# Drag coefficient

Some lie awake at a night stuff for like minded individuals. Is drag coefficient ever measured and published for kayaks? It would be good to know if kayak A took 1.5X the power to move along as kayak B. Some random thoughts on how: (assuming ideal test conditions and equal weight on board).

1. tow with a boom from boat to a given speed. Release and measure deceleration (I read one study where this was used)
2. tow with boat and boom to a given speed. Attach a scale in cable, or at fulcrum point on boom on boat. Measure average resistance/weight for given speed.
3. using a bridge, tower, or building next to water, attach a weight high in air to cable and pulley, to a fixed pulley at water level, with cable attached to kayak at a distance from pulley. Drop weight and measure time and speed to pull kayak over given distance, or determine weights needed to pull different kayaks at the same speed.
4. find a place where water is flowing steadily through a sluice. Tether kayak in water flow to fixed point with cable and scale. Measure weight/resistance reading for different kayaks.
5. attach an electric motor and propeller to kayak. Measure kayak speed for given motor output, or measure power needed to achieve certain speed.

Probably the easiest and most obvious I have failed to envision, or perhaps there is a standard method out there. Someone kindly enlighten me. Thanks.

Different speeds
Don’t forget that the correlation between speed and resistance is not linear. One boat might be more efficient than another at low speeds and less efficient at higher speeds.

There are calculations to figure this out. Even better there is software that will crunch the numbers for you. But you need to learn the software and have a digital copy of the hull in order to get the numbers out of it.

Here’s an article on the subject. I haven’t bothered to read the whole thing so I can’t vouch for its accuracy. Nor do I understand the subject in depth enough to spot most inaccuracies if they are there.

Alan

Wow !
And for the past twenty years, I have been wasting my time looking at turtles, sharks, manatees, water snakes, star fish, and beautiful sunset and sun rise horizons !

Jack L

LOL
any formula goes right out the window with the first scratch. Also the depth of water matters. Assuming a virgin paddlecraft in deep water

Resistance to Friction

http://www.greenval.com/shape_part1.html

Rf = 0.97 x Cf x Sw x V^2

Sea Kayaker magazine published

– Last Updated: Oct-23-15 10:36 AM EST –

The magazine when it did reviews on kayaks used to published this info in what was called Winters/KAPER and Broze/Taylor numbers. To bad the magazine went under.

Here are the numbers for a QCC 700x

Speed Winters/KAPER-------Broze/Taylor
2 knots------0.96-------------- 0.98
3 knots------2.01---------------2.01
4 knots------3.51---------------3.69
4.5 knots----4.67---------------4.72
5 knots------6.39---------------6.38
6 knots------11.62--------------11.12

I don't know of anyone else who ever had this kind of info.

Weight, rope and pulleys
I kind of like the weight, rope and pulleys arrangement, but I’m not sure of the practical application except maybe for designers testing different shape configurations.

If the point of the study is to compare performance data when making a kayak selection, it wouldn’t rate very high on my list of considerations. There are so many things that have to be checked off before you get around to ultimate speed potential. I have come to the realization that I will never wring out the last bit of speed of any boat without the real danger of hurting myself.

not with a ten foot GP

More crude but quicker . . .

– Last Updated: Oct-23-15 5:25 PM EST –

Is speed the flip side of drag? You're wondering about power, I'm talking about speed, but maybe it will still help you out a bit.

Based on formulas I found online, I devised a kayak hull speed calculator and put in on a Google doc at http://tinyurl.com/oy5nwz2 Please only modify the green shaded boxes (or save a copy to your own Google drive and then do whatever you want). As stated on the document, other factors that influence speed such as hull shape, hull stiffness, and boat weight, are not accounted for.

Based on my experience length, width, and weight of the boat (in that order) are generally the three factors that have the greatest influence drag, efficiency, and speed. Except in extreme cases, hull shape and hull stiffness seem to have lesser effects than the factors mentioned above.

If anyone has suggestions for improving the calculator, I'm all ears.

this is why
you are curmudgeonly.

Speed is the big factor…
Shorter boats are more efficient at lower speeds then hit a wall at higher speeds. Measuring drag on a displacement hull is not as simple as putting a car in a wind tunnel.

Did you do an in depth research
of the origin and language of that word or did it just pop into your gray matter, or is that “brown matter” ?

Guy

Wasting time?
Not at all, and it sounds like those who designed and built your kayak weren’t wasting theirs either.

Compared to bikes.
When I thought I would try cycling, I went out and bought a mountain bike. Later I asked a neighbor to come ride on the Blue Ridge Parkway. As we started down a gradual descent, I was pedaling fairly vigorously, while he was lightly tapping his brakes to maintain the same speed. Of course, he was on a road bike. I had no idea rolling resistance made such a huge difference.

If one’s intent is to cover a lot of territory or keep up with others, I would think knowing the drag would help a bit in selection.

Which brings me to another variation. Using a motor and propeller with known controllable output, kayaks are propelled over distance for a certain time. Have controls at each end of the spectrum, the fastest known yak in existence at one end, and maybe a pumpkin at the other. Your boat is rated along the continuum at an index of the fastest, for example my Tarpon SOT might rate a .4. (Yes, it’s fraught with error and I am fantasizing such knowledge would ever be found in a review.)

The Blue Ridge Parkway !
My favorite training place for the “Assult on Mt. Mitchell” -back when I was a youngster

Sorry! I got carried away when you mentioned the Parkway.

