Slides from Sunday talks

[JohnHuth]

Dear All:

I've posted my slides from Sunday's presentations. You can download them from the following URL's:

http://huhepl.harvard.edu/~huth/On%20the%20go%20navigation.pdf

http://huhepl.harvard.edu/~huth/Turning%20strokes.pdf

There is also my Primitive Navigation website, where there are a few other talks I gave - my other NSPN talks, a talk to the Cape Cod Astronomical Society, my Freshman Seminar and an Applied Mathematics class (not as intimidating as it might sound). There are also a few time-lapses. If you're curious about the funny figure on the website, it's called an analemma

http://huhepl.harvard.edu/~huth/Primitive%20Navigation.html

[spuglisi]

John,

That's just awesome! Thank you so much for posting this information and for putting on the presentation.

[djlewis]

Hi, John:

Gee, I did not realize you'd be doing the physics of turning -- would have come for sure!

Did anybody ask -- or do you know -- why the sweet spot for a hanging draw changes as the boat slows, and you have to move the paddle to maintain it.

Sorry, but as I sit at my desk after a winter off the water, I cannot recall for the life of me which way this goes -- does the boat yaw toward the paddle as the boat slows, requiring you to move the paddle backwards, or the other way. A google search is inconclusive. This video seems to show the paddle being moved forward. But I've also seen it said the other way.

Note -- I'm not talking about where the sweet spot is on different boats -- I know it varies -- but how it changes fore/aft as the boat slows, which ought to be the same direction for all boats, right? Some folks have told me it does not change, but I was taught it does and it happens for me every time.

http://www.viddler.com/explore/pesdapress/videos/4/

[djlewis]

http://huhepl.harvard.edu/~huth/On%20the%20go%20navigation.pdf

This one appears to be corrupted or not there.

[spider]

It comes up ok for me. It took a long time bringing it up for Safari, but shorter with Google Chrome.

(mine might be a bit slow now that I did away with the high speed connection to regular speed connection)

ton's of good info...thanks John ..much appreciated.

[JohnHuth]

David -

Let's see....

First the file is quite large, so it could be a time out problem in the process of downloading. Certainly on two kinds of browsers it worked for me. If there are still problems let me know.

On the "sweet spot" issue - here's what I think's going on.

First, let's take a simpler case. We have a classic problem in freshman physics where we set up the following situation: a stick is lying on a sheet of frictionless ice. A hockey puck is moving along in a direction perpendicular to the stick with some velocity, and strikes the stick at a distance from the center of mass of the stick. What's the ensuing motion?

Ans: It's a combination of linear motion and rotation of the stick - that is to say, the center-of-mass of the stick slides, but the stick also gets some rotational motion.

There's the related problem of how to swing a baseball bat. There's a sweet spot where the ball should hit to get the maximum effect.

A hanging draw at some distance from the center of mass of the kayak will create some translational motion and rotation al motion. In general, the further from the center of rotation, the more rotation and the less translation you get.

Now, there are three issues associated with the water flow - the first is the flow around the paddle which gives it lift, the second is the flow around the hull which tends to lock in forward motion the faster you move, and give some asymmetry between the bow and the stern. Finally, there's the possibility of an interaction between the hull and the paddle itself because of the diversion of the water flow. I assume the water flow around the paddle doesn't really change the sweet spot with speed, but the flow around the hull and the interaction of the flow around the hull and the paddle could very well shift the sweet spot.

I have to work this through, but it's probably some combination of the effects.

[djlewis]

I understand the simple issue -- plain rigid-body mechanics,right?

And I'm glad to hear that you don't have a quick answer for the way water speed changes the sweet spot. It's clearly a flow dynamics problem, and I can usually work those out intuitively for kayak turning issues. But darned if I have been able to figure out this one, nor have I heard a convincing explanation.

Let us know when you have a chance to work through it. Also, observe it on the water and see what happens for you. Maybe playing with it will help understand what's going on.

