SHEATHING OF WOODEN VESSELS

KISHOR KUNAL

FISHERIES COLLEGE AND RESEARCH INSTITUTE,

TUTUCORIN,TANUVAS

SHEATHING OF FISHING VESSELS

BY:KISHOR KUNAL

BFT08011FC & RI

TOPICS :

What ?

Why ?

Where ?

How ?

What is sheathing ?

• A lamination or plastic skin or any other

covering employed to protect the under

cover material from external

environment.

• Process of putting sheath or covering.

Need or importance of sheathing :

Protection from decay

Prevention of leakage

Protection from marine borers & foulers

Prevention of corrosion

For structural strength

Cosmetic

..a sheath’s main purpose is to protect. A secondary function of a sheath is cosmetic -to improve appearance. Note that we do not assign a structural function to the sheath:

it should not be expected to strengthen the boat appreciably.

What to sheath ?

should begin at a point above the load waterline.

should completely cover the hull including the keel, dead wood, stern post, the wooden skeg & all other parts exposed, when the vessel is at her deepest draft.

other parts which are prone to rot or corrode.

Development

Older sheathing methods – use of 3 materials• White stuff –train oil + rosin + brimstone• Black stuff – tar + pitch• Brown stuff – tar + pitch + brimstone According to marine archeology• 15th century – copper and lead sheathing by

Romans, Greeks, Spanish & Portuguese navies.• 1514 – John Sebastian, lead sheathing, England• 1670s – lead sheathing, extra planking,

England

Development continued….

• 1708 – Charles Perry, copper sheathing• Early 18th & 19th century – further

development in copper sheathing. Use of iron or zinc alloy with copper.

• 20th century & today – aluminium and alloy sheathing. Galvanic series sheathing. Glass fibre or reinforced polyster sheathing. Ferrocement sheathing. Composite sheathing material

Types of sheathing…

1. Lead sheathing

2. Copper sheathing

3. Aluminium sheathing

• The galvanic series

4. Ferrocement sheathing

5. Fiberglass sheathing

6. Composite sheathing-e.g- woolen–nylon

sheathing

Lead sheathing

Lead sheathing, whilst more effective

than wood in its stated purpose, reacted

badly with the iron bolts of the ships

causing sometimes severe damage.

Not in practice.

Copper sheathing

Cold rolled sheets of 18 to 26 gauges are used.

Copper sheathing nails (tacks) of 25 mm length or less may be used according to thickness of planking.

Tack should be 3mm less than plank thickness, should be driven at about 25mm interval, 12mm in from edges with 3 no. of nails at 100mm interval.

Sheets should overlap by 25mm.

Merits of copper sheathing

No need of antifouling paint When in contact with water, the copper

produces a poisonous film, composed mainly of oxychloride, that deter the marine creatures.

Furthermore, as this film is slightly soluble it gradually washes away, leaving no way in which marine life could attach itself to the ship.

Good life-span. (6-8 years)

Demerits of copper sheathing

The copper bolts used to hold the plates

to the hull react with the iron bolts used

in the construction of the ship, rendering

many of them near useless.

High cost.

Average strength

High rate of corrosion in tropical waters.

Aluminium sheathing

Aluminium alloy sheets, Indian standard

(I.S) 737 : 1955 no.5 is considered better

due to low cost.

Aluminium screws or tacks can be used

for fixing.

Copper, brass or other mettalic

fastenings should never be used.

Merits of aluminium sheathing

Cheaper (less than 50% that of copper).

Lightweight (about 1/3rd of copper).

Less rate of corrosion in tropical waters.

Demerits of aluminium sheathing

Lesser life-span or durability. (3-4 years)

Less strength.

Bronze, copper or brass fittings need to be

avoided to prevent galvanic action.

Galvanised iron, mild steel, cast iron or

aluminium alloys can be used rather.

Needs anti corrosive & antifouling painting.

The galvanic series

Galvanic action between metals can be

set on when they are immersed in

seawater, as seawater is a good

conductor of electricity.

When either copper or aluminium

sheathing of hull is done, some under

water fittings like the keel shoe, rudder

forks stem bands should be fabricated

suitably to avoid galvanic action.

When copper sheathing is done :

Use following for fabrication –• Stem band - - brass or bronze• Keel shoe - - bronze• Rudder forks & fittings - - bronze• Rudder shaft - - brass• Seacocks - - bronze• Strainers - - copper or brass

When doing aluminium sheathing

Use following as underwater fabrication

• Stem band – galvanised iron

• Keel shoe – mild steel painted/galvanised

iron/cast iron.

• Rudder fork and fitting – cast iron

• Rudder shaft – mild steel

• Seacocks – cast iron

• Stringers – aluminium alloy

• If brass and bronze fittings can be avoided

like propellers, stern tubes housing

etc.,care should be taken to see that bi-

mettalic contacts are avoided using rubber

or wooden pads.

