Skeg Design

[Glenn Engel]

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

[Matt C]

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

[James HS]

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!

[lin...@gmail.com]

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

[James HS]

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!

[Glenn Engel]

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?

[carl]

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

[Matt C]

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.

[James HS]

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!)

[Andy McKenzie]

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.

[carl]

Agreed in every particular, Andy.

[carl]

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.

[Steve S]

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.

[James HS]

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

[Glenn Engel]

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

[James HS]

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

[carl]

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).

[Mark Liddell]

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.

[carl]

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.

[Glenn Engel]

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?

[carl]

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.

[Glenn Engel]

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?

[carl]

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.

[Glenn Engel]

> 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

[Bob]

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.

[Steve S]

At the Canadian masters nationals in Montreal a few years ago, I noticed that almost all the quads had no rudders, just big fixed skegs. It drew my attention because a couple of weeks prior toeing errors had caused my team to loose first place at the USA masters nationals.
Hugh Hudson was looking after the Hudson stand at the regatta and I asked him about the Canadian practice. He told me they use fins designed for 8s. We ordered one for our club's quad, installed it, and pulled the rudder and toeing mechanism.
It has proved to be a wise choice for 1k masters events. Why? Because our quad racing crews are usually stitched together at the last minute, often as composites with scullers from other small clubs, and it is unlikely that anyone in these crews has the talent to handle effectively a toe mechanism that is new to him or her.
We've made a useful adaptation: now the bow is careful about calls to keep the quad tracking along the middle of the lane, minimizing the excursion from straight line. These calls are more frequent, sometimes much more frequent, than would be the case with if the person with the toe was experienced and dexterous.

[Bob]

Steve,

I can see how that works well in a sprint race. Our res is circular with buoys that force us along the irregular shoreline.

It is interesting that you mention Hudsons. We have a few Hudson doubles in the club and they are hard to turn.
In fact, I won't row them but I don't have to as I'm part owner of an old Vespoli double that turns on a dime and is a dream to row
on the Reservoir. I'm sure it is slower than the Hudson on a straight course. Most of my previous experience is in whitewater where our boats are meant to turn easily. They are upturned and rounded at the ends.

I suspect the hull design is most important in how easily a boat turns but that fin design can improve turning for a given design?

A further anecdote in support - before becoming a kayaker I learned to whitewater (open) canoe on the Potomac River in DC. I had purchased an open canoe that was advertised as a downriver canoe. The instructor stood on a rock telling us to head for it and avoid it at the last instant by making drawing and prying strokes. We did as he said and hit the rock dead on. The third time I was prying so hard I broke my paddle. The boat would not turn. A true whitewater craft turns easily and is symmetric . A downriver canoe - not so much. That has to be hull design.

Do you need separate boats for sprint and head racing?

Your point about frequent calls to stay centered in the lane is really interesting. A boat like a canoe or kayak, designed to turn, requires an adjustment on every stroke or it is in danger of spinning out. With frequent calls you only need small changes that can be accomplished by going harder on one side without reducing the opposite
side effort. I suppose crews that have been together do that automatically.

[Dick White]

Steve, the mention of Hudson doubles is interesting to me also. Our club has a collection of Hudson doubles (no other brands) that are a chore to turn. Our reservoir is 11 miles of twisting former river with lots of wild life to make the long rows enjoyable. Saw two bald eagles today and an osprey diving for its lunch. But those turns are a chore. Do you have any sense of what the difference is between your Hudsons and your delightful Vespoli?

[Bob]

On Thursday, October 7, 2021 at 7:59:03 PM UTC-6, Dick White wrote:
> Steve, the mention of Hudson doubles is interesting to me also. Our club has a collection of Hudson doubles (no other brands) that are a chore to turn. Our reservoir is 11 miles of twisting former river with lots of wild life to make the long rows enjoyable. Saw two bald eagles today and an osprey diving for its lunch. But those turns are a chore. Do you have any sense of what the difference is between your Hudsons and your delightful Vespoli?

Dick,

It is a shame that clubs such as ours don't make more informed decisions when purchasing boats for our particular venues. In retrospect, the Hudsons may not have been the best boat for both our needs. I just volunteer coxed Novices for their first time in a Vespoli eight and I had to sit on top of the deck. The cox seat was so narrow that most Masters rowers could not fit in it. Most cox seats are a tight fit but we manage in our other eights. There are many questions that should be asked about boats other than price and availability when deciding on a purchase for a Masters club. For example, is a bow coxed four really a good choice for Masters rowing with "volunteered" coxswains?

Regarding the special features of the Hudson hull that make it want to go straight vs the Vespoli hull, there are more knowledgeable people than myself who can answer that better. Maybe your question will draw them out so that we can all become better informed?

Bob

[Andy McKenzie]

In our recreational club we won't have any bow loader fours, and definitely make purchase decisions based on cox comfort. I once scrapped a wooden eight that we were given that had a wooden strake at exactly hip bone height. No idea why everyone got so huffy when i suggested planing 6 inches off their hips....

[carl]

I knew of an eight into the bow seat of which only the snake-hipped
could fit. ISTR that it was known as "The Bacon Slicer".

Oar-steerability of a shell depends on a complex mix of: fin design, fin
position, rudder (if any) design, hull form, bow shape & weight
distribution. Also waves and cross-winds.

IMO, all shells down to 2x should be rudder steered, but all crews would
do well to learn the sensitivity needed to control the boat with their
oars. That last was demonstrated a few years back in the Head of the
Charles, when the Great Eight lost its rudder but those top-class
scullers took that set-back in their stride.

In the Moscow Olympics I heard that the GB eight broke a rudder wire, &
cox, Colin Moynihan, had to reach behind himself to grab the steering yoke.

But regarding the comments in an adjacent thread - on sitting the boat
for "masters" rowers - it seems to me that adults of advancing age
ought, at the very least, to appreciate the need to sit their own bit of
the boat. Otherwise, how dare they call themselves masters, & why
should anyone wish to row with them?

[Steve S]

> oars. That last was demonstrated a few years back in the Head of the
> Charles, when the Great Eight lost its rudder but those top-class
> scullers took that set-back in their stride.
>

The race that Carl mentions was a classic. The woman cox of the Great Eight was an English graduate student or post doc at Harvard who had a sterling reputation from coxing v. successfully on the Tideway. When they lost their rudder she aided the steering by immersing her left or right hands as appropriate.
As I recall, it was snowing at the time.

[carl]

Steve - I'd forgotten that bit! Thank you.

And, of course, the human hand, held vertically and thumb-first into the
flow, is not so bad an approximation to an aerofoil or a wing (as any
kid who ever held a hand out of the car window will recall). Effective
but rather draggy.

There should be a book on rowing with a chapter on how coxes saved the
day, regardless of personal cost.

[Bob]

Here is the rec sport recap on the whole affair:

https://rec.sport.rowing.narkive.com/Q2YuGLOa/great-8-steering-at-the-charles#post2

Fascinating and germane to the start of this thread before I got it off track.

Bob

[Glenn Engel]

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

[Andy McKenzie]

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.