zone
Staying in the ___ keeps you out of the danger zone and gives you a way out
Maneuvering zone
It is preferable not to make opening and closing maneuvers when ___
Lines are over (except the heaving line)
Avoid making correcting maneuvers on the ___
Face of a wave
Tow veers to one side or the other
Yaw
Ways to minimize yaw include ___
- Change scope
- Adjust trim
- Decrease speed or alter course
- Deploy a drogue
- Use a bridle
Max safe towing speed
1.34 x square root of length at waterline x 0.9
4 conditions encountered when towing in current
- Head current
- Tail current
- Cross current
- Combo of currents
Depending on the velocity of the head current and the speed of the tow, SOG may be ___
Reduced, stopped, or even reversed
If the current opposes wind and seas the seas get ___
Steeper and break more readily
A head current increases ___
The strain on the tow rig
When towing in a waterway with bends and turns, the greatest current will be to the ___
Outside of the bend
At a bend the water will be deepest ___
On the outside of the bend
At a bend, the towed vessel may ___
Sheer or yaw to the outside of the bend
Prevent towed vessel sheer by ___
Reducing the length of towline prior to entering narrower sections of a waterway
Current flowing the same direction as the course steered
Tail current
Compensate for tail current by ___
Taking early action
In a narrow channel if the towing vessel gets into an area of lesser current than the towed vessel with a following current, the tow ___
Sheets off along the axis of the current, possibly causing slack in tow rig, loss of firm control, and potentially overrunning the towing vessel
A current that is flowing from either side, across the intended track
Cross current
A cross current will require the towing vessel to ___
Adjust heading for set and drift for both vessels
During a cross current the towed vessel should ___
Steer into the current if able
Cross current can be minimized by ___
Shortening tow, offsetting track up current, and the towed vessel steering into the current
Two methods of switching to a side tow
- Drop tow method
2. Transition method
When a tow is about to sink you should ___
Break the tow and rescue the people
If the sinking boat is unable to disconnect the tow you should ___
Cut towline or slip the towline by breaking the bitt
If you have to cut the towline, cut it ___
Directly aft of the tow bitt
If there were no onboard the tow, the water is shallow (less than the length of towline), and it is safe, a sinking tow should ___
Be allowed to pay out until the tow reaches the bottom. A floating object should then be attached to the end of the towline
Do not attempt to break the bitt if there is ___
A strain on the towline
When shortening tow in heavy weather, set a course of ___
10 to 15 degrees off the seas with the tow to windward
When shortening tow in heavy weather, divide your attention ___
80% aft and 20% forward
When a boat is pulled sideways by an opposing force
Tripping
Static forces of towing
- Inertia
- Moment of Inertia
Dynamic forces of towing
- Momentum
- Angular momentum
- Frictional resistance
- Form drag
- Wave-making resistance
- Wave drag
- Spray drag
- Wind drag
Prior to towing, what environmental information should be collected?
- Marine forecast
- Currents and tides
- Daylight and darkness
What communication should be made with the tow prior to arriving?
- ETA
- Advise to wear PFDs
- Deck layout, fittings & backing plates
- Ask for info the distressed crew thinks is important
- Ask if anything has changed since last comms with OPCON
- Ascertain heightened sense of urgency
- Advise further comms will be conducted on scene
- Set up comms schedule
What communication should be made with the tow once on scene?
- Concerns
- When & how equipment will be sent
- Explain plans and pass safety instructions
- Describe the approach
- When & how the tow rig will be passed
- Tow rig instructions
- Inquire about type/condition of tow connection points
- Emergency breakaway procedures
- Emergency signals
What should be evaluated on the vessel to be towed prior to passing gear?
- Compare vessel’s drift rate to your own
- Confirm # of POBs & PFDs
- Circle vessel to see condition
- Look at vessel shape and how it lays to the seas
Opening-by-Pivoting
- Pivot bow 45° towards boat
- Back away till bow even with boat’s bow
- Bow back into the elements
- Return to optimum
Opening-by-Backing
- Back square to conditions
- Pivot away from boat after bow is clear
- Move ahead to optimum
- Pivot square into the elements
Closing-by-Backing
- Back square to the conditions
- Pivot bow towards boat
- Move ahead to optimum
- Pivot square into elements
Closing-by-Quartering
- Pivot away from boat
- Tend line off quarter
- Square up in optimum
