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  skeg measurement trials
  Posted by: gjf12 on Jan-27-13 5:53 PM (EST)
   Category: unassigned 

I paddled my Seda Ikkuma inside the bay, at about 4 kts, in a steady 10 kt cross wind and very light chop. The Ikkuma weather cocks very slightly, as a good design should, and is easy to control directionally with slight leans and slight sweeps.

However, my GPS indicated an increase in speed of 0.1 to 0.2 kts was achieved by deploying the skeg. Without the skeg slight sweeps and leans are required every 4 or 5 strokes, while with the skeg deployed even slighter corrections are required every 20 or 30 strokes. I repeated these skeg up and down trials multiple times with the same results.

The Ikkuma skeg is small, with high aspect ratio, but quite effective. Any drag increase due to the skeg is overwhelmed by the benefit of greater stroke efficiency.

YMMV. Don't know if this applies to any boat and skeg.

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Messages in this Topic

 

  sounds about right
  Posted by: edzep on Jan-27-13 6:24 PM (EST)
I think the "debate" begins when people talk about using skegs NOT for wind, but to compensate for a hull that may be too loose, or for a paddler's technique causing side-to-side veer. Or, in my case, just to coast in a straight line when not paddling -- messing with a camera. Some of us have found that the skegs cause drag and impede speed. I often forget to put my skeg up, after taking photos. I soon notice that the boat feels sluggish.
 
 
  low drag
  Posted by: gjf12 on Jan-28-13 12:27 AM (EST)
Using my gps on calm water I have not been able to detect any drag/speed difference with the skeg deployed or not. Perhaps your skeg is very large or you are more attuned to small drag differences.
 
 
  Faster/less effort with effective skeg
  Posted by: Celia on Jan-28-13 7:51 AM (EST)
At least that is what I have found the few times I have paid attention. The times that I have it more than partially deployed, I am usually focused on other things. IMO, the skeg and drag thing is way overrated as an issue for anyone other than people racing. The keel on my first sea kayak was a lot more distracting than the skeg has ever been (which is not always healthy for a skeg when landing).
 
 
  Really depends on the hull shape
  Posted by: FrankNC on Jan-28-13 11:39 AM (EST)
Like most racing boats NEED the rudder, many standard kayaks perform better the skeg.

I like that you took the GPS and made your own discovery with it. I learn the most about my different boats by trying to pay attention to how it responds in different situations, but I'd do better if I kept notes.

Right now, my newest boat does not respond well to an outside edge but really turns with an inside edge which is opposite of what I do most of the time. It is really taking me a long time to adjust. I hope to become unconsciously competent with it, but then I wonder if I'll be able to go back to a sea kayak and paddle using mostly the outside edge.

 
 
  Design consideration...
  Posted by: Celia on Jan-28-13 2:08 PM (EST)
Yes, it still goes to the designer's intent. My first sea kayak really was intended to work with a rudder even in its stowed position - it was part of how that hull was planned to deal with wind. Since I found the rudder to be a PITA for my own purposes and always ended up taking it up again, that meant a different boat when I decided that I'd rather be living with a skeg. So I got a boat that was designed with a skeg in mind.

QCC tends to offer a choice of either on their boats, which can muddle the point for newbies that a given hull is often designed with an assumption about whether rudder or skeg. So we end up talking about these things artificially, as though you can take any hull and successfully pop one off and the other on. But for some boats that can be a fairly bad idea - if you want the hull to behave you really have to get the device that normally comes with it.
 
 
  Which boat?
  Posted by: booztalkin on Jan-28-13 3:48 PM (EST)
My first kayak turned towards the inside edge. It really messed me up because everyone was telling me it was supposed to turn away from the leaned (down) side. After paddling it hundreds of hours (weird thing named Kanoe Latvija), I concluded it had three turning positions, slight, medium, and heavy lean, which turned the boat in, out and in. It did mess me up. I got rid of it.

I am wondering what boat you have that is giving you the fit.

~~Chip
 
 
  Sorry, your test proves nothing.
  Posted by: scott_f on Jan-29-13 1:18 AM (EST)
That small margin of difference could have easily been your mindset. Without doing a blind test, that 2.5%-5% difference means nothing. Not trying to be harsh, just looking at this scientifically.
 
 
  proof?
  Posted by: gjf12 on Jan-29-13 3:00 AM (EST)
I agree that nothing was proven. However, I tried it enough times to lead me to deploy the skeg in wind and chop if I want to go a bit faster or with less effort.
 