--David

[Suz]

I understand the simple issue -- plain rigid-body mechanics,right?

And I'm glad to hear that you don't have a quick answer for the way water speed changes the sweet spot. It's clearly a flow dynamics problem, and I can usually work those out intuitively for kayak turning issues. But darned if I have been able to figure out this one, nor have I heard a convincing explanation.

Let us know when you have a chance to work through it. Also, observe it on the water and see what happens for you. Maybe playing with it will help understand what's going on.

--David

I will give you the answer why the sweet spot changes in lowly non scientific terms:)

The bow is under pressure when you are under way like this: <--- The stern is loose as obvious in my little diagram. As you slow, there is less pressure generated on the bow and therefore you must change the position of the draw by moving the blade back a bit towards the direction of the stern. All the while NOT changing the angle of the blade.

Things that change the placement of the draw would be:

- how much pressure there is on the bow (generated by either forward momentum or current)

- the shape of the hull of the boat (this is why in some boats you need to be further forward/aft to have same effect)

[Pintail]

David, John did actually announce beforehand that he would probably be covering the subject of the dynamics of stern draw strokes! Your loss...

Suzanne is very much on the right track; but you need to consider the sides of the kayak (in plan form) as a wing -- it has roughly the same sort of shape and is subject to the same laws and influences. The "centre of lift" is that point through which the total lift appears to act. It is dependent on speed (speed is a function of its value). As your craft slows (be it wing or marine hull) that centre of pressure moves backwards or forwards and you have to move your paddle accordingly to remain in the region of the CofP, where the force of the paddle is most effective.

Quite simple, really; but that gives you a place at which to start. John will do the maths for you, if need be, I am sure.

[EEL]

As your craft slows (be it wing or marine hull) that centre of pressure moves backwards or forwards and you have to move your paddle accordingly to remain in the region of the CofP, where the force of the paddle is most effective.

All very interesting, but I dimly recall needing to move the paddle forward to prevent a turning as opposed to sideslip motion as the boat slows in every boat I have tried. Then again, my memory isn't....well...what was I saying?

Ed Lawson

[Suz]

All very interesting, but I dimly recall needing to move the paddle forward to prevent a turning as opposed to sideslip motion as the boat slows in every boat I have tried. Then again, my memory isn't....well...what was I saying?

Ed Lawson

Interesting... Will need to try it in the pool on Saturday night as I won't be on the water prior.

Of course, if you "lean" to your paddle or away from your paddle, that would change everything. My explanation is flat boat as it would be if you are doing a text book hanging draw. BODY rotated and flat boat.

Haven't been on the water with Ed for donkeys years and so I don't know what your rotation is like but definitely a draw stroke done w/o rotation can cause the boat not to be flat and if you edge the boat either away or towards the paddle, it will have an effect on the stroke.

Edge "up" that is closest to the paddle will cause the boat to turn towards the paddle. In order to maintain straight, you would need to have a slight prying action further back to the stern.

Edge "up" that is furthest from the paddle will cause the boat to turn away from the paddle and then moving the blade further towards the bow would correct for that and cause the boat to draw straight.

I will try and remember to report in after the pool.

[djlewis]

...As your craft slows (be it wing or marine hull) that centre of pressure moves backwards or forwards and you have to move your paddle accordingly to remain in the region of the CofP, where the force of the paddle is most effective.

A, Sir G -- "backwards or forwards" -- which is it?! You couldn't remember either!

OK, I get it now. The answer is backwards. When going forward fast, the bow is pinned and the stern loose, as Suz notes, so the pivot point is near the bow. As you slow, the bow loosens, the pivot point moves backwards, and so must your paddle.

[JohnHuth]

My problem, as a scientist, I have to predict the effect before I test it out and then report whether I was right or wrong. Bummer.

Anyway, I'm working on it, but I'm not going to report until I think I have a cogent explanation.

