Design consideration…
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?
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.
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?
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
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?
Accuracy of gps ?
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
I would tend to agree, and…
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.
You are probably correct regarding
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
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.
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
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…
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.
Sure enough. All I did was point out …
... 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.
Correct, but it’s even easier than that
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.
There is a reason it hasn’t been done
Skegs and gps have been around for 2 decades,
no one has done the experiment "well enough" to publish.
Not that complicated
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
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
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
Over a 60 minute time frame
- yeah, 500ft is pretty marginal.
No, you don’t get it.
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.