Haul-Out Tech 1

Before you start

How this course works

The practical course for what lives under the waterline — corrosion, through-hulls and bottom paint. Enough ABYC to know the difference between galvanic and stray-current corrosion and how to stop it, plus the hands-on skills: measure a through-hull right, remove and replace it, service the seacocks, and read the paint. Each chapter ends in a quick check; an 80% final unlocks your certificate.

Chapter 1

🚤Why Underwater Metals Sink Boats

The haul-out is where you catch the thing that puts boats on the bottom.

Before any theory, the reason this course exists. The data on sinkings is blunt: for every boat that sinks at sea, about four sink right at the dock — quietly, while nobody is aboard. And when you look at why dockside boats sink, the water almost always comes in below the waterline through a fitting.

This is the practical ESI manual for the metal and plastic that lives under the waterline: corrosion, through-hulls, and bottom-paint inspection. It covers enough ABYC corrosion theory to explain galvanic corrosion, DC and AC stray current, galvanic isolators and isolation transformers, and the US-vs-European bonding difference — but it is not the full ABYC corrosion-specialist course. When a job goes past what is here, you will know to escalate (Chapter 13).

Sources: BoatUS Marine Insurance / Seaworthy claim studies (≈4 boats sink at the dock for every 1 at sea; ~50% of dockside sinkings begin at an underwater fitting, stuffing boxes leading); ABYC framing.

Chapter 2

🧪The Galvanic Series & the Metals on a Boat

Which metal eats which — and why stainless is a trap below the waterline.

Put two different metals in seawater and connect them, and you have built a battery. The galvanic series ranks metals from active (anodic / least noble) to noble (cathodic). The rule is simple: the more active metal corrodes to protect the more noble one, and the further apart they sit on the series, the faster it happens.

Stainless steel is confusing because it lives in two places on the series. In clean, oxygen-rich, moving water it is passive (a thin oxide film) and very noble. But starve it of oxygen — buried in a crevice, packed in a stuffing box, under marine growth, or inside a through-hull — and the film breaks down. The metal goes active and suffers crevice corrosion and pitting, often hidden, often without warning. That is why stainless is the wrong choice for below-waterline through-hulls and seacocks.

Sources: standard marine galvanic series (ABYC / Steve D’Antonio / Practical Sailor); stainless active-vs-passive crevice-corrosion references.

Chapter 3

⚡Galvanic Corrosion & the Anodes That Stop It

The slow, normal corrosion every boat fights — and the zincs that win the fight.

Galvanic corrosion is the everyday, low-voltage corrosion every boat lives with. It needs three things, and removing any one stops it:

1. Two different metals (e.g. a bronze through-hull and an aluminium saildrive),
2. An electrolyte connecting them (seawater — brackish and salt conduct best),
3. An electrical connection between them (the bonding wire, the shaft, or just both touching the same water).

Close that loop and the active metal gives up millivolts of potential and slowly dissolves. It is normal. We do not eliminate it — we redirect it onto a metal we are happy to lose.

A sacrificial anode is a lump of a more active metal bonded to the metals we want to keep. Because it is the most active thing in the circuit, it corrodes first and the protected metals do not. Everyone calls them “zincs,” but the metal depends on the water:

• Replace at ~50% wasted — do not run them to nothing.
• Never paint an anode and never let bottom paint creep onto it — paint insulates it and it stops protecting.
• Clean metal-to-metal contact at the stud; a corroded or loose connection means no protection even with a fat anode bolted on.
• Shaft, rudder, trim tabs, saildrive, and a hull/bonding anode each protect their own metals — confirm each is present and connected.

Sources: BoatZincs / Performance Metals anode-by-water guidance; ABYC E-2-2025 cathodic protection; standard marine anode practice (50% rule, never paint, clean contact).

Chapter 4

🔌Stray-Current Corrosion — DC & AC

The fast killer: not millivolts over years, but volts over days.

Galvanic corrosion is slow and measured in millivolts. Stray-current corrosion is the abnormal one — driven by stray volts from the electrical system or the dock — and it is brutally fast. A stray-current fault can eat a through-hull or a prop in days to weeks, not years. When you see rapid, localized metal loss, think stray current.

