• Skeg Design

    From Glenn Engel@21:1/5 to All on Wed Sep 1 10:27:03 2021
    I've been 3D printing replacement skegs for some of the shells at my local rowing club as we get them knocked off by partially submerged debris in the river quite frequently. So far I've just been replicating the prior designs. One potential advantage
    to 3D printing in addition to cost is I can design in a weak spot that will allow the skeg to break before it rips a hole in the boat when it hits a log, making replacement an easier task.

    I've noticed quite a range of designs from fat or skinny, angled back side, straight back side, chamfered edges only in front or both front and back, curved top etc. With all the variations I'm beginning to think it's just random designs thrown on boats
    so they look good or perhaps easy to manufacture.

    Since I'm 3D printing these I can make them however I wish so I'm wondering if there are any known good designs that are recommended or that have been properly simulated and tested.

    What's the best skeg design to use?

    --
    Glenn

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  • From Matt C@21:1/5 to Glenn Engel on Thu Sep 2 22:46:39 2021
    Hi Glenn

    I did a little research in this area for windsurfer fins back in the day. There are standard foil shapes available. Effectively all of them have a lower coefficient of drag for a given width or cross sectional area compared to a flat plate, with or
    without bevels. Generally, foil shapes benefit in performance from a high aspect ratio.

    I also recall some research into boat rudders from 40 years back that found that a square trailing edge up to 1.5 mm wide had effectively the same drag as a sharpened trailing edge. Rounded or bevelled trailing edges had higher drag.

    Good on you for raising this. It might not be the biggest influence on boat speed, but I suspect that there is more than a bow-ball in it. The idea of putting a weak spot into the skeg itself is brilliant.

    Matt

    On Thursday, September 2, 2021 at 3:27:04 AM UTC+10, Glenn Engel wrote:
    I've been 3D printing replacement skegs for some of the shells at my local rowing club as we get them knocked off by partially submerged debris in the river quite frequently. So far I've just been replicating the prior designs. One potential advantage
    to 3D printing in addition to cost is I can design in a weak spot that will allow the skeg to break before it rips a hole in the boat when it hits a log, making replacement an easier task.

    I've noticed quite a range of designs from fat or skinny, angled back side, straight back side, chamfered edges only in front or both front and back, curved top etc. With all the variations I'm beginning to think it's just random designs thrown on
    boats so they look good or perhaps easy to manufacture.

    Since I'm 3D printing these I can make them however I wish so I'm wondering if there are any known good designs that are recommended or that have been properly simulated and tested.

    What's the best skeg design to use?

    --
    Glenn

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  • From James HS@21:1/5 to Matt C on Mon Sep 6 23:22:55 2021
    On Friday, 3 September 2021 at 06:46:40 UTC+1, Matt C wrote:
    Hi Glenn

    I did a little research in this area for windsurfer fins back in the day. There are standard foil shapes available. Effectively all of them have a lower coefficient of drag for a given width or cross sectional area compared to a flat plate, with or
    without bevels. Generally, foil shapes benefit in performance from a high aspect ratio.

    I also recall some research into boat rudders from 40 years back that found that a square trailing edge up to 1.5 mm wide had effectively the same drag as a sharpened trailing edge. Rounded or bevelled trailing edges had higher drag.

    Good on you for raising this. It might not be the biggest influence on boat speed, but I suspect that there is more than a bow-ball in it. The idea of putting a weak spot into the skeg itself is brilliant.

    Matt
    On Thursday, September 2, 2021 at 3:27:04 AM UTC+10, Glenn Engel wrote:
    I've been 3D printing replacement skegs for some of the shells at my local rowing club as we get them knocked off by partially submerged debris in the river quite frequently. So far I've just been replicating the prior designs. One potential
    advantage to 3D printing in addition to cost is I can design in a weak spot that will allow the skeg to break before it rips a hole in the boat when it hits a log, making replacement an easier task.

    I've noticed quite a range of designs from fat or skinny, angled back side, straight back side, chamfered edges only in front or both front and back, curved top etc. With all the variations I'm beginning to think it's just random designs thrown on
    boats so they look good or perhaps easy to manufacture.

