Lightning mitigation and an unusual cruising sail cat

Lightning strikes are a significant danger to equipment and lives on board larger sailing cats. It is one of the top 10 reasons for insurance claims, and one of the top 10 causes of accidental fatalities/serious injuries. Multihulls are almost twice as likely to be struck than monos, and the risk increases with LOA.

Lest you think I’m making this up, I include some sources. I find insurance sources to be credible and easily understood.

Striking Lightning Facts - Seaworthy Magazine - BoatUS https://www.boatus.com/seaworthy/magazine/2015/january/lightning-facts.asp

Lowering The Lightning Odds - Seaworthy Magazine - BoatUS https://www.boatus.com/seaworthy/magazine/2014/july/lowering-the-lightning-odds.asp

Modern Lightning Protection On Recreational Watercraft - Seaworthy Magazine - BoatUS https://www.boatus.com/seaworthy/magazine/2016/january/lightning-protection.asp

Seeing that I’m contemplating a cruising sail cat 7m BOA and 12.9m LOA, the risks of lightning strikes are fairly high.

No proven method exists to prevent lightning strikes. The only options are to hope to be lucky, or provide lightning a more direct route to ground bypassing one’s expensive equipment and priceless crew. Even so, induced current may fry electronics.

The hypothetical cruising sail cat I’m contemplating is unusual, due to some unusual requirements. Relevant requirements for this discussion include petrol outboards in bridgedeck wells for auxiliary propulsion, an air draft modifiable to less than 4.7m by two crew on relatively calm waters, and 20+ years of durable antifouling (occasional bottom scrubs are acceptable, haulouts for antifouling are not). The boat is intended for liveaboard on the hook, with no need (or desire) for a marina.

For durable antifouling, I’d use 1-2mm thick C70600 (~90% copper, ~10% nickel) epoxied to the hull below the (fresh) waterline. The use of outboards (raised out of the water when not in use) and avoidance of marinas limits concerns with exposure to sacrificial anodes (which inhibit copper’s antifouling properties), galvanic corrosion, and electrical corrosion. Incorporating this cladding into a lightning mitigation system seems simple. Relevant sources regarding the feasibility of cupronickel cladding include https://www.sciencedirect.com/science/article/pii/B9781845692414500058 Boat and Ship Hulls https://www.copper.org/applications/marine/cuni/applications/hulls/ Copper-Nickel : Biofouling and Corrosion of Foil Sheathed Hulls https://www.copper.org/applications/marine/cuni/applications/hulls/biofouling_and_corrosion.html .

The air draft requirements are the tricky part. I’ve decided against a compression mast with Bermuda sloop rig; I just don’t see it being safely lowered by 2 crew on calm waters, or being reliable enough after raised again. Too many things to go wrong. So I’ve envisioned a freestanding (unstayed) carbon fiber mast in a stout tabernacle with a “new generation” battened lug soft wingsail. A good discussion of such rigs (admittedly for heeling monohulls, not a stable cat) is The design of soft wing sails for cruising https://www.boatdesign.net/threads/the-design-of-soft-wing-sails-for-cruising.49425/ (my hat off to you, David Tyler). Either one large sail, or two smaller biplane sails, would be partially balanced and use the bridgedeck salon roof as an end cap. These rigs offer little in the way of trimming options, are self tacking, but most importantly can be reefed immediately and singlehanded at any point of sail. Performance is secondary to safety and convenience for this boat. After determining the feasibility of this rig, the future addition of a traction kite control system for sailing off the wind would warrant consideration.

Which brings me back to the start of the thread, lightning mitigation. Attaching a lightning terminal above the mast, attached to a heavy-gauge (2 AWG?) copper wire running the length of the mast, dangling directly in the water and also connected to the cladding would seem a safe and simple pathway to ground. Provided, of course, the system is sufficiently isolated to prevent the lightning from using the carbon fiber mast as part of its pathway.

Your thoughts on this unusual solution?

Can you think of any ways to use the superb heat conductance of C70600 without raising corrosion issues or mucking up the fairness of the bottom? All my ideas have strayed too far from KISS principles.

Thanks!

 

I would also take a look at the assertions on this site: Marine Lightning Protection Inc. http://marinelightning.com/index.html

They focus on providing a path to ground (the water) at the waterline through electrodes through the hull. The claim being that even in a bonded boat, the lightning will find a path to the water through the hull at the waterline in preference to continuing to immersed electrodes or the keel. They aim to make that easy and "safe". I'm not sure there is enough evidence to conclude that this is the right way, but it certainly seemed to be well thought out and sounded reasonable to me.

