| I have a piece of foam. I measured it and it's big enough for me if I
cut it on a slope. So I measured it around on the slope, making a line on
all four sides.
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| This is a picture of the Universal Hovercraft lift fan press. They
make a stack of blanks at a time, and I can't imagine any method I've heard
of or thought of that would make a better blank. When people order a prop,
they grab a blank and carve it out.
The basic process is to cut out some boards to appropriate lengths, plane
them to consistent thickness, then glue them together. UH uses epoxy. I
like plastic resin. Both seem to work well.
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| I took some metal flats and clamped them on the foam so that when I
hotwire the foam, the wire rides on the flats. The clamp jaws have to be a
bit from the edge, since the hot wire will sag a bit on the outside from
the pressure you use to keep it on the flats.
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| I can't take a picture while I'm using the hot wire, so I took this
picture with the wire in approximately the right position.
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| A few seconds with the wire and we have two pieces.
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| There are a couple steps here. First I eyeballed the foam on the
centerline of my hull and pressed it down so the hull made a mark on the
foam. That part was cut out using a different hotwire. That hotwire is a
wood handle with wire wrapped around it, with a heavy weight on the other
end of the wire for tension. I used whichever wire as needed during this
process. The weighted wire gets you a cut straight down.
The impression made by my hull to show where to cut is on the bottom of the
foam. The top of the foam is sloped, and since the foam is upside down
right now I had to brace it up with something so my weighted wire would cut
straight down and give me a vertical wall.
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| Now I'm trying to cut out more volume of foam. Anything to keep the
beads from getting out of hand.
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| That's done. I used the "bow" hot wire, with two straightedges clamped
on the lines.
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| Now I carved some grooves in there so i could fit it into the area to
be filled. I want a tight fit so I took into account the overlap over the
lift duct.
The yellow rectangle is the hole for the battery box.
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| Here's Marquis doing prop work.
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| Lots of stuff done now. The battery box is in (just resting) and I
went through lots of contortions with the foam. I had made the blocks so
they would sort of fill the space within 3/4 inch of the plywood. There's
a stringer holding the plywood up and I wanted to get as much coverage as i
could. In the process I cut the 3" or so gap you see here. Finally I
realized I wasn't going to get the foam in when the battery box was there,
and the battery box is in part held down by the foam. So I hotwired the
3/4 inch span of foam so the blocks would slide in.
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| Another view of that. In the long run it would have been easier and
faster (and probably better) to leave the stringer space empty and not have
to leave this gap.
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| Foamed the battery box in. I'm using some two-part pour foam here. I
don't really like this stuff but don't have the scratch to buy a decent
two-part spray foam, they're about $300 a shot and I need to get a load of
other stuff for this thing.
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| I epoxied the foam on now. The hole near the duct is where the battery
cables will go through, and the other gaps were very disappointing to me.
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| This is where the left-over epoxy goes...Waste not want not.
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| Marquis watered his prop enough it got bigger...
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| Still not happy with the gaps, I crammed bits of foam into the spaces
and was much happier.
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| Anything heavy gets drafted into holding foam down.
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