USCG Masters Captains License 25/50/100 Ton Upgrade

SHIP CONSTRUCTION - Ship's Contruction

Ship Construction - Module 2

Ship's Construction

The United States Coast Guard is responsible for inspecting commercial vessels insuring that they meet rigorous specifications for their design and equipment.  The plans are reviewed by the Coast Guard prior to construction and periodic inspections during construction insure that the plans and specifications are being followed.

A vessel may be classed for insurance and financial purposes.  This process, which entails additional plan review and inspection, is carried out by a classification society.  Although there is no requirement that a vessel be classed, the service assures owners, shippers, and underwriters that the vessel is fit for sea duty.

The building of the keel is the first step in construction.  This is the main structural member giving longitudinal strength to the vessel.  The traverse frames give shape to the hull and add strength athwart ships (across the vessel).  The longitudinal frames provide longitudinal (lengthwise) strength.  A vessel’s quarter is the section which is on either side of the stern.

Ship's Construction

Rows of steel plates called strakes form the hull plating, which produces a watertight boundary. The joint formed when two steel shell plates are placed longitudinally side to side is called a seam.  The sheer strake, the uppermost strake of the hull, joins the main deck.  At the turn of the bilge where the hull makes the sharp upward turn is the bilge strake.  The A-strake, or garboard strake is the plating next to the keel.  The margin plate is the outboard strake of plating on each side of an inner bottom.

Ship's Construction

Longitudinal deck beams which run fore and aft and transverse deck beams which run athwart ships, support the decks.  In addition, these deck beams also give additional strength to the ship along its length and breadth. Seam welds, unlike tack welds, are 100% efficient as if it’s all one piece.  A weld does not reduce stress.

On the main deck or weather deck deck plating forms the final side of the watertight block of the ship.  It also provides rigidity to the structure.  The maximum length of a plate is the permissible length. 

Ship's Construction

The main deck of the vessel being built is slightly arched to aid water removal from the deck.  This is called camber. The deadrise is the distance that the bottom of the ship rises from the keel to the turn of the bilge.  The angle the bottom of the hull makes with a horizontal line is the slope of the bilge.  Flare is the outward turn of the hull as the sides of the hull come up from the water line.  Sheer is the curvature of the main deck from stem to stern.  Reserve buoyancy is the additional floatation provided by the flare and sheer.  Chine is the turn of the boat’s hull below the waterline.  Chine affects the vessel’s speed and turning characteristics.

The diagram below shows that the top-most deck that runs from the bow to the stern is called the main deck.  The space between any two continuous decks is called the tween deck (short for between decks), with the space between the main deck and the next deck down called the upper tween deck.  The Orlop deck is the partial deck in the hold.  The manger is the perforated bottom that allows the anchor chain to drain and dry.

Ship's Construction

The bulwark is a strong solid metal fence around the edge of the main deck.  The function of the bulwark is to help keep the deck dry.  Holes called scuppers or freeing ports are cut in it at the main deck level to allow water to run off.

The entire forward area of the main deck is called the forecastle (pronounced fok’s’l).  The stern is the cutting edge of the bow and the hawse pipe is the tube that leads the anchor chain from the main deck down through the hull plating.  The superstructure the deck structure located above the main deck contains the crew messing and berthing areas as well as the bridge.

Ship's Construction

The freeboard of the vessel is the distance between the main deck and the water line.  The draft is the distance between the keel and the water line, and can be read at the draft marks painted at the bow and stern of the vessel.  The numbers are placed at one foot intervals and are measured from the bottom of one number to the bottom of the next number.  Each of the numbers is six inches high.  The draft of the vessel in the diagram is 14’6”.

Ship's Construction

The rudder is used to steer the ship while the propeller provides the means of moving the ship through the water.  The propeller shaft connects the main engine to the propeller.  A strut may be used to reduce vibrations if there is a large amount of the shaft outside the hull.

When a vessel floats it moves or displaces a certain amount of water out of the way.  One way to describe a ship’s size is by its displacement, which is the amount (weight) of water the vessel moves out of the way.  The displacement is measured in long tons (2240 pounds) in comparison to short tons (2000 pounds).