With respect to the Cu-Ni sheathing, why are you proposing so thick 1mm+? The article you reference used .15mm foil on a commercial vessel and seemed to have no problems with the thickness?

 

What I’m proposing is a plumb-line copper cable dangling from the tabernacle to the water, with connections to the copper cladding in the event the energy is too much for the plumb line, and to mitigate side flash. I don’t think I’m describing a situation where it will be a shorter distance for lightning to penetrate the FRP hull.

I’ve examined the available information regarding lighting mitigation like marinelightning.com and strikeshield.com (currently offline it seems) but these focus on retroactive installation, while it seems easier, and possibly more cost-effective to address the issue during initial construction. Correct me if I’m wrong, but isn’t the Sidearc a hull penetration at or below the waterline? One of the things I like about my proposal is the lack of hull penetrations.

I wonder if the cladding would function as part of a Faraday cage, protecting contents from induced current?

One of the articles I cited did use 0.15mm foil. Another links to a study of 2mm cladding. Environmental effects on cupronickel cladding indicate 0.02mm may be lost in a year, which would not give the foil 20+ years of durable antifouling desired. In the event of damage from collision/grounding etc. I would definitely use the foil for repair, but it would not be my first choice for initial build. That, and I doubt 0.15mm foil would be capable of conducting a meaningful amount of current in the event of lightning strike. The weight is a concern, but at least it’s low. The cat will have a high center of gravity due to shallow draft.

 

Interesting thoughts! I would considder a side by side biplane. Should have several benefits suiting your concept. 1) 2 mast make for shorter individual mast length. In turn, a) reduce risk of lightning, b) closer to hullside, and shorter easier path to route lightning rod. c) easier to handle lowering and rasing. d) less heeling moment, suitable both for safety and performance. e) easier construction not having significant (compression) loads on bridgedeck but on hulls. (Depending on stayed vs unstayed).

 

I’ve been debating the mono vs biplane question. As you say Niclas, biplane would lower the air draft, lower the center of effort, decrease the likelihood of lightning strikes, reduce heeling moment, make the sails more manageable, and allow better bury of the masts. Biplane might allow something like a cruising chute to be rigged, which I wouldn’t trust to a freestanding monoplane setup.

My biggest concern with biplane is “wind shading”. At 90 degrees apparent wind (a favorite point of sail) one would be better off dropping the leeward sail, as it produces drag but little lift. Monoplane will be more efficient than biplane, and probably make better use of the salon roof end cap. Rigging only involves one or two sheets, and the halyard, which would need doubled for biplane. Not a big deal, but still a consideration.

I picture monoplane as a 14m mast, the bottom 2m of which is buried in the bridgedeck tabernacle (1:7 bury, I’ve seen cited as the minimum). Basically rectangular, 5m wide, results in ~60m2 of cambered soft wingsail with a 2.4 aspect ratio. I picture the partial balance to put ~.5m of the sail on one side of the mast axis and ~4.5m on the other. Maybe 6 2m high panels/reefing points and 7 carbon fiber wishbone battens.

I picture biplane in tabernacles near the fore corners of the salon. Mast 10m above the salon roof, biplane would allow at least 3m of bury, more than adequate. 3.2m wide, ~32m2 with a 3.125 aspect ratio. Perhaps 5 2m panels and 6 wishbone battens, ~0.3m on one side of the mast axis and ~2.9m on the other. Clearly lots of simulation and experimenting required.

Biplane seems better for working upwind, but for a cruising cat I’m uncertain how important this is. On paper monoplane seems adequate upwind, better for working off the wind, which is a more likely point of sail.

Mark, regarding the cladding, I should mention that I’m not considering 2mm C70600, even if that’s what is required to conduct a lightning strike. Even 1mm will have significant weight. 0.5mm will be enough for 20+ years of durable antifouling, which is more important for me than using it for lightning mitigation. Multipurpose things are good, but sometimes dedicated purpose things work best; lightning mitigation can be addressed separately if needed. The foil is intriguing. Sure, It may well not last 20+ years, or conduct meaningful amounts of current, but it would be easier to apply, repair or replace.

Thanks for the input.

 

Another easier to apply option on the long-term antifouling is CopperCoat: Coppercoat Anti-Fouling Epoxy: much more than a marine bottom paint. Coppercoat is a multi-year bottom treatment for your boat or yacht. Save time, money, and haul-outs with Coppercoat. http://www.coppercoatusa.com/

I haven't used it myself, but came across quite a few British boats in my travels that seemed very happy with it.