Somewhere on board, DC current is escaping its wire and taking a shortcut through the water. A classic source: a bilge-pump or accessory wire with chafed insulation sitting in bilge water, or a corroded connection, leaking current into the bonding system. The current flows out through one underwater metal, through the water, and back — and the metal where the current leaves into the water erodes ferociously.

• Pattern: very fast, very localized loss; anodes vanish in weeks; one fitting destroyed while its neighbours are fine.
• Hunt it: a multimeter / clamp meter to find current on the bonding wire or a circuit that should be dead; switch loads on and off and watch for leakage. Isolate the faulty circuit and the corrosion stops.

The moment you plug into shore power, the green safety ground wire connects your boat’s underwater metals to the dock and to every other boat on it. Two things ride that shared wire:

• A neighbour’s stray current can travel down the common ground and corrode your metals — the boat three slips away really can eat your props.
• AC leakage current from a fault on board or on the dock. AC’s slower at eating metal than DC, but it is the lethal one for people: AC leakage into the water is the cause of Electric Shock Drowning (ESD), which is why an ELCI/RCD belongs on the AC system.

Sources: ABYC E-11-2025 (AC/DC), ABYC corrosion guidance; Steve D’Antonio / BoatUS on stray-current and ESD; marine electrical references.

Chapter 5

🛡️Shore-Power Protection — Galvanic Isolators vs Isolation Transformers

Two ways to break the dockside loop. One cheap, one complete.

From Chapter 4: the green shore ground links your metals to the whole marina, carrying galvanic current and stray current onto your boat. You cannot just cut the safety ground — it has to stay connected for AC fault protection. So we interrupt the corrosion path while keeping the safety path. Two devices do this.

A galvanic isolator sits in the green ground wire. It uses diodes that will not conduct below about 1.2–1.4 V — so the small galvanic voltages (millivolts to ~1 V) that drive dockside galvanic current are blocked, while a real AC fault (which easily exceeds that threshold) still passes through to keep the safety ground working.

• Blocks: low-voltage galvanic DC coming up the shore ground.
• Still passes: AC fault current (safety preserved).
• Must be a fail-safe type (passes fault current even if it fails) and properly sized — ABYC A-28.
• Cheap, light, easy. Every shore-power boat should have at least this.

An isolation transformer takes shore AC into one coil and induces it into a second, separate coil that feeds the boat. There is no metallic connection between dock wiring and boat wiring — the energy crosses magnetically. That single break does it all:

• Completely isolates the boat’s ground from the dock ground → kills galvanic and stray-current paths from shore.
• Best defence against AC leakage / ESD — the boat makes its own ground reference.
• Also fixes reverse-polarity and dirty-dock issues.
• Heavy and expensive — but complete. The right call for metal boats, persistent corrosion, or owners who want maximum safety.

Sources: ABYC A-28-2024 (galvanic isolators) & E-11 (AC systems / isolation transformers); Victron / Blue Sea / ProMariner technical notes on isolators vs transformers.

Chapter 6

🌍Bonding — US (ABYC) vs European (ISO)

Two philosophies under the floorboards. Know which boat you are on.

A bonding system is a heavy green wire that ties the underwater metals — through-hulls, seacocks, shaft, rudder, struts, engine — together, and to the sacrificial anodes (and, per the standard, to DC negative/ground). It exists so all those metals share cathodic protection from the anodes and so stray current and lightning have a defined path. Here is the part that surprises techs: the US and Europe disagree on whether to bond at all.

US boats built to ABYC generally bond the underwater metals to a common bonding system tied to anodes.

• Pro: the anodes protect all the bonded metals; one cathodic system; a defined path for fault and lightning current.
• Con: every metal is now electrically joined — so a single stray-current fault or a galvanic mismatch can reach all of them through the bond, and a wet wiring fault can drive current through the bonded fittings.

Many European builders (think Beneteau, Jeanneau, and others, to ISO 13297 practice) leave the underwater through-hulls UNBONDED — each bronze fitting stands electrically alone, relying on good-quality bronze’s own corrosion resistance, while the engine/shaft/saildrive get their own anode.

• Pro: no shared wire means no shared path for stray current — a fault on one circuit cannot feed every through-hull; an isolated bronze seacock in clean water simply corrodes very slowly.
• Con: those isolated fittings get no cathodic protection from anodes and have no defined fault path.

