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Last fall, I purchased a 1993 Mac 26 S that had been kept in saltwater (Outer Banks). When I bought the boat, it had a 'grounding plate' on the starboard side of the boat that has since, come loose. Before leaving OBX, I was told that it could simply be cleaned up and reglued to reattach it to the hull. Since bringing it back to the freshwater region of the midwest and planning on keeping it in freshwater, my question is what to do with it? Do I need it? Should I simply clean it up and reglue it? As it is already attached to the boat, it seems like the simplest thing would be to reattach it to the hull and leave it be. Suggestions? (Thanks Scott for the picture below.)

I am having trouble understanding what the 'grounding plate was for"

looks like a mighty big conductor to transfer current.......

and nothing RF needs to be grounded outside the boat....... so can you describe any internal connections??

salt water is such a good conductor it is hard to imagine what that plate aids.

Are there any cracks or repairs under it? Perhaps a hasty hull repair.

Re: DO I NEED THIS?
by waternwaves » Thu May 13, 2010 4:36 pm

I am having trouble understanding what the 'grounding plate was for"

looks like a mighty big conductor to transfer current.......

and nothing RF needs to be grounded outside the boat....... so can you describe any internal connections??

salt water is such a good conductor it is hard to imagine what that plate aids.

Claiming all ignorance on the subject, I found an article by Don Casey on 'sacrificial zincs.' In part he said this:

"Hull plates
Bonding is a different subject altogether, but boats with all underwater fittings bonded together electrically are typically fitted with one or more zinc plates bolted to the hull. The mounting bolts for these anodes are connected by heavy-gauge electrical cable to the bonding circuit. If these anodes are allowed to deplete or if the electrical connection deteriorates, other underwater metal, such as bronze through-hull fittings, will begin to corrode. "

The link for his article is: http://www.boatus.com/boattech/casey/23.htm

It was my understanding from the prev. owner that it was called a 'grounding plate' and was peculiar to salt water boats.

Report this postReply with quoteRe: DO I NEED THIS?
by Scott » Thu May 13, 2010 5:19 pm

Are there any cracks or repairs under it? Perhaps a hasty hull repair.

No, from what I can tell, it is still attached at the forward end of it and serves some sort of an electrical purpose, perhaps as the zinc hull plate that was mentioned by Don Casey.

Without going into electronegativity, conductivity, ground planes, and polar molecules.............

as a fair to middlin degreed.............electrical and mechanical engineer.......forensic engineer.....and a licensed operating engineer.........an occasional marine engineer, surveyor...................... mechanic............fabricator..............welder............. and all around good fer nothin tirekicker..........not to mention the biochemical and nuc engineering.......

sumpon ain't right here.

either that or it is the worst sacrificial anode I have ever seen in my life.......(and I have seen 10's of thousands)

and if someone was looking for a ground plane for HF VHF communications........ it just simply a terrible way to do it......in an even worse location.

and it is not sufficient for lightning protection charge diffusion.......

so.....that leaves us......a couple of possibilities

a)mounting for a megawatt ultrasonic transducer?
b)hull armor for protection from newly found heavily armed Dungeness......(couldnt resist the pun)?
c) documented case of 1/8" thick non navy bottom paint........Honey could you bring me the really really big paintbrush... no the bigger one?
d) the worst patching job ever done on fiberglass



e) someone who didn't quite understand what they were doing?

My guess is that it was a previous owner's mod to ground the mast to for lighting enhancement. We don't get struck very often by lightning, and some folks seek to rectify that by grounding the mast to attract more lightning.

Others feel that the boat's metal fittings are not corroding fast enough and bond them all together and then to a submerged ground plate in the hopes of enhancing their corrosion experience. Sometimes they will also bond this to the dockside ground and put a bilge pump in with a badly insulated level switch for an extra special corrosion experience.

My 92 26s came with the same grounding plate in, it looks to be, close to the same place. The only thing grounded to it was the Loran C unit that one of the POs had installed. My bet is if you could search back through your Mac's history you will find it once had a Loran C. I took out the Loran but left the plate.

My only concern is there is a through hull somewhere that should be inspected to make sure it isnt leaking.

Report this postReply with quoteRe: DO I NEED THIS?
by gordl » Fri May 14, 2010 12:45 am

My 92 26s came with the same grounding plate in, it looks to be, close to the same place. The only thing grounded to it was the Loran C unit that one of the POs had installed. My bet is if you could search back through your Mac's history you will find it once had a Loran C. I took out the Loran but left the plate.

Since my 'grounding plate' has pulled loose, would you recommend securing it back to the hull or removing it? I have it in the shop for some glass work right now, and this would be the perfect time to remove it. I can have the shop determine what is being grounded to it.

Was your boat in salt water as well, as in is this a salt water preference for the Macs?

loran C was possible,

however the hack asymetric mounting, below water surface radiation, extensive saltwater conductivity/ corrosion, and general poor propagation, reflectivity, radiation etc........tells me BS.....

I have installed 7 loran receivers over the years.....Better ground planes/radiators existed even then, for less cost. . Someone read something someplace and never measured performance.

not to mention plain bottm painting over metal..........ughhhhhhhhhhh

My plate is heavier than yours and bolted through the hull. In your case I would remove the plate, patch whatever holes result and be done with it.

Some HF (SSB) radio antenna installations require a ground plate to saltwater. Unless you are a ham radio operator, or feel the need for marine SSB communications (not VHF), I would discard it and seal the connection that must be going through the hull. It is useless for lightning protection.

~Rich