(Sorry, I can't work out how to embed the link without the advertising text showing up automatically)

 

Only problem with copper is it's toxicity to all marine life. As far as I know it's baned in Sweden for use on non comercial craft. At least in the Baltic sea. Think it was not only popular for its effectiveness and longevity but also its hard durable and fast slippery surface... or at least that is my impression of what I've heard. No personal experience of the stuff. Leaning towards skipping the toxic stuff all together and use bottom mat or regular scrubbing. Difficult with deep keel. But seems easy enough for daggerboard boats.

 

I’m familiar with CopperCoat, but the firsthand reviews I’m given seem pretty mixed. Most seem to indicate that it doesn’t seem to increase the time between needed bottom scrubs, but that haul-outs for sanding down to a fresher layer are easier and quicker than repainting. I’m curious to know if the boats experiencing better outcomes with CopperCoat have some way of limiting exposure to sacrificial anodes.

The widespread toxicity of copper to marine life is not proven; cupronickel is regularly used for aquaculture containment systems, because in the absence of sacrificial anodes it greatly reduces the weight of biofouling on the system and reduces parasite/pathogen exposure. Seeing as aquaculture is concerned with money above all else, and they seem to prefer this expensive equipment, that carries some weight with me; if it were killing the fish they are raising for market, I’m sure they wouldn’t bother. Copper is a necessary trace element for most multicellular organisms.

I’m aware of attempts by governmental and regulatory bodies to prohibit copper-based antifouling (including paints) in the US and EU, but most of these attempts have been struck down when confronted by the lack of evidence that copper creates any issues beyond its immediate proximity. I’m a believer in not poisoning the earth, otherwise tributyltin would be an option (nasty stuff that!). If you have links to good science showing the widespread toxic effects of copper on marine life, I’d be most appreciative for sharing. Articles I’ve found like Toxic substance profile: Copper http://www.ukmarinesac.org.uk/activities/water-quality/wq8_7.htm do not support the idea that copper is severely toxic to marine life at anything but extreme concentrations (although that particular article draws those conclusions from reviews of previous studies).

 

Thank's for interesting posts. I don't have specialized knowledge in that field myself. Just a laymans interest. But I do appreciate meta studies, since they often give a good overview on the current state on any subject. I'll take a peek at the one you linked. Thanks!

 

I sailed a boat with a Hasler designed junk schooner rig across the Pacific, and I liked it. My masts were solid, and I used my lighting protection gear just once, when crossing the equator. I used the idea of a traditional burgee rig, using an aluminum tube, tied to the spare halyard at the middle and bottom of the tube, with a 14 gauge cooper wire attached, with about the last 10 feet stripped of insulation and dropped into the water. This rig enabled the tube to stick about 4' above the mast head. I was down below, and I heard a crackling noise, so I went to find the source. There was ball lightning dancing at the head of my mainmast! I like the twin rig concept, and the wingsail / junk hybrid rig. When the wind is abeam, you free the windward sail, and place the leeward sail in a close reaching position. Seen from above, the net effect is like the camber of a single sail cut with camber.

Copper is used for bottom paint BECAUSE it is toxic. As the old adage goes, it is the dose that makes the toxin. I have no idea of cupronickel's longevity in fresh water, but I am dubious of its longevity in salt water. Don't forget electrolysis! Monel is durable, but it is over half nickel.

 

Hi Bigcat. Thanks for the description of your rig, I like your solution for a beam reach. I’m aware of the fact that copper is antifouling because it is toxic, I’m just pointing out that the toxicity is a localized effect (unlike tributyltin). For every organism on earth except except some bacteria and archaea, copper is a required trace element used mainly in enzymes, oxygen transport and electron transport. The toxicity of copper is not well understood. I would guess it is due to the fact that in high concentrations it will be incorporated into enzymes, structural proteins and nucleic acids that require a different element to function, which is the toxic basis of heavy metals.

Several studies have been performed on the corrosion of cupronickel in seawater, and the results are impressive. Some of the results of these studies, with links to more, are found at Copper Nickels : Seawater Corrosion Resistance and Antifouling https://www.copper.org/applications/marine/cuni/properties/corrosion/corrosion_resistance_and_antifouling.html#cor2 . Cupronickel is used extensively in marine engineering and marine aquaculture.