Sources: ABYC E-2-2025/E-11 bonding; ISO 13297; Steve D’Antonio and Practical Sailor on the “to bond or not to bond” US-vs-Europe difference.

Chapter 7

📟Measuring Protection — the Reference Electrode

Stop guessing whether a boat is protected. Read it in millivolts.

You can argue about anodes all day, or you can measure. A silver/silver-chloride (Ag/AgCl) reference electrode hung in the water, with a multimeter on millivolts connected to the bonding system, tells you the boat’s hull potential — exactly how well-protected the underwater metals are.

Under-protected (not negative enough): the metals are still corroding — add/parallel anodes or fix the bond. Over-protected (too negative, past about −1100 mV, especially on aluminium and wood): you start stripping paint, generating hydrogen, and causing alkali damage. Over-zincing is a real failure mode — more anodes is not always better.

• Reference electrode + multimeter (mV): hull potential / cathodic protection level.
• Multimeter / clamp meter: hunt DC stray current — look for current on a circuit or bonding wire that should be dead; switch loads and watch.
• Record the readings on the haul-out sheet so next season you can compare.

Sources: BoatZincs Corrosion Reference Electrode guide; Steve D’Antonio reference-cell testing; ABYC E-2-2025 protected-potential ranges (Ag/AgCl: ali −950/−1100 mV, fibreglass −750/−1100 mV; 200 mV shift; unplug shore cord to test).

Chapter 8

🚰Through-Hull Types — Bronze, Marelon, and What to Condemn

Real bronze, composite, and the dezincification test that saves boats.

Proper silicon or aluminium bronze is the classic below-waterline metal. The danger is the look-alike: high-zinc yellow brass plumbing valves and cheap import fittings. In seawater the zinc leaches out of brass — dezincification — leaving a porous, weak, copper-coloured shell that crumbles and can fail without warning.

Marelon (Forespar) is a glass-reinforced polymer composite, not a metal. Forespar describe it as offering complete freedom from corrosion and the ravages of electrolysis, above and below the waterline. For ESI that means a lot:

• Never dezincifies, never corrodes galvanically, draws nothing from the anodes, needs no bonding for corrosion.
• Immune to the stray-current and shore-ground problems that eat metal seacocks.
• Trade-off: it is a serviced part — it must be exercised and lubricated on schedule (Chapter 12), and only with the correct non-petroleum lubricant. Threads must match the system (Chapter 9).

Sources: Forespar Marelon material data; Steve D’Antonio / Practical Sailor on dezincification and seacock vs ball-valve vs gate-valve; ABYC H-27-2021 seacock practice.

Chapter 9

📏MEASURE THROUGH-HULLS CORRECTLY

If you remember one chapter, make it this one. Measure the bore and the thread — not the outside.

More wrong parts get ordered — and more seacocks leak or shear — from bad measuring than from anything else under the waterline. Through-hull sizing is genuinely counter-intuitive, so slow down and do it right.

A through-hull or seacock is named by its nominal bore / pipe size — the hole the water flows through — not by the outside diameter of the threads. A “1-inch” through-hull has roughly a 1" bore / 1" pipe thread, but its thread OD measures larger (around 1.3"). Measure the OD with calipers, call it “1¼-inch,” order that, and the part is wrong.

Never mix NPS and NPT. A tapered NPT valve on a straight NPS through-hull engages only a turn or two of thread — it looks tight, then leaks, strips, or shears off below the waterline. European boats are frequently BSP/metric; measure and match the standard, do not force a US fitting on.

1. Bore / nominal size — measure the inside diameter of the flow passage; that is the size you order.
2. Thread OD + pitch — calipers on the thread diameter, a thread pitch gauge on the threads per inch (or mm). Identify NPS vs NPT vs BSP.
3. Hull thickness (+ backing block) — this sets the through-hull barrel length you need so the nut and seacock land properly.
4. Hose ID at the barb — match it so you do not choke flow, and so the hose and double clamps fit.
5. Write it all down before ordering; better still, bring the old part.

Sources: Groco / Forespar / Marelon sizing data and ABYC H-27-2021; Compass Marine and Steve D’Antonio on through-hull sizing by bore and NPS-vs-NPT thread matching.