A vessel can also be measured and described by its gross tonnage and net tonnage.  Rather than being a measurement of weight, they are a description of volume, with one ton equal to 100 cubic feet of space.  The gross tonnage of a vessel is the space inside the watertight bulkheads, including machinery and crew spaces LxBxD/100.  In contrast, the net tonnage is the usable space inside the watertight bulkheads, excluding the machinery, crew, and certain other spaces exempted by regulation.  Length overall (LOA) is another measurement of the vessel’s length, which is the distance from stem to stern in a straight line and does not change according to the way the boat sits in the water.  Draft is the depth of the boat from the actual water line to the bottom of the keel.

Machinery spaces are collectively all the major compartments in which machinery are located.  The engine room is the compartment in which the main propelling unit or units are installed.  The engine operating station is the location or compartment from which a main propelling unit is operated.  The fireroom is a compartment containing boilers and the station for firing or operating the boilers. The boiler room is a compartment containing boilers but not containing the station for firing or operating the boilers.  The boiler operating station  is the station from which boilers are operated.

Watertight compartments are specified by letters and numbers.  The ship is divided into three divisions from the forward aft and are lettered A, B and C.  Machinery compartment includes fire, boiler and engine room, main machinery space and compartments where propelling machinery is located.  All compartments and spaces that are completely watertight, oil-tight, airtight, or fume-tight are numbered.  Doors, hatches, and manholes are numbered by label plates and tags. These numbers are combined with that of the compartment(s) in which they provide access.  Hatches operate with quick acting devices such as wheels or handles and must be watertight if they are exposed to water or weather.

The hull is the main body of the boat.  They can be constructed of many different materials such as metal or fiberglass.  The three basic types of hull forms are displacement hull, planing hull, and semi-displacement hull.  At rest, all three hulls displace water around the vessel.  The main difference is that when underway, the planing hull skims along the surface of the water, while the displacement hull always forces the water around it.  The semi-displacement hull is a combination of characteristics of both displacement and planing hulls.

Trim is a relative term that refers to the way the boat sets in the water in relationship to the vessel’s stability and buoyancy.

Load Lines and Plimsoll Marks

Loadlines or Plimsoll mark indicates the maximum draft permitted for a vessel, depending on the season of the year and the waters a vessel may be traveling.  46 CFR 175.122 states that a vessel of 24 meters (79 feet) in length or more that is on a voyage other than a domestic voyage is subject to load line assignment, certification and marking under CFR Subchapter E.  Load Line Certificates certify that the load line marks are correctly assigned to each vessel.  These marks are permanently inscribed and then painted at the appropriate places on the vessel’s plating.  You need to record the position of the load line and draft marks in your logbook prior to leaving the dock.  The load line, used in conjunction with the Plimsoll Mark, indicates the maximum allowable draft and trim for that vessel in different circumstances and seasons.  46 CFR 42.13 covers load line regulations.

The top two lines show the maximum draft permitted in tropical fresh (TF) and (F) water.  The lines to the right of the vertical line show the maximum drafts permitted in salt water areas designated tropical (T), summer (S), winter (W), and winter North Atlantic (WNA).  The summer line and the horizontal line through the disk are the same distance down from the deck line, which is at the edge of the main deck.

Ship's Construction

A vessel loading cargo in the Great Lakes, prior to an ocean voyage, is permitted to submerge its loadline because when it sails into the denser salt water of the ocean it will ride higher thereby complying with the maximum allowable draft.

Machinery Operations

Gasoline Engines

Most small pleasure craft have gasoline engines, which operate on a four stroke cycle.  The fuel is drawn from the tank by a fuel pump and led to the carburetor through a fuel filter.  In the carburetor, the fuel is mixed with air to make an explosive mixture.  This mixture is then drawn into the cylinder on the intake stroke of the piston and then compressed on the compression stroke.  Just as the piston reaches the top of the compression stroke, the spark plug ignites the mixture.  The burning of the mixture causes an expansion of gases resulting in the power stroke.  As the piston returns to the top of the cylinder on the exhaust stroke, the burnt gases are forced out and the engine is ready for another cycle.  The spark is provided through a distributor which sends the spark to the right cylinder at the right moment, and a coil which jumps the voltage from a low supply to high output.

Maintenance and Emergency Repairs
Routine maintenance on a marine gasoline engine is much the same as on your automobile engine.  Check the oil before starting the engine and maintain the spark plugs and points in good condition.  Check the level of coolant (if a closed freshwater system) and don’t forget to check your fuel level before leaving the dock.

