Interesting discussion! May I first respond to James' comments on the
fin in his boat? We bond them in with polyurethane mastic, James, never
silicone. And we rely on ligaments of PU embedded in keyhole slots in
the fin root to provide excellent retention but with loss of fin rather
than damage to boat if you indulge in heavy-duty dry-land sculling.
NB Nothing bonds well to the hard-anodised finish on our fins, so those
PU rubber ligaments are vital to fin retention. Silicone rubber is soft
& doesn't have what it takes to resist the normal knocks, let alone the
PTRC hard groundings at low tide ;)
Now to Glenn's comments:
The leading edge should be sloped backwards sufficiently to minimise the
risk of the fin retaining grass, weeds and man-made detritus. Also to
enable it to hop over floating debris with minimal damage to anything.
You're welcome to look at fins in our web shop to what we find works well.
If the leading edge is sloped, then it makes sense to slope the trailing
edge as you want to preserve a reasonable amount of area out near the
tip, which is doing a lot of the work out there in the undisturbed main
flow. It is the right length from leading to trailing edge (the minor
chord) that matters for fin performance (all other things being equal),
as the area thus provided has to resist side loads (i.e. generate lift)
until the sculler wants to make a significant change of direction
(whereupon it should stall). The slope of those edges is based on other
considerations, as indicated.
Fin depth (major chord, as with a wing) & area are largely down to
experiment & experience.
Thickness does matter as a fin that's too thin is easily damaged,
impossible to fully straighten & is too thin at its leading edge for
good performance (see below).
Thickness matters for other reasons. A plate fin must generate enough
lift when at a small angle to the flow to control the position of the
stern & the boat's direction under normal the off-axis forces, but be
capable of stalling if the sculler wants to change direction. And the
flow must reattach to pull the boat straight once the course correction
is done. A ~2mm aluminium plate works well & resists damage. The
leading edge should be radiused, never sharp - & not just to protect
fish & swimmers. A radiused leading edge better accommodates slightly
off-axis flows without premature stalling & minimises drag. But the
trailing edge should be moderately sharp, to prevent generation of
trailing vortices which generate a buzz & may diminish fin efficiency.
There are no standard shapes or locations for fins. You could say it's
an under-researched topic, but I might disagree ;)
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
& now on Facebook @ CarlDouglasRacingShells
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