Me or the boat?

Builder's Forum
1

I am new to kayaking, thought I'd take up the sport. I got a plastic kayak for cheap (had a hole, I fixed it) while I finish saving up to build a real one (C17). I went out paddling today and all I could basically do is turn left, even if I'll I did was paddle on the left. I would get going semi striaght and then spin left, a few times to right but almost always left. I am lost on the problem, I don't know if it is the boat or the user (me).

Here are pics of the boat:

 http://picasaweb.google.com/107267314409083170158/BlueKayak?feat=directlink

 

Any ideas would help, thanks!

2

I am not an expert but i would say you. Thank yak is gonna be hard to make go straight with out good from. The bottom is round and will be very sensitive to "twisting" of your lower body at the hip while you are paddling. It is a white water boat and made to "not" track straight thus making it more manuverable(sp?) i had just the reverse problem with my millcreek 13, i had a hard time not going straight as the CLC boats are "hard chined" boats and are designed to track straight. i hope what i said helps…just work on form alittle and youll get use to it.

3

Yeah, that looks like an old Dancer - a whitewater kayak. It is absolutely designed to turn, not go in straight lines - just what you need for whitewater river running, rather than paddling on a lake.

So, you will always have a problem going anywhere on flatwater, in that boat. As for only turning left, well, was the wind blowing? Weathercocking (the boat always wants to turn into the wind)is a common occurrence in many kayaks, and might well be especially pronounced in a ww boat like yours. Lee cocking - the boat wanting to turn away from the wind is another, less common occurrence.

It might just have been your stroke, too, though a combination of all 3 (hull shape, weathercocking, and techniques) issues is the most likely problem.

Perhaps try a strap-on skeg if you really want to enjoy paddling that boat on flat water.

 Good luck!

Dave Gentry

4

Thanks, I figured it was me. The water was perfect glass and no wind at all. It the water hadn't been 40 degrees I'd probably would've jumped in for a swim. I'll have to try the skeg idea. Give me something to work on today while it is snowing  :) . Any good ways to work on my technique mainly hip control/ I did figure out leaning could tighten or widen the turn, yet I am not very good at it.

 

Thanks for the help guys!

5

Boats weathercock when the center of effort is displaced from the center of balance. If the center of effort is behind the center of balance, you cock into the wind. If its ahead, you cock away from the wind.

The center of effort is where the "sail" area forces act. On a kayak, the sail is the hull and the paddler. You can't do much about the hull and the paddler's position is usually fixed.

The center of balance is where the boat would balance if set on a knife edge. A gallon jug of water can be used to easily change the center of balance.

If you need to go into the wind, put the jug up front. If you want to travel with the wind behind you, put the jug in the back. If there's no wind, keep the jug close to the center, or just dump out the water.

A skeg is just an underwater sail, used to move the center of effort backwards.

Laszlo

 

6

Thanks for that, Laszlo. That explanation is a big help.

7

Finally got my skeg made and strapped onto my kayak. I took it for a quick paddle and man did it make a difference, I can actually get somewhere now. Thanks for the help and ideas.

So on the whole water jug concept that Laszlo is talking about, if I were to take a jug of water and but it the back of the kayak would it work the same way?

 

Jason

8

Jason,

I know what you went through.  In addition to the kayak I'm building, we have an inflatable kayak that will not track straight on flat water...unless the skeg is attached.  Once the skeg is attached, it tracks straight as an arrow.

9

One minor correction to the above explanation about weathercocking/leecocking.

Boat turns when the center of lateral air resistance is offset from center of lateral water resistance.

Center of  balance is not relevant.

10

One minor correction to the minor correction.

It's all about the center of gravity (or center of mass, to be totally accurate)

When a boat (or anything) moves, it's in response to a force. So sayeth St. Isaac. The difference between moving in a straight line and turning is the distance between where the force gets applied and the object's center of gravity. If the force is offset from the center of gravity, it's a torque (indicated by force times distance) and the boat spins. If it's exactly at the center of gravity, the distance is 0, so the torque is 0 so the boat doesn't turn. The boats moves in a straight line.Torque in front of the center of gravity is the opposite sign as torque behind the center of gravity, so the boat spins in opposing directions, depending on where the torque is applied. So if the boat is weathercocking, the trick is to move the torque the other way.

The center of lateral air resistance is where the sail forces are applied. The center of lateral water resistance is where the underwater forces are applied. If you take their vector sum, then it's the same as if you had just one force being applied at one point. That was the center of effort I was referring to above. The distance between that point and the center of gravity (or balance or mass, different names for the same thing) is what determines if there's a torque.

Camper is right in that if the boat is currently balanced and then you offset the centers from what you started with that the boat will turn. But that's because the vector sum of the centers change, introducing a new torque which still acts through the center of gravity.

Hope everyone is still awake,

Laszlo

 

 

11

Dang.  Well, we will just have to take up the theoretical physics argument at Okoumefest next year, where the well-balanced can stay out of earshot, and where we have the option of pistols at 30 paces if calm discourse fails us.

12

The theoretical physics argument should occur at CLC’c  open house after the food competition.  If after three minutes there is no obvious winner (volume not to be a factor), the participants will resort to the best two out of three attempts to make the prettiest smoking hockey puck.  Entrants will be allowed to bring their favorite cups and mixing sticks, but will have to pay a small fee to CLC to offset the costs for epoxy and hazardous waste cleanup.  The winner will be required to take the winning puck home and display it prominently in his/her workshop.  The puck to be returned to the following Okoumefest as a perpetually renewable trophy for the next arguement, the subject to be decided by CLC’s staff during appropriate interludes in the small cubicle to the far left of the Shop Cam’s view.  Happy building.

