Stern Tube / Propeller — Most Asked Questions
14 questions asked about stern tube / propeller across the General and Fn 3 and Fn 4b papers, heaviest first.
Learn the system first
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1 A 3 blade propeller has the following characteristics
Answer
Read a propeller by diameter, pitch, blade area and direction: a 3-blade wheel trades smoothness for simplicity.
- Diameter sets the disc swept; pitch at 0.7 radius states the advance per revolution without slip.
- Blade-area ratio — developed area over disc area — states how much blade grips the water against cavitation.
- Three blades run rougher than four or five but cost less and suit many merchant hulls; blade count also dodges hull resonance.
- Hand, skew, section shape and pitch-to-diameter ratio complete the description the surveyor wants.
Surveyor follow-up: same diameter, finer pitch — what happens to slip, thrust and rpm for the same speed?
Full theory: The Propeller — Fixed Pitch, Controllable Pitch and Cavitation -
2 Apparent and true slip?
Also asked: “Apparent and real slip”
Answer
Apparent slip uses ship speed; true slip uses wake speed — the wake correction turns one into the other.
- Apparent slip compares pitch-times-rpm advance against speed over ground, ignoring the wake.
- Wake fraction states how much the hull drags water along — true speed through water is ship speed over one-minus-wake.
- True slip recomputed on wake speed is the real working slip; textbook wheels run about 7–15% apparent, 12% and up true.
- Worked example: 90 rpm on 5 m pitch with 15% apparent slip and 0.10 wake gives true slip near 6% once wake is removed.
Surveyor follow-up: apparent slip negative with a following sea — ahead or astern wake, and is the wheel ventilating?
Full theory: The Propeller — Fixed Pitch, Controllable Pitch and Cavitation -
3 Types of propeller and a brief notes on each
Answer
Wheels split fixed, controllable, skewed and ducted: fixed is cheap and tough, CPP manoeuvres, skewed runs quiet, ducts pull hard.
- Fixed pitch: blades cast solid on the hub in copper alloy — robust, cheap to make and run, but manoeuvring suffers.
- Controllable pitch: pitch angle changes under way through hub hydraulics — full thrust either way without reversing the shaft.
- Skewed and high-skew wheels sweep the blades back to cut vibration and cavitation on fast hulls.
- Ducted Kort nozzles wrap the wheel to boost bollard pull for tugs and trawlers at the cost of open-water speed.
Surveyor follow-up: CPP stuck at full ahead pitch with the engine stopped — how do you bring her alongside?
Full theory: The Propeller — Fixed Pitch, Controllable Pitch and Cavitation -
4 How to measure propeller drop
Also asked: “How to measure propeeler drop”, “Propeller drop measurement”
Answer
Propeller drop is read with a poker gauge through a capped access: shaft sag from worn sternbush against 0.2–0.3 mm limits.
- Remove the copper-gasketed plug at the special access and drop the depth gauge to the shaft.
- Worn liner and bush grow clearance, lose sealing, admit seawater and let the heavy wheel bend the shaft down.
- Maximum allowable drop runs 0.2–0.3 mm; bearing clearance itself sits in the same band when new.
- Grooved liners, lost sealing and rising leakage precede the drop — measure before the seals fail completely.
Surveyor follow-up: drop 0.4 mm with leakage normal — sail, watch or dock, and what do you log?
Full theory: Stern Tube Seals — Lip, Floating Ring and Radial Face -
5 In a Controlled Pitch type propeller, it is possible to alter the pitch by rotating the blade about its vertical axis by means of mechanical and hydraulic arrangement. This helps in driving the propulsion machinery at co
Also asked: “In a Controlled Pitch type propeller, it is possible to alter the pitch by rotating the blade about its vertical axis by means of mechanical and hydraulic arrangement. This helps in driving the propulsion machinery at constant load with no reversing mechanism required as the pitch can be altered to match the required operating condition. Thus the manoeuvrability improves and the engine efficiency”
Answer
CPP turns blades, not shafts: hub servo twists each blade on its axis for ahead to astern at constant rpm.
- Blades rotate about their vertical axes through yoke, crank pin and hub servo fed down the shaft.
- Constant shaft speed keeps auxiliaries and shaft generators happy while pitch sets thrust.
- Full astern by pitch reversal without stopping or reversing the engine — manoeuvring in blade angle, not rpm.
- Cost is a bigger boss limiting blade area with complex oil-tight hub machinery power-limited by design.
Surveyor follow-up: pitch stuck ahead with the shaft turning — hydraulic, control or seizure, and what first?
Full theory: The Propeller — Fixed Pitch, Controllable Pitch and Cavitation -
6 How will you order bolt?
Answer
Order coupling bolts to formula: shaft diameter first, then bolt diameter, flange thickness, length, thread and count.
- Quote shaft diameter with class K factor — bolt diameter follows the prescribed formula from it.
- State flange thickness, taper, bolt count, shank length, thread pitch and fitted-versus-clearance type.
- Name material and liner practice so seawater never reaches the joint — composition fills every non-fitting strip.
- Give position: tailshaft-to-intermediate or propeller coupling, with reamer size if holes are finished together.
Surveyor follow-up: bolts ordered to diameter only with threads cut short — fit, grip or fatigue failure first?
Full theory: The Propeller — Fixed Pitch, Controllable Pitch and Cavitation -
7 Type of Seals used in stern tube and the construction of Seals
Also asked: “Types of seals on Stern tube( In depth”
Answer
Stern tubes seal oil in and sea out: Simplex floating-ring lip stacks aft and forward with header tanks setting the pressure.
