• Short fins vs long fins on 8s

    From Jake Frith@21:1/5 to Walter Martindale on Wed Jan 6 12:45:57 2021
    On Thursday, November 12, 2020 at 8:32:33 PM UTC, Walter Martindale wrote:
    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.

    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.

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  • From Henry Law@21:1/5 to Jake Frith on Thu Jan 7 10:02:30 2021
    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

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  • From Jake Frith@21:1/5 to Henry Law on Fri Jan 8 07:31:20 2021
    On Thursday, January 7, 2021 at 4:02:32 PM UTC, Henry Law wrote:
    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

    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.

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  • From carl@21:1/5 to Jake Frith on Tue Jan 12 14:26:22 2021
    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.

    Cheers -
    Carl
    --
    Carl Douglas Racing Shells -
    Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
    Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
    Find: tinyurl.com/2tqujf
    Email: carl@carldouglasrowing.com Tel: +44(0)1932-570946 Fax: -563682
    URLs: carldouglasrowing.com & now on Facebook @ CarlDouglasRacingShells

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  • From Jake Frith@21:1/5 to carl on Tue Jan 12 08:01:41 2021
    On Tuesday, January 12, 2021 at 2:26:02 PM UTC, carl wrote:
    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.
    Cheers -
    Carl
    --
    Carl Douglas Racing Shells -
    Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
    Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
    Find: tinyurl.com/2tqujf
    Email: ca...@carldouglasrowing.com Tel: +44(0)1932-570946 Fax: -563682
    URLs: carldouglasrowing.com & now on Facebook @ CarlDouglasRacingShells

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    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.

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