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Old 03-10-2005, 21:43   #1
Kai Nui

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External Ground Plate

On our trimaran, we have a hollow keel on the main hull. THe boat currently has no electrical system. I am considering screwing and bedding copper plate to the keel for a ground plate. How big should it be?, will it stay on?, are there any other inherent problems to this set up? I will be connecting to the plate through the hull by way of a copper stud through the bilge, to a flat strap extending from the plate. I intend to attach a second plate to the other side of the keel exclusively for an RF ground. Do not know if this is necessary, but it seemed logical to seperate the two grounds to some degree. Just so current is not going to flow between the two and create a problem. (Yes I know salt water will conduct between the two, but with some resistance)
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Old 09-10-2005, 00:51   #2
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Is This Also Part Of Lightining Protection?

Hey Kai,

Were you planning on adding lightining protection for your tri? That could be hooked up to the plate? I have another idea. But, I'll first hear your response first. Ok?


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Old 09-10-2005, 02:14   #3
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A good Ground requires a direct and permanent immersion in seawater. It must also have sufficient area, mass, and edging to adequately dissipate the strike energy. Electrical energy dissipates better through edges and points, that it does through flat surfaces.

Minimum design criteria for this ground plate by ABYC-E4 standards are:

• Minimum area – one square foot totally submerged for salt water (more for fresh water)
• A Ground Strip is preferred to a rectangular plate.*
• Thickness – minimum of 3/16"
• Width – minimum of 3/4" (for a grounding strip)
• Materials – copper, copper alloy, stainless steel or aluminum. (others have suggested adding monel or navel bronze)

Other important guidelines relative to designing the external ground plate are:
(1) the ground plate should be located as close to directly under the down conductor (mast) as possible;
(2) the edges must be sharp and exposed, and not caulked or faired into the adjoining hull surface;
(3) *the grounding strip, if used, should extend from directly below the mast towards the stern and be electrically connected to the aft end of the engine (a minimum strip length of 48" is recommended, with an appropriate width to yield at least 1 sq. ft of area); and
(4) a pair of thru-bolts should be installed at each end of the strip to prevent it from twisting. Intermediate bolts may be used as necessary.

Good resources:

Ewen Thomson, Ph.D (Marine Lightning Protection Inc):

“Lightning& Sailboats” Florida Sea Grant

“A Critical Assessment of the U.S. Code for Lightning Protection of Boats”
IEEE - Transactions on Electromagnetic Compatibility

“Comments on ProBoat Article”
In the December/January 2003/2004 issue of Professional Boatbuilder , pages 26 - 43, Nigel Calder gives a comprehensive analysis of lightning protection. While Calder's article was particularly well written and accurate, we felt that some of his scientific comments could benefit from some elaboration. Specifically, the role of ground plate edges has suffered from a drift in interpretation over time. Consequently, we submitted a letter that was subsequently published in the 2004 April/May issue, pages 4 - 6. The exchange concerning lightning dissipaters on pages 6 - 10 is also well worth reading.

Lightning Grounding Systems - Basic Theory & Practice ~ by Johnathan Klopman

Non-Member Price: $40.00
Member Price: $20.00

Gord May
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"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

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