Avoidance of galvanic action

Use zinc (99.6% pure I.S.I.209 ) blocks.

Keep maximum distance.

Insulation or fabrication.

Proper painting.

Ferrocement sheathing

Done with iron and cement. The structure of ferrocement is the metal

netting embedded in it. Three layers of 1 in sq 22g welded mesh

are stapled, and then plastered with a very rich mix of cement, pozzolan (fly ash) and basalt sand.

Cement sheathing does not adhere to the vessels planking, it incases it.

Merits of ferrocement sheathing

Very much durable in rough conditions.

If applied correctly it will flex without

fracture.

If designed appropriately it can replace

the failing structural integrity of the

vessel.

Needs very less maintenance.

Demerits of ferrocement sheathing

Makes the vessel quite heavy.

As the metal netting is embedded in cement,

the are hard to inspect.

Renovation is much difficult.

Quite costly.

Poor impact resistant.

Fiberglass sheathing

What is FRP ?

• The term FRP is generally accepted to mean

fiber reinforced plastic. The names fiberglass

reinforced polyester, resinglass, and glass

reinforced plastic (GRP) are also used.

Reinforcing fibers may be short and

discontinuous such as asbestos, chopped

glass, or flock; or they may be continuous

filaments as in woven cloth or spun (non-

woven) fabrics. Other types and

modifications of fibers exist in profusion, but

woven fabrics are the reinforcements of

major utility for sheathing.

In its preferred form the wooden hull is covered with a high strength fiberglass skin which completely covers at least that portion of the hull wetted by, and in contact with the water. The fiberglass skin not only protects the hull against rot and marine organisms, but also increases the life and strength of the boat as well as dramatically reducing the cost of maintenance. The fiberglass skin for added strength is preferably secured to the hull by a combination of mechanical fastenings, and an elastomeric adhesive which chemically bonds the fiberglass skin to the hull.

The fiberglass skin, after it is secured and bonded by the use of adhesive to the hull is generally, and preferably, saturated with resin, covered with a chopped strand mat to provide added strength, a fairing compound is applied to the outside surface, or exterior of the skin, and the surface is then sanded and painted to provide a smooth, and even a glossy finish. This can be done without any necessity of tipping the boat on its side to complete the construction; which is a particularly preferred advantage when laminating large boats.

Fiberglass is useful for the construction of FRP boats since it is a moderately stiff fiber. In primary structure, stiffness is much to be desired. Also, of interest, is a property known as “tensile elongation.” This is the amount the fiber will stretch before it breaks. For fiberglass this value is about 3 per cent or about l/32 of an inch per foot of length.

Advantage of FRP sheathing

Reduction of maintenance No caulking, no leaks. No plank shrinkage when laid up Rot proof and resistant to borers. FRP is

non-organic and will not rot. As a plastic it cannot be eaten by marine borers.

Corrosion and electrolysis reduced. FRP is inert. As a plastic it will not corrode.

Simpler construction Reduction of skill levels required once a

basic training is received.

Demerits of FRP sheathing

Prone to fire and chemical hazards.

Few non degradable materials used make it

hazard to environment.

Needs skilled and qualified technicians for

application and repair.

Chemicals used posses health hazards.

Composite sheathing

Widely used where the external surface is

exposed to liquid water but inner surface

is exposed to moisture vapour.

It allows moisture vapour to pass but

prevent the passage of liquid water.

The composite sheathing material according to the present invention comprises a core layer of paperboard, a first layer of paperboard having a first layer of a permeable resin on a surface thereof, said first layer of paperboard being adhered to the core layer of paperboard by means of a first adhesive layer therebetween, and a second layer of paperboard having a second layer of a permeable resin on a surface thereof, said second layer of paperboard being adhered to the core layer of paperboard by means of a second adhesive layer therebetween. The first and second adhesive layers each comprises a controlled distribution of thicker and thinner adhesive portions.

General procedure of sheathing

Complete caulking & seaming before start.

Mark the design waterline.

Denote top of sheathing line 15 to 25 cm above the waterline.

Maintain moisture content from 8-12 %

Plane all the surfaces to be sheathed.

Fill all the holes made for nails, rivet heads etc with seaming compounds. e.g.-white putty, malabar dammar, lime powder, neem oil or coal pitch.

It is very important to note that all the fastening heads should be at least 6mm deep into the wood so that there is ample clearance between the sheet or metal.

The hull should be coated with one coat of coal-tar or tar still or any other suitable bituminous compound like bituminous paint applied.

A sheathing of underlay is placed between the sheathing & the hull planking.

Apply another coat of coal tar or other bituminous compound over the underlay.

Fix the sheath.

THANK

YOU