Thanks for the discussion
A few of you mentioned making balance corrections without thinking. You correct balance on a moving bicycle by making slight steering adjustments. When you hop on one foot you come down on a slightly different spot each hop to correct balance. When I was in my new boat on a bench seat for the first time I was surprized how much I had used the tractor seats in my Prism and Rendezvous to lock in and allow me to use hip movements more effectively to balance. On the bench seat when I started moving balancing was definitely easier. I’m still not sure what the techical explanation is but I’m glad it happens.
A few of you said that after some time in my boat the tippiness feeling would go away. That too and I put a tractor seat in my new boat.
Is vector force analysis applicable?
Dynamic stability is certainly real. If you want to less at the tumbling mercy of waves, currents and jumbled water, speed the heck up.
Is this phenomenon related to the vector summation of forces?
Suppose you have a billiard ball (your boat) at rest in Kansas. The cue ball (wind, waves) shoots in from due east and hits you. The billiard will shoot due west toward wonderful Carmel, California.
But suppose the billiard ball in Kansas is not at rest; it is moving at speed toward the north. Then it gets hit by the cue ball at exactly the same place and with the same force. It will not be so abruptly lurched to Carmel; it will less abruptly be angled northwest to downtown Spokane.
Wouldn’t the same be true of any force that hits the moving billiard, be it a force from underneath or at some sort of rotation-inducing glance angle.? The very fact of the northward motion of the billiard should make the directional effect of the cue ball less than it would be on a cue ball at rest.
If this is all so, the sideways-rotational tippiness of a resting boat caused by the lateral forces of wind, waves and the slightly left-right twitching of the paddler’s body mass CG, should be reduced in effect if the boat is moving forward at a right angle to these tipping forces.
hmmmm…thinking of that
vectors make sense to me.
For sure it is harder to turn when underway than while stopped. I think the same sort of vector thinking applies.
And when you stop(to turn) in some wave conditions because you cannot turn with any speed underway, there are some tense tippy moments.
Decoupled forces
In the billiard ball analogy, the forward motion happens independently of the lateral forcing due to the cue stick. The motions are completely uncoupled, which is to say that although it is a 2D situation, each dimension can be analyzed independently of the other.
To whit: a ball moving horizontally and a motionless ball both take the same time to hit the ground if released from equal heights. The vertical motion due to gravity is independent of its horizontal motion.
In the kayak situation, drag forces which are primarily longitudinal (i.e. in the direction of motion) have a very small (probably negligible) ability to influence the heeling of the kayak, which is rotating about its longitudinal axis. Any transverse drag forces caused by the rotation itself (very small) would be the same whether the kayak was moving forward or not. The fact that drag depends on velocity gradient means that lateral drag is uncoupled from longitudinal drag (because the velocities are uncoupled).
To me this implies that dynamic stability occurs because of the paddler and paddle, and is not due to unbalanced hydrodynamic forces on the hull.
Rightfully so !
Looks like I was taken taken to task and rightfully so. The winter’s doldrums have set in. 28’F here. No excuses and I apologize.
Decouple me again, please
Carl, just on the semantic level before we get to physics, you seem to be saying the billiard ball situation is one of decoupled (uncoupled?, independent?) forces. But then you say the same thing about longitudinal and lateral drag -- decoupled.
You conclude the physics aren't the same. I thought you were going to say that one example had coupled forces and the other had decoupled forces, and that was why they are different.
Let me try a different thought (gedanken) experiment.
An empty canoe is at rest on calm water. No wind. No waves. No paddle. No paddler. You fire a billiard ball at the gunwale with just sufficient force to tip the canoe over.
Second case: The same canoe, having been pushed, is gliding across the same water under the same conditions. The same billiard ball hits it in the same place with the same force. Does the canoe tip over?
I'm not sure, but I don't think so.
No harm no foul
Draw the vectord then let us know
If you can’t draw them, then you’re not really sure, are you?