    Since I'm 3D printing these I can make them however I wish so I'm wondering if there are any known good designs that are recommended or that have been properly simulated and tested.

    What's the best skeg design to use?

    --
    Glenn

    Older Empacher skegs were held in by a screw at the sternward end, and this was a small piece of aluminium and broke before the fin box was damaged - so a 'fuse' section is a useful element to include!

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  • From lindig@gmail.com@21:1/5 to James HS on Tue Sep 7 08:39:50 2021
    On Tuesday, September 7, 2021 at 7:22:56 AM UTC+1, James HS wrote:

    Older Empacher skegs were held in by a screw at the sternward end, and this was a small piece of aluminium and broke before the fin box was damaged - so a 'fuse' section is a useful element to include!

    The idea of a tear-off line is also explored in this design: https://theflyingboatman.co.uk/product/breakaway-fin-1x/

    -- C

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  • From James HS@21:1/5 to lin...@gmail.com on Wed Sep 8 02:21:35 2021
    On Tuesday, 7 September 2021 at 16:39:52 UTC+1, lin...@gmail.com wrote:
    On Tuesday, September 7, 2021 at 7:22:56 AM UTC+1, James HS wrote:

    Older Empacher skegs were held in by a screw at the sternward end, and this was a small piece of aluminium and broke before the fin box was damaged - so a 'fuse' section is a useful element to include!
    The idea of a tear-off line is also explored in this design: https://theflyingboatman.co.uk/product/breakaway-fin-1x/

    -- C

    Interesting

    I drive a CD

    have lost my fin a few times, and as it is held in with silicone it unobligingly pops out

    fortunately I can later wander the shoreline and retrieve :) - and a bit of silicone later it is back in, ready for my shoreline adventures!

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  • From Glenn Engel@21:1/5 to All on Wed Sep 8 19:39:47 2021
    How about the design of a skeg?

    - Matt C mentioned there is no difference between a 1.5mm and sharp trailing edge. What about the leading edge?
    - Should the back edge be angled back or perpendicular to the boat?
    - Is there an ideal angle on the leading edge?
    - What about height? How is the appropriate height determined?
    - Does thickness matter?
    - Are there standard shapes documented somewhere?

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  • From carl@21:1/5 to Glenn Engel on Thu Sep 9 23:34:07 2021
    On 09/09/2021 03:39, Glenn Engel wrote:
    How about the design of a skeg?

    - Matt C mentioned there is no difference between a 1.5mm and sharp trailing edge. What about the leading edge?
    - Should the back edge be angled back or perpendicular to the boat?
    - Is there an ideal angle on the leading edge?
    - What about height? How is the appropriate height determined?
    - Does thickness matter?
    - Are there standard shapes documented somewhere?


    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 ;)

    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 Matt C@21:1/5 to All on Thu Sep 9 21:18:49 2021
    Like Carl, I don't know of a standard fin shape. Perhaps we could revert to the well documented standard aerofoils? Here's a nice site on the basic NACA 4 digit profile: http://airfoiltools.com/airfoil/naca4digit?MNaca4DigitForm%5Bcamber%5D=0&
    MNaca4DigitForm%5Bposition%5D=40&MNaca4DigitForm%5Bthick%5D=5&MNaca4DigitForm%5BnumPoints%5D=81&MNaca4DigitForm%5BcosSpace%5D=0&MNaca4DigitForm%5BcosSpace%5D=1&MNaca4DigitForm%5BcloseTe%5D=0&yt0=Plot

    Just as an opinion, I think something like the NACA 0006 would be a good start.

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  • From James HS@21:1/5 to carl on Thu Sep 9 23:48:58 2021
    On Thursday, 9 September 2021 at 23:34:07 UTC+1, carl wrote:
    On 09/09/2021 03:39, Glenn Engel wrote:
    How about the design of a skeg?

    - Matt C mentioned there is no difference between a 1.5mm and sharp trailing edge. What about the leading edge?
    - Should the back edge be angled back or perpendicular to the boat?
    - Is there an ideal angle on the leading edge?
    - What about height? How is the appropriate height determined?
    - Does thickness matter?
    - Are there standard shapes documented somewhere?