The highest observed rate of corrosion of 90-10 Cu-Ni in clean non-stagnant seawater is 0.02mm in the first year, with both sides of the plate exposed to seawater, slowing rapidly as a passivating film of oxides form. This passivating film is a mixed blessing, as it slows the production of biocides, but it seems to eventually slough off or be easily scrubbed off. Certain conditions can cause localized corrosion; stagnant water supporting the growth of sulfate-reducing bacteria, and certain (chloride based, if memory serves) industrial pollutants. Fortunately neither is an issue where I plan to go, and the passivating oxide film offers some protection. Sacrificial anodes appear to slow corrosion further, but inhibit the production of biocides. Galvanic corrosion is only an issue with dissimilar metals electrically connected; avoiding inboards with their sacrificial anodes constantly in the water limits these concerns.

After crunching some numbers I’m steering away from cladding, but strongly considering foil. A reputable supplier (in China, of course) quoted me a price of $12-14/kg (not including shipping or tariffs) for 0.2mm thick 305mm wide C70600 foil. 8.94 grams of foil will cover 5cm2, 17.88 kg will cover 1 square meter (provided my math is correct). It’s a notable weight penalty, but 10+ years (probably closer to 20+) of durable antifouling makes it tempting. Years of never needing to haul out ($) to repaint ($$) or sand and repaint ($$$). A pity it probably won’t tolerate beaching or collisions well, but repair should be easy. Provided my math is correct, a boat with 25m2 wetted surface would require 447kg of foil (ouch). If the price delivered were $20/kg, $8940. I think CopperCoat would cost more and not last as long, although save weight. Going with the 0.15mm foil used in one study would weigh 335.25kg and cost $6705, and probably still give 10+ years of durable antifouling.

Since the ability to safely raise and lower masts on calm waters is a big consideration, I’m still thinking carbon fiber mast with an isolated copper conductor, connecting the conductor to a copper hull penetration at the bottom instead of mucking about with the cladding.

Any lightning experts out there? Carbon fiber is a conductor, just not a good one. A lightning terminal and wide gauge wire should prevent lightning from using the mast as part of its pathway, and the carbon fiber would act as a Faraday cage preventing induced current in other parts of the boat (like electronics). Would this induced current produce enough heat from conductive resistance to damage a mast?

Thanks all,

 

You're off by one order of magnitude. 1m x 1m x 0.0002m x 8700k = 1.74k However, 0.2mm is very thin and not likely to last.

 

Hi Gonzo, I liked your answer better and I had to check where my math went wrong. As it turns out I had solved it for 2mm, not .2, which I agree is likely too thin. What I get for posting before fully waking up, and my background is more biology than math. Plus my dog ate my homework. Now 0.5mm may be adequately durable, and I’m certain 1mm would be. I’ll try to solve for 1mm.

8.94g=1 cm3
At 1mm thick, 1cm3 would cover 10cm long by 1cm wide, or 10cm2 of surface.
1m2=100cm x 100cm= 10,000cm2
10,000cm2/10cm2= 1,000.
1000 x 8.94= 8940g/m2= 8.94kg/m2

My math perfessers would be disappointed.

For a hypothetical boat with 25m2 wetted surface, 1mm cladding would weigh 223.5kg. In the unlikely event corrosion took a full 0.02mm per year, that’s many years before it became too frail to withstand the rigors at sea. 1mm might not be adequately flexible to apply as foil, but 0.5mm should be, and approaching reasonable at 111.75kg, $2235 @$20/kg.

Thanks for checking my math, any other thoughts on this?

Cheers,

 

Hi, Markusik

I can't find an article I remember, but it was an analysis of a damaged, grounded carbon fiber mast. The conclusion was that there must be NO angles, bends or curves of any kind in the path from the top of the lightning rod to the ground. If the lightning leaves the grounding system, it bounces around the boat, seemingly at random, though it does like to seek out the larger bits of metal in a boat. Anchor windlasses, engines, and through hulls, are infamous for this, but there us usually damage to non-conductive structures, despite the good grounding power of through hulls and prop shafts.

When Lightning Strikes: Following Standards Minimizes Boat Damage - Professional BoatBuilder Magazine https://www.proboat.com/2016/04/3530/
Carbon masts and lightning https://www.boatdesign.net/threads/carbon-masts-and-lightning.26861/

The fact that a trace amount of metal is used in an enzyme is no guarantee that larger doses won't kill you. Copper Sulfate http://pmep.cce.cornell.edu/profiles/extoxnet/carbaryl-dicrotophos/copper-sulfate-ext.html

BTW, putting a sheet material on a hull bottom will be easy only if it is a hard chine boat.