Remember, vectors are defined by direction & length (on the piece of paper). So, in a moving canoe, which vector will change directionn and/or length compared to astill canoe?
Wavemaking
Hi Carl,
I started this same topic elsewhere last year I think. I don’t have all the answers, would I think take a towing tank to get them and the costs are prohibitive. But think about what happens with the waves when you’re paddling, it sounds like the boat would become less stable as the speed increases because the bow and stern become more immersed and the center of the hull, where it’s widest, falls more into the trough between the bow and stern waves. Only problem is the boat becomes more stable as the speed increases. The only force that could be doing that is that the bow and stern are more flat sided than the center of the boat and the water being forced up the bow and stern by the bow and stern waves (buoyancy has to come from somewhere) offers the support that would normally be provided by the hull midships at rest. Probably could be answered better if I had more sleep .
Bill H.
not linear momentum
How does conservation of linear momentum affect lateral stability? Maybe you should follow your own advice? 
Yep
OK, cranky in the winter — wonder what that’s like? Your weather is heading downstate, guess I’ll know soon…
Jargon
Sorry for the jargon-y post, I'll be clearer. Yes, I think in both cases the motions are independent.
In the falling ball case, the horizontal and vertical motion are independent because gravity operates only in the vertical direction.
In the kayak situation, the straight-line, longitudinal drag is created by the forward velocity. Transverse drag, which occurs while the hulls is rolling to one side, is caused by a (very small) transverse velocity due to the rotation of the hull surfaces about a longitudinal axis, presumably somewhere near the geometric center of the hull. That drag is based solely on the transverse velocity, independent of the forward motion - this is a fundamental tenet of physics, and is what permits the decomposition of vectors (e.g. analyzing a 2-D vector as a sum of x and y components separately).
In your gedankenexperiment, the capsize should happen regardless of the forward motion. I've been sketching 3-D velocity vectors and considering momentum balances, and I'm pretty sure this is correct. I will definitely try it experimentally when I'm able.
Bow wave!
That’s right!
When the boat is at rest, the water displaced by the boat on both side of the boat is holding it upright.
When the boat is moving, it push forward and the water in front of it got push to the side (forming bow wave we can actually see). Now there’s more water on the side of the bow. That will hold the boat upright more firmly than then when the boat is at rest.
Yes, saw that thread
Hi Bill, you night-owl. The thread you started is linked above, lots of interesting hypotheses, some sound plausible, others not so much, not a lot of conclusions.
Your bow and stern wave idea sounds plausible, but I just don’t think the forces resulting from such wave action will result in the steadying we all feel while paddling. Not only are the wave heights modest, but the resulting pressure forces will be perpendicular to the hull surfaces. Assuming the rolling axis is located somewhere within the hull cross section, that means the moment arm for a restoring torque caused by those pressure forces will be quite small.
On the other hand, a buoyant paddle blade (like a WRC GP) laid in the water out to the side of the boat can generate a restoring torque of several foot-pounds just by floating there. I often lean on my GP stretched out to one side of the boat while at rest, as many people do.
For another gedankenexperiment, how about paddling a perfect cylindrical boat, with zero static stability. I think a telephone pole would be more stable while being paddled than sitting still, don’t you? The pressure forces couldn’t possibly help in that case.
Re: the experiments, I am finalizing the design of a water channel which should be finished sometime over the summer. I’m assembling a list of simple, interesting experiments for myself and my students to start off with. This is definitely on the list.
Transverse drag can’t be the culprit
Assuming a boat in motion is more rotationally stable than when at rest, I really don’t think transverse drag, as you are calling it, could be the reason. Hence, I agree with what I think you are saying – that the transverse drag would be the same under motion as at rest. But I never thought tansverse drag had anything to do with dynamic stability.
The transverse drag of bicycle tires against the road is not any greater (except maybe trivially) when the bike is stable under speed than when it is unstable at rest. The transverse drag of a tilting ice skate blade on the ice is not any greater when a skater gliding on one skate is in motion than when she is at rest. Yet that skater is much more stable at speed than at rest. (And in the case of the skater, there is no supposed “gyroscopic effect” of wheels, as is said – dubiously, IMO – for bicycle dynamic stability).
