JohnHuth

Without getting into the ethics argument, there's an interesting lesson on how long a person can last in the water. The story seems to suggest that the person was in 44 degree water for 10 minutes and would've been close to hypothermia. This sounds about right from my experiments in cold water (yes, I deliberately immersed myself in cold water). Also, the general tables you can find. This works against the 50-50-50 rule: you have a fifty percent chance of survival after swimming 50 meters in 50 degree water. I've personally swam about a mile in 50 degree F water. I was cold, to be sure, but I wasn't seriously hypothermic, no shivering yet. I'm not advocating going out in a T shirt and shorts in 44 degree F water, but the 50-50-50 rule is just not consistent with experience, and I wish instructors would drop it, because it can destroy their credibility, and not give good guidance on the conditions that bring on hypothermia.

Thanks for that. I'm sure someone's thought of this - but it leads to the interesting concept of extending the trail south, through New Hampshire and into Mass. The logistics might be more difficult because of population and the nature of the terrain, but if people can camp on relatively unpopulated parts of beaches, I suppose it could be managed. I haven't thought through all the issues - somewhat hairy points like the southern tip of Monomoy, how to handle Buzzard's Bay, etc, but it's intriguing. On the other hand, MITA does have some exposed sections, like Schoodic Head, and the peninsula jutting out toward Petit Manan Is (these are the ones I'm familiar with, there are surely more).

The distinction between phase velocity and group velocity only has meaning if you have a 'wave packet' - that is to say, the wave is the result of the addition of different wavelengths. I'd think that a progressive wave would be dominated by the wavelength of the tide, but depending on the width of the opening, I suppose shorter wavelengths could be created. In any case, the real power delivered comes from the group velocity, not phase velocity.

I was going to draw a diagram of the hydraulic current condition, but then realized that it appears in the literature, so I scanned one of the pages and attach it. It shows how you can get different slack tides when you have a large embayment next to a constricted opening. For what some others said - yes, the mean height of the water in Great Bay is going to have an effect on the timing. At spring runoff, I would reckon that slack might be delayed further after ebb and pushed earlier after flood. On the difference between progressive and standing waves. A progressive wave is a traveling wave, where the crest of the high tide moves up an estuary or narrowing embayment and the maximum current coincides with the high tide. A standing wave happens when water moves in, but reflects off an obstacle, so that there's effectively a rise and fall of the water level, but the "wave" of the tide doesn't really transfer any energy. Another example of a standing wave is called a 'seiche' - which is a standing wave that can occur in a trapped body of water. You often see these in harbors and they have a period of several minutes, which is dictated by the size of the harbor. Here's an animation of a seiche http://earthguide.ucsd.edu/earthguide/diagrams/waves/swf/wave_seiche.html Generally, when you have standing waves, there's a phenomenon called 'resonance', where an embayment has a natural frequency and responds to the 'pushing' of the water from the tides associated with the moon. This is called a 'forcing function'. When the natural frequency of the system coincides with the frequency of the moon passing overhead, it creates the strongest tides. This is why the tides are large in the Bay of Fundy and the Gulf of Maine - they have natural frequencies around 1/13 hours^(-1), while places like Nantucket have natural frequencies that are higher than the moon - something like 1/8 hours^(-1). The resonance response of embayments is classic for a standing wave, where you expect the highest current to be midway between high and low tide. On long, narrow estuaries, you tend to get more of a case where there's a progressive wave and the highest current is at high or low tide. The case of Great Bay and the constriction is such that there's no direct forcing because the constriction is too small, hence no standing wave, but because it's not really long narrow estuary, it's not really a progressive wave either, so the category of an hydraulic wave seems to fit best. Tides can get rather complicated if the bodies of water are linked together. The flow into Long Island Sound and Nantucket Sound are linked and have a common source around Newport, RI. This is how we get oddities like the flood moving east past Monomoy Island, when you might naively think it would flood west.

Yes, that matches quite well with the definition of an hydraulic current. It's the difference between the height of water at the constriction and in the embayment that drives the current flow. There's perhaps one more rare case that is not mentioned in those NOAA definitions, that's the case of a tidal bore, which I'd even hesitate to call a 'current'. When tidal differences become very large in a steadily narrowing estuary (really a transition from an embayment into a river), it can create a solid wall of water. This happens in parts of the Bay of Fundy, the Severn River in England and few other locations.

Any time there's a narrow opening into a large embayment, there's usually a lag time between high and low tides and the time of slack water, that's because it takes time to fill up the basin and a 'full basin' won't correspond to the time of high tide at the opening. The timing of the lag is related to the width of the constriction and the size of the basin being filled. In this case, Great Bay is the culprit.

"wave-schmave" - more water to displace means more force is required to move it. The bow wave puts more water up alongside the hull toward the front to be pushed out of the way. Force = mass*acceleration. More mass means it takes more force to change motion.

I think it's something like the following - the bow wave heaps up some distance behind the bow. In order to turn, you have to displace more water closer to the bow than further behind. The faster you move, the longer the wavelength of the bow-wave, and so you have a larger amount of water to displace in order to turn. Of course, speed will help in terms of a "lift" for a heeled hull and for a hanging draw. I'm beginning to see how a "sweet spot" can get generated, based on the speed and the wavelength of the bow wave. Getting there....