[djlewis]

My problem, as a scientist, I have to predict the effect before I test it out and then report whether I was right or wrong. Bummer.

Anyway, I'm working on it, but I'm not going to report until I think I have a cogent explanation.

Gee, I thought we had a good theoretical hypothesis -- the bow loosens as you slow -- and an easily testable prediction from it -- the boat will yaw toward the paddle and correct if you move the paddle backwards. Perhaps we could use some more predictions, experiments to show effects we have not already observed.

How about this. While the boat is still moving fast, turn the hanging draw into a slight forward sweep with the paddle's power face (as opposed to the usual forward sweep which uses the back face), or maybe just open the power face a bit. (This is like the little extra turn you can put on the end of a bow rudder). Feel how much pressure it requires to yaw the boat. Now do it again after the boat has slowed, and it should yaw more with less effort/pressure. This might also give a direct sense in your muscles of the magnitude of the pinning of the bow.

[JohnHuth]

I know that the "bow is locked" and the "stern is slippery" is the gospel explanation offered for a lot of kayak phenomena, but I suspect that stern wave generation may create some stern-locking as well. I have two kayak buddies who are fluid dynamics experts and I have a query into them about the "bow is locked" and the "stern is slippery" gospel.

I know there's more than simple turbulence in the wake going on.

[JohnHuth]

If you look at slide 11 from my second presentation, you'll see some of the waves created by both the bow and stern waves, and the wake from each. This is from a computer simulation. I suspect that the "bow gets more locked" effect comes more from the pattern of bow and stern waves, rather than the"bow gets locked" and the "stern slops around in a bunch of turbulence" explanation. I think it has much more to do with the generation of the wake and the nature of the bow wave and stern waves.

So, the coarse effect might be the same, but I suspect that the 'stern slops around in a bunch of turbulence' statement is suspect (in my mind).

[JohnHuth]

Here's what my fluid dynamics friend told me about this explanation:

------------------------------------------------------------

This explanation, while put forth by many kayak instructors and seemingly knowledgeable people, is fluid dynamically incorrect. The issue is non-trivial to understand and I have not properly written up a better explanation for it, but I believe I understand the physics on this. I will try to let you know what I know about it.

You can imagine that, from a pressure loading standpoint, the forces around the bow and stern are symmetric -- this debunks the "tight" bow and the "loose" stern, which are poor terms to use also (given that water is incompressible at these pressures).

------------------------------------------------------------

More later....

[JohnHuth]

OK, after some consultation, here is what I any my fluid dynamics buddies agree on:

1.) The stern turbulence argument makes no sense - it's not consistent with fluid dynamics. So, one doesn't say that the stern slips around - just doesn't work.

2.) Bow 'lock' - a bit closer, but not exactly what happens. The water displaced by the bow slicing through the water creates a wave that is a bit behind the bow itself, but inhibits turning. So, the center of rotation moves forward.

[spider]

I know you folk are knee deep into dynamics but thought I would ask something more along the navigation end of the deal.

Taking John's recommendations I'm drawing the magnetic lines on my charts mostly just the north south, but one I set up more as a UTM grid with each square marked off in mile grids just to try it out etc.

Any way I'm practicing plotting coordinates whenever I see a reference to them...practice, practice, practice.

on this NOAA link http://forecast.weather.gov/MapClick.php?lat=42.805476827860275&lon=-70.79864501953125&site=box&smap=1&unit=0&lg=en&FcstType=marine

I see the Isles of shoals as lat 42.97 N

and long 70.62 W

When I plot them out I'm off about 22 miles from Isle Of Shoals

grabbing a coordinates right from the Map Tech... Cape Ann to Cape Elizabeth

I get approx the same lat but the longitude is way off (W70.37.22)

So my questions are...

Am I doing something basically wrong?

or

are the coordinates from the link the location of some weather station out at sea

and not the coordinates for Isles of Shoals though it seems to state it as such ?