    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 ;)

    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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    Thanks Carl for the info on the mastic - perhaps that is why they come out so easily now as I have been using silicone - will seek out something more durable :) (oh, and anticipate beeches better!)

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  • From Andy McKenzie@21:1/5 to Matt C on Fri Sep 10 01:44:55 2021
    On Friday, 10 September 2021 at 05:18:50 UTC+1, Matt C wrote:
    Like Carl, I don't know of a standard fin shape. Perhaps we could revert to the well documented standard aerofoils? Here's a nice site on the basic NACA 4 digit profile: http://airfoiltools.com/airfoil/naca4digit?MNaca4DigitForm%5Bcamber%5D=0&
    MNaca4DigitForm%5Bposition%5D=40&MNaca4DigitForm%5Bthick%5D=5&MNaca4DigitForm%5BnumPoints%5D=81&MNaca4DigitForm%5BcosSpace%5D=0&MNaca4DigitForm%5BcosSpace%5D=1&MNaca4DigitForm%5BcloseTe%5D=0&yt0=Plot

    Just as an opinion, I think something like the NACA 0006 would be a good start.

    Scary to enter a conversation on fin design with Carl in the room, but aerofoil sections might not be the best option for a fin. You want a fin to resist turning forces up to a point to keep you straight, and then stall, so the boat can be slewed easily
    when you want to turn. In general the more the fin resembles an aerofoil the more angle will be needed to get that stall. I also think aspect ratio is a design criteria where you are making engineering tradeoffs. From a hydrodynamic perspective a long
    narrow fin (think glider wing) should give less drag than a squat fin of the same area. But equally the risk of collecting floating debris and of damage from impacting the river bottom or solid objects on land goes up with aspect area, as does the
    stresses at the fins base - so we compromise. The extreme end of this compromise is the 'restricted' or touring boat designs which dispense with a fin and have a full length keel.

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  • From carl@21:1/5 to Andy McKenzie on Fri Sep 10 12:41:31 2021
    On 10/09/2021 09:44, Andy McKenzie wrote:
    On Friday, 10 September 2021 at 05:18:50 UTC+1, Matt C wrote:
    Like Carl, I don't know of a standard fin shape. Perhaps we could revert to the well documented standard aerofoils? Here's a nice site on the basic NACA 4 digit profile: http://airfoiltools.com/airfoil/naca4digit?MNaca4DigitForm%5Bcamber%5D=0&
    MNaca4DigitForm%5Bposition%5D=40&MNaca4DigitForm%5Bthick%5D=5&MNaca4DigitForm%5BnumPoints%5D=81&MNaca4DigitForm%5BcosSpace%5D=0&MNaca4DigitForm%5BcosSpace%5D=1&MNaca4DigitForm%5BcloseTe%5D=0&yt0=Plot

    Just as an opinion, I think something like the NACA 0006 would be a good start.

    Scary to enter a conversation on fin design with Carl in the room, but aerofoil sections might not be the best option for a fin. You want a fin to resist turning forces up to a point to keep you straight, and then stall, so the boat can be slewed
    easily when you want to turn. In general the more the fin resembles an aerofoil the more angle will be needed to get that stall. I also think aspect ratio is a design criteria where you are making engineering tradeoffs. From a hydrodynamic perspective
    a long narrow fin (think glider wing) should give less drag than a squat fin of the same area. But equally the risk of collecting floating debris and of damage from impacting the river bottom or solid objects on land goes up with aspect area, as does the
    stresses at the fins base - so we compromise. The extreme end of this compromise is the 'restricted' or touring boat designs which dispense with a fin and have a full length keel.


    Agreed in every particular, Andy.

    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 carl@21:1/5 to James HS on Fri Sep 10 13:01:44 2021
    On 10/09/2021 07:48, James HS wrote:
    On Thursday, 9 September 2021 at 23:34:07 UTC+1, carl wrote:
    On 09/09/2021 03:39, Glenn Engel wrote:
    How about the design of a skeg?

    - Matt C mentioned there is no difference between a 1.5mm and sharp trailing edge. What about the leading edge?
    - Should the back edge be angled back or perpendicular to the boat?
    - Is there an ideal angle on the leading edge?
    - What about height? How is the appropriate height determined?
    - Does thickness matter?
    - Are there standard shapes documented somewhere?

