Quick answer: A galvanic isolator blocks the small DC voltages that drive galvanic corrosion at the dock and costs a few hundred dollars. An isolation transformer breaks the electrical connection to shore entirely — stopping galvanic corrosion, stray-current electrolysis, and reverse-polarity problems — and costs several thousand. An ISO-Boost transformer adds voltage compensation for older marinas where shore voltage sags under load. For boats kept in a marina more than a few weeks a year, isolation is not optional.
Every season we pull boats with $5,000 to $20,000 worth of stray-current and galvanic damage that never had to happen. Pitted bronze, sacrificed shafts, riddled outdrives, and in the worst cases, fiberglass blistering from electrolysis-driven coatings failure. The common factor: shore power was connected for years without proper isolation between the boat's grounding system and dockside utility ground.
If you spend time at a marina — any marina — you need to understand three pieces of equipment that sit between your boat and the dock pedestal: galvanic isolators, isolation transformers, and the often-overlooked ISO-Boost transformer. Each addresses a different failure mode. Each has a place. Choosing the right one (or the right combination) can be the difference between a boat that holds its value and one that quietly dissolves a few hundred dollars at a time.
The hidden problem: why marinas eat boats
When two dissimilar metals are connected by an electrolyte — say, your bronze through-hulls and a neighbor's aluminum outdrive, both sitting in saltwater — a small voltage develops between them. Electrons want to flow. In open water that's not a problem because nothing connects the two boats. At a dock, your shore power green wire (safety ground) is bonded to your engine, your engine to your bonding system, your bonding system to your underwater metals. Your neighbor's boat has the same path. The dock pedestal ties both green wires together.
Now there's a complete galvanic circuit running through the saltwater between your boats, and through the dock's grounding system. The less noble metal — usually aluminum or unprotected steel — becomes the anode and starts giving up ions. If that's your boat, your shaft zincs disappear in weeks, your prop turns pink, and your through-hulls develop pinholes. If it's your neighbor's boat, your shaft is the cathode and you're fine — until the new boat shows up next slip over.
There's a second problem layered on top: stray AC current. Faulty wiring on someone else's boat, a worn shore cord, or a problem at the pedestal itself can put leakage current onto the green wire. That current looks for a path back to the utility neutral, and one of those paths is through the water around your hull. The result is accelerated electrolysis — not garden-variety galvanic corrosion, but the much faster kind driven by an actual voltage source. Aluminum components can be reduced to powder in a single season.
These are not theoretical risks on the Chesapeake. Brackish water makes the bay slightly less aggressive than full-salt environments like the Florida Keys, but the long boating season and the high density of older boats with marginal wiring at municipal marinas more than makes up for it.
Three lines of defense
The solution is to break the path that allows DC galvanic currents (and stray AC currents) to flow between your boat and the rest of the marina, while still keeping the safety ground intact for fault protection. There are three approaches, and they're not interchangeable:
- Galvanic isolator — blocks low-voltage DC up to about 1.4 V while passing AC fault current. Inexpensive, code-friendly, the right answer for most coastal boats.
- Isolation transformer — fully separates your boat's electrical system from shore via magnetic coupling. No metallic connection at all. The gold standard, especially for aluminum-hull boats, metal trawlers, and yachts kept in marinas with questionable wiring.
- ISO-Boost transformer — an isolation transformer with the added ability to boost low dockside voltage. Critical for older marinas where 30 A pedestals routinely sag to 105 V or below under load.
Galvanic isolators: the baseline
A galvanic isolator is a passive device — typically a pair of diodes in series-opposed configuration, sometimes paired with a capacitor — that sits in the shore-power green wire between the dockside pedestal and your boat's grounding bus. Below about 1.0 to 1.4 V, it blocks DC current entirely. Above that, it conducts normally so an AC fault can clear a breaker the way it's supposed to.
ProMariner ProSafe is the dominant brand in this segment for good reason. The ProSafe FS (Fail-Safe) line — including the ProSafe FS 30, FS 50, and FS 100 — meets the ABYC A-28 standard and includes built-in fault monitoring that confirms the isolator is still working. That last part matters. A galvanic isolator that has failed shorted offers no protection at all, and one that has failed open opens your boat's safety ground. Both failures are silent without monitoring. ProMariner units include LED status indicators and can be wired into an alarm panel.
