When you do a hanging draw, as the boat slows down, it seems (and I was taught) you have to move the paddle forward to keep the boat sideslipping perfectly. Otherwise, the the bow yaws away from the blade.
The correct starting forward/aft pivot point varies a bit with the boat, but the need to move the paddle forward as the boat slows is always there, at least for me (most recently in an Avocet and an Aquanaut).
Can anybody explain why?
–David.
pivot point
the pp of the boat changes as you slow. moving the paddle fore/aft during a sideslip (I’m not a fan of ‘hanging draw’ terminology) is just basic adjustment for where the pivot point IS and where it’s moving.
I can show you some tricks with a mid-ship pry that shows/explains this concept brilliantly.
steve
Pivot point…
… yup, I get that. But why does it work the way it does? Every time I try to think through the forces, I get the opposite effect, namely…
boat speeds up --> bow tightens --> pivot point moves forward
boat slows down --> bow loosens --> pivot point moves aft
…which says to me that you should move the paddle ~backwards~ as the boat slows in a hanging draw/sideslip.
Or more directly, with a moving boat, the bow is somewhat pinned and resists sideways movement in both (left and right) directions. So I should have to place my hanging draw more forward to keep from yawing the stern. In empirical fact, I have to place it a bit aft, and move it forward as the boat slows.
But obviously, I’ve got something conceptually backwards.
Can you describe your tricks with a mid-ship pry. I was out today trying various maneuvers to understand this, and am feeling frustrated.
Thanks. --David.
Relative velocity
Let’s say you’re coasting at 4 mph and put your paddle in to start a sideslip. If the sideslip velocity is 1 mph, the hull sees an apparent velocity that is at an angle to the bow - the angle is arctan(1/4) which is about 15 degrees. OK.
As the boat slows, the angle of the apparent velocity will change. When the forward velocity has reduced to 1 mph, the apparent angle is arctan(1/1) which is 45 degrees. Essentially, the hull now feels like it is coasting at 45 degrees to the prevailing current.
The increase in apparent angle of the current tends to push the bow sideways more vigorously as the boat slows, so moving the paddle forward fights against that tendency by giving more leverage; i.e. reaching ahead of the pivot point of the boat with the paddle blade gives you a bigger moment arm to fight the unwanted rotation of the hull.
I’m fairly sure this is what’s going on, but hey, you never know.
Carl
Interesting analysis at the bow, but…
… it also applies to the stern and to every point along the length of the boat. So if there were no pinning effect on the bow – which reduces lateral freedom of the bow more than it does the stern – then the sideslipping forces should be the same everywhere no matter what the speed.
But your account does make me think it has something to do with complex flow at the bow due to the sideways motion, turbulence and such. That will definitely be different at the stern. So it may be quite tricky.
I had a conversation with a physicist over the weekend, and he thought it may be complex fluid dynamics rather than simple force vectors.
–David.
easier to show but…
paddle fwd. when you’re moving along fairly straight, with one hand, jam your paddle blade in right next to the boat so it’s 1. dead vertical 2. parallel with the waterline. now open up the back of the blade distance to the boat causing the front edge to ‘jam’ the boat to the offside. now adjust the fwd/aft position to maintain straight heading.
you dig?
steve
Yes, I think that’s right
I believe you your physicist friend are correct. The bow of the boat should generate a vortex due to the off-angle flow, similar to the vortex generated over the leading edge of a delta wing.
On the wing, the vortex causes low pressure on the top of the wing, thus increasing lift (and also helps to keep the boundary layer attached). On the boat bow, the vortex would cause a low-pressure zone on the downstream side of the bow, effectively pulling it to the side faster than the stern.
So I think you are correct that in order to explain the pivot behavior, some 3D fluid flow is involved. Otherwise, as you say, the skewed current would just push the boat laterally, and pivot direction would depend on the lateral area distribution of the boat below the waterline, as is the case (mostly) with weather cocking due to wind action on the boat above the water line. I wonder if there is any significant vortex contribution during weathercocking - I think there might be at higher wind velocities.
Nigel Foster’s explanation (I think)
I recently purchased Foster’s DVD series, which is excellent. He explains this observation in terms of pressure. Forward speed increases the pressure upon the bow (due to the bow wave), making it resist sideways movement. The stern becomes relatively looser since it is now in turbulence. This can cause weathercocking.
