Short fins vs long fins on 8s

[James HS]

Fins on 8s seem to be getting longer and longer (I should say deeper) to the point that or standard 8 bay racking now seems not to work without damage between boats.

Someone told me there were 'summer' and 'winter' fins - though no idea what that meant.

Does someone that knows more about this than me know what the difference in handling would be between the standard (less tall) fin than the taller one.

I am on the tideway - and the steering portion of the fin dies not change in height, so presumably this is to keep the rear end from slewing?

I am trying to work out why we do not change back to the less deep design that neatly fits our current racking - or should we alter the spacing?

Empacher and Filippi

And I know an aerow would be the correct way to go :)

James

[carl]

Hi James -

The summer/winter fin idea will be news to most of us. A deeper fin
will have more effect than a shallower one, & depth matters more than
area. But why you'd want different summer & winter depths is a puzzle.

The job of the conventional flat-plate main fin is to keep the stern
from slewing, which is its natural tendency (the hull's centre of drag
being well ahead of the boat's centre of gravity). The job of that
little rudder is to over-ride the natural action of the main fin but,
being small & operating within the boundary layer of slower & more
disturbed flow close to the hull, it is not very effective at the best
of times.

Unfortunately, the way the steering then works is that the main fin
resists the efforts of the little flat rudder until cox has applied
rather a lot of rotation (causing significantly increased drag), but
then stalls (loses a fair bit of the "lift" with which it had hitherto
resisted the rudder's input), whereupon the stern swings. So when cox
seems to have difficulty in holding a good course, it might not be his
or her fault as they're having to ride a knife-edge between over- &
under-steer.

And, yes, there is better kit

Cheers -
Carl

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Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
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[Phil]

I'd hazard a guess that the less deep fin was named for 'summer' river depths...
I once saw a couple of aluminium skinned (yes, really!) 1x that had sprung, retracting fins.
Phil.

[James HS]

Carl,

Would there be a dis-benefit to reverting to the 'shorter' fins - they are still substantially longer than the steering rudder (and so I understand bad) as it seems strange that all the manufacturers have suddenly switched to this longer fin when we only used to total our 8s through bad judgement - now we do it every time we rack them!

The longer fins also make a more regular visit to the bottom of the river as careless coxes mis-judge the 'shallows'. The fact that it becomes 'shallow' on the tideway twice a day seems to leave them bemused!

James

[Andy McKenzie]

[Andy McKenzie]

On Monday, 2 November 2020 at 08:09:58 UTC, James HS wrote:

I thought that the variants of fins were about weed? A long thin fin presumably has less drag and slightly more stabilising effect, but is more likely to collect weed, and summer rivers are weedier!

[carl]

Andy -
I take your point on weeds, but don't understand why fins should be
getting deeper - which is what James says is happening.
A more sweptback shape, without the sharpening (it's pointless - it
doesn't reduce drag but does impair performance) of the leading edge,
would be better at shedding weeds, not to mention the polythene bags &
other detritus

James -
There's a solid case for a deeper rudder, but that's hard to arrange
with a plate fin.

Good steering gear, by keeping the boat under better control & inducing
less steering drag, is likely to give a crew more speed than any other
change you might make (say 1 to 5 seconds over 2k, depending on crew,
coxing skills & conditions).

[Lucy]

Is the summer/winter thing perhaps related to straight line racing in summer, where a deep fin might be helpful, vs. head racing in winter, with more bends?

[Walter Martindale]

On Monday, 2 November 2020 at 05:37:52 UTC-7, carl wrote:

> I take your point on weeds, but don't understand why fins should be
> getting deeper - which is what James says is happening.
> A more sweptback shape, without the sharpening (it's pointless - it
> doesn't reduce drag but does impair performance) of the leading edge,
> would be better at shedding weeds, not to mention the polythene bags &
> other detritus
>
>

I recall one day on the Avon in Christchurch, NZ, when one 8+, rowed by the OURC men, shredded three coxswains' hands right to the tendons when they reached under the boat to clear weeds off the sharpened fin. Not my program but I went over with a file and took the edge off the fin. Oddly nobody else cut their hands. The crew still won their race if I remember correctly (it was pre-2000). One of the stupidest things I've ever seen is people sharpening a fin on a boat.

[Jake Frith]

Sharpening the leading edge of a fin is hydrodynamically daft. If you were going to do anything, a gentle, curved radius would be best, but unnecessary as it's pretty close to that anyway. There is some small (imeasurebly, pathetically small) benefit to sharpening the trailing edge of underwater appendages but not worth even getting your file out when they are only the size of the fin on a rowing shell.

