"Ian White, G3SEK" wrote in message ...
SpamHog wrote:

My balun box is small, at most 3/4" deep, all metal contacts are
already coated with thin epoxy, and the transformer wire is PVC coated
too, so a momentary contact with water spray, soon to be soaked up by
the PU, should be OK.

Agreed. However, the PU foams that we buy in DIY stores do tend to soak
up water into the outer layer, especially after the smooth surface has
been cut to trim off any excess.

Some foams are notably better than others in this respect, and it might
be better to look for the special 'closed cell' foams that are
specifically rated for use in for flotation aids.

I'll do another test in a small plastic box containing some wiring, all
pre-wetted. As the foam keeps expanding as it sets it may be a good
idea to avoid closing and bolting the box lid before it cured
completely, or at very least to provide some escape holes. More later.

Before you do, you'd better read this:
http://www.diyfaq.org.uk/humour.html#foam

The link brings up fond memories of a very similar debacle I saw first
hand. This one wasn't "engineered" by a home shop canoe builder, this
one was put together by none other than the U.S. Navy.

My first job out of school 40+ years ago was as a civilian mechanical
engineer with the U.S. Naval Air Engineering Center. The group I
worked in was responsible for the design and ongoing development of
carrier-based steam aircraft catapults.

Catapults are individually calibrated and periodically recalibrated by
launching minimum-budget wheeled vehicles of various weights called
"deadloads" off the catapults in into the water when the carrier is in
port. The idea is/was to create weight vs. end speed vs. steam
pressure curves which are vital bits of info when twisting the
catapult steam pressure control knobs below to get the airplanes off
the end of the catapults flying nicely instead of into the drink or on
the other end of the scale not torn apart by excessive g forces.

Deadloads are, or were at least back then, simply big steel boxes of
various weights to simulate the range of aircraft weights the
catapults dealt with. Each corner of the box was equipped with a wheel
salvaged from junk aircraft. They were divided up into internal
"watertight" compartments and hopefully they floated after a test shot
and could be reused. Some deadloads floated, some leaked or came apart
and sank. An average deadload weighed 35,000 pounds and was launched
at perhaps 115 mph.

Along came this self-proclaimed engineering genius from the next group
yonder who decided that his route to military fame and glory was to
become the guy who came up with the concept of filling the deadloads
with foamed flotation compounds and save all deadloads from the briny
deep.

At that point in time DIY foamed flotation plastics were experimental
and were not yet on the general market. But he managed to acquire some
samples of a two-part foaming compound for test purposes. He also
managed to get hold of an old deadload and parked it on the tarmac off
to the side of the main hanger and went to work drilling holes in all
the compartments. Then he went back and poured carefully measured
amounts of part 1 and 2 into each compartment.

I had no idea what was going on out back at the hanger, I wasn't
involved in it. Until a lieutenant commander who worked in our area
came down the main aisle with tears of laughter rolling down his
cheeks hollering "You gotta see it to believe it!" So off we trooped
to find out what was going on. The genius had slipped a decimal point
with his slide rule and the deadload had been loaded with ten times
more foaming agent than it should have had.

The deadload had turned into a steel ball 10-12 feet in diameter with
it wheels neatly arranged around it's equator, plastic shoots were
flying dozens of feet into the air like fireworks and all over the
tarmac, beyond priceless.

I keep wondering what that guy would have done to a balun box . .

For 'canoe', read 'kayak'... and don't say that nobody warned you!

w3rv