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.

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....

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).

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.

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.

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.

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

You're quite welcome. I'll try to post the slides to my website in the next day or so and then send around a link to it. I had an additional thought on the drive home. For those of you who have an aversion to compasses: Try them out on dry land. If you can get a topographic map of some local region, or even a map that shows features, such as buildings and has a decent north orientation, you can practice your skills. If you have a compass-skilled friend nearby, invite him or her along and see if your readings agree. It's partly a matter of habit. I carry a compass in my backpack and take bearings every so often for fun (not in line at Starbucks, however, although I suppose this might be a conversation starter). If you need assistance finding a local topographic map, please contact me and I can help find one of your local area for you to work with, and happy to help out! huth (at) physics (dot) harvard (dot) edu John H.

Go to: http://tidesandcurrents.noaa.gov/station_retrieve.shtml?type=Tide+Data Click on your state of interest, and then click on the "map data" button. You can then click on a station to get tide information. I can't seem to do the same thing with current, however, which was your original request, I know.

For long portages, I use a portable yoke. Old Town makes some decent ones. I like the portable yokes because they're much more compact for stowage. I've gone on 2 miles portages with these yokes and they work pretty well. It also gives you a lot more options on terrain - you can't always guarantee a nice easy portage trail that will allow for tires. As far as dolly - I don't know, however.

Are there exceptions for the BCU 5*? Well, anyway, I was driving home last night and nearly ran into a guy on a bike. He was riding - without hands, in the middle of the lane, without reflectors, without lights without anything. I had to slam on the brakes and all of the stuff in my car (including my paddle) slammed forward with huge effect. If they're going to enact legislation like that, they really should consider bicyclists, canoeists, pedestrians, and if I put some thought into it, a few other folks who wander around at night. Racoons? Opossums?

I personally could do the 20th, if there was a strong sentiment for that - but I don't know about the availability of the center, other folks' plans etc.

Doug - Sure. I'd like to hear what people might be interested in. I have a longer presentation on weather predictions if people are interested. You can see from my website some of the material I offer. One area of interest is the topic of hull-paddle interaction in things like the stern rudder and certain kinds of draw strokes. There's also the question of the hull, trim, etc in weather cocking, turning. Use of stars and the sun and 'on the go' navigation: natural bearings and ranges, estimating range, blind navigation Those are some thoughts off the top of my head. I can explain any more in depth, if there's some interest. John

I posted my slides at the following website: http://huhepl.harvard.edu/~huth/Primitive%20Navigation.html The first two tags are the presentation slides. If you would like more information, please contact me. Best, John H.

Bill - I'm not sure this is what I posted, but it does have slides to download, some of which bear on dead reckoning. http://huhepl.harvard.edu/~huth/Primitive%20Navigation.html Let me know if this is helpful. John H.

Here are two bonafide squall line photos. The leading edge of these produce very strong downdrafts, part of a gust front. It turned calm waters into a churning mess. The photo shot inside the squall line toward the gust front allows you to see some of the churning of the surface of the water in the distance and then the ensuing rain and lightning. The conditions preceding this were interesting - I also got some shots of the cloud sequence. There were four extremely hot and humid days, where I was wondering why there were no thunderstorms produced. The air was very calm. I was walking on the beach and my wife, who is now wholly acquainted with my weather obsession, pointed to the sky and said "oh look, mackerel scales, we're going to have some rain." The next hour or so showed a very rapid progression of the clouds associated with a warm front of an approaching hurricane. Then, in the distance you could see many flashes of lightning associated with this continuous band of smooth clouds. When the gust front hit, it really churned up the water and there were lightning strikes all over the place, followed by extremely heavy rain.

Here are three that I recommend - unfortunately they're all pretty thin. Weather Forecasting by Michael Hodges, Globe Pequot Press 2nd ed. (1999) The Weather Wizard's Cloud Book by Louis D. Rubin Sr. and Jim Duncan Algonquin Books (1989) Marine Weather Forecasting, by Frank Brumbaugh, Bristol Fashion Publications, (2000) They all have their plusses and minuses, and present a lot of similar information. I found the Weather Wizard's Cloud Book to be the best of the three. One of the exercises I did with my students this year was to ask them to track their ability to predict weather against a concept called "persistence". Persistence is the idea that tomorrow's weather is like today's. Now, it is said that this is correct 80% of the time, but that 80% refers to a very specific and obscure kind of measurement. We cooked up a figure of merit: what fraction of tomorrow will it rain? We tested persistence against our ability to predict the weather using this figure of merit. This forced the students to make observations, commit to a forecast and then track it over the course of a week. Invariably they improved as the week wore on, and it's an easy thing to do at home. Unless you force yourself to make a prediction and then observe the following day, you can't really gain the skill. Like most skills, it takes practice. It also varies depending on the season - e.g. thunderstorms can crop up on random hot days in the summer, but clearly it's not very likely in the winter.