(in which case never use the listed coordinates thinking they are the numbers for the actual place you're looking up)

thanks

[gyork]

Am I doing something basically wrong?

or

are the coordinates from the link the location of some weather station out at sea

and not the coordinates for Isles of Shoals though it seems to state it as such ?

Try using this google link:

http://marine.geogarage.com/routes

It gives me coordinates of 42.59.187N 78.36.929 for the tower on Appledore I.

The coords you listed for the weather buoy likely reflect the buoy position, which is NOT IOS.

Good idea to practice, practice, practice with paper charts, dividers, ruler, and orienteering compass.

[spider]

Thanks I'll check it out.

(I'm thinking the lat has to be 70 something...thinking the 78 is just a typo)

[spider]

Thanks I'll check it out.

(I'm thinking the lat has to be 70 something...thinking the 78 is just a typo)

[EEL]

So my questions are...

Am I doing something basically wrong?

or

are the coordinates from the link the location of some weather station out at sea

and not the coordinates for Isles of Shoals

My answers are (for what they are worth):

You are doing something wrong, and the coordinates are not for a weather station out to sea.

http://www.ndbc.noaa.gov/station_page.php?station=iosn3

Here is a little secret.

You might find the following software fun to play with. As I read the EUA, it is free for continuing use and only some advanced features stop working after the 5 day trial period. It is heavy duty software and enables you to find exact coordinates on charts as it directly reads them from pointer. It also does a great many other tasks for plotting trips. Great stuff for this never ending winter.

http://rosepointnav.com/CoastalExplorerExpress/default.htm

Ed Lawson

Who is eagerly awaiting more info on the stuck bow and loose stern matter.

[djlewis]

OK, after some consultation, here is what I any my fluid dynamics buddies agree on:

1.) The stern turbulence argument makes no sense - it's not consistent with fluid dynamics. So, one doesn't say that the stern slips around - just doesn't work.

2.) Bow 'lock' - a bit closer, but not exactly what happens. The water displaced by the bow slicing through the water creates a wave that is a bit behind the bow itself, but inhibits turning. So, the center of rotation moves forward.

I buy the "stern turbulence argument makes no sense" thing intuitively -- always bothered me. Seems to me more that it's simply that the stern does not get bow-lock, bow-lock being something special up there where the water is hitting hard, with no wave to spread things out. IOW, the stern is actually in kind of a quiet zone, compared to the bow.

But what how does a wave a bit behind the bow inhibit turning? That needs quite a bit more explanation.

In fact, I think there are (at least) two different effects here. As we all know, once you get a boat turning enough in current (or with momentum), it turns ~more~. This is the issue of keeping ferry angle in current low enough to prevent a forced turn downstream. It also (I think) is behind the familiar effect that a long-boat paddler gets when they first paddle a whitewater boat -- at first it spins uncontrollably. I can see a simple force diagram for this effect -- turn right; the left side of the bow is even more diagonally across the oncoming flow and the right side of the bow is both somewhat shielded from that flow and more parallel to it. So the bow really takes off sideways to the right, hence uncontrolled spinning. There may be more to the spinning effect than this, but the above seems intuitively at least a part of it.

But before the bow turns enough for the spinning effect to take over, at least in a long boat, there is the locking effect, which is the opposite. This is what we are mainly talking about, right? But how does it work? Still a mystery to me. Wave-schmave! What's happening in terms of forces?

Of course, all this is somewhat beside the original question, about why a hanging draw has to move backwards as the boat slows. That seems definitely due to bow-lock, or reduction thereof, no matter what the reason for bow-lock.

--David.

[EEL]

John:

Just to dial some more factors. When the kayak is side slipping or whatever during a hanging draw, is there a disparate force on either side of the bow from the bow waves? Does that force change with the kayak's velocity? If so, what impact would that change have on the kayak's heading?

Neat stuff, the slides were great, and the "putting it all together" pictures/text of final slides was especially nice.

Ed Lawson