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Marine Lightning Protection



This page was prompted by this recent devastating fire on board the charter cat House Cat in Abacos Bahamas

Catamarans have their own special set of issues:

In this page we present strike statistics based on Boat US insurance claims, explain why catamarans get struck twice as often, and outline our approach for lightning protection.

Strike statistics

First of all let's deal with the statistics.  A number of years ago I was on an IBEX panel with Bob Adriance who presented the first statistics showing catamarans were struck twice as frequently.  Now Boat US has published the latest numbers, and here they are, courtesy of Bob, for 1998-2008.  Double these frequencies for Florida .  See the full discussion on p.15 in the October 2008 issue of Boat US Seaworthy.
Chances per 1,000
$ Severity (10=Highest)
Multihull Sail
Aux Sail
Sail Only
Bass Boat

So, not only are sailing multihulls struck with twice the frequency, they also sustain more damage per strike than sailing monohulls with auxiliary power.  

We can explain why catamarans are struck twice as often, which has nothing to do with having twice as many hulls or elevated bridge decks.  However, it has a lot to do with being twice as wide, which means they get less shielding from nearby boats when they are in a marina.  Also, multihulls, because of their wider beam, are more likely to be docked at the end of the dock, in which case they have no shielding at all on the open-water side.  When many boats are docked side-by-side the attractive effect of the mast is about the same for all so that the strike probability is proportional to the area exposed to the sky.  This is explained in more detail below.

Strike probability

Consider as an example these boats on the same dock as House Cat.  Since these boats are close to one another and with different mast heights, we might expect their strike probabilities to vary depending on their relative likelihood of initiating the attachment streamer that determines the final lightning path.  See our Science and Air Terminals pages for details of this process.

While there are several ways to estimate the attractive distance of a lightning rod, one that is easy to visualize is the following that is based on the rolling sphere model used in lightning protection standards for planning placement of lightning rods.  Since the upward, or attachment, streamer has a length of about 100', we can imagine the attractive zone for a mast as being a sphere with a radius of 100' centered at the top of the mast.  If the boat is not close to any other boats or tall objects, any lightning that intercepts this sphere is diverted to the mast.  According to this model, the probability of lightning striking an isolated sail boat in Florida is about 3%.   While of comparable magnitude to the boat US statistics, this is in fact 50% higher than the catamaran statistics (~2 % per year) and three times the monohull numbers (~1% per year). We attribute the difference to the shielding effect of nearby boats.  

Now if we add other boats within 100', the other masts compete to initiate the attachment streamer, lowering the probability.  In this case, we assume that the streamer is launched from the closest mast.  So a lightning that would have struck somewhere along the dock is diverted to the closest mast as shown below.  For example, a lightning that would have connected to ground within the two vertical dashed lines instead intercepts the green sphere when it is 100' away from the green mast and so attaches to the green mast.

Note that the dashed purple sphere associated with the smaller purple mast is completely below the two spheres from the adjacent taller masts and so, according to this model, is shielded from the strike.  However, the smaller blue mast on the right-hand side is not completely shielded by the taller mast next to it.  Of course, this simplistic explanation comes with the warning that, while it is useful for explaining large scale trends such as catamarans being struck twice as often, it cannot be relied upon. In practice when lightning strikes very close by it is common for objects to initiate sparks off their highest points that can also cause damage.

Why this can result in a doubling of strike probabilities for catamarans is illustrated in the following set of diagrams.  Specifically, we find that doubling the spacing between boats in a crowded marina doubles the effective attractive area, or footprint, for each boat with the notable exception of those on the outside.

In the first two diagrams we place identical boats at a spacing of 15'.  As in the above discussion, we assume that lightning strikes a particular boat if it intercepts a sphere with radius 100' centered on the top of its mast, as shown on the left.  The right hand diagram is a plan view that includes the attractive area for boat.  This varies from 3000 ft2 (0.3%per year probability in Florida) for a boat on the inside to 17000 ft2 (1.7 % per year) for one on the outside.


In the next two diagrams we increase the distance between masts to 30'.  Now the attractive areas are 6000 ft2 (0.6% per year) for a boat on the inside, and 19000 ft2 (1.9 % per year) for one on the outside.  So while the strike probability has not changed much for the outside boats (~2% per year), it has doubled for any on the inside.


Also note that the probability of a strike increases by a factor of 3-6 when a boat is moved from the inside to the outside, and by a factor of 5 -10 when it moves from an inside slip to anchoring out.

Catamaran lightning protection systems

Complete system

Having been involved in a number of a catamaran projects (e.g. Tag Young 60' sail cat , Domino 20 60' power cat) we have come to grips with all of the special issues for catamarans.  For example, here is the layout for Tang, the first Young-design 60' cat built in the new Tag shipyards in South Africa.  Hull #2 also is having the same system installed. In keeping with our exoterminal technique, this system has lightning conductors near the outside, ten grounding electrodes,  and a one-square foot immersed strip outside the prop on each hull.  Note the two electrodes directly below the mast base to provide secure sparking points below the bridge deck.  Our systems are based on standards published by the National Fire Protection Association, NFPA780, that has been honed over many decades from observations of lightning damage to buildings.  In keeping with the philosophy of NFPA, we first identify where the predominant lightning damage is and then place oversized conductors to divert the current along this predetermined path. Lightning prevention is not the objective: providing a conducting path from the strike point to the water surface is.

See also our Systems gallery page for systems on other multihulls.

New bridge-deck electrode

While a comprehensive system is the best remedy, in practice it can very difficult, time consuming and expensive to undertake a project such as this after the yacht has been built. And the reluctance of many builders to acknowledge and take on the responsibility of addressing this issue may border on negligence. In particular, as the damage to Housecat testifies, the major hazard in an unprotected cat is just below the mast base where spark formation is highly likely. However, the fact that a catamaran mast base is suspended above the water is actually an advantage once the physics of the situation is taken into account, and points to a straightforward solution that can be implemented without the need for a haul out.

First let's understand the concepts concerning lightning grounding from an unprotected cat. Briefly, once current starts flowing from the top of the mast, the mast, and everything connected to it, develops a high voltage that is sufficiently large to form sparks of any length. The attractive charge for these sparks, which is also the charge eventually neutralized by lightning, resides on the surface of the water. So the sparks tend to head for the water surface. In fact, the preferred mechanism for neutralization of this charge, is via surface discharges in the air just above the water but not all boats have suitable electrodes to initiate these sparks. So alternative fittings such as propellers, hull-mounted transducers,bonded through-hulls, etc, become unsuspected lightning conductors. Or the spark can just form directly from the base of the mast, sideflash to any nearby conductor, and blast its way through the bridge deck to make the connection to the water surface. Apparently in the case of Housecat enough heat was generated from these sideflashes to ignite a fire.

Now the solution: to address these major issues at the mast base and bridge deck, we are in the process of developing a bridge deck electrode. This is connected via a flexible conductor so that it can be lowered to just above the waterline, for maximum grounding effectiveness, or all the way up to the bridge deck, for clearance while cruising. We plan to package this electrode with a kit for connection to an aluminum mast or stainless mast support. If you may be interested in this approach, please email us with details of your cat so that we can assess its feasibility for your particular geometry.