Roger Long wrote:

Thanks, this is very helpful. I'd planned on the flanges being
arranged so that the nut of the through bolt is sort of in the end of a
box. By extending the flange against the hull and making the top and
bottom edge flanges triangular, I could get in two or more bolts.
OK, Keep the box section but widen it vertically against the hull to
allow for two bolts. Add a rib down to the flange between the bolts.
After a trial fit has been done and all fixing bolts tightened to
conform it to the hull, take it out and braze or silver solder any joints.


My problem is that this all has to be done from one side so I can't put
bolts forward of the mast strut or main vertical plate.

Difficult. I presume there is a bulkhead in the way.
Can you get a strap round the front of the mast bolted to the copper
bracket either side sufficiently far out that it doesn't have sharp
bends in it? It wont have as good contact to the support post but should
help prevent flashover at the angle from the post to the bracket for
relatively little cost and difficulty of installation.

If I keep extending the bolt flange along the hull back to add
additional bolts, do I run into a situation of diminishing returns?
Yes. Two bolts a side is practical, three would be possible, more would
be of little benefit. Also the bolts should be as close as possible to
the main body of the flange and the same distance from it or the closest
will hog the current and the others be less effective.


It would be nice to simply duplicate the arrangement on the other side
but I have wires and plumbing running through that side of the bilge.
There is no reason why the other copper bracket cant have some big holes
in it for plumbing and other services, Just bolt braze or rivet a strap
of cross sectional area equivalent to the thickness times hole diameter
to both the top and bottom edges of the bracket.


I know I'll still have a lot of damage in event of a major strike but
I'd like to be figuring out my next move sitting aboard the boat than
swimming or sitting in a dinghy waiting for the next bolt. Most boats
that I have heard of being struck in this part of the world had only
electronics damage. even without good protection, so something like this
should heavily weight the odds in my favor.

Is there any advantage or downside to making this conductor out of
multiple laminations of 1/16" copper sheet?

Corrosion and getting even current sharing between the layers are
against you. Better to go thicker rather than thinner.

Dont loose sight of the fact that you *KNOW* you dont want any current
flowing down through the blocks of ballast in the keel and exploding the
possibly somewhat damp resin between them and blowing chunks of the skin
off. It also seems advisable to review your chain stowage. If its too
close to the foot of the support post, you will get side flashes out
through the hull via the chain.

I've looked over http://www.marinelightning.com/ and they dont seem to
say anything aboout the problem of a keel stepped mast (or a metal mast
support post) and the monohull installation link leads to a single
roughly annotated photo - not encouraging. Some good ideas and info
though especially for those of us with deck stepped masts without a
metal support post.

IanM wrote:

Can you get a strap round the front of the mast bolted to the
copper bracket either side sufficiently far out that it doesn't have
sharp bends in it?

I can't get to the front of the bracket without major surgery that would
compromise the boat's structural integrity as well as appearance.

I'm beginning to realize that this subject is so complex that only tests in
a high voltage chamber (which would cost enough to simply buy a high end
boat with protection already built in) will really answer the question but,
do you think this is worth putting in?

http://home.maine.rr.com/rlma/Ground.jpg

This is the earlier drawing with a top view added. The horizontal brackets
would be top and bottom. I recognize that the long tail is probably useless
for the primary current flow but will assist in attaching the copper outside
the hull and give me a point to lead bonds from the toe rail and other items
to.

I may be cooked anyway. The mast post ends in a plate lagged into the top
of the fiberglass ballast encapsulation so four sharp pointed lag screws
lead right down close to the encapsulated lead. I'm can't imagine now that
there won't be enough current flow left over, regardless of what I do, to
prevent something gross happening down in the keel area.

--
Roger Long

Roger Long wrote:

IanM wrote:

Can you get a strap round the front of the mast bolted to the
copper bracket either side sufficiently far out that it doesn't have
sharp bends in it?


I can't get to the front of the bracket without major surgery that would
compromise the boat's structural integrity as well as appearance.

I'm beginning to realize that this subject is so complex that only tests
in a high voltage chamber (which would cost enough to simply buy a high
end boat with protection already built in) will really answer the
question but, do you think this is worth putting in?

http://home.maine.rr.com/rlma/Ground.jpg

This is the earlier drawing with a top view added. The horizontal
brackets would be top and bottom. I recognize that the long tail is
probably useless for the primary current flow but will assist in
attaching the copper outside the hull and give me a point to lead bonds
from the toe rail and other items to.

I may be cooked anyway. The mast post ends in a plate lagged into the
top of the fiberglass ballast encapsulation so four sharp pointed lag
screws lead right down close to the encapsulated lead. I'm can't
imagine now that there won't be enough current flow left over,
regardless of what I do, to prevent something gross happening down in
the keel area.

I see little benefit in the long diagonal strap. Current sharing with
the short strap wont be anywhere near equal. I know you have pipes and
wires the other side, but the big advantage of keeping the copper
bracket reasonably symmetrical (apart from one or more holes for the
wires etc.) port and starboard is the inductance from the change of
direction will partially cancel. If you try to take it out one side
only with too sharp a bend it *will* arc over to the other side and down
through the ballast. If its already got a heavy copper path out the
other side, it probably wont.

Several square feet of ground plate each side and you will have reached
the point of sharply diminishing returns. Just try not to leave the
boat in fresh water in storm country. If you need to do so and its
going to be on a shallow berth, take a strap down the side of the keel
to the bottom each side and pray.

As to the lag bolts, if there is any other way you could secure the
compression post foot like bonding it into place with Epoxy, do so.
Otherwise you are just going to have to gamble that you've provided a
good enough diversion path unless you want to bore through the ballast
and tap studs into it so its electrically bonded as well, then tap more
studs into it through the sides of the keel.

As long as nothings caught fire you couldn't put out, you have a means
of determining a course to make port (figuring your electronics is toast
and all compasses aboard have been magnetically damaged and are
untrustworthy) the underwater damage is less than you can cope with a
manual bilge pump, and you can still either make sail or get the engine
going you've succeeded in saving your boat, even though you may have to
stay on board pumping till you can be hauled out. Plenty of yachts have
been struck and survived. If your grounding system significantly exceeds
industry standards, with chain plates, toe rails etc. bonded, the odds
are in your favour. No guarantees though.

OTOH if you were designing a production series of yachts it would be
prudent to call in a specialist to do some heavyweight numerical
modelling and scale model testing to prove that it is effective enough
protection for 99.{as many 9's as you need)% of recorded lightning strikes.