Sizing is straightforward: match the rated current of your shore inlet. 30 A inlet, FS 30. 50 A inlet, FS 50. 100 A inlet (twin 50 A or a single 100 A feeder), FS 100. Galvanic isolators are inexpensive — typically under $400 — and they install in the engine room or electrical compartment on the shore-side of the main breaker, in series with the green wire only.
The limitation: a galvanic isolator does nothing for stray AC current. If a neighbor's bilge pump is leaking current to ground, that current still flows through your green wire, into your bonding system, and out into the water. The diodes only block the small DC galvanic potential. For boats kept in busy marinas with mixed-age neighbors, or boats with aluminum components below the waterline, this is often insufficient.
Isolation transformers: complete separation
An isolation transformer takes the shore-power input and re-creates it on the boat side magnetically. There is no copper path between dockside ground and boat ground. The shore-side neutral and ground stay on the dock; the boat-side neutral is freshly bonded to the boat's ground at the transformer. Stray AC current has nowhere to go because there's no longer a circuit. Galvanic DC current has nowhere to go for the same reason.
ASEA Power Systems and ANG (Atlas Marine Systems) are the two specialist names on the high end of this market. ASEA is widely specified on production trawlers and semi-displacement cruisers — their 30 A and 50 A units are sized to fit in typical engine-room spaces and operate quietly. ANG is the choice on larger yachts and commercial vessels where 100 A and dual-input configurations are needed; their units handle the heavy continuous loads of full-time liveaboards and yachts with electric galleys and air conditioning.
Beyond galvanic and stray-current isolation, isolation transformers solve two other problems:
- Polarity correction. European 230 V wiring, reverse-polarity outlets at older marinas, and miswired pedestals all become non-issues. The transformer's output is correctly polarized regardless of input.
- Voltage conversion. A 230 V to 120 V isolation transformer lets a U.S.-wired boat plug into European marinas (or vice versa). For boats that cross the Atlantic, this is enabling equipment, not a nice-to-have.
The trade-offs are real: isolation transformers are heavy (a 50 A unit is 80 to 120 pounds), expensive (roughly $2,500 to $6,000 installed depending on size and brand), and they generate heat that has to be ventilated. They also have an inrush current at connection that can trip pedestal breakers if not properly soft-started — both ASEA and ANG units include inrush limiting that addresses this.
ISO-Boost: when the dockside voltage is the problem
A specific subset of isolation transformers — the ISO-Boost — adds tap-changing capability to compensate for low shore voltage. This matters on the Chesapeake more than people realize. Older municipal marinas, marinas at the end of a long utility feed, and marinas where every slip is running air conditioning on a summer afternoon routinely show 105 V or less at the 30 A pedestal. Plenty of boats see 95 V or below.
Low voltage is harder on equipment than high voltage. Inductive loads — compressors, motors, transformers themselves — draw more current to make up the power deficit. A 12,000 BTU air conditioner that is happy at 120 V can pull 30 percent more current at 105 V, heating windings and stressing capacitors. Battery chargers either undercharge or thermally cycle. Inverter-chargers sometimes refuse to accept the shore input at all.
An ISO-Boost transformer adds a 10 percent boost tap (and sometimes a 5 percent buck tap for high-voltage conditions). When the dock is reading 105 V, the boat sees 115 V. When the dock sags to 95 V, the boat still sees a usable 104 V. Combined with the isolation function, the result is one device that protects against galvanic corrosion, stray-current electrolysis, polarity errors, and voltage sag — all of which are common in real-world dockage.
ANG offers ISO-Boost variants across multiple amperage ratings, with 30 A, 50 A, and 100 A configurations the most commonly specified. For boats with full-time air conditioning kept in older marinas, this is the configuration we recommend most often. ASEA's equivalent product line is similar, with slightly different tap-changing logic.
Choosing the right approach
The honest answer is "it depends on the boat and the slip." Some guidance:
- Daysailer or weekender, 30 A inlet, transient docking — a ProMariner ProSafe FS galvanic isolator is appropriate. Inexpensive, code-compliant, addresses the most common failure mode.
