-----Original Message----- From: Nick Schade <schade_at_guillemot-kayaks.com> To: paddlewise_at_lists.intelenet.net <paddlewise_at_lists.intelenet.net> Date: Tuesday, May 25, 1999 8:39 AM Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form" >At 8:55 AM -0400 5/24/99, John Winters wrote: ><snip A whole bunch of stuff> >> >>The flow indicates where the water goes. To assume that it doesn't go where >>it is going kind of boggles. >> The flow over the hull is the resultant of the motion of the hull and the direction the water is actually moving. If the water moves 2" down while the boat moves 200" forward the flow past the hull would appear to be at a slight downward angle but the water would have moved only 2" downward (and slightly forward due to the friction with the hull). So the water is mostly moving up or down (and out to the side during a crest and back in the trough) with the waves lifting it up and down and being pulled along at some speed between as fast as the boat (or even faster for spray flung forward at the bow) and not forward at all some distance from the boat (the area where water is being set into forward motion gets further from the hull (in all liquid directions) as the boat moves past resulting in the wake behind the boat. >One demonstration of water diving below the boat can be seen in shallow >water. Most people have probably experienced the big wake and slow progress >you get when paddling in water 1 foot deep or so. The easy explanation for >this is waves like deep water so the stack up like surf at a beach. But, >your boat is not effected by the size of the wave once the wave leaves the >boat. You don't go slower just because your wake encounters shallow water. >There must be some other explanation. I disagree. Your boat is affected by both the size and the length of the wave created