    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 ;)

    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

    ---
    This email has been checked for viruses by AVG.
    https://www.avg.com

    Thanks Carl for the info on the mastic - perhaps that is why they come out so easily now as I have been using silicone - will seek out something more durable :) (oh, and anticipate beeches better!)


    So the first loss took one heck of a bang while every subsequent (how
    many!!?) was much too easy? You will have a problem, however, unless
    you fully eliminate all traces of silicone rubber as it's an excellent
    release agent for anything else.

    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 Steve S@21:1/5 to All on Mon Sep 13 22:29:41 2021
    Hi Glen

    I have experience with the latest 1x and 2x fins from Empacher and Filippi. In both cases they are excellent modern designs, both in plan form and section, and I’d guess they entail substantially less drag than what for years preceded them. These
    companies have surely invested a lot in testing things like enough rake so that weeds are shed. They are a very useful first approximation for your project.

    A caveat: these designs have less lateral resistance, so in the event of a beam wind, one’s boat has more of a tendency to turn towards it.

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  • From James HS@21:1/5 to carl on Tue Sep 14 01:49:35 2021
    On Friday, 10 September 2021 at 13:01:41 UTC+1, carl wrote:
    On 10/09/2021 07:48, James HS wrote:
    On Thursday, 9 September 2021 at 23:34:07 UTC+1, carl wrote:
    On 09/09/2021 03:39, Glenn Engel wrote:
    How about the design of a skeg?

    - Matt C mentioned there is no difference between a 1.5mm and sharp trailing edge. What about the leading edge?
    - Should the back edge be angled back or perpendicular to the boat?
    - Is there an ideal angle on the leading edge?
    - What about height? How is the appropriate height determined?
    - Does thickness matter?
    - Are there standard shapes documented somewhere?

    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 ;)

    Cheers -
    Carl

    --
    Carl Douglas Racing Shells -
    Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
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    Thanks Carl for the info on the mastic - perhaps that is why they come out so easily now as I have been using silicone - will seek out something more durable :) (oh, and anticipate beeches better!)

    So the first loss took one heck of a bang while every subsequent (how many!!?) was much too easy? You will have a problem, however, unless
    you fully eliminate all traces of silicone rubber as it's an excellent release agent for anything else.
    Cheers -
    Carl

    --
    Carl Douglas Racing Shells -
    Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
    Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
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    It is not all user error :) (but quite a bit is)

    At low tide, the inshore zone becomes quite narrow, and those using the (correct) part of the fairway are quite close - so I do have a tendency to occupy a safe zone close'ish' to the bank.

    Often what looks like a proper depth of water has a shock (Granite sets that hold the wall up have crumbled and migrated into the inshore zone) so you can have blade depth on both blades one minute, then your riverside blade will hit a rock (some 3-4
    metres from the shoreline) and then your shoreside blade (in what looks to be the proper depth), and then your fin

    Normally I know where they are, but occasionally when I am concentrating on technique (yeah, fairly rare) I forget.

    Go further from shore - no chance, the buoys which define the inshore zone are there to remind you, and navigation of others is often more perilous than losing a fin.

    I once lost a fin on an outcrop (this one I should have known better) and did my 12K outing (boy does it point out any power imbalance) - returned to the club, wandered to the now dry outcrop, retrieved my fin and replaced it!

    Will replace the silicone :)

    James

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  • From Glenn Engel@21:1/5 to All on Tue Sep 14 22:04:25 2021
    Wow, this is great info about design factors. Thanks everyone.

    Carl, I looked at some of the fins on your site to get an idea of shape and they remind me considerably of shark fins. Probably no surprise mother nature has perhaps figured out some hydrodynamics as well. I'm curious what material is used on your
    AeRowFin hinge. In one direction it must compress while in the other it must stretch. I understand if this is a proprietary element of your design.