- Cruising sailboat or motoryacht, 30/50 A inlet, kept in a slip 6+ months a year — isolation transformer (ASEA 30 A or 50 A). Pays for itself the first time you avoid replacing shaft hardware or refitting a fuel tank.
- Trawler, motoryacht, or aluminum-hull boat, 50 A or 100 A service, full-time liveaboard or long-term dockage in mixed-age marina — ISO-Boost transformer (ANG or ASEA). Handles voltage sag in addition to isolation.
- Boat with European or international cruising plans — isolation transformer with voltage conversion, full stop.
The decision is also influenced by space and weight. A sailboat under 40 feet rarely has the engine-room volume for an isolation transformer, and the weight penalty matters; a galvanic isolator is often the only practical choice. A 50-foot trawler with a dedicated electrical compartment has no such constraint.
ABYC compliance and what surveyors look for
ABYC standard A-28 covers galvanic isolators and isolation transformers in detail. The key requirements: galvanic isolators must be fail-safe (i.e., must fail to a state that maintains safety ground), must have monitoring, and must be rated for the inlet's full current. Isolation transformers must have appropriate inrush limiting, must bond the boat-side neutral to ground correctly, and must be installed with adequate ventilation.
Surveyors increasingly note the presence (or absence) of galvanic isolation in insurance surveys, and at least one major marine insurer now requires it on boats over 30 feet with permanent shore power. If you're refinancing or changing insurers, this can become a closing item.
On the Chesapeake specifically, we recommend isolation transformers on any boat kept at a public or municipal marina for more than a season at a time. The combination of brackish water, dense slip configurations, and a mix of well-maintained and poorly-maintained neighbors makes galvanic and stray-current exposure higher than the diode-only solution can handle.
When in doubt, measure
Before specifying any of this equipment, the right first step is a corrosion survey: a reference electrode and a calibrated voltmeter to measure the actual potential between your hull and the surrounding water at the slip you're in. The reading tells you whether you have a galvanic problem, a stray-current problem, both, or neither — and it tells you whether you need the basic galvanic isolator solution or the full transformer.
We carry ProMariner ProSafe galvanic isolators and order ASEA and ANG transformers (including ISO-Boost variants) on a per-boat basis after sizing and survey. Installation is straightforward but unforgiving of mistakes — the boat-side bonding has to be done exactly per the ABYC diagram or the transformer creates new problems instead of solving old ones. For the broader shore power picture — inlets, GFI breakers, AC distribution — see our marine shore power installation page.
Frequently Asked Questions
Do I need both a galvanic isolator and an isolation transformer?
No. An isolation transformer makes a galvanic isolator redundant, because the transformer provides complete electrical separation between the boat and shore. Most boats use one or the other based on the use case, not both.
Will a galvanic isolator stop electrolysis from a wiring fault on a neighboring boat?
No. Galvanic isolators only block low-voltage DC up to about 1.4 V. Stray AC current from a faulty neighbor still flows through your green wire and bonding system. For protection against stray AC current, you need an isolation transformer.
How often should a galvanic isolator be tested?
ABYC recommends annual testing. ProMariner ProSafe FS units self-monitor continuously and indicate status with an LED, but a manual test with a meter at haulout is still good practice.
Can I install an isolation transformer myself?
The transformer itself is a hardware install that an experienced DIY owner can handle, but the boat-side bonding and neutral connection must follow ABYC A-28 exactly. Errors here can create more corrosion than they prevent. We recommend professional installation or, at minimum, professional review of the wiring before energizing.
Why is my dockside voltage so low?
Long utility runs, undersized dock wiring, and high simultaneous load during summer air-conditioning season are the usual causes. ISO-Boost transformers compensate by boosting the boat-side voltage approximately 10 percent when the input sags.
What's the difference between ASEA and ANG isolation transformers?
Both are high-quality marine isolation transformers. ASEA Power Systems is widely specified on production trawlers and cruisers in the 30 A and 50 A ranges. ANG (Atlas Marine Systems) is more common on larger yachts and commercial vessels where 100 A and dual-input configurations are needed. Both offer ISO-Boost variants for voltage compensation.