Mast earthing?

[cgw]

Can anyone please comment on what is considered normal (good) practice for keel-stepped masts on non-metal boats? The options seem to be:-

1) left floating (and so can then be used as a long/medium wave antenna)
2) bonded to 12V DC negative
3) bonded to an external anode if present, or alternatively the keel bolts

I know there's a much debate regarding lightning protection but my question is not inspired by that. I'm more concerned at present about interference.

I'd be grateful for any thoughts.

Regards,


<hr width=100% size=1>Chris

 

[ccscott49]

I know one thing, DO NOT connect it to 12v negative! Unless you do want a massive explosionn if you do get a lightning strike!! Even St. Elmos fire, might be enough to blow everything sky high! Not sure what an aluminium mast connected to a zinc anoode would do, but I wouldnt do that either. IMHO

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[johnsomerhausen]

IMHO, the best solution would be to connected the mast to a keel bolt, using thick battery wire with the proper terminals for it, with a bolt into the base of the mast and an additional nut on the keel bolt. Thje is a web site of the Univeresity of Florida where an assistant professor has made a study of lighning protection and suggests that solution (the West coast of Florida, near Tampa, has the highest rate of thunderstorms in the USA, so these blokes should know something on this subject)
john

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[William_H]

Lightning is the major concern. If you connect to earth via negative wiring then to engine the wiring will vaporise with a strike and the voltage will then seek some other path to the sea. If you do not earth the mast in a robust manner the aluminium will simply invite the high voltage of the lightning into the cabin where it will undoubtably find its way elecrically to the sea with arcs and moltennmetal. Thats why you give it an easy path via keel bolts. Of course lightning strike may be rare in your location but if it is easy to connect to keel bolts assuming steel keel then it is worth it. If you have a fibreglass encapsulated keel then if there is any leak of sea water to the conductive balast it would blow off the fibreglass as it boils the moisture. That would probably be a lesser eviil. At least 2 square feet of metal in the water is regarded as a substitute for a keel connection but this may be more trouble to provide. I would suggest that the mast will not be such a good antenna as in any heavy sea it will tend to have a low resistance to the sea via spray etc and a high caspacitance to the other mast wiring. If you have no wiring to the mast and consider lightning a low risk then no connection would be best. IMHO will

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[bruce]

ground strap from mast to keel should be very heavy, check out wire used to ground lightning in homes which is about 1/4 inch solid copper. am not sure that the mast can be used as an antenna except for receiving only, insulated backstays are the gen rule for transmitting antennas.

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[mhouse]

On my Dufour 2800 with a deck stepped mast,there was a heavy earth strap to the keel via a very large fuse (at least 1/2" dia) all this must have been factory fitted.

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[cgw]

Thanks everyone for your useful comments. I'm not surprised that concerns regarding lightning feature strongly in the comments, although it does not seem to be a part of "standard or good practice" by boat builders.

I recently had to replace both the wind speed and wind direction modules at the top of my mast (ST60's) which failed simultaneously and expensively on a lovely sunny morning in the Walton Backwaters. It was this that set me thinking about whether the mast should be earthed rather than floating, even though there was no reason to suspect lightning activity (and the radar was on standby too). I also use the mast chainplate for LW/MW and navtex reception, so I'm reluctant to just earth the mast to the lead keel.

Maybe a heavy current cable between mast and keel via a spark arrestor device or suchlike might be possible: I could then have protection and a Rx antenna!



<hr width=100% size=1>Chris