 
  I agree about a blind test
  Posted by: adbass on Jan-29-13 8:37 AM (EST)
and about the lack of definitive proof, but really...
...a skeg is usually used to counter the effects of the wind, such that fewer correcting strokes are required from a paddler to hold a straight course. Is it not inevitable that a boat covers a greater distance with less effort with its skeg deployed?
 
 
  I would tend to agree, and...
  Posted by: scott_f on Jan-30-13 5:06 AM (EST)
I would say that is a good hypothesis. I'm not sure the efficiency of a skeg has ever really been tested. It would take an incredible amount of resources to really prove the efficiency. There are simply so many variables (paddle style, stroke style, paddler size and ability, skeg shape, hull shape, water conditions, wind, etc). I guess it's reasonable to go along with the experience of someone like Paul Caffyn who agrees that skegs over distance are better (and rudders are better yet), but I tend not to firmly believe anything without testing by scientific method. And even then, a theory is only correct until it's proven wrong. My experience shows that a skeg is more efficient for beginners in high winds because they don't have the skills to cope without an aid to prevent weathercocking, and some boats also have a strong need of an aid to prevent broaching. But when it comes to splitting hairs over 2.5%-5% efficiency, I really don't know. Either way, it's an interesting thing to think about.
 
 
  Accuracy of gps ?
  Posted by: willi_h2o on Jan-29-13 7:56 PM (EST)
-- Last Updated: Jan-29-13 8:16 PM EST --

The GPS processes the incoming satellite signals
and calculates the difference in timing and
equates that to distance over time which equals speed.

I don't believe the equipment and conditions can
be set up or calibrated in a way to truly record
differences that can't be explained away by the
inaccuracies of measurement.

Try mowing your lawn and see if the GPS records
every turn, every variance, every nuance of your
moves in an extremely small area like a residential lot.
Handheld GPS units don't like slow speeds with small movements

 
 
  You are probably correct regarding
  Posted by: adbass on Jan-30-13 8:33 AM (EST)
GPS accuracy if the original poster is talking about measurements of "instantaneous" speed, but not if the average speeds were calculated over sufficiently longer distances.
I don't know which is the case here.
 
 
  Side to side movement
  Posted by: willi_h2o on Jan-30-13 12:57 PM (EST)
GPS simply won't pick up that very minimal ziggy-zag
motion that actually increases total distance.
A 1 meter gps accuracy = 3 foot of sloppiness in data
 
 
  But there's no need to measure that.
  Posted by: guideboatguy on Jan-30-13 2:44 PM (EST)
-- Last Updated: Jan-30-13 5:13 PM EST --

All you really want to know is how long it took your boat to get from Point A to Point B, while paddling as straight as practical.

If two cars drove the same course in exactly the same amount of time but one of them had a bent wheel (thus it wobbled a little the whole time), would you say that the car with the bent wheel had traveled faster? Why bother? Each car covered the same distance in the same time, and delivered its passengers in the same amount of time, so by any definition that matters, they both went the same speed. Same goes for a boat for which every last bit of wig-wag can't be eliminated. Just worry about how long it took the boat make the trip and be done with it.

If the little wig-wags are due to poor technique, the paddler can effectively go a little faster by learning to go straighter (or in this case, deploying the skeg in windy conditions might make you faster for the same reason). After all, in a race that's what would matter. You couldn't take second place and then argue that you really should be awarded with first place because you unintentionally zig-zagged slightly more than the other guy.

 
 
  Everything has limits
  Posted by: willi_h2o on Jan-30-13 3:55 PM (EST)
-- Last Updated: Jan-30-13 4:03 PM EST --

Handheld GPS can't account for everything
- there are caveats and exclusions.

A receiver compares the bit sequence received from the satellite
with an internally generated version.
By comparing the rising and trailing edges of the bit transitions,
modern electronics can measure signal offset to within
about one percent of a bit pulse width.

Since GPS signals propagate at the speed of light,
this represents an error of about 1 to 3 meters.

I doubt few people could ""accurately"" measure
skeg_up vs skeg_down over a set distance and
get repeatable results.