Chapter 10

🔧Removing & Installing Through-Hulls

Heat the stuck ones out, prep the hole for a real bond, and bed it in the right Sikaflex.

A through-hull is held by its bedding, and the fastest way to release bedding is to soften it with heat. ESI’s preferred method on a stuck fitting is to heat it — not to fight it cold or jump straight to cutting.

• From inside first: back off the seacock or the backing nut while the mushroom head is held outside.
• Hold the mushroom: a step-plug / through-hull (“step”) wrench or T-handle engages the slots in the mushroom from outside while you turn the nut inside — a two-person job.
• Heat the stuck ones (preferred): warm the fitting with a heat gun (or carefully a small torch on bare metal) to soften the bedding, then break it free and back it out. Heat the metal fitting, work in passes, and keep heat off the gelcoat and away from fuel/hoses. Heat releases the bond with far less collateral damage than grinding.
• Cut only as a last resort: if heat will not free it, cut the barrel with an oscillating multi-tool or sawzall, split the nut with a chisel, or cut the mushroom head off outside and push the stub through.
• Protect the boat: never gouge the laminate or crack the gelcoat; support the area.

The new fitting is only as good as the surface you bond it to. Take the hole back to clean, sound, dry laminate and give the adhesive somewhere to grip.

• Clean back to sound glass: remove all old bedding and any soft/wet/delaminated material; grind/sand the bonding faces bright; solvent-wipe clean and let it dry — Sika wants the surface clean, dry and free of grease, oil and dust.
• Rout a small bevel (chamfer) around the hole edge in the glass. A light bevel routed into the laminate at the hole rim increases the bonding area and gives the Sikaflex a fillet to key into — a bigger, stronger, better-sealed adhesive footprint than a flat square edge. This is the ESI standard prep for a new through-hull.
• Cored hull? Over-bore, dig the core back from the hole and fill the ring with thickened epoxy, then re-drill to size — so the fastener and fitting clamp on solid laminate, not wet core. Then bevel as above.

ESI beds below-waterline through-hulls in Sika Sikaflex polyurethane. Pick the grade to the job:

• Surface: clean, dry, grease-free; check the Sika pre-treatment chart — some substrates need a Sika primer/activator for a lasting bond.
• Apply: lay a triangular bead for a uniform bondline, butter the bevel and the fitting, and seat it within the open time (292 skins ~40 min / open ~30 min at 23°C) — never join adhesive that has already skinned.
• Set it: snug the fitting to squeeze out a full bead all round, tool the squeeze-out, and let it cure before splash (PU cures with moisture; do not fully immerse 291 until cured).

1. Backing block: through-bolt the flanged seacock to a solid backing block (fibreglass / G10 or glassed-in hardwood — never plain plywood that wicks and rots).
2. Align: set the seacock so the handle swings fully open/shut within reach.
3. Bond per the boat: connect or deliberately leave isolated per the boat’s bonding philosophy (Ch 6) — do not change it without reason.
4. Hose & plug: double-clamp the hose with all-stainless clamps and tie a soft wood plug at the fitting (hose & clamp detail in Chapter 11).

Sources: Sika Sikaflex-291 & -292i product data sheets (291 above/below waterline; 292i structural, seawater-resistant, rated for through-hull fittings below the waterline; clean/dry/grease-free prep, triangular bead, open/skin time); ABYC H-27-2021; Compass Marine / Practical Sailor through-hull replacement (clean to sound glass, bevel for bond area, cored-hull epoxy ring).

Chapter 11

🧵Hoses, Clamps & the Below-Waterline Connection

Half of all dockside sinkings start here — at a hose, a clamp, or a stuffing box.

Back to the data from Chapter 1: about half of all dockside sinkings start at an underwater fitting or hose, with stuffing boxes leading. The seacock is only half the connection — the hose and the clamps are the other half, and they are a wear item. This is pure sinking-prevention.

Hose has a service life: it hardens, cracks and lets go. Check every below-waterline hose at haul-out.

• Hardening / brittleness — good hose is firm but supple; glazed, stiff or crazed hose is end-of-life.
• Cracks & checking — especially at bends and at the clamp lines.
• Soft / spongy / bulging spots — delamination or saturation inside; condemn.
• Chafe, kinks, collapse — anywhere it rubs, bends tight, or has flattened.
• Weeping / staining / odour — seepage at a joint, or permeation smell on sanitation hose.
• Squeeze test — it should feel firm and spring back; brittle or mushy = replace.