If you were to perform this stroke going backwards, everything would be in reverse, since the higher pressure created by the “bow” wake would now be at the stern, and the bow would be relatively loose.
Foster uses this concept a lot in teaching course corrections etc. It really made a lot of sense to me and explained a lot.
Simple theory
I am pretty sure it has very little to do with the fact you are slowing down and much more to do with the time from your last correction stroke. No kayak I have ever paddled will go straight for very long without paddle or rudder correction, you will need to correct sooner or later.
To avoid the kayak turning towards the paddle (due to drag)while side slipping, you are starting with a stroke that makes the bow turn very slightly away from the side that you will be drawing towards. This is effectivly initiating a turn, albeit badly and will eventually become a more powerful turn unless countered by another turning force (putting the kayak on an edge while side slipping, as many of us do will amplify this process) This is why you are always having to correct by moving forward to bring the bow back and not the reverse.
The turn theory…
– Last Updated: Sep-13-07 11:38 AM EST –
... is very interesting. Of course, there is no actual turn, since a good hanging draw / sideslip will send the boat perfectly sideways. But your idea makes sense to me this way...
Say for a sideslip to the right, the bow is effectively turning right, but the stern is turning ~left~ (that is, it is slipping right, which is what it does in a left turn). Maybe as the boat slows (or simply as the two "turns" progress in your theory) the right "turn" at the bow weakens more than the left "turn" at the stern, so the boat yaws left.
Of course, I'd really like to understand why a boat once turned tends to continue to turn. So far I've come up with these but haven't digested them in detail...
http://www.qcckayaks.com/resources/controllability.asp
http://www.greenval.com/jwinters.html#technical%20articles
--David.
While we’re on this subject…
Maybe I am doing something “wrong,” but when I am about to do a hanging draw/sideslip/whatever, I practice it three ways.
Let’s say I am going to do the hd/ss on my right, going forward. If my last forward stroke was on the left, it is harder to prevent the bow from turning to the right a little bit than if my last stroke was on the right–the same side. Also, I practice it by moving the blade–still in the water–directly from the last right-side forward stroke to the hd/ss position (quickly slice and move forward towards the cockpit, then open up blade angle).
Is there supposed to be one “correct” way to set up for the hd/ss? I have assumed that it’s good to practice all three ways for each direction and side.
Yes, good to practice all ways…
And here’s another – take a draw-on-the-move on the side you want to slip towards, but as you finish with the blade near the boat, move the paddle and blade into the hd/ss position. That way you get a head start on sideslipping.
As for small tendencies to yaw one way or another, just make a quick correction by moving the paddle (in hd/ss position) quickly forward or backward – forward to turn toward the blade, backward vice-versa. When you get good at that, you can make such a smooth, quick correction that it will barely compromise the nice parallel sideslip.
And after a while, you get to know exactly where to put the paddle in any situation. Of course, when you change boats, that changes a bit too, as it does with different speeds, etc.
–David.
Pressure on the bow
"Of course, there is no actual turn, since a good hanging draw / sideslip will send the boat perfectly sideways" Perfectly side ways but with the bow slightly off set i.e. slightly diagonal to the direction of forward movement and away from the direction of sideways movement. This may be slight enough to apear non existant but is there. This puts pressure on one side of the bow which balances the paddles drag tending to turn the kayak.
“…once turned tends to continue to turn” and the turn may decrease in radius. As the turn starts the kayak becomes diagonal to the direction of forward travel, this creates pressure on the bow pushing it side ways and reinforcing the turn, as the bow gets pushed sideways it becomes more diagonal and thus gets pushed more powerfully sideways, kinda snow ball effect. Plus all the eddies, vortices, high and low pressure areas etc. As long as the kayak is moving the pressure remains the extent to which the turn “snow balls” seems dependant on hull shape design/edge and speed.
Don’t get it; but conclusion maybe right
> "Of course, there is no actual turn, since a good
hanging draw / sideslip will send the boat
perfectly sideways" Perfectly side ways but with
the bow slightly off set i.e. slightly diagonal to
the direction of forward movement and away from
the direction of sideways movement. This may be
slight enough to apear non existant but is there.
Sorry, but I don’t get your point. If the boat sideslips perfectly, why is there ~any~ actual diagonal positioning away from the direction of forward travel.
