Drag coefficient test on the cheap?

[MechaNik]

For A load sensor you should ask around the larger rigging and sail makers. This is where a friend of mine borrowed one from (quietly on the weekend), it was wireless and recorded constantly. He was trialing a "friction less spray coating" on his 12ft skiff so needed a fair size scale.

[ancient kayaker]

Originally Posted by Alik

Yes, this works, we did and it works.
The question is how high should be towing point on kayak.

The lower the better presumably, too high would depress the bow and affect the test results. The towline needs to stay out of the water of course but this kind of test should be done in flat water to be valid.

You can test a kayak to see if it has a yawing tendency: just paddle fast in a straight line then stop paddling. If it is going to yaw it will veer off course after a few seconds instead of coasting in a straight line.

[ldigas]

Originally Posted by Alik

This is qualitative testing of 1:10 scale model of 16m catamaran:
http://www.youtube.com/watch?v=FvYlP...1&feature=plcp

Is that a life vest on top ?!

[Alik]

Originally Posted by ldigas

Is that a life vest on top ?!

Yes, it works as emergency floatation

[Submarine Tom]

Originally Posted by Alik

This is basics: normally models are tested without appendages. Why? Because Reynolds number (Rn) on small scale appendages are low, so they can not be modelled that way. Usually appendages are tested separately at higher Rn or added as result of calculation.

For testing real size kayak it might work to test with skeg.

No mention was made of any model.

-Tom

[Alik]

Originally Posted by Submarine Tom

No mention was made of any model.

-Tom

But I was talking about possible problems of MODEL testing. Because we test models this way sometimes. Clear?

[Submarine Tom]

Originally Posted by Expandacraft

Hello members,
I'm new here and hope this is the right place to pose this question. I'm the designer and manufacturer of Expandacraft modular boats and wanted to know if there is an inexpensive way to determine drag coefficient. My bows are wave piercing and it is my theory that while they may not be as slick as some kayaks in flat water, they are in fact more efficient into the waves.
My goal is to actually measure and document the data, not simply to calculate a number. I'm thinking of some sort of devise which I can attach to a motor boat and test in (real world) situations.

Just not sure why you brought up MODELS when in the OP there is clearly no mention of them.

Oh, Happy New Year Alik

-Tom

[messabout]

While you are at it, you will surely want to test at different pitch angles. At least one bow down test and another bow up. Phil made reference to that situation above.

To complicate matters a bit, you need to consider aerodynamic drag as part of the total drag figure. The connecting structure on the cat planform will have some influence on your derived numbers. Towing upwind, downwind and crosswind will provide some clues, but only clues, about the aero factors. As you will be using various ballast items to simulate actual sailing or motoring displacement conditions, it needs to be as well hidden from the airstream as you can make it. Why is everything that has anything to do with boats so damned laden with variables?

[tspeer]

Take a look at Frank Bethwaite's "High Performance Sailing". He describes the tests they did with towing dinghy hulls to get the drag of one vs another. They towed two hulls from opposite ends of a pivoted cross beam to get a sensitive measure of the difference in drag between the hulls.

[ancient kayaker]

Originally Posted by tspeer

Take a look at Frank Bethwaite's "High Performance Sailing". He describes the tests they did with towing dinghy hulls to get the drag of one vs another. They towed two hulls from opposite ends of a pivoted cross beam to get a sensitive measure of the difference in drag between the hulls.

That is a simple and accurate way to determine if a design or other change is good or bad. Simplicity is important here. Expandercraft are clearly not aimed at performance-oriented markets, so there's no need for anything more complicated . . .

[rwatson]

Yes AK, this 'drag testing' might be over complicating the evaluation of little boats.

I would be tempted to start with a few people to just paddle comparative boats, and see if anyone can tell the difference ( maybe even have a few informal races), swapping between boats.

If you cant tell the difference - it doesn't matter. If people report discernible performance differences - that's all you need to know.

But, if you still need to 'quantify' the results, I think the only true test is to stick a battery powered trolling motor on two boats, and measure the current draw at the same boat speed in the same conditions. (with passengers, and swapping motors for averaging results of course.)

[ancient kayaker]

Originally Posted by rwatson

. . . I would be tempted to start with a few people to just paddle comparative boats, and see if anyone can tell the difference ( maybe even have a few informal races), swapping between boats . . .

It's surprising what can be learned from such a simple test as that. A buddy and I did swap tests between a rotomolded kayak whose manufacturers claimed a racing background and a lightweight home-built (and designed) plywood canoe. We were surprised to find that a cruising level of effort would put the 11.5' canoe ahead about a length per minute but in an all-out race the 13.5' kayak would take the lead. This happened every time, didn't matter who was in which boat, although we're a fair bit different in weight and height. Length and wetted area matter even between very different designs. You could hear the canoe during the speed tests but the kayak was still silent - we probably did not have the strength or skill to get the kayak up to the speed of which it is capable so it was wasted on us.

Since the ply canoe was intended for relaxed cruising, both boats clearly met their intended purpose, but now everyone wants the ply boat, which also revealed something about ourselves - we're lazy! So, since the ply canoe is half the weight of the plastic kayak, the rule now is, you carry the other guy's boat to the water . . .

[JotM]

Here's how drag coefficient testing is done to determine the handicap of open water rowing craft in NL.
The setup is drawn schematically in slide 7 of the presentation here:
http://www.federatiesloeproeien.nl/f...cap-pres-1.pdf


In order to determine the power exerted during a race the parameters A and B of a total drag coefficient-function is determined for the average rowing speed over the last season, which is considered to follow the function Cw=A/(1-(v/B)^2) -> power P = Cw*v^3 <=> P = A/(1-(v/B)^2)*v^3

The crew for which the average power / number of rowers is the highest, wins a race, where the average power is determined by multiplying the average speed over the entire race to the power of 3 with the drag coefficient for that speed and dividing by the time rowed.
(Pav,n=P = A/(1-(v_av/B)^2)*v_av^3/(()*n) - where Pav,n is the average power divided by n rowers, v_av is the average speed over the entire race and n is the number of rowers)

Regards,

Jaap