The increased rotational stability of a canoe has to be caused in part by the forward motion of the canoe.
Isn’t the same dynamic stability present in a power boat? Are you more stable in confused seas with your motor off or when you are powering forward in a straight line?
Don’t know
OK, we both agree that drag, vorticity, whatever isn’t the cause of increased dynamic stability while in motion. I think altered pressure distributions due to the motion are not the answer either. If it’s not caused by the surface forces on the hull, what’s left?
“Yet that skater is much more stable at speed than at rest. (And in the case of the skater, there is no supposed “gyroscopic effect” of wheels, as is said – dubiously, IMO – for bicycle dynamic stability).”
agreed, and I think the stability at speed is the result of the skater’s constantly moving body position and continuous corrections to the force magnitude and direction applied to the ice via the skates.
“The increased rotational stability of a canoe has to be caused in part by the forward motion of the canoe.”
If it is, what is the mechanism? I can’t think of a plausible one, so I can’t agree. One may turn up, I suppose, if we think about it long enough.
“Isn’t the same dynamic stability present in a power boat? Are you more stable in confused seas with your motor off or when you are powering forward in a straight line?”
That’s a good question, I don’t know. It seems like it would be so, but I would guess it is due to the shift in frequency of the confused sea due to the forward motion. When traveling through chop of a given wavelength, the boats forward motion effectively increases the frequency of impact of the waves, so the boat experiences chop with a shorter effective wavelength while moving. If this is farther away from resonant oscillation frequencies of the hull (likely), it should feel more stable. But the dynamic stability effect is felt in a kayak or canoe on flat water too, so that’s not the whole answer.
Re: the bicycle, if you let a bicycle coast without a rider, won’t it tip over just as soon, on average, as if you just let go of it while motionless? I agree that the gyro effect is probably pretty small, but I don’t think forward motion is the cause either - it’s probably the rider. I really would like to test this stuff out.
stability
I’m not familiar with all of the forces acting on a moving boat but I can think of one that would help keep it upright.
Think of the front of a canoe…looking straight at the bow from the front, any boat with front rocker presents sort of a wedge shape to the water - which should try to lift the bow during motion (think of an airboat hull…simple rectangular profile but like a wedge in front…that helps it lift over obstacles). The bow pushes the water out of the way and the water pushes back…and this longitudinal force has a component (vector) that tries to slow the boat down and a component that tries to lift the bow. If you lean the boat a bit then there’s effectively a bigger wedge on the side towards the lean, which means there’s also a bigger vertical force pushing up on the side you’re leaning on. This would help keep the boat vertical.
I’m not sure if a boat with zero rocker would benefit from this self-correcting force…it may depend on the exact hull shape and whether it generates some bow lift. If we hollowed out a telephone pole and had no bow shape (just a blunt end) I wonder if the boat would still be more stable moving than at rest. I don’t think it would.
Other than that I can’t think of how forward inertia would affect stability (rotation around the axis of boat travel)…but I haven’t studied the links that kayamedic provided.
Maybe the forward motion also helps keep the paddler more stable and centered(“a body in motion tends to stay in motion unless acted upon by another force”)?
water channel
Hi Carl,
Cool, though you are out where the towing tanks are. Closest one around here that I know of is at the UM campus in Michigan.
Btw, I work nights, the reason for these late night posts, at least for another few years.
Bill H.
gp
Oh yeah btw, I do the same thing with my GP, floats like a cork 
Also btw, the K1 I tried to paddle last season had about the same stability as your telephone pole. It’s what started my thread.
Bill H.
tests
Carl,
Btw, back when I was asking about this I even asked at BoatDesign.net and even the degreed NA’s didn’t have a good answer about this question. Would be an excellent test for your students
There may be an answer out there somewhere, but I’m starting to think it’s a question that’s never been answered.
Bill H.