    Regarding accidents - our river has a tidal influence and we see swings up to 12 feet with the current sometimes going upstream. Given the number of old pilings, trees, and other junk that floats up and down the river, it's a different hazard situation
    every time we row and sometimes a new hazard appears out of nowhere it seems and others stick around for months coming and going with the tide. This of course leads to broken fins, flipping of small boats, and once I recall a double getting high
    centered on a large log with the coach standing on the log pushing the boat off. We even had a Native American totem pole floating down one day... The stories are endless. Needless to say we never row without a safety launch near the boats.

    Glenn

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  • From James HS@21:1/5 to Glenn Engel on Wed Sep 15 00:50:58 2021
    On Wednesday, 15 September 2021 at 06:04:26 UTC+1, Glenn Engel wrote:
    Wow, this is great info about design factors. Thanks everyone.

    Carl, I looked at some of the fins on your site to get an idea of shape and they remind me considerably of shark fins. Probably no surprise mother nature has perhaps figured out some hydrodynamics as well. I'm curious what material is used on your
    AeRowFin hinge. In one direction it must compress while in the other it must stretch. I understand if this is a proprietary element of your design.

    Regarding accidents - our river has a tidal influence and we see swings up to 12 feet with the current sometimes going upstream. Given the number of old pilings, trees, and other junk that floats up and down the river, it's a different hazard situation
    every time we row and sometimes a new hazard appears out of nowhere it seems and others stick around for months coming and going with the tide. This of course leads to broken fins, flipping of small boats, and once I recall a double getting high centered
    on a large log with the coach standing on the log pushing the boat off. We even had a Native American totem pole floating down one day... The stories are endless. Needless to say we never row without a safety launch near the boats.

    Glenn

    It is definitely the flotsam that presents the biggest challenge - i flipped last week (first time in 12 years) as I was moving at rate and left hand blade hit a floating tree. Just when I have been loosening my grip :)

    I could stand at the bank, but slimy mud made climbing back in tricky, so (after a bit of thinking) leg over the boat (straddle) and then I could work my way yo feet on the shoes, and seat under - was not elegant, but I was never not in control of the
    boat :)

    If on my own I tend to boat 2 hours after high tide as there is more shoreline showing :)

    James

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  • From carl@21:1/5 to Glenn Engel on Thu Sep 16 01:13:34 2021
    On 15/09/2021 06:04, Glenn Engel wrote:
    Wow, this is great info about design factors. Thanks everyone.

    Carl, I looked at some of the fins on your site to get an idea of shape and they remind me considerably of shark fins. Probably no surprise mother nature has perhaps figured out some hydrodynamics as well. I'm curious what material is used on your
    AeRowFin hinge. In one direction it must compress while in the other it must stretch. I understand if this is a proprietary element of your design.

    Regarding accidents - our river has a tidal influence and we see swings up to 12 feet with the current sometimes going upstream. Given the number of old pilings, trees, and other junk that floats up and down the river, it's a different hazard
    situation every time we row and sometimes a new hazard appears out of nowhere it seems and others stick around for months coming and going with the tide. This of course leads to broken fins, flipping of small boats, and once I recall a double getting
    high centered on a large log with the coach standing on the log pushing the boat off. We even had a Native American totem pole floating down one day... The stories are endless. Needless to say we never row without a safety launch near the boats.

    Glenn


    Glenn -

    I should explain first that, unlike the usual stuff with which coxes are expected to steer, our AeRowFin steering foils require never more than
    20 degrees of rotation at the hinge to achieve a powerful turning
    effect, & normally we are talking about just 1 or 2 degrees. We tell
    coxes to steer by millimetres, not inches, for a smoothly graduated and proportionate response.

    So the rubber seals provide a smooth transition between the main part of
    the AeRowFin & its steering element - the trim-tab. So the seals
    should never experience serious levels of stress or deformation.

    Fin profile has much to do with not providing a bluff edge for detritus
    to hit & wrap around. In principle, a forward-raked fin would work
    fine, but be a real weed collector. 20 years ago we created
    forward-raked dagger-boards for the C-Class catamaran that we had built,
    but the purpose there was to reduce the chance/degree of aeration that
    could otherwise cause flow separation & stall on a heavily-loaded
    dagger-board - through atmospheric air getting sucked down the low-
    pressure face of the foil when its upper end broke the water surface.