A measured course on water is tough to set-up.
Bouys move and each gps will measure just
a bit differently from another one.

Tooo many variables in human, nature, equipment combo

 
 
  agree, results will vary...
  Posted by: jcbikeski on Jan-30-13 4:33 PM (EST)
-- Last Updated: Jan-30-13 4:40 PM EST --

If one really cared you may need to do say a one mile course ten times alternating between using the skeg and not. Be careful you don't make wind direction skew your results. Your speed would vary each lap but if the difference was big then one would win by a bit most times. If the difference was small you may see either skeg or no skeg win at any time but perhaps one wins a bit more often.

The part of about a fixed course though is easy. You just pick some pier, point, home, etc. and measure with the GPS or map. It's nice to know the approximate distance but you don't need to know the distance at all. You care that one trial takes say 4% less time than the other.
Time should be measured carefully on some fixed distance and that's pretty easy to do. You could also use a fixed time and measure the distance but it's easier to accurately measure time.

 
 
  Correct, but it's even easier than that
  Posted by: guideboatguy on Jan-30-13 5:05 PM (EST)
-- Last Updated: Jan-30-13 5:19 PM EST --

The GPS may not be accurate enough to indicate very minor speed difference across very small distances, but the amount of error it introduces in measuring the length of a fixed course of substantial distance is very small. For example, fairly large error in location determination, such as being off by 50 feet at times, hardly matters at all when measuring the distance between two points that are one or two miles apart. Even that much error over a one-mile distance will allow calculation of overall course speed with a precision that is well within 0.1 mph of the actual figure, which should be good enough for most of us (in fact except for extreme errors in opposite direction, thus being additive, precision will be not far off from 0.001 mph, which is MUCH more precise than average people would care about). The greater the distance, the less this kind of error matters.

Also, don't forget that these kinds of errors are not instantaneous random events. If they were, a stationary GPS would indicate sporadic high-speed movements, but that never happens. The error is more like a slow drift, with the error changing slowly enough that a stationary GPS always knows that it is not moving, and by the same token, one that IS moving at a steady speed will not suddenly indicate drastic speed variation (except perhaps when reception has been obstructed for a little while, but that's another issue entirely). Anyway, none of that even matters for overall speed calculations over great distance, and I only mention it to put Willi's earlier complaint in perspective.

 
 
  Sure enough. All I did was point out ..
  Posted by: guideboatguy on Jan-30-13 4:53 PM (EST)
-- Last Updated: Jan-30-13 5:37 PM EST --

... how totally pointless it is to worry about trying to measure non-contributary movements when doing so serves no purpose related to the desired outcome. Bringing up an entirely new way of saying that no method of measurement is perfect in all ways doesn't change the fact that being unable to measure extremely minute course deviations is totally irrelevant with respect to determining the time needed to cover the distance between two points.

Of course, the biggest issue of all is figuring out a way to insure that you actually exert the same degree of effort during comparative trials. Measuring speed accurately enough is easy compared to this part.

 
 
  There is a reason it hasn't been done
  Posted by: willi_h2o on Jan-30-13 7:23 PM (EST)
-- Last Updated: Jan-30-13 7:27 PM EST --

Skegs and gps have been around for 2 decades,
no one has done the experiment "well enough" to publish.

 
 
  Not that complicated
  Posted by: gjf12 on Jan-30-13 11:26 PM (EST)
And publishing is not required. Just mount your GPS on the deck and paddle at, say, 4 kts for a minute holding the speed. Then increase effort till speed increases to 4.1 kts, easily done. You will see that a 0.1 kt change can readily be seen, and maintained. In 10 kts cross wind on a lake or bay, you can readily see a 0.1 difference between skeg up and down.

Out on the ocean with swell and wind waves the GPS readout is noisy and cannot be used to hold a constant speed, or determine the benefits of the skeg, one way or the other.

 
 
  Enjoy the experiment
  Posted by: willi_h2o on Jan-31-13 12:25 AM (EST)
0.1 knot = 0.1 mph
one tenth of 5,280ft per hour
528 feet over 60 minutes

Hopefully everyone gets their course measurements
more accurate than 500 feet to see the differences.

 
 
  The whole point
  Posted by: gjf12 on Jan-31-13 12:57 AM (EST)
is not navigation, but to verify, at least to myself, a marginal increase in speed to be obtained by deploying the skeg, at least in moderate cross wind.
 