Use marine-grade, reinforced hose rated for the application — never thin clear vinyl below the waterline.

• Raw-water intake (suction) hose must be wire-reinforced so the pump’s vacuum cannot collapse it.
• Match the hose to its service — raw-water/intake, sanitation, bilge and exhaust each have their own marine hose (sanitation is permeation-rated; exhaust is heat-rated, below).
• Size the hose ID to the barb, and support runs so they cannot chafe or sag onto something hot.

A clamp is only as good as its worst part. The ESI standard is the all-316L stainless, non-perforated, worm-drive band clamp — we use PYI (the same solid-band design as AWAB/ABA).

316 vs 304/18-8 — the screw is the tell. Most hardware-store “stainless” clamps are 304 (18-8), and the worm screw is often the cheapest part of the lot. In salt water 304/18-8 has far less resistance to chloride pitting and crevice corrosion than 316, so the screw rusts, streaks the hull and seizes solid exactly when you need to back it off. Below the waterline ESI uses full 316L — band and screw (PYI/AWAB). Quality 316 marine clamps are non-magnetic and marked; do not trust the word “stainless” on the bag.

Every below-waterline hose connection gets two clamps wherever the barb is long enough to land both fully.

• Both clamps fully on the hose-over-barb, behind the barb bead — never on the taper or hanging off the end.
• Stagger the screw housings ~180° apart so the hose is squeezed evenly all round.
• Push the hose fully home past the barb shoulder before clamping.
• If the barb is too short for two clamps to bite the hose-over-barb, fit a longer barb/adapter — do not fake it with one clamp on the taper.

Wet-exhaust hose is large, soft and runs hot. A T-bolt clamp (constant-torque, solid band) delivers high, even clamping force around big soft exhaust hose that a small worm band cannot — so ESI uses T-bolt clamps on exhaust connections.

• Double-clamp the exhaust at the mixing elbow / riser and at the water-lift muffler (ABYC calls for two clamps on exhaust).
• Use quality all-316 T-bolt clamps — cheap T-bolts can carry a spot-welded or lower-grade part that fails; inspect the band and trunnion.
• Exhaust hose hardens and cracks with heat and age — look for soft/charred spots and replace on a schedule; silicone exhaust hose suits higher-temp runs.

• Torque it right. Snug the clamp to seal and grip without cutting or distorting the hose — follow the maker’s torque (PYI/AWAB publish a figure; small-band marine clamps land around a few N·m). Use a nut driver or torque driver — never an impact gun.
• Do not over-torque: too much crushes and cuts soft hose, distorts or strips the worm, and can shear the band — and a hose cut under a tight clamp still leaks.
• Re-check after the hose takes a set: retighten after the first heat/run cycle and at every haul-out; a clamp that was tight last year backs off as the hose relaxes.
• Mind the sharp band end. A cut clamp band is a blade, and seacock hoses live in dark bilges you reach into blind. Fit clamp end protectors / band-end caps (or use rolled-edge clamps) to cover the cut end, and orient the screw housing so a reaching hand meets the cap, not the edge.

Sources: PYI Inc. 316L non-perforated solid-band hose clamps; AWAB/ABA all-316 clamp data & Practical Sailor long-term clamp test; Compass Marine double-clamping practice; ABYC H-27-2021 (seacocks/hose) & P-1 (exhaust); SAE J2006 wet-exhaust hose.

Chapter 12

🛢️Lubricating & Servicing Seacocks — Marelon & Bronze

A seacock you cannot turn is no safety device at all.

Every seacock has one job in an emergency: shut. A seized one is useless. So the most important maintenance under the waterline is dead simple — exercise every valve through its full open/close travel, regularly, and lubricate it on a schedule. Forespar’s guidance is a good universal rule: work the handle through the full path every ~30 days, and do a proper lube service twice a year.

• Bronze ball valves: mainly exercise them; at haul-out you can swab a waterproof grease (water-pump/winch grease) or LanoCote on the ball.
• Tapered-plug seacocks (Groco/Wilcox-style): disassemble at haul-out, clean the plug and body, grease the plug with waterproof grease, reassemble and set the plug tension so it turns firmly but fully.