- Simplex-type floating lip rings ride the liner with spring-loaded rubber lips in series chambers.
- Oil header tanks hold seal pressure above sea pressure with cooling circulation through the aft peak.
- Cedervall, Boor and Johnson variants ring the same idea — staged lips with drains and leak alarms between.
- Prove by header levels steady, drains dry and cooling flowing — weeping aft seals earn docking, not topping up.
Surveyor follow-up: aft seal weeping with header level steady and cooling proved flowing — lips, liner or pressure first?
Full theory: Stern Tube Seals — Lip, Floating Ring and Radial Face -
8 Dirrence between apprent slip and true slip
Answer
Apparent slip is measured against the ship’s speed; true slip accounts for the wake the propeller is actually working in.
- Apparent slip is the difference between the propeller’s pitch speed and the ship’s speed, taken as a percentage.
- The propeller works in the wake, so its inflow is slower than the ship’s advance — that is the wake fraction.
- True slip uses the water speed at the propeller, the ship’s speed reduced by the wake fraction, and is always positive.
- Apparent slip is greater than true slip, and the difference is a measure of how much the wake is helping the propeller.
Surveyor follow-up: apparent slip quoted where the question asked for true slip — which speed goes in the denominator, and which fraction?
Full theory: The Propeller — Fixed Pitch, Controllable Pitch and Cavitation -
9 From engine to propeller what arrangements was there
Answer
Engine to propeller is a line: crankshaft, thrust and intermediate bearings, shaft coupling, the shaft in its bearing, and the stern tube.
- From the engine the crankshaft turns the shafting through a flexible coupling that absorbs the small angular misalignment.
- The shaft is carried by a thrust bearing taking the propeller thrust and by the intermediate shaft bearings along its length.
- At the stern the shaft enters the stern tube, where the tube assembly and the aft end bush form the bearing on the aft side of the hull.
- Past the tube the propeller is fitted on the shaft tail by taper, key or flange, and it is the propeller that makes the thrust.
Surveyor follow-up: shafting removed and the aft bearing with heavy blueing — is it out of alignment or the aft bearing?
Full theory: The Propeller — Fixed Pitch, Controllable Pitch and Cavitation -
10 Pitch of the propeller
Answer
Pitch is the distance the propeller would advance in one turn at no slip: the advance per revolution that sets the speed of the ship.
- Pitch is the distance a propeller advances in one revolution if there were no slip at all — a geometric figure.
- A high-pitch propeller is efficient at speed; a low-pitch one gives more thrust at low revolutions and is used on ships that start and stop often.
- Pitch ratio pairs that with the diameter, and the propeller is chosen from the resulting curves for the ship’s service.
- Coarse and fine pitch denote the same: a coarse propeller has a low pitch ratio and a fine one a high pitch ratio.
Surveyor follow-up: pitch quoted as a distance travelled per revolution when the ship is making way — geometric or actual?
Full theory: The Propeller — Fixed Pitch, Controllable Pitch and Cavitation -
11 What is pitch ratio,prismatic coefficient,block coefficient,sheer strake , and tomble home
Answer
Two are hull form, two are the hull above the waterline: prismatic and block describe the underwater body, sheer strake and tumble home the topsides.
- The block coefficient is the ratio of the underwater volume to a box of the same length, breadth and draught — it says full or fine.
- The prismatic coefficient is the ratio of the displacement to a prism of the ship’s length with its midships area — high Cp means U-form sections.
- The sheer strake is the strengthened strake along the upper edge of the side, the main longitudinal strength member at the deck edge.
- Tumble home is the topside sloping inward above the waterline; above the point of maximum breadth, the sides must come inboard.
Surveyor follow-up: Cp high with Cm low — what shape of body sections does that combination give the ship?
Full theory: Ship Hydrostatics: Buoyancy, Displacement and Trim -
12 Wt is keelson propeller....and how do u fix the propeller to shaft
Answer
The keelson is the heavy transverse bar on the ship’s centreline carrying the propeller shaft through the deadwood, and the propeller is forced on the shaft.
- The keelson is a deep transverse girder on the centreline, stiffening the flat of keel and carrying the propeller shaft bossing.
- The shaft passes through it in a stuffing box at the aft bulkhead, which is why the stuffing box appears where it does.
- A fixed-pitch propeller is fitted on a tapered shaft tail and forced on by a nut; the taper was originally used with a key as a safeguard.
- Keyless fitting relying on the interference fit is now common because it removes keyway problems; many propellers are flange-mounted and bolted.
Surveyor follow-up: keyless propeller that will not stay on the taper — is it the fit, the taper angle or the nut?
Full theory: The Propeller — Fixed Pitch, Controllable Pitch and Cavitation -
13 how is a ship moving in water. What is the propeller doing
Answer
The propeller pushes water aft; the water pushes the ship forward — thrust, and a larger thrust than the tow rope resistance.
- The propeller accelerates a mass of water astern, and by reaction on the ship it delivers forward thrust.
- That thrust is greater than the tow rope resistance, because the water has to be given momentum as well as the ship moved.
- Thrust minus resistance gives the net force, and net force divided by mass gives the acceleration the ship actually has.
- The propeller is also an interaction device: the hull the propeller sits in augments resistance, so the required thrust is more than the bare tow rope figure.
Surveyor follow-up: ship making way with the propeller cleared on the card — is that acceleration, resistance or power?
Full theory: Ship Resistance, Hull Form and Propulsion -
14 what is fitted bolt?
Source: 123 written papers and surveyor oral banks, 24,613 entries, merged into 17,210 topics. Ticks are stored on this device only.