You can get "winchers" for around £35 which most chandlers will be able to get. They always seem to be make out of a soft blue plastic and fit onto the top of the existing winch. However, I've seen reports on them which indicate that they don't last long and aren't very reliable.
I thought about them but ended up getting some shiny new Lewmar self tailers. Wonderful! I got some of my money back by selling the old winches.
I've had those winchers. They do work, and are much better than nothing, but they are not a patch on genuine self-tailers. However at £35 for the winchers compared to >£500 for a new pair of winches they are very good value for money.
Older winches cannot be converted. I believe that some more modern ones use the same shaft in both versions, so could theoretically be converted. Whether this would be economically worthwhile is another question.
My personal view of Winchers is that they are close to useless. The main problem is that coils are not stripped from the drum, so this has to be done manually, with the consequence that only a single turn can be made between stops. For a single hander this might offer some small advantage but for a crewed boat there is none.
I have added 'Winchers' to the old style Centaur winches - bottom drive on a ratchet as you would otherwise bark your knuckles on the guardwires or dodgers. Seem to work OK as long as you do not expect them to be as good as proper selftailers. Someone obviously thought they were useful as they tried to remove them from my boat, but they are very stiff to get on and even more difficult to get off without destroying them!
In practice, I found them virtually useless and all the users with whom I've since discussed it since seem to agree.
In my case I tried to convert my Enkes 8, reefing winch. All my other winches being self-tailers and nearly all my sailing being single-handed.
After one season I relegated the Enkes to a snubbing winch on the genoa furling gear and bought an Antal 16 (the bottom-end self-tailer) for the reefing winch.
I'd advise saving up to get real self-tailers - the Antal range is very well-priced, well-engineered and (being the old Harken subbie) identical, except for label and price to Harken.
A word of warning, Harken (and hence Antal) do not use the same nomenclature as Lewmar and other winch manufacturers - their winches are all a half-size smaller.
Is there somewhere where you can find out which winches use a common bolting pattern (and presumable be of similar size), so I don't have to consider re-drilling the deck when I get round to replacing my old Barient sheet winches?
Did have Winchers installed, but they were so old, that they proved only capable of leaving blue debris on the sheets and in the cockpit.
The previous owner of my Moody had fitted winchers to the secondary Lemar 30s. They were absolutely useless i9n every respect. I changed over to new ST30s last year and they have been superb. Best to put the £35 on the 6 Dog at Walthamstow...
If you do bite the bullet and buy new winches take a good look at the andersen winches, yes they are pricey but there build quality is second to none, you need less turns on the drum than a normal winch (i thought the ribs on the drum were just a sales gimmick, but they work) and they're nearly as light as an alloy winch.....keith