    There is a relevance in that to the rowing stroke - the near-surface
    oarblade will suck air into the interface between its convex back and
    the bulk of the water, thereby breaking the tension which plays a large
    part in resisting face-first blade slip (& consequent energy dissipation
    & loss of propulsive efficiency).

    Cheers -
    Carl

    --
    Carl Douglas Racing Shells -
    Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
    Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
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  • From Mark Liddell@21:1/5 to All on Mon Oct 4 15:17:28 2021
    I've not researched this but I'd caution looking at naca sections and windsurfer fins to try, especially for small boats. I'm sure that the fins are often flat plates because they need to stall to be able to steer effectively, the problem is a different
    problem.

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  • From carl@21:1/5 to Mark Liddell on Tue Oct 5 15:49:10 2021
    On 04/10/2021 23:17, Mark Liddell wrote:
    I've not researched this but I'd caution looking at naca sections and windsurfer fins to try, especially for small boats. I'm sure that the fins are often flat plates because they need to stall to be able to steer effectively, the problem is a
    different problem.

    Hi Mark -

    You'll find, from my past postings here, that I recommend not using
    aerofoil sections for the "dumb" fins on singles for exactly that reason
    - that we need to be able to stall the fin at quite small angles of
    attack when making a turn or course correction.

    Cheers -
    Carl


    --
    Carl Douglas Racing Shells -
    Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
    Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
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  • From Glenn Engel@21:1/5 to All on Tue Oct 5 11:15:27 2021
    At what point does a 'dumb fin' flat plate become an aerofoil? Is it when it becomes fatter with a feathered leading edge or when it both starts fatter and tapers down on the tail?

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  • From carl@21:1/5 to Glenn Engel on Tue Oct 5 22:45:33 2021
    On 05/10/2021 19:15, Glenn Engel wrote:
    At what point does a 'dumb fin' flat plate become an aerofoil? Is it when it becomes fatter with a feathered leading edge or when it both starts fatter and tapers down on the tail?


    Even a flat plate is, in effect, an aerofoil. But it acts well as a
    foil (as a wing) over only a rather narrow range of angles of attack.

    Aerofoils proper (for this application, because in supersonic conditions
    other rules apply, & we never go there when rowing!) have radiused
    leading edges, get thicker until you pass a certain point (maybe 25-35%
    from the (now radiused) leading edge and progressively taper to very
    little at the trailing edge. There is a huge range of aerofoil sections
    in the published literature, but each has its own particular
    characteristics and the trick is in choosing one relevant to your needs.
    In the case of the 1x fin/skeg, a flat plate around 2mm thick & with a radiused (NOT sharpened!) leading edge & a tapered (slightly sharpened)
    last bit to its trailing edge is what you need for a responsive yet
    effective fin, not an aerofoil section fin.

    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
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  • From Glenn Engel@21:1/5 to carl on Tue Oct 5 19:36:09 2021
    On Tuesday, October 5, 2021 at 2:45:16 PM UTC-7, carl wrote:

    In the case of the 1x fin/skeg, a flat plate around 2mm thick & with a radiused (NOT sharpened!) leading edge & a tapered (slightly sharpened)
    last bit to its trailing edge is what you need for a responsive yet
    effective fin, not an aerofoil section fin.

    This is interesting. My local club has a number of quads where the steering has been disabled to keep it simple for inexperienced bow seats. Does this mean those quads would be better served with a flat plate design over a disabled aerofoil?

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  • From carl@21:1/5 to Glenn Engel on Wed Oct 6 14:37:15 2021
    On 06/10/2021 03:36, Glenn Engel wrote:
    On Tuesday, October 5, 2021 at 2:45:16 PM UTC-7, carl wrote:

    In the case of the 1x fin/skeg, a flat plate around 2mm thick & with a
    radiused (NOT sharpened!) leading edge & a tapered (slightly sharpened)
    last bit to its trailing edge is what you need for a responsive yet
    effective fin, not an aerofoil section fin.

    This is interesting. My local club has a number of quads where the steering has been disabled to keep it simple for inexperienced bow seats. Does this mean those quads would be better served with a flat plate design over a disabled aerofoil?


    And your post above intrigues me, Glen. The idea of an inexperienced
    quad without steering seems a little exotic, & potentially exciting in
    all the wrong kinds of way.