Marelon is corrosion-proof, but the moving parts still need lubrication and exercise. The critical rule is what you lubricate it with:

• Use Forespar MareLube — a non-petroleum lubricant (PTFE micro-powder in a biodegradable vegetable-oil base) made for Marelon ball valves and seals. Forespar also accept waterproof grease or LanoCote swabbed on the ball.
• Cadence (Forespar): work the handle full open/close every 30 days; full lube service twice yearly, ideally one at haul-out.

Sources: Forespar Marelon Valve & Seacock Maintenance tech tip and MareLube product data (non-petroleum PTFE, exercise every 30 days, lubricate twice yearly); Groco tapered-plug service; standard waterproof-grease/LanoCote practice.

Chapter 13

🎨Bottom-Paint Inspection & “Flowering”

Read the paint and the boat tells you exactly where the trouble is.

Most antifouling is copper-based (cuprous oxide). Beyond keeping growth off, the paint is a free corrosion survey — if you know how to read it. The headline tell is flowering.

Flowering is paint that lifts, bubbles, or blisters in a star/halo radiating out from a through-hull, prop, strut, or fitting. It is the boat flagging an electrochemical reaction at that spot — corrosion or stray current. At a cathode the reaction makes gas and alkali that push the paint off; at an anode the metal itself is going. A flower of lifted paint around a bonded fitting often points to current or over-protection; around an isolated fitting it can mean a developing corrosion cell.

Copper antifouling is noble. Put it in direct contact with a bare active metal — an aluminium outdrive or saildrive, a steel hull — and you have built a galvanic cell that eats the metal fast.

• Never put copper antifouling on aluminium drives/legs. Use an aluminium-safe, copper-free antifouling and a proper barrier/primer coat.
• Keep paint off the anodes — a painted anode is a dead anode.
• Maintain the barrier coat so copper paint is not sitting on bare metal anywhere it should not be.

Sources: Practical Sailor / Steve D’Antonio on paint blistering & flowering as corrosion/stray-current signals; antifouling makers (Pettit/Interlux) on copper paint vs aluminium and barrier coats.

Chapter 14

🧭Diagnose, Document & Escalate

Put it together into a repeatable haul-out survey — and know your limit.

1. Read the paint & anodes at the wash: flowering, halos, blisters; anode % wasted and actually attached (Ch 13, 3).
2. Every through-hull: bronze colour (gold vs pink), dezincification scratch/tap test, seacock operation, backing & support, hose condition & double clamps, bonding wire intact or deliberately isolated (Ch 8, 10, 11, 6).
3. Shaft / prop / rudder / drive: pitting, grooving, crevice attack on stainless (Ch 2, 4).
4. In the water: reference-electrode hull potential with the shore cord unplugged; multimeter/clamp leakage hunt for stray current (Ch 7, 4).

Condemn (replace now, do not “watch”): dezincified/pink bronze; a cracked or weeping seacock; a seized valve; a fitting more than ~50% wasted; stainless below the waterline showing crevice attack.

Escalate to a corrosion specialist / ABYC corrosion-certified tech when: loss is persistent and unexplained; anodes burn through repeatedly; you suspect stray current you cannot isolate; the boat is metal-hulled or has an ICCP system; or the bonding is non-standard/mixed (Ch 6). That is where this practical course hands off to ABYC E-2-2025.

Sources: ABYC E-2-2025 / A-28 / E-11 / H-27; Steve D’Antonio and ABYC corrosion-survey practice; ESI haul-out standard.

Field reference

🛠️Common Problems & Fixes

Field-tested: the symptom, the usual cause, and the fix. Always diagnose before you replace parts.

Final certification

🎓Final exam

A short check ran at the end of each chapter. Pass this final at 80% to unlock your certificate.

ESSENTIAL SHIPYARD UNIVERSITY

Certificate of Completion

This certifies that

has successfully completed Haul-Out Tech 1 — underwater metals, through-hulls & bottom-paint inspection — and passed the final examination to the ESI standard.

Jason Knott

President

Essential Shipyard Industries

★CERTIFIED

Date Issued

Essential Shipyard University