Compound guns?

That's a good question and probably someone (maybe in the Med) has made some protos of what you call a 'true compound' but generally, the term indeed seems to sometimes be used for any pulley/polispast/demultiplied gun like you suspected.

Another question, and very much one of semantics, is whether the word 'compound' can be said to necessarily have to have anything to do with eccentric cams? In general, compound just means "two or more parts". Maybe in physics, it means something about a non-linear force delivery as in your eccentric pulleys? I am far from certain but I don't think so. Will be interesting if someone knows the answer:-).

But the more I think about it, the more I tend to think 'compound' can not be said to equal the eccentric cams parts of a bow. Maybe compound in a bow is the use of both pulleys and cams and maybe the bow folks kinda just annexed the word.

And maybe the spearo people didn't even think of compound guns when they started using the term, as there is such a thing as a compound pulley system already - which I guess is as close to home for a pulley gun as a bow is to a speargun.
In a mechanical pulley system - as with ropes and pulleys - I think the word 'compound' is used to define a special mix of pulleys where the tag line of the main sysem has another system of pulleys acting on it (again, two or more parts). I can't explain it well, sorry:-(

Now, a lot of pulley guns are actually just straight pulley systems, so maybe it is wrong to call some of them compounds but perhaps not because they are lacking eccentrics cams:D;)

To the first part of your question: I have searched for the past 20 mins because it is is an interesting question and I know I have seen designs like this before but can't find them again. Maybe from an Italian who experimented with it. I would think, Pete might have something bookmarked. In the meantime, I will keep searching.
I guess the new DreamAir can be said to have eccentric loading and power delivery though not with cams and rubbers but with air pressure and conical rollers, but tapping into the same idea of redestributing the force curve.

And I also agree that in the context of bows, 'compound' infers the use of a cam. I just happen to think you can use the same term in a speargun without using eccentric cams, without non-linear force distribution and without it being a misused term:-).
(If this website can be believed then there is such a thing as 'compound pulley systems' without any cams and without any change in speed or loading effort: Rope Rescue Calculating MA by Counting the Lines)

Quote from sharkey

I'm more interested in hearing from those with experience in both compound bows & pulley guns or who understands the concept. Compound doesn't just infer two different parts in this context, it implys two different pulls. That is, that it starts off hard & when the cams roll over there is a let off anywhere from 35/65%, depending on the cams. Two levels of effort. This also means that a spear can be lighter/thinner as it starts more gently before accelerating. Is anyone who is claiming "compound" actually using eccentric cams?

The closest thing to a compound bow is the "Sea Archer" shooter which is something like a Hawaiian Sling in that there is no trigger mechanism. The draw effort changes to a lower level as you hold the spear back ready to shoot due to the cam action offered by a pair of counter-rotating pulleys mounted on a vertical axle. The power comes from rubber bands at the front while the draw is provided by thin cables that wrap a pulley track on the drum or pulley peripheries that wind the cables in during the shot and the cables connect to a tail cap that pushes the spear out of the guide tube.

The cable pulls from the edge of each pulley, so it is operating at a constant radius, while the band pulls against the rim of a raised "fence" on the pulley and changes to a smaller radius as shown in the schematic when it comes off that fence. This then changes (reduces) the torque applied by the bands at maximum draw and allows the operator to hold against the band pull with a much reduced effort.

In the very early days, and that was the 1930's, 1940's and 1950's, no one was really sure what would work underwater and so they tried everything. But the only spearguns that worked and kept on working were the very simple ones, so all the weirdo guns vanished leaving pretty much what we have today.

A "eurogun", such as the first 1943 Champion "Arbalete", is like a slingshot or catapult held out at arm's length, only it is mounted on the end of a long pipe, like a camera on a selfie stick. Rather than your other hand holding onto the stretched band end and the spear tail we have a grip handle and trigger mechanism, so that is how the band powered speargun came about.

The "Sea Archer" was created to provide a speargun that was not a "gun" as at one time there was a concern that mechanical spearguns would be banned in certain places. Believe it or not there are some people who would still like to achieve that outcome. To maintain this weapon at maximum draw the compound bow concept was used, but bands replace the bow limbs and the tip pulleys/cams are in essence stacked on top of each other. In some ways it is a compound bow collapsed in on itself.

Sharkey, what do you mean by eccentrics?

2.
a disc or wheel mounted eccentrically on a revolving shaft in order to transform rotation into backward-and-forward motion, e.g., a cam in an internal combustion engine.

Origin
late Middle English (as a noun denoting a circle or orbit not having the earth precisely at its center): via late Latin from Greek ekkentros, from ek ‘out of’ + kentron ‘center.’
Translate eccentrics to
Use over time for: eccentrics

I believe he's refering to def # 2.

Cheers, Don

The "Dreamair" speargun has inner and outer winding drums that are in a sense continuously variable cams as the outer drums increase their diameter and the inner drum gets smaller in its diameter by virtue of spiral cut cable track grooves in the drums which are keyway mounted on the same shaft which spins. The relative radius that pulling forces are applied at in any given instant on each drum is somewhat constrained by what can be fitted in the available space. I estimated that 4:1 is about the largest ratio that can be achieved, but it may be more as the axle has a minimum diameter and the outer drums have to pull at about the level of the top deck/spear guide track.

You need to read this thread, particularly post #17,
http://spearfishing.world/spea…ion-roller-dyneema-2.html

The shape of the drums changes the leverage by pulling at one radius on the axle and applying it to hauling the piston against the air pressure in the reservoir tube from a smaller radius in order to gain a mechanical advantage. By manipulating the length of the lever arms the force profile can be altered, however the retrieval rates also change as a large radius wraps a longer cable length than a smaller one. The graph shows the desired outcome, but whether the gun actually achieves this we don't know yet.

Quote from sharkey

Interesting a 50% let off. I'm a bit slow with schematics & I am really going to need to see one & pull it apart & put it back together several times myself before I fully understand this new "dream air" concept. The videos of it working are awesome, it would also be nice to see the speed & distance qualified in them. Either way, its an exciting invention & its great to see ( in a large part thanks to yourself ) these creative people investing so much brain power & energy in spearfishing. Hopefully its not just a 50 million dollar space pen, to replace a pencil.

Apparently it was not really that much for the space pens, but I know what you mean.
The Truth Behind The Million Dollar Space Pen Hoax