Far from help, know her hull, what keeps her upright, and how her systems fail.
1 · Hull
Materials and the haul-out
What causes osmosis blisters on a GRP hull, and how is it cured?
Osmosis: water through the gelcoat reacts with the laminate, raising sour blisters below the waterline. The cure is to peel the gelcoat, dry the hull and coat it in epoxy.
Osmosis: water through the gelcoat reacts with the laminate, sour blisters below the waterline; the cure is peel, dry, epoxy. A cored deck rots round unsealed bolt holes: dull thud, flexing, damp; re-core, pot the holes. Star cracks at a fitting: a backing plate.
Material
Enemy
Cure
Steel
Rust under broken paint
Blast, epoxy-prime; renew thin plate
Aluminium
Copper paint, stray DC
Copper-free paint; find the leak
Wood
Rot: fresh water, still air
Cut to sound timber
Slings go on the builder's marks, clear of saildrive and transducers; the keel takes the weight on long blocks, chained stands only steady her. Enter a slipway cradle on the rising tide; a grid needs tide to float off too. Fast hulls need hard antifouling; painted anodes protect nothing; a weeping rudder is wet inside.
2 · Structure and rig
Keel, rudder, rig, seacocks
How do I know if my yacht's keel bolts are corroding?
Rust weeping at a nut: stainless keel bolts corrode unseen in a wet bilge, so draw one or X-ray it. A nut that turns without the torque rising means the bolt is yielding.
Stainless keel bolts corrode unseen in a wet bilge: rust weeping at a nut, draw one or X-ray it; a nut turning without the torque rising is yielding. After a grounding, survey the keel grid. A spade rudder hangs on its bearings alone; a skeg adds a lower bearing and fends off lines.
Shroud loads go through chainplates into a bulkhead: cracked tabbing and a door jamming on one tack mean it moves; unload the rig, get in. A leak along the lee side only with the rail under: the hull-to-deck joint. Below the waterline one metal, bronze or DZR, or a rated composite: brass dezincifies.
Rigging
Watch for
Wire
Broken strand at the swage: replace
Rod
Fatigues at the heads unseen: re-head on schedule
Dyneema
Creeps, chafes: re-tension, replace if cut
3 · Stability
The righting lever and its curve
What is a yacht's angle of vanishing stability?
The heel at which the righting lever GZ, past its peak, falls to zero; beyond it she is stable inverted. It assumes a closed hull, so an open companionway's downflooding angle is the real limit.
Weight acts down through G, buoyancy up through B; heeled, B moves to the low side and the arm between them, GZ, is the righting lever: moment = displacement × GZ. Small angles: GZ ≈ GM × sin θ, GM = KM − KG.
Stability booklet, loaded
KM 3.20 m, KG 2.60 m: GM 0.60 m
At 10°: GZ ≈ 0.60 × sin 10° = 0.10 m
Δ 30 t: righting moment 30 × 0.10 = 3 t·m
Against heel GZ rises to a peak, then falls to zero at the angle of vanishing stability; past it she is stable inverted. It assumes a closed hull: the downflooding angle of an open companionway is the real limit; washboards in. A catamaran's GZ peaks as the windward hull lifts, then collapses: no heel warns you, reef for the gusts.
RCD category
Designed for, at the plate's crew
A ocean
Beyond force 8 and Hs 4 m
B offshore
Up to force 8, Hs 4 m
C inshore
Up to force 6, Hs 2 m
4 · Weights
Stability you change yourself
Why are half-full tanks bad for a yacht's stability?
Sloshing liquid raises G like weight aloft: the free surface loss grows with the tank's width cubed, and every slack tank adds its own. A centreline division cuts it to a quarter; run tanks down one at a time.
Adding or raising weight moves G towards it: rise of G = weight × height ÷ displacement; GZ falls by that rise × sin θ. A weight on a crane acts at the crane head. Before a gale, heavy gear low and central.