 
  500ft over an hour
  Posted by: willi_h2o on Jan-31-13 2:06 AM (EST)
Over a 60 minute time frame
- yeah, 500ft is pretty marginal.
 
 
  No, you don't get it.
  Posted by: guideboatguy on Jan-31-13 11:31 AM (EST)
-- Last Updated: Jan-31-13 2:12 PM EST --

First of all, you are going about this calculation of error backward. When someone says the error in MPH is well within 0.1 (actually, approaching 0.001 in a lot of cases), that doesn't mean you can just use that very conservative figure of 0.1 MPH error to come up with an error in distance measurement that adds up the maximum possible error over time. That kind of logic in reasoning tells me this isn't worth my time, but what I wrote below, I wrote earlier so I'll leave it pretty much alone.

The distance measurement is determined via location determination over time, not via the calculated speed during travel, so a generous assignment of 0.1 mph error isn't your base data when making error-in-distance calculations. Any place your GPS makes a location determination, it may be off by a certain amount - no argument from me about that - but these errors are not continuously additive, because the machine does not rely on previous measurements to determine its current location as time goes by. Also, plus and minus errors relative to the direction of travel average out over distance so they really are not cumulative. Errors to the right and left, relative to your direction of travel, have a much smaller effect on the measurement of distance traveled than those in the plus/minus direction (it's just geometry), but further, since simply observing location readings on the machine suggest that the error is more of a slow drift than totally random, right-left errors due not automatically insert zig-zags into your route that are as severe as the total possible error. Your way of adding up this error suggest you expect that worse-case situation, but in actual fact, one does not see this happening when monitoring course.

So, assuming once again that determination of location might be off anywhere from 0 to 50 feet in any direction, if you use the GPS to determine straight-line distance between two points, it uses ONLY THOSE TWO POINTS to determine the total distance between them. If those two points are a mile apart and the error is 50 feet or less, the most the error can possibly be over that straight-line distance is 100 feet, and most of the potential combinations of distance-direction error will result in a total error that is much less than that (geometry again).

I really can't understand why you make these errors out to be so huge, and the things you say makes me think you've never played with one of these machines or compared the location readings to actual locations on a map (you can plot points on modern topo-map programs to within a few feet pretty easily). My GPS is the cheapest model I could get, and I've taken it on plenty of walks in my neighborhood and found the accuracy to be enormously better than you describe (I go for lots of walks in the evenings and use to take the GPS with quite a bit). If I mark waypoints along the route at the centers of intersections (for easy map reference later) and plot them on a topo map afterward, they are almost always accurate to within half the width of the street I was walking on, and almost never off by more than the full width of the street (that's about 50 feet). Also, when walking the same one- or two-mile route many times over many days, the total distance always comes out to within 100 feet or so. Try it yourself and see. Error is not additive in the way that you described, but it does add up in the way I described above, which turns out to be much less severe than your assumptions.

Finally, I really can't imagine you have actually tried to calculate any of these errors you are talking about when a recent post of yours shows that you wouldn't even think twice about interchanging knots and mph as if they were equivalent. Interchanging units is not a sign of familiarity with this topic.

 
 
  At very slow speeds - equality
  Posted by: willi_h2o on Jan-31-13 2:17 PM (EST)
-- Last Updated: Jan-31-13 2:29 PM EST --

It does indeed equal 0.1 knot = 0.1 mph
Higher speeds the decimal differences add up.

1 knots = 1.15077944802mph - go nuts for knots at low speed

Paddling involves distance so you'll probably be out for an hour

Accuracy at speed (dynamic) vs.
accuracy standing still (static)
are two different animals of error

I still say the _skeg effect_ is tough to measure accurately
and repeatedly in a natural environment with handheld gps units

If the unit ""smooths out"" the track then its
simply not recording true distance.

 
 
  Tough to measure yes...
  Posted by: guideboatguy on Jan-31-13 2:28 PM (EST)
... but there's no need to greatly exaggerate distance-determination errors, especially not by erroneous reasoning, just to make this point. And the difference in mph vs. knots becomes very significant when using those values interchangeably to determine distance traveled over an hour's time. Calculation of distance traveled by this method is backwards and inappropriate in this context (it makes no sense to calculate something based on a calculated result if you already have more basic data), but that is what you were espousing, and therefore the difference between these units matters a lot.
 
