There is a bundle of severed copper cables in the bilge of Crux, a 1994 Moody 44 now lying in the Dominican Republic. They once tied every large piece of metal on board into a lightning protection system fitted after the boat left the factory. The previous owner, an electrician with 40 years in the trade, cut them on purpose. The current owner, a South African sailor cruising the Caribbean with his family, credits those cuts for lightning missing them off Honduras. German boating magazine float and its writer Jens Brambusch built a two-part series, published on 18 September and 7 October, around the argument Crux stands for: should a yacht be grounded against lightning or left isolated?
Grounded or isolated: what each side believes
Why some owners cut the cables
Crux ran the 40 miles from Utila to Roatán in squalls, thunder and hail, with strikes hitting the water close by. "The passage was sheer horror," the owner told float. The boat was not hit. He keeps his own count: of 13 sailors he has met whose boats took a direct strike, eight sailed catamarans, four had grounded monohulls and one had an ungrounded boat. float itself notes that a count like this says little.
What the physics says
A strike begins with a stepped leader, a channel of charge working down from the cloud. High points send an upward leader to meet it, and when the two connect, the main discharge passes in microseconds. Whether a masthead launches that upward leader depends above all on how strongly the electric field gathers at its tip, and the sceptics say a connection to the water makes it stronger: a well-grounded boat often has less than one ohm of resistance to the sea, so charge reaches the masthead faster. Other experts think the effect is too small to count. Brambusch's conclusion: "Which approach you trust is more a question of philosophy than of physics."
float also cites experts who warn that bad grounding can sometimes be worse than none: a cable that is too thin, bent sharply or fixed to corroded terminals can melt and set the boat on fire.
The engineer who wants both
A German mechatronics engineer in his first Caribbean season aboard Serena, a Jeanneau Sun Odyssey 45, wants grounding because he fears a fist-sized hole blown through the fibreglass below the waterline. "There is no such thing as 100 per cent protection against a direct strike, grounded or not," he says. What worries him on a permanently grounded boat is galvanic corrosion between the keel, engine and metal fittings. His answer comes from fuel depots: a spark gap that insulates in normal conditions and conducts within nanoseconds when lightning strikes. Industrial units start at 80 euros, and he plans to try one on Serena.
Why no rule settles the question
Since 2020, ISO 10134 has no longer been an international standard: ISO withdrew the 2003 edition on 27 May 2020 and replaced it with ISO/TR 10134, a nine-page technical report on "established practices" for boats up to 24 metres. The American document, ABYC TE-4, is also a technical report, and its text says "complete protection from equipment damage or personal injury is not implied".
In practice, even new production yachts sometimes leave the factory with only the simplest protection, if any. Asked a list of questions by float, Hanse answered in one sentence: "We use a system from ELNA Hamburg, which the customer can choose from the optional extras price list."
The systems and what they cost
A classic system puts a thick stainless or copper rod on the masthead, above the aerials. The aluminium mast or a heavy copper cable carries the current to the keel bolts or a porous ground plate, and the rigging, engine, tanks and seacocks are bonded in.
| System | How it works | Price | What is promised |
|---|---|---|---|
| Grounding with bonding | Leads the strike from masthead to water | No published price; an extra on a Hanse | ABYC: complete protection not implied |
| Spark gap | Insulates normally, conducts during a strike | Industrial units from €80 | Used in industry; one owner plans to try it |
| Dinnteco DDCE, sold by ELNA | Bleeds off charge so a strike does not form, says the maker | €3,500–6,000 for a 42-foot (12.8 m) boat, per float | Pantaenius accepts it if properly installed |
| Forespar Lightning Master | Masthead brush that dissipates charge | Around €210 on average at EU and US retailers | Fewer direct strikes; full protection not promised |
Holger Flindt, head of claims at insurer Pantaenius, told float the company does not know how many boats carry DDCE. "But we do have one example where it didn't," he said. In that strike the system worked like ordinary grounding. "But there was still some damage."
What the insurers' numbers show
A ten-year claims analysis by BoatUS, published in 2015, puts the US average at 0.9 lightning claims per 1,000 boats a year. Sailing monohulls come in at 3.8, and boats of 40 to 65 feet (12 to 20 metres) at 6. BoatUS finds multihull sailboats almost twice as likely to claim as monohulls; UK and US insurers quoted by Yachting World say two to three times. Over 75% of claims came to less than 30% of the insured value, mostly electronics.
"Over the past 15 years, the total number of such loss events has tripled in our statistics," Jonas Ball of Pantaenius told Yachting World in 2020. Flindt told float that lightning now accounts for about 10% of insurers' total losses, and up to 50% in the Caribbean. The Mediterranean's share of Pantaenius lightning cases was 10–12% in 2020, up from under 5% a decade before. float counts up to 140 thunderstorm days a year in the tropics, about 15 on the Baltic and two to three times that in the Mediterranean.
Sometimes the consequences only come to light several months later. Intle Breeze, a Leopard catamaran of the Leopard 50 model built from 2017 to 2025, was hit at anchor off Puerto Barrios, Guatemala, in June 2025. Instruments, engines, pumps and the generator failed one after another, and after she was hauled out in Panama the damage list ran to 59 items. A year later and a few miles away, SV Cuba, a Beneteau Oceanis 461 Clipper, came out with 36 holes in the hull and its entire electrical system wrecked. Its policy did not cover lightning, and a crowdfunding appeal for 16,800 euros raised 10,000 euros more than its target.
What to do on board in a thunderstorm
The US National Weather Service advises staying in port when thunderstorms are forecast, and notes: "The vast majority of lightning injuries and deaths on boats occur on small boats with NO cabin." Advice from the NWS, BoatUS and Yachting World comes down to these steps:
- Head for a sheltered harbour early. If you are caught out, "drop anchor and get as low as possible".
- If you are in port, disconnect radar and aerial cables, switch off the batteries and unplug shore power.
- Put handheld devices in the oven or microwave, which works as a Faraday cage.
- Stay in the cabin, away from the mast, chainplates, engine and steering pedestal.
- On deck, keep your hands off the wheel, guardrails and rigging. On an open boat, crouch in the middle.
- "Stay off the radio unless it is an emergency!"
- Wait 30 minutes after the last thunder, then check the electrics and compasses.
Someone struck by lightning carries no charge, so start CPR straight away. After a strike, check the policy as well as the boat: Flindt says a direct hit always leaves traces, while indirect damage is hard to prove weeks later.
Photo: Atef Ouni / Wikimedia Commons, CC BY-SA 4.0. Source: float (Jens Brambusch, 18 September and 7 October 2026), ISO, ABYC, BoatUS, Pantaenius, Yachting World, US National Weather Service.