But I do see your overall point, since even if the ss is perfectly parallel, the bow ~thinks~ it is turning toward the paddle because of the sideslipping itself. IOW, the hydrodynamics at the bow are the same or very similar to a turn to the right (assuming sideslip to the right), so the bow tends to accentuate the “turn” once started. At the stern, however, the hydrodynamics are for a left turn (that is, the stern sideslipping to the right) and a left turn gets accentuated. So the two are working against each other to keep the boat more or less parallel to its original course.
So the ultimate effect is the same as you claim, though for a slightly different reason. But it’s a terrific insight anyway, and seems like a real candidate for part of the explanation.
If so, the rest of the explanation involves the hydrodynamics of turns in general and of moving forward in general (as many have noted here), combined with boat speed. The trick will be to analyze the different turning effects at the bow and stern at different speeds.
–David.
Draw-on-the-move?
– Last Updated: Sep-14-07 1:16 AM EST –
Do you mean a sculling draw? (Seems like a weird way to go about it.) Or do you mean what I think of as just a "draw" but while moving forward? That seems like it would work better than the sculling draw. Hmmm, gotta compare those next time I go paddling.
Draw-on-the-move…
– Last Updated: Sep-14-07 8:16 AM EST –
... not sculling draw. While moving, just reach out and do a single, aggressive straight draw with a neutral paddle angle.
Actually, a hanging draw is in a sense the equivalent of a sculling draw on the move, with the motion of the boat providing the movement for a continuous "scull" in one direction. So, no need to keep changing direction as in true sculling. In fact, a strict sculling draw would be very difficult when moving, as you'd have a hard time with the backward direction of the scull, at least until the boat slowed considerably.
It's a useful blend, however, to start with a hanging draw and finish with a sculling or straight draw. You might use this to come alongside someone for a rescue or other purpose, where the hanging draw doesn't quite get you there because of the initial positions and motion.
--David.
My present hypothesis
– Last Updated: Sep-15-07 8:56 AM EST –
After reading and pondering, here's my latest thinking, fwiw...
* when you apply turning power to a bow or stern, it continues to turn because of vortices forming on the inside of the turn, which decrease water pressure; call this the "turn effect"
* the turn effect is stronger at the bow, because the oncoming water is "clean", while water passing the stern is already swirling somewhat because it's in the boat's bow wake.
* when you sideslip (say to the right), the turn effects at the bow and stern come into play, but the bow acts as if it's turning right and the stern left (that is, sideslipping right).
* since the bow turn effect is stronger, the bow wants to go right more than the stern does. To correct for that, you have to place the drawing (or prying) paddle a bit aft of the usual neutral point, to exert more rightward force on the stern (a slight stern draw, in effect).
* as the boat slows, both forward and rightward, the turn effect differential between the bow and stern decreases and the boat starts to turn left. To fix that, you need to move the paddle forward to exert more rightward draw (or pry) force on the bow and less on the stern.
* that's not simply a matter of uncompensating the prior paddle placement; the boat actually exhibits a turn effect to the left, so you have to move the paddle even forward of the neutral point.
What I mean by the neutral point, btw, is the spot you'd have to draw from if the boat were not moving, usually right at the hip.
So, the bow pressure effect does come into play, as many pointed out, though in a somewhat different way than with a simple turn. And the turn effect is crucial, as slowcoach was the first to note.
Make any sense?
--David.
Perferction
"If the boat sideslips perfectly, why is there ~any~ actual diagonal positioning away from the direction of forward travel."
This description of perfect is kinda like describing a perfect forward stroke as having no turning effect, its impossible there will always be a small ammount of turning effect involved. Perfection is in minimizing the negative and maximizing the positive.
If zero diagonal is perfection then very few if any of us are side sliping / hang drawing perfectly. Try getting someone to video you and see. Where did you get this description from anyway? Can’t seem to find it on any of the better known sylabuses.
It may be very slight but the angle/pressure is there. If it isnt the kayak will turn towards the paddle and you will have to put the paddle in far back and end up turning the kayak diagonal anyway.
Perfection is in doing the minimum diagonal angle to resist the turning effect and produce minimum turning effect with the paddle. Some over exagerating of the diagonal will make it easier for students to achieve some initial success and alow them to refine the technique.