 
  Display nonsense
  Posted by: willi_h2o on Jan-31-13 2:34 PM (EST)
-- Last Updated: Jan-31-13 2:35 PM EST --

Just because a display has lots of decimals on it
- doesn't necessarily mean its highly highly accurate -

http://wiki.xkcd.com/geohashing/GPS_accuracy

 
 
  accuracy
  Posted by: jcbikeski on Jan-31-13 2:41 PM (EST)
for a GPS for this purpose if you have a mile course and the GPS is accurate to say 50ft then you have 1% error possible on distance. But the good news for measuring things like skeg effectiveness is you DON'T really need to know the distance at all. If you have a fixed route (say one pier to another) then all you have to do is measure TIME accurately to determine that say a skeg is 2% faster or slower. So measuring is pretty darn accurate -- the problem is now the paddler which will vary by fatique, motivation, etc. For that problem the best you can do is have many trials alternating between skeg or not. The variations in the results will give a clue to how accurate you can declare your final conclusion to be.
 
 
  Yes, but
  Posted by: adbass on Jan-31-13 3:37 PM (EST)
don't forget that weather conditions can change from minute to minute and so represent a factor, other than paddler fatigue and motivation, that will affect the accuracy and reproduce-ability of any measurement.

Nevertheless I am very sympathetic to the idea that deploying a skeg in a cross wind makes it easier for a paddler to paddle a further distance in a limited time period (i.e., increase his/her average speed) since fewer corrective strokes and leans are required.
 
 
  agree
  Posted by: jcbikeski on Jan-31-13 3:41 PM (EST)
the hope on weather/wind is that if you do say ten trials in a row (alternating skeg or not) that the weather changes happen gradually and so you still see a trend. Far from perfect of course.
 
 
  You never even read what I wrote.
  Posted by: guideboatguy on Jan-31-13 4:12 PM (EST)
I'm not going to go back and illustrate how stupid it was of you to bring up this crazy idea that anything I said is based on what's seen on the display.

I should have known better than to get hooked into this, since it's your nature to sidestep anything that makes a point and "counter" by saying something that's either outlandish or irrelevant. In this case, it's both. I can't waste more time discussing things with someone who glosses over the words and just hears what they want to hear.
 
 
  I seem to remember these discussions
  Posted by: CapeFear on Jan-31-13 2:19 PM (EST)
I think I remember several discussions of this sort years ago on this site. I remember the gist of it being whether it is more efficient to achieve directional control through edging and strokes, or through use of a skeg or rudder.
I personally still don't use my skegs and rudders, but would be surprised if I wasn't more efficient using them than not where directional control attention becomes evident. Keeping the entire waterline of the kayak in it's most efficient position and handling directional control with a strategically placed fin just seems to me to offer greater efficiency potential. Submerging an unbalanced portion of the hull below water seems like it would create more unuseful sources of turbulance and drag not contributing to the desired effect.
The very end of the stern is where the least resistance would be required to prevent directional changes. Any paddle stroke has the end of the lever in the cockpit, the area where the most force is required to create or prevent rotation. So that hardly seems worth considering by itself.
Maybe when it gets into more extreme directional changes, putting the kayak up on edge with the ends released perhaps takes less energy to quickly turn 90+ degrees than leaving the entire waterline engaged and having a rudder force that entire waterline into rotation?
I would guess you found ways to try to mitigate all the extraneous contributors to the best of your ability, and I'm not surprised you found the kayak more efficient using the skeg. I'm full aware of the efficiency benefits of traveling in a Soltice GTS or Sirius on a windy, gusty day vs. a more weathercocking prone hull, so I'm pretty confident that the drag created by constantly edging and regular directional control strokes is significant.
How do you like that Ikkuma in the surf? Do you think you could give these highly rockered Delphins and Romanys a run for the money?
 
 
  best test
  Posted by: radiomix on Jan-31-13 3:31 PM (EST)
For this nonsense is the following.

Take two boats out with two people. First boat, non ruddered or skeged boat and an experienced paddler in said boat. Second boat the opposite and a paddler of equal experience. Go out on a windy day. See who complains first. My money is on guy without correction.

Ryan L.
 

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