Emergency Towing Flashcards
What is one of the most dangerous evolutions conducted in the marine environment
Towing
No two towing evolutions are
exactly the same
Except int he simplest, easiest and least risky of all situations, towing should be
a last resort and then only if Coast Guard or commercial sources aren’t readily available.
When should you tow someone
If the Coast Guard or commercial sources can’t or won’t be available in time to keep the vessel and its occupants safe
The most important concerns is
safety
through communication
Realistic towing training based on standardized techniques, critical analysis, and mission debrief will contribute to
risk management and the development of a towing risk management plan.
For non-emergencies, rather than take another vessel in tow the best choice may be
for the disabled vessel to simply drop anchor and await assistance from commercial sources.
Other forms of assistance to a boat include
Transferring fuel (if it can be done safely), providing a communications relay, or standing by and monitoring until assistance arrives
The moment you attach a line to another vessel, you become
responsible for it and all persons onboard.
Your first responsibility is to
your own vessel and crew, and keeping them safe is paramount.
If the time comes when you must tow another vessel (or be towed), what is critical?
a fundamental understanding of the issues involved is critical.
All personnel on the vessel being towed should be
wearing a suitable PFD.
If a tow line fails under load it will snap back in the direction
of the pull.
The preferred tow lines are usually
double-braided nylon of suitable circumference and length for the vessel to be towed because it offers a good combination of strength and elasticity.
When towing a small vessel only a short distance in protected waters, what kind of rope can be used
polypropylene (but NOT a ski tow rope!) can be used.
Offshore or in heavy weather, more tow line may be needed to keep a towed vessel in step and to minimize the effect of shock-loading due to seas.
keep a towed vessel in step and to minimize the effect of shock-loading due to seas.
The tow line should always be the _______ ____ in the towing gear.
“weakest link”
Towline should be stowed on
tow reel whenever possible, with the bitter end secured to the reel with smaller line. The line will lie evenly on the reel. More importantly, to quickly release the towline in an emergency, just cut the smaller line with a knife, and the bitter end will run free.
When putting new towline in use, splice an eye at both end to allow an “end-for-end” switch before part of the towline is beyond useful service.
to allow an “end-for-end” switch before part of the towline is beyond useful service.
Things the da
Cuts
Chafing
Flattening
Fusing (caused by overheating or over-stretching)
Snags
Hardening (heavy use will compact and harden a towline and reduce its breaking strength)
If a line shows any of these characteristics, do not use it as a towline.
It is not always possible, or safe, to attach a tow line directly , in these cases a
bridle might be the best coalition. The bridle attaches to the eye of a tow line and is made fast to the deck fitting(s) on the towed vessel. Like the tow line, double-braided nylon makes the best towing bridles for most small to medium-sized
vessels.
For a towing bridle to equally distribute the load and provide a center pull,
both legs of the bridle should be even length.
To reduce west and chafing at the towline end, use a
pendant
Use a long bridle when
the best attachment points of the towed vessel re well aft to either side of the deck, but maintain a fair lead forward to reduce chafe.
The angle of a tow bridle’s left should be
less than 30 degrees
The legs of a towing bridle should be long enough to reduce
towed vessel yaw
protect bridges with
chafing gear when nescessary
Where should thimbles be used in two bridles
in the leg eyes where they mmeet
When shackled to the towline, remember to _____ the shackle pin.
Mouse
What is a length of light line used to carry a larger line or hawser between vessels?
a messenger
The following may serve as a messenger for small vessel towing evolutions
heaving line and heaving ball
float line
fender
What line is made if light, flexible line with a monkey’s fist at the thrown end?
Heaving line.
Heaving line must be at least __ feet long.
75
What protects towlines, bridles and pendants from wear caused by rubbing against deck edges, gunwales, bulwarks, chocks, taffrail or two bars
Chafing gear placed at friction points.
Three common items can be used to help with the towline or bridle wear to prevent chafing
old fire hose
heavy canvas
lether
steel or galvanized iron, grooved, heart-shaped insert that is fitted into eye splices to provide strength and to minimize wear and tear due to chafing action.
A thimble. Thimbled splices permit joining two lines with shackles.