Launching the tender
Δ 40 t, GM 0.80 m; tender 1.0 t
Lifted 4.0 m: G rises 1.0 × 4.0 ÷ 40 = 0.10 m
GM now 0.70 m
Swung 2.8 m outboard: G shifts 0.07 m tan θ = 0.07 ÷ 0.70: list about 6°
Sloshing liquid raises G like weight aloft: the free surface loss goes with the tank's width cubed, so a centreline division cuts it to a quarter, and every slack tank adds its own. Run tanks down one at a time; stop flooding spreading. A slow, lazy roll means GM has fallen.
5 · The diesel
Fuel, air, cooling
How do I bleed air out of a yacht's diesel fuel system?
Bleed along the fuel path, in order: primary filter and separator, lift pump, fine filter, injection pump. Then crack each injector union while cranking.
A diesel fires on the heat of compression: fast cranking, sound rings, air, clean fuel. A bad seal on the suction side draws in air without a drip: runs, then falters. Rising filter vacuum: sludge and diesel bug stirred up by the sea; switch filters, polish the fuel.
Fuel path, and the bleeding order
Tank → primary filter and separator → lift pump → fine filter → injection pump → injectors
Bleed in that order, then crack each injector union while cranking
Smoke
Means
Black
Too much fuel for the air: overload, shut vents
Blue
Burning oil: rings, turbo seals
White
Sweet: coolant, head gasket; grey after fuelling: water
Strong flow yet overheating: scaled exchanger tubes; a keel cooler fouls outside. The anti-siphon valve keeps the sea out of an engine below the waterline; locked solid with milky oil, turn it by hand, never the starter. Service by hours or months, whichever first.
6 · Drive train
Gearbox, propeller, speed
My engine never reaches rated revs at full throttle: what's wrong with the propeller?
Too much pitch: the propeller is overpropped. If it reaches rated revs already at part throttle, it has too little pitch.
A gearbox slipping, hot and smelling burnt: low oil; top up, find the leak. Vibration growing over a season with a weeping stern gland: a worn cutless bearing; align the engine afloat. A saildrive's diaphragm alone keeps the sea out: renew it at the maker's interval. One engine dead, the bow swings towards it: counter-rudder, moderate revs.
Vibration, falling revs, black smoke after fishing gear: a fouled propeller; stop, clear it by daylight. Rated revs at part throttle: too little pitch; never reached at full throttle: too much. Hull speed ≈ 1.34 × √LWL in feet. Litres per mile = litres per hour ÷ knots: a planing yacht goes furthest slow, below the hump.
7 · Electrics
Batteries, generator, anodes
How big a battery bank do I need for my daily amp-hours?
Twice the day's amp-hours for a lead-acid or AGM bank, used only to half charge. Lithium gives about 80 %, but its BMS can cut off mid-charge: protect the alternator.
Add up the day's amp-hours: a lead-acid or AGM bank, used only to half charge, needs twice that; lithium gives about 80 %, but its BMS can cut off mid-charge: protect the alternator. Charging runs bulk, absorption, float; an hour a day never finishes absorption and the bank sulphates. A shunt monitor drifts until a full charge resyncs it: trust the resting voltage.
Motors start on three times their running load: shed loads. A generator run nearly unloaded glazes its bores: load it. Rising salinity with clean prefilters: a fouled membrane; unused, flush or pickle it. A galvanic isolator in the shore earth stops the pontoon eating your anodes.
Water
Anode
Sea
Zinc or aluminium alloy
Brackish
Aluminium alloy
Fresh
Magnesium
Practice questions
Questions from this topic, as the app asks them. Open one to see the answer.
Haul-out after six years afloat
Blisters 5–15 mm, below the waterline only
Pricked, they weep a sour, vinegary liquid
Topsides and deck unaffected
You haul out a 14 m GRP yacht after six years afloat and find this below the waterline. What is it?
Osmosis — water in the laminate behind the gelcoat
Galvanic corrosion eating into the laminate
Delamination of a balsa core under the skin
Air trapped under the last coat of antifouling
Show the answer
A. Osmosis — water in the laminate behind the gelcoat
Water diffuses through the gelcoat and reacts with the resin: acid fluid in blisters below the waterline is osmosis.
All 100 questions of this topic are in the app: practice by topic with the reason after every answer, your mistakes brought back, and a timed mock exam. Free, no sign-up, in the browser or in Telegram.