If repairs are needed, try to track down the problem.  If you suspect the engine isn’t getting fuel, check to see if your tank is empty.  If your fuel is not a problem, trace the system to the carburetor, making sure that it is getting to each step.  If you suspect the spark plugs aren’t working, check to see if the spark is getting to the plug.  If the points are fouled, file them with a fine file.  Don’t use sand paper or emery boards because they leave a residue.

Diesel Engines

Diesel engines are generally safer than gasoline engines because the fuel is less volatile (flammable).  The operation of a diesel engine also differs from a gasoline engine in that air is drawn into the cylinder and compressed before the fuel is injected.  This compression generates heat.  The ignition of the fuel takes place because the compressed air reaches temperatures of 1000 degrees F. 

In the two-stroke diesel engine, compressed air rushes in through the intake ports when the piston is at the lower of its stroke.  As the piston starts up, it covers the intake ports and compresses the air.  As it reaches the top of its motion, the fuel is sprayed in and ignites, creating great pressure from expanding gases.  The pressure forces the piston down on the power stroke.  The exhaust valve then opens, allowing the burnt gases to escape.  When the intake ports are uncovered, the compressed air forces the rest of the burnt gases out and the engine is ready for another cycle.

Maintenance and Emergency Repairs
The fuel system should be the first thing checked in a diesel engine.  The pressure for the injectors must be correct or too much or too little fuel will be supplied.  The fuel line could be clogged and not enough fuel could be getting to the cylinders.  If the engine over speeds and runs out of control, clogging the air intakes will prevent combustion.


Both diesel and gasoline engines use batteries for starting.  Maintenance involves keeping the electrolyte (fluid) level within ¼ inch of the bottom of the filler plug and keeping contacts clean.  The battery compartment must be well ventilated because the battery gives off explosive gases as it charges.  Because the battery contains acid, it must be kept on a lead plate or other substance resistant to the effects of the acid.


Ventilation of the engine compartment is required by Coast Guard regulations.  The ducts should extend to the lowest part of the bilge so that vapors heavier than air, such as gasoline vapors, can be removed.  The compartment holding the fuel tank should be ventilated directly to fresh air.  After fueling, you should ventilate the entire vessel for 5 minutes prior to starting the engine to remove any vapors that may have accumulated during fueling.

Fire Protection Devices

Because the carburetor contains an explosive mixture, a flame arrester is required on every carburetor on a gasoline engine to prevent backfire flames from entering the carburetor and igniting the mixture.  Vents leading to the fuel tank compartment must also have a flame screen of at 30 by 30 mesh over the openings to prevent flames from entering the tanks.  Finally, a dry exhaust system must be equipped with a spark arrester to keep sparks from being discharged (drip collector under carburetor).


All passengers should be off of your vessel during fueling.  Smoking is prohibited.  When pumping the gas, touch the fuel nozzle to the side of the fill pipe.  This prevents sparks from the building up of static electricity.  When topping off, leave a little room in your tank to allow for expansion of the fuel when it gets warm.  Be very careful not to spill any fuel into the water, as you may end up facing a pollution fine.

Vessel Alterations, Repairs and Hot Work

46 CFR 176.7 discusses regulations regarding vessel alterations, repairs and hot work.  Repairs or alterations to the hull, machinery, or equipment that affect the safety of the vessel must not be made without the approval of the OCMI, except in an emergency.  When repairs are made during an emergency, the owner, operator, or master shall notify the OCMI as soon as practicable after such repairs or alterations are made.  Repairs or alterations that affect the safety of the vessel includes, but are not limited to: replacement, repair, or refastening of deck or hull planking, plating and structural members: repair of plate or frame cracks; damage repair or replacement, other than replacement in kind, of electrical wiring, fuel lines, tanks, boilers and other pressure vessels and steering, propulsion, and power supply systems; alterations affecting stability; and repair or alteration of lifesaving, fire detecting, or fire extinguishing equipment.  The owner or operator shall submit drawings, sketches, or written specifications describing the details of any proposed alterations to the OCMI.  Proposed alterations must be approved by the OCMI before work is started.  Drawings are not required to be submitted for repairs or replacements in kind.   The OCMI may require an inspection and testing whenever a repair or alteration is undertaken.

46 CFR 176.710 covers inspection and testing prior to hot work.  An inspection for flammable or combustible gases must be conducted by a certified marine chemist or other person authorized by the OCMI in accordance with the provisions of National Fire Protection Association.  If the inspection states that the operation can be untaken safely, a certificate that states that must be written by the certified marine chemist or authorized person before the work is started.