What is a triangular shaped heavy steel plate with three holes, one at each corner?
a fishplate
The three-fold purpose of the fishplate is to
add strength at the connections
at weight to the make-up
maintain a horizontal position underwater while towing to prevent twisting of the hawser and bridles
What are attachment or fair lead points on vessels for towlines, anchor lines, and mooring lines.
Fittings
For towing, only use attachment points and fair leads designed
for horizontal loads. Common fittings include mooring bitts, towing bitts, cleats, chocks, and Samson posts. A tow bar or taffrail acts as a fair lead.
What hook has a quick release safety buckle and snap hook clip that can be attached directly to the boathook handle.
Skiff hook
To use a skiff hook you
Attach the skiff hook line to a towline with a shackle or double becket bend
Use the skiff hook assembly to reach down and place the hook into a small disabled vessel’s trailer eye
The hook is snapped into the eye and the handle is slipped off the round stock and pulled back
What is a frougue
a device that acts in the water somewhat the way a parachute works in the air. The drogue is deployed from the stern of the towed vessel to help control the towed vessel’s motions. Operators must familiarize themselves with the operating characteristics and effectiveness of available drogues. Test drogues under various conditions to learn drogue capabilities. The time to learn about a drogue is before you need to deploy one.
Drogues are typically used when
he tow is shortened as in preparing to tow into a bar or inlet. With a short hawser and large swells on the stern, the drogue is deployed to prevent the towed vessel from running up the stern of the towing vessel and keep tension on the towline to help prevent the towed vessel from “surfing” down the face of a wave.
PREPARE THE DROGUE GEAR
Visually inspect the drogue rig for worn rusted, or corroded fittings and swivels, correct
size shackles, and untangled bridles.
Ensure that the drogue rig has 200 feet of two-inch, double-braided nylon line properly
attached to the bridle swivel using a correctly sized shackle. Make sure it has no sharp
fittings or exposed wires, and is stowed in a manner that will keep it intact until it is deployed.
Provide all necessary equipment with a drogue rig such as extra shackles, bridles, straps,
and chafing gear to achieve the best possible connection on the stern of a tow.
Place all equipment in a gear bag along with laminated written instructions and illustrations on
how to rig a drogue, both with and without a bridle. At night attach a chemical light to the bag
and include a flashlight inside.
Attach flotation to the bag, usually a fender, and two lines, each 40 feet in length, to the
handles of the gear bag. Bend a heaving line or buoyant line onto one of the lines.
Fenders
rotect a hull when maneuvering in close proximity to other vessels, docks or pilings. Fenders have either eye(s) or a longitudinal hole for attaching lines. They can be spherical, cylindrical, or rectangular prisms. Fender size varies greatly, and the appropriate size should be used depending on the situation. Use fenders that will keep space between vessel hulls or rub rails and hulls.
What type of sources cause a vessel to resist motion
Static forces such as mass of the vessel and its displacement.
What are the two different properties of statice forces which cause resistance in towing vessels
Inertia and the Moment of Inertia
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.
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 disabled vessel ahead and change its heading at the same time.
Both interim and the moment of inertia strain the towing equipment so
deal with them separately whenever possible
Once making way, the static forces
lessen
Dynamic forces occur once
the towed vessel is moving
the dynamic forces forces when a vessel is being towed depend on the
vessel’s characteristics (Shape, arrangement, rigging)
the motion caused by the towing vessel
the effects of waves and wind
Once a vessel is moving in a direction its _____ wants it to keep moving in a direction
momentum
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.
FRICTIONAL RESISTANCE
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. 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.
DRAG
The less water a hull shape has to push out of its way, the easier it will move through the water. A deepdraft, 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.
WAVE MAKING RESISTANCE
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.
Never tow a vessel faster than
the design speed of its hell
A boat cannot go faster than about
1.4 times the square root of its waterline lengt, in knots.
”Shock-load” or “shock-loading” is
the rapid, extreme tension on the towline, which transfers through the tow rig and fittings to both vessels.
frictional forces act on
the hull, topsides, superstructure, and rigging.
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 when towing 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.
When towing in following seas be aware that
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
Spray Drag
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.
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.
Because of the potential dangers, the tow vessel must use various techniques to prevent or counteract shock-loading, or reduce its 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”
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 shockload source.