No serious water vehicle should ever have a flat plate fin, and definitely not a flat plate rudder. Though again, when a rudder /fin is as small as they are on rowing shells, the drag improvements in going to a slightly smaller symmetric foil fin are very small. If you replace a flat plate with a proper foil, it will produce more lift (in the case of a fin lateral resistance) for its surface area so can be made a bit smaller, so a bit less drag in a straight line. When steering though the foil is vastly superior to a flat plate- it can maintain laminar flow across its surface at much higher angles, this means a win/ win of both better steering and less drag!
Note that foils for rudders and foils for fins should have a different profile. Foils for fins are (or should be) thicker (than, for example in the case of a racing dinghy the foil for the centreboard). That's because a thicker foil may have a tiny bit more drag in a straight line than a thinner foil of the same depth and chord, but a thicker foil keeps attatchment and laminar flow across it at a higher angle of attack (so will stall less readily)- so is much better as a rudder. It's leading edge would not be remotely sharp, for most of the same reasons the leading edge of the tail of a Boeing 747 is not sharpened by guys with files on stepladders after every flight!

When a rudder is placed just behind a fin, as Carl says, the two are to an extent fighting each other- and because they have (or should have) different profiles to do different jobs, working out what is going on gets very messy, although computational fluid dymamics would probably be able to do it for you. However, why bother? No racing yacht has had a skeg hung rudder in about the last half century for this reason. Surely the solution is a balanced spade rudder of the correct size that gives both the lateral resistance you require in a straight line and the steering power you need when altering course? It has the profile it needs to work as a rudder, and when you turn it you turn the whole thing. Your rudder in a straight ahead position is your fin.

I like dipping rudders. One profiled and angled for turns to port, the other starboard, You want to make a little turn to port, you dip a little bit of port rudder tip, big turn to stbd, you immerse your whole stbd rudder etc. (They both slip out of the water totally when your steering is straight ahead- so zero drag). Make them bigger than you need, so if you ever need to do a BIG sudden turn- for tactical or safety reasons its taken care of, but you're not paying any drag penalty for that superpower when you're not using it. Easy to set up with rudder lines working against bungees, so for the cox it's the same, pull one line to go one way, the other to go the other! But I digress. I expect FISA have them well and truly banned. Or probably would if someone turned up with one. I always wonder about them when I watch the Boat Race or Henley though, as not sure how closely these events' elderly stewards run them to FISA's dreaded 'Definitions of rowing'.

Back to your NZ crew. I bet they also sanded the hull down with 1000 grade sandpaper, or didn't sand it down, or sanded the aft half of it, or polished it up to a mirror finish, or coated it with some hydrophillic, or hydrophobic etc. etc. snake oil product displaying a distinct lack of knowledge re: boundary layer theory.

[Henry Law]

On Wed, 06 Jan 2021 12:45:57 -0800, Jake Frith wrote:

> No serious water vehicle should ever have a flat plate fin, and
> definitely not a flat plate rudder

But if the boat is yellow all that scientific malarkey is rendered null
and void. With your considerable knowledge of fluid dynamics you must
surely realise that?

--
Henry Law n e w s @ l a w s h o u s e . o r g
Manchester, England

[Jake Frith]

Yellow's so last year! Most watersports now favour spread tow carbon fibre under a clear gelcoat.

I made a typo in my missive above a couple of paragraps in, in the unlikely event anyone's read it. It's the rudder that should have the thicker profile.

[carl]

On 08/01/2021 15:31, Jake Frith wrote:
> No serious water vehicle should ever have a flat plate fin, and
> definitely not a flat plate rudder

Hi Jake -

Might I respectfully disagree with your first point?

A (non-steering) fin - as on a 1x - serves to prevent the boat from
yawing between strokes. (Yawing is a shell's tendency to swing
off-course due to its centre of drag being ahead of its centre of mass.)

Over a rather narrow range of angles of attack a flat plate is as
effective as any aerofoil-section fin in generating lateral lift. But
once that limited range of attack (maybe +/- ~2 degrees) is exceeded it
stalls, progressively losing lift.

This is just what you want with an oar-steered boat - a system that
stops resisting when you need to change direction where an
aerofoil-section fin keeps resisting all efforts to turn or manoeuvre.

For any steering element, then I do agree that you need a
properly-shaped thick foil, the last thing you need being for your
rudder to stall. And for the flat fin (say 2mm thick) you should still
have a radiused leading edge & a sharpened trailing edge.

[Jake Frith]

Yes, Thanks Carl, I made an oversight in that aspect. I got too bogged down in the airfoil theory and forgot what the things are actually for. I should know as at one point I made a small clark-Y section fin with a dolphin profile for my coastal racing single. My idea was it would make buoy turning much faster as when you want to make a big, sharp turn (like the 180 degree turn that they annoyingly put in the middle of a coastal sculls race), it would 'let go' more spectacularly, allowing the boat to spin in a much sharper and faster radius, and provide less resistance when getting dragged sideways through the water in the main bit of the turn (one blade locked in deep, the other pulling like crazy), as it was of smaller area. But for the reasons you cite above, it didn't really work like that. It would 'let go' at inopportune moments during smaller course corrections on the straight bits or in sudden side gusts, and for the tight turning bit it felt just the same as the flat plate fin (I imagine because a long, thin boat gets so much of its resistance to sharp turning from its shape ex appendages) The boat has had a flat plate fin since.