Jack L

For that purpose, …
… it should be good enough to learn some general principles of boat performance, and pick one (or three or four). Take two reasonably similar boats and put two reasonably similar paddlers in them, and the differences between the boats will be drastically eclipsed by the range of power output that’s within the comfort range of the paddlers. That’s because ordinary paddlers don’t exert themselves at a level such that they finish their trips completely spent, and wishing they could have arrived at their destination 20 minutes sooner. If you are a racer, getting every extra bit of speed out of your boat puts you farther ahead, and even a few seconds might make the difference, but you’ll also be exerting yourself at a level that puts you well into the range of least efficiency (“hitting the wall”, as one person already described it). Ordinary paddlers aren’t trying to push through that “wall” the way racers do. Don’t forget that any significant advantage of top speed (and you’ll have to work you butt of to make use of that) or efficiency at normal speeds will carry with it some disadvantage in other performance factors. Take that to an extreme, with the fastest boats available, and not many people even have the skill to keep them upright.

gnaw
none of these ‘methods’ will survive the inherent variables.

see the Navy’s tow tank online and in utube.

The tinyurl method is ok for personal choices…as we would imagine several personal choices would compose a distinct and definable hull category linked together by the person’s choice…right?

a problem with speed at a high performance speed spectrum end is that hull shape for many of us has energy losses in balancing the hull thru ongoing variable conditions.

that hull looks fast. in practice is less so. There are testimoanials.

Little difference in similar boats
When it comes to just keeping up with the group or another paddling partner during normal paddles the difference between similar hulls doesn’t amount to much, IMHO.

Yes you can see the efficiency difference on paper but at low speeds (3-4mph) the effort required to hold those speeds is quite low. The skill, technique, and strength of the paddler will far and away make the biggest difference.

The differences become more apparent when you start racing or pushing hard for hours on end. Even then the skill of the paddler counts the most but they’ll be better rewarded with a more efficient hull.

That’s not to say it isn’t fun to try and ferret out this sort of information and that it can’t be valuable. But it’s best to have reasonable expectations of how it correlates to on the water performance.

Alan

Terms etc

– Last Updated: Oct-25-15 12:43 PM EST –

The two most important forms of resistance to a hulls passage through water are frictional resistance or drag, and residual resistance which is wave making.

FR, Drag, is a function of skin surface area, skin condition and speed.
Longer boats have more skin at any given burden. New gel coat has 1 mil surface roughness paint, badly roughed up hulls ~ 40 mils. Drag is an arithmatic function and increases steadily with speed.

RR, or Wavemaking Resistance is mostly a function of hull length. As speed increased, transverse waves initiate along the hull's length. As speed increases those waves spread apart and deepen until their are only two, near each end of the hull. That spread stops when the stems can no longer be supported on the wave, the hull settling down between the bow and stern transverse wave. Longer hulls allow the waves to spread farther so are faster if enough power is available. This is theoretical hull speed, a realistic maximum. Craft can exceed TMS with excessive power and cadence; witness a marathon race canoe climbing it's bow wave and surfing the wave. Residual Resistance is a geometric function. Combining FR and RR yields a geometric curve of total resistance increasing with speed.

Read Winter's The Shape of the Canoe for more and better information.

Testing is another bugaboo for paddlers. The David Brown test facility being generally unavailable. Strain gauges might approximate the data but are pricey and both yield kinda worthless numbers as they are measuring static hulls while in real life paddlecraft are always in yaw, driven to either side by off-center power application[s] that are cyclical, always accelerating and then slowing.

And we will never find paddlers similar enough to provide reliable comparisons.

It is a matter fit of an basilisk, more so because there isn't enough profit generated to justify scientific study.

Drag
The test methods proposed by the OP were all tried by the early ship model testers – Froude in the UK and Taylor in the US. Modern methods generally use force transducers to measure the relatively small hydrodynamic forces on a model – either on a carriage towed through still water (tow tank) or held stationary in a moving stream of water (flow channel).

Froude developed the drag coefficient to relate his model tests to full-size vessels. The idea is that if a model is tested in such a way that the important dynamic parameter (Froude number for ships) is the same for the model as for the full-size vessel, then the drag coefficient of the model and vessel will be the same. The measured coefficient for the model can then be used to estimate the drag on the full-size vessel. The drag is equal to the propulsive thrust needed for cruising at constant velocity, so determines the vessel power requirement.

The drag coefficient is defined as the actual drag force on a vessel divided by a fictitious force equal to the dynamic pressure times a reference area. It was mentioned in a post above that it is wetted surface area. That is one common definition, but some drag coefficients use the frontal area instead. Either can be useful, depending on what you want to characterize about the hull.

For a kayak, which can be tested at full size, there is no need to use the drag coefficient – the actual drag force is much more useful. In the Sea Kayaker data referred to above, the right-hand column is in units of pounds-force, directly translatable to the actual propulsive force that the kayaker would have to deliver (on average) to cruise at the given velocity. However, these values are simply numerical predictions given by regression formulae – Broze/Taylor and Winters/KAPER. Both are for kayak hulls in straight line motion at constant velocity. Early issues of SK published actual tow tank tests from University of BC, but they were discontinued early on – they were too laborious to maintain, I believe.

In any case, CEWilson and Alan Gage make some very good points – drag forces on kayaks are pretty small, and kayaks are almost never traveling straight ahead through motionless water without surface waves. The intermittent power application of the paddler is a major discrepancy not covered by any modeling software, not to mention yawing, pitching and rolling. It’s likely that the skill and body motion of the paddler is as big a determinant of hull performance. So while the quantification of hull performance is interesting to think about, there are a whole lot of other things going on that are probably more important.

a skilled paddler can make a poorly
designed boat shine while a beginner will struggle in a high performance boat.