Keels provide lift upwind and in fact most displacement mode sailboats use NACA foil templates for their keels. Downwind, keels are drag and in some boats, you pull up the centerboard to reduce that drag.
>in some boats, you pull up the centerboard to reduce that drag.
Oddly enough I'm writing from such a boat right now :). Though in our case the main purpose of having it retractable is to allow navigating shallow waters (like smaller marinas).
I believe that the boats where the keel lifts up do not rely on the keel weight to stay balanced. They are also easier to capsize (regardless of whether the keel is lifted) than the ones with a fixed heavy keel that lowers the center of the mass.
Probably less, as it would go sideways rather than tip.
In fact, lifting the keel/centreboard can be a good technique for manoeuvring towards a peer; often you have to keep the bow pointing a certain direction, but want to move a little sideways.
Common usage would have you believe that all keels need a keel weight, but many catamarans have sharper structures on the bottom of their hulls that are called keels. Generally a keel is not retractable, a centreboard is.
Are these functionally distinct from normal centreboards? I'm trying to recall the term and what comes to mind is a keel that's hung like a centreboard to reduce grounding damage.
The Catalina 22 is available with a swing keel that is about 550 lbs. Those models have no other keel. By contrast, my Morgan 24 has a full-length shallow keel with 1900 lbs of ballast, plus a fully retractable centerboard that is not particularly heavy. I think the only common distinction between the terms "swing keel" and centerboard is whether it provides appreciable ballast.
In theory, a centreboard is a type of keel, when describing its purpose and action.
However, in practice, nothing that is not a fixed structural element of the hull is ever referred to as a 'keel' [edit:1]. Thus, a reference a keel can be taken to mean a structural keel.
A centreboard will only ever be a pivoting blade that rotates down from (and up into) a housing inside the hull, and such a blade will only be described as a 'centreboard', unless the meaning is otherwise clear. [edit:1 redux, 'swing keel'.]
More rarely, dinghies will have simple 'daggerboards' that slide straight down, and likewise they will only be referred to as 'daggerboards' in any circumstance where ambiguity might occurs.
Keels are usually weighted, of course, but there's no distinct word for an unweighted keel, structural or otherwise.
----
1: nicwolff reports, as you might have noticed, that a (deliberately) heavy centreboard is referred to sometimes as a 'swing keel', a term with which I was unfamiliar. So, matters are less cut and dried I'd initially represented them to be. However, I think this leaves the presumption that a keel will be structural largely intact.
Keels provide lift upwind and in fact most displacement mode sailboats use NACA foil templates for their keels. Downwind, keels are drag and in some boats, you pull up the centerboard to reduce that drag.
You can always make things more complex.