Ship Construction Definitions -

Introduction

This article explains common ship terms in two groups: the terms for the hull and its dimensions, and the terms for the propeller. Each term has a short definition and, where it helps, what it is used for.

Hull Definitions

Base Line

A line drawn on top of the keel plate, used as a reference to measure moulded dimensions.

Aft Perpendicular (AP)

An imaginary perpendicular line drawn at the point where the summer load line waterline intersects the aft side of the rudder post, or the centre of the rudder pintles (the devices that hold the rudder).

Forward Perpendicular (FP)

An imaginary perpendicular line drawn at the point where the forward stem meets the summer load line waterline.

Length Between Perpendiculars (LBP)

The length measured between the forward and aft perpendiculars, along the summer load line.

Length Overall (LOA)

The length measured between the extreme forward edge and the extreme aft edge. This includes the outer tip of a bulbous bow if it extends beyond the bow.

Length on the Waterline (LWL)

The length measured along the present waterline from forward to aft.

Figure 1. AP and FP are at the ends of the summer load waterline. Sheer is the rise of the deck at the ends.

Midship

A point midway between the aft and forward perpendiculars.

Extreme Breadth

The breadth measured from the outer edge of the hull plating (the shell plating) on either side.

Moulded Breadth

The breadth measured from the inner edge of the hull plating on either side.

Rise of Floor (Deadrise)

The upward slope of the bottom hull plating towards the ship’s side. It helps to drain water from the double bottom.

Camber

The curvature of the deck in the transverse direction. It helps to drain green water (sea water on deck) off the deck.

Figure 2. The extreme breadth is measured over the shell plating, the moulded breadth to its inner edge.

Sheer

The height of the deck at the aft and forward ends above the deck at midship. It provides more buoyancy at the ends, and helps the ship ride over waves.

Flare

The outward curvature of the side shell. It helps to clear the anchor and to avoid sea spray.

Tumblehome

The inward curvature of the side shell. It narrows the ship towards the deck, and is seen on some warships and older vessels.

Figure 3. Flare widens the hull towards the deck, tumblehome narrows it.

Parallel Middle Body

The length along which the midship section remains constant in area and shape. It makes the ship simpler to build.

Other Terms You Will Meet

Draught: the vertical distance from the waterline down to the keel.
Moulded depth: the vertical distance from the base line to the top of the deck beam at the ship’s side, amidships.
Freeboard: the vertical distance from the waterline up to the deck line. The load line article shows how it is measured.
Displacement: the weight of water displaced by the ship, which equals the total weight of the ship and everything on board.

Propeller Definitions

A right-handed propeller turns clockwise, seen from astern, when the ship is going ahead.

Blade Back

The suction side of the blade, facing forward.

Blade Face

The pressure side of the blade, facing aft.

Figure 5. The face pushes water astern, and the back is the side where the pressure is lower.

Pitch

The theoretical forward movement of a propeller during one revolution.

Slip

The difference between the actual distance and the theoretical distance covered by the propeller through the water.

Figure 4. Slip is the part of the theoretical advance that the ship does not achieve.

The theoretical distance in a period is pitch × revolutions. As a percentage:

Slip % = (Engine distance − Ship’s distance) / Engine distance × 100

For example, a propeller of 6 m pitch turning at 100 rpm advances 6 × 100 × 60 = 36,000 m, or 19.4 nautical miles, in an hour. If the ship makes good 16 knots, the slip is (19.4 − 16) / 19.4 × 100 ≈ 17.7%. Slip is a useful check on the condition of the propeller and hull. It rises with a fouled hull, heavy weather or a heavy load, and can even be negative when a following current helps the ship.

Rake

The degree to which the blades slant forward or backward in relation to the hub, seen from the side. Aft rake is common, and it helps to give clearance between the blade tips and the hull.

Skew

The transverse sweeping of a blade. Viewed from forward or aft, the blades show an asymmetrical shape. Skew helps to reduce vibration and noise.

Aft skew (positive skew): the blade sweeps in the direction opposite to the rotation.
Forward skew (negative skew): the blade sweeps in the same direction as the rotation.

Figure 6. Rake is the lean of the blade along the shaft. Skew is its sideways sweep.

Conclusion

These terms are the basic vocabulary of ship construction and propulsion. Learn the reference lines first, because lengths, draughts and stability figures are all measured from them.

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Ship Construction Definitions