Friday, March 30, 2012


I got into a discussion the other day with someone about the likelihood of a boat getting struck by lightning. Lightning strikes on boats ... especially big yacht targets, are not uncommon. Statistically you are more likely to get struck by lightning at anchor. This may be the case because of several factors:

- boats tend to be out in the open while at anchor

- boats in a marina have lots of other company and most are plugged in and well grounded to the marina electrical system

- 70% of lightning strikes occur between noon to 7 pm




Negative charges repel negative charges and attract positive charges. Positive charges are trying to reach the negative charge up in the clouds. The positive charges accumulate at the top of the highest conductive object. The better the contact with water the more easily the positive charge can enter the object. When the electrical potential between the positive and negative are great enough to overcome the insulating values of the air, lightning occurs. The electrical potential can be 100 million volts. A lightning strike is the flow of electricity from negative to positive and usually to the highest positive conductor in the area. Side flashes are frequent as the charge tries to follow it’s path to ground – that means anything that is conductive along the way, including humans, risks being a conductor to the ground.

No boat is lightning proof. Sailboats masts on fiberglass boats are especially vulnerable even when the mast is bonded with heavy copper cable to the lead keel in the water. If the potential charge is great enough it can overwhelm the conductor resistance and look for multiple paths. An ungrounded not bonded mast is like waving a flag – hit me ... hit me.

Power boats are not immune either and tower equipment like VHF antennas are prime targets. Grounding needs to be at least 8 AWG copper to ground. To some degree you are in a cone of protection inside a boat provided you sit low in the center of the boat and don’t touch anything electrical or conductive – but that doesn't mean the lightning won’t blow the bottom out of the boat if it cannot find sufficient path to the ground (water), and the boat could sink. If you are underway in a storm it is especially dangerous to be touching two different metal objects with different potential (like throttle controls and VHF). Best to plot a course and go on auto pilot while touching nothing conductive. Better yet get into a safe harbour before the storm if possible.

If your really wanted to be lightning preventative you could use a long pair off jumper cables clipped on your anchor rode chain (you need chain right to the boat from water and clip the other end to the highest conductive point (like a VHF antenna or mast) but how many people are willing to come prepared for that ... and even so, no guarantees of side flashes following your very conductive “full of water” body. A living spark plug so to speak.

When all else fails, I recommend toasting the God of the sea - King Neptune ... and pray for salvation. Lightning is scary stuff.

Posted by at 7:40 AM