Related discussion from yesterday:

https://news.ycombinator.com/item?id=23795913

At the altitudes these are flying at (10km+), the air is bone dry. Unless moisture is encountered on the ascent, there is no need for conformal coating.

P.S. - I've done a fair number of these flights and can speak in more detail about this hobby on request. Great fun!

It amazes me that the balloon can last that long. That it's not losing it's loft over time.

Maybe they just put on a thin layer of conformal coating?

There are several approaches that can be taken with balloons. By design, they do not expand like a weather balloon does. One wants to put in enough lifting gas to make it fly high (mostly to avoid clouds that will bring down a balloon with such little free lift), but not enough that it will burst.

In the referenced article, an SBS-13 balloon is used. It's made from a clear Mylar-like material and will fly at 40000ft+ when filled with helium (higher with hydrogen). This is the 'gold standard' of balloons and can fly for months at a time as it's over most weather. The downside is that they cost close to $200.

Another approach is to buy a cheaper 36" diameter balloon made of similar material from AliExpress that cost about $1. These are often pressurized to a certain differential pressure to 'stretch' the balloon before launch and gain some extra volume, as well as to QA the quality of the balloon (the QA on a $1 balloon isn't extensive). If filled with hydrogen, these can fly at 35000 ft+, a bit lower for helium. Usually two of these cheap balloons are used for a launch for more lift, though a single one can be used and a lower altitude reached.

A light payload is desirable as it will increase altitude. I've seen as light as 5 grams and the heavier ones are closer to 20g. One can calculate the free lift where the differential pressure of the balloon at altitude will pop it, but it's usually around 7g (so the balloon + payload + 7g will neither rise or fall before launching).

I've launched one of these balloons before. Made it from Colorado to the Gulf of Mexico and started making the turn up the coast before losing contact overnight.

The balloons are standard mylar balloons, but slightly bigger than average. About 2.5 feet in diameter. In order to allow the balloon to climb to ~30k feet, you need to leave a lot of room for the gas to expand. I'd say our balloon was only about 15-20% full at launch. You put just enough helium in the ballon so that it has about 1 gram of lifting force.

Airborne transmissions of this sort are not legal over the UK, North Korea, or Yemen. Guessing it's a leftover rule from WW2 in the case of the UK, not sure.

These transmitters typically have geofencing routines that turn off the transmission when flying over these countries.

In the US, a balloon with a total payload weight of less than 4 pounds doesn't require any kind of clearance or approval.

You can, but you may not get the results you're looking for.

APRS is a line-of sight signal. In a balloon, you might be heard by transmitters 200 miles or so away. On a boat, I'd expect that range to drop to 50 or less. So if you're out at sea, it's unlikely you'd be heard when you're much outside the view of land.

WSPR has a very long range, but the density of information transmitted is very low. Each transmissison takes about 2 minutes and the tx rate is around 5 baud (not 5kb...5 baud!). The protocol was developed to test the range of ham radio antennae and it doesn't give you much more than the transmission voltage and the 'maidenhead' coordinates (which will give your location within a roughly 80 mile box depending on your latitude).

If that rough estimate of your position is good enough, WSPR may be useful to you for tracking a boat. Some balloon trackers also transmit a secondary signal under a different callsign with some more advanced telemetry information, such as altitude, speed, more granular position within a 2 or 3 mile box.

Also, you'll need an antenna of some length. My balloons transmit on the 20m bands, which means it has a 17 foot antenna both above and below the tracker constituting a half dipole (it's 36 gauge magnet wire so weights almost nothing. If your boats are big enough to cross oceans, you can likely mount them in a way where you'd be heard.

I’m guessing it’s possible. I think WSPR supports a wide selection of bands so you can have over the horizon comms. APRS is usually VHF/UHF so it’s mostly line of sight. On land, there are APRS repeaters that rebroadcast your signal at higher power so I’m guessing it’s more suited for land.

I love Leo Bodnar's stuff. I made a 'flight sim' joystick and pedals from one of his boards

These are run by solar panels that only transmit during the day. The capacitor just gives a half a minute or so in the event of brief cloud cover, panel not facing the sun at a good angle, etc. Not strictly necessary, but since a WSPR message takes almost two minutes to send, a good idea to not have it abort in the middle of the transmission due to lack of power for a couple of seconds.

As the other person responded, basically a battery back-up.