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.
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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.
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.
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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.