skeg measurement trials

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.

– 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

– 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.

I seem to remember these discussions
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?

Tough to measure yes…
… 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

– 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 -

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.

best test
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.

Yes, but
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.

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.
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.