    Do they keep going in roughly straight lines, stopping before anything
    untoward happens? I'd have thought that having a competent steersperson
    in the boat might be quite a good idea.

    I guess they steer by someone shouting for more work on one side, in
    which case a flat plate could make that easier but, unless I've
    misunderstood, I'd say there could be a flaw in your club's approach.

    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
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  • From Bob@21:1/5 to Glenn Engel on Wed Oct 6 08:04:22 2021
    On Wednesday, October 6, 2021 at 8:26:58 AM UTC-6, Glenn Engel wrote:
    And your post above intrigues me, Glen. The idea of an inexperienced
    quad without steering seems a little exotic, & potentially exciting in
    all the wrong kinds of way.
    Hah! That sounds about right. I don't know the source of the notion as it's been part of the club since I started rowing about 12 years ago.

    This is only used with masters groups and it's mostly for pick-up boating where there isn't a fixed lineup. Generally the bow seat will maintain straight lines or mild turns by powering up on one side or the other for a few strokes much like they would
    in single or double. For more serious turns the boat is instructed to power up for a few strokes on one side. It works fairly well in practice for recreational outings but I find it totally unsuitable for head racing as with speed the boat seems to
    resist turning. I've seen boats get off course enough to need to power down significantly to turn.

    Glenn
    I partly own a quad (with steering). Our normal bow steers it like a race car around the reservoir. When I bow I try to minimally steer. The club with which I'm also associated has quads with disabled steering on a circular reservoir with buoys. I've
    tried to get the coach to put the steering back in to no avail. The bow calls the steering. Two seat calls the workout. They steer by pressure calls and by having one side feather across the water for more serious turns. I was put in the bow of one
    of those quads in the dark of early morning and it was not a pleasant experience. While they haven't smashed any boats or injured anyone so far, that doesn't mean anything. I just reread Richard Feynman's book that includes the Challenger rocket
    disaster. He pointed out the flaw in the NASA argument on space shuttle reliability that used past performance an indication of reliability. He had a term for that logic. It's called Russian Roulette. In their favor the club has converted some of the
    coxed fours into coxed quads for novices.

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  • From Glenn Engel@21:1/5 to All on Wed Oct 6 07:26:55 2021
    And your post above intrigues me, Glen. The idea of an inexperienced
    quad without steering seems a little exotic, & potentially exciting in
    all the wrong kinds of way.

    Hah! That sounds about right. I don't know the source of the notion as it's been part of the club since I started rowing about 12 years ago.

    This is only used with masters groups and it's mostly for pick-up boating where there isn't a fixed lineup. Generally the bow seat will maintain straight lines or mild turns by powering up on one side or the other for a few strokes much like they would
    in single or double. For more serious turns the boat is instructed to power up for a few strokes on one side. It works fairly well in practice for recreational outings but I find it totally unsuitable for head racing as with speed the boat seems to
    resist turning. I've seen boats get off course enough to need to power down significantly to turn.

    Glenn

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  • From Glenn Engel@21:1/5 to All on Sat Jul 2 08:23:43 2022
    Adding another question to the topic of design. Carl mentions the usefulness of sloped leading and trailing edge. How does height affect performance? Is taller better performance or is it a tradeoff between catching and hopping over debris and
    performance? I suppose taller would also be more likely to bend as well.

    --
    Glenn

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  • From Andy McKenzie@21:1/5 to Glenn Engel on Mon Jul 4 02:26:58 2022
    On Saturday, 2 July 2022 at 16:23:45 UTC+1, Glenn Engel wrote:
    Adding another question to the topic of design. Carl mentions the usefulness of sloped leading and trailing edge. How does height affect performance? Is taller better performance or is it a tradeoff between catching and hopping over debris and
    performance? I suppose taller would also be more likely to bend as well.

    --
    Glenn
    In a general sense a long thin fin will 'perform' better than a short and broad fin of the same area. High aspect ratio fins generate less drag and more lift to counter rolling. But like any engineering decision there's a compromise to be made - a long
    fin is more likely to be damaged, will be more flexible, and will be more likely to catch and collect weeds etc.

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