Some notes from a marrow rum cynic - I hold to my position that it does not work (at least, not as some of you think).
Common to all the comments is that the result is sweet. That means that not all of the sugar has fermented out, so full alcoholic content has not been achieved - so don't expect super-strength. Lots of you report a sludgy liquid with no fermentation. I'd count yourselves lucky. Unless you add yeast, any fermentation could be caused by anything which floats by - including some particularly nasty bacteria. In the end, all of you end up fermenting the resulting liquid in a container - so why not put it all into that container in the first place? What you're doing in reality is allowing a marrow to rot in the presence of a lot of sugar. As the marrow cell walls break down, releasing the cell contents, the sugar liquifies. Any fermentation which occurs within the marrow can only be at the margins of the sugar/marrow contact zone, where the sugar concentration is less. This is because of an unalterable fact of life with yeast - it cannot function if the concentration of sugar is too high. Then again, nor can it function if the concentration of alcohol is too high - so you're never going to achieve more than 15 to 18% ABV - half the strength of rum at best. If you use baker's yeast, you're not even going to attain that - more like 10%.
When the sludge is in the container, fermentation cannot begin until the strength of the sugar solution falls below a certain level i.e. when enough rotting-marrow-liquid gets in there. It's distinctly possible that that will never happen - as a few of you have discovered. Unless you add your own liquid, of course, but then you're simply making marrow wine.
I think you're all wasting your time and some expensive sugar, but I'm not trying to put you off - I'm all for experimentation. However, I worry, sometimes, for your health. If you must do this, then please do it in a safe way (which, basically, means avoiding those nasty bacteria and avoiding the fruit flies). Chop up your marrow and put it into a clean demijohn. Add some yeast and give it a shake (that bit's important as you don't want all of the yeast sitting on top of the sugar). Now add the sugar - demerara will certainly give you a more "rummy" taste than anything else - in the same amounts that would have been stuffed inside the marrow. Put an airlock over the whole thing and then just leave it. There's absolutely no difference between this and what you were doing before, except that now you're doing it under controlled conditions. The marrow will rot and liquefy and the sugar will go into solution. The yeast will, if it's able, begin pockets of fermentation. What you may end up with is a very sweet but LIGHTLY alcoholic something. Or you may find that no fermentation has occurred at all - but that's no different from what some of you have reported anyway.
For what it's worth, the marrow is irrelevant. You could achieve the same (actually, much better and more dependable) result by fermenting a solution of demerara sugar with nothing else added except a lot of yeast nutrient. You'd need a hydrometer and you'd have to learn the technique of feeding the must with sugar solution, but it's possible (just) that way to achieve 20 or 21% ABV. At least that method's safe.
To sum up, a few guarantees ...
Using the "hang a marrow" method, at least 50% of you will fail to produce anything but sludge. Another 20% of you will produce a dangerously infected jar of brown sludge. Yet another 20% will come to your senses and throw the muck out. 10% of you will produce a very sweet table wine.
There are easier ways of making alcoholic drinks.
I know that some of you are going to insist that the traditional method works and that you end up with something like rum. But I'm afraid that yeast chemistry is now a well-understood science and it's not possible to make normal yeasts perform miracles - certainly not in such an uncontrolled and unhygienic manner.
The secret of life is to aim below the head (With thanks to MMM)