13

I accept!  I've completed slightly fewer boats than L. (actually, my first boat is sitting idle and unfinished, due to distractions like work and money).  But at acheiving undesirable effects with epoxy, I think of myself as pretty accomplished.

And Okoumefest will give me a deadline to finish the boat!

Actually, I have to kind of admit that a few fuzzy facts from college science courses have drifted back into my brain from past centuries, and upon further thought, I saw a teeny tiny bit of error in my "correction", and realized that L.'s  explanation was maybe just ever-so-slightly right.

But any flaws in my position will never be noticed by the judges.  As we do at work when we get ourselves into a bind like this, I'm going to bring a powerpoint presentation, with animation and lots of different fonts.

14

Laszlo pointed out above that you can reduce weathercocking by moving the center of gravity aft, for example by adding weight in the stern.

Since Laszlo is right (as usual) I have decided to bow out of the great debate/epoxy puck competition currently planned for next year's Okoumefest. 

I'm not saying the Sharpie will be done and I will be sailing. More likely I will be doing quality assurance on the cheeseburgers. 

Note: my quibble was only with the "why" part.  But "why" it works doesn't matter when trying to make building and kayaking more enjoyable, which is what this forum, and OkoumeFest, is all about.

Note: if there is anyone here who shares my incurable obsession with "why it works", I wrote my own physical explanation of weathercocking, too lengthy, unreadable, and off-topic to post here.  To understand my quibble about Laszlo's "why", it would be best if you read it first.  So, if I figure out how to post it somewhere else I'll put the link here!

 

15

So, this got kinda deep over a skeg. Good concepts I learned something I think. Anyway I think I just fill an old milk jug with water and chuck it in the stern of my boat and see what happens, maybe I won’t need the skeg. I understand how it works, I believe. Let you know how it works.

16

Sailors usually pitch this argument in terms of center of effort vs center of lateral resistance and neither is anything but indirectly correlated with center of gravity.  Keels skegs and rudders are used to adjust the center of lateral resistance in the water and mast location(s), rig tension and sail trim reposition the center of effort above water.  Nowhere is this more evident than in board sailing where the sailor must position both the above and below water forces (and do it both instinctively and reflexively) to keep the board on any given course. (In this example the rails can also play an important role in controlling lateral resistance.) It is true that if you shift your weight back in a kayak (this is easier to do in a canoe) you will lift the bow thereby increasing the windage and allowing it to fall  off the wind. You are, at the same time burying the stern and moving the center of lateral resistance aft.  However, you can accomplish the same thing by dropping the skeg without moving a pound.  Drop it too far and you will fall off downwind but get just enough of it in the water and you can keep a course across the wind.  Who needs a rudder? For the purists who would scorn even a skeg, my suspicion is that the only reason the Aleuts did not put deployable skegs in their boats is that they could not figure out a good way to do it with the materials at hand. 

Bob R. 

17

Bob has the physics exactly right.  (Now I don't need to figure out how to post the explanation!)

18

You guys are right as far as you go, and Bob spotted the simplification I used of ignoring the buoyancy. But the CG location is very directly correlated with the amount and direction of weathercocking. It's what determines whether you have positive, negative or neutral stability.

The relationships of the centers of effort and lateral resistance determine the net turning force. The relationship between the the net turning force and the center of gravity determine the net torque. The net torque determines whether the boat turns into or away from the wind, and how quickly.

If it was just the relationship between the centers of effort and lateral reistances, then what would happen when you didn't have both? Think of an arrow fired into a pond. It starts off with no center of lateral resistance, then it gets both, then it ends up with no center of effort, but it always has a center of gravity and it always flies point first.

Submerged submarines and flying zeppelins also only have one or the other of the centers with the net force acting through the center of gravity.

Now I'm getting the puck outta here before I put people to sleep again.

Laszlo

 

19

You wrote:

"The net torque determines whether the boat turns into or away from the wind, and how quickly."

Exactly correct.

"The relationships of the centers of effort and lateral resistance determine the net turning force."

Exactly correct, if by "net turning force" you mean "torque".  "Turning force" is often given as the definition of torque, although it is imprecise.

"The relationship between the the net turning force and the center of gravity determine the net torque."

Nope.   The two forces are equal in magnitude when the boat is underway.  (If they weren't equal at some instant, then the boat would accelerate laterally and the "lift" of the hull--the lateral force--would adjust quickly to make them equal; the net sideways force would quickly go back to zero and the boat would again be moving in a straight line, with the orientation of the hull and the direction of motion differing by the "leeway angle".  The lift of the hull depends on the speed, coefficient of lift, and leeway angle or "angle of attack" of the hull).

In fact, the two forces form a force couple and it is this couple which determines the torque on the boat.  This torque is proportional to the (equal) force, and to the moment arm (the distance between the two centers of effort).  It is independent of the position of the center of mass with respect to either COE.  Both COEs might be aft of the COM, or ahead of it, or straddling it.  The torque is the same.

To visualize this, imagine an 8 foot 2x4 sitting on a frozen pond.  If you got at one end and twisted it--your hands being say .25 m apart and the equal lateral forces being 10 Newtons--it would start to rotate, with the angular acceleration being proportional to the moment of inertia and the torque.  If you got over the COM and twisted it, it would rotate with the same angular acceleration.  If you moved to the other end, it would start to rotate with the same angular acceleration.  It doesn't matter where you apply the two forces with respect to the COM. 

But you are correct that if the boat is standing still and the angular velocity is zero, and suddenly a cross wind hits the boat,  then for an instant before the boat begins turning, there is no lateral force from the water, and the angular acceleration is determined by the moment arm between the (unopposed) force of the wind and the center of mass. 

20

Where I said...

"If you got over the COM and twisted it, it would rotate with the same angular acceleration."

...I should have said

'...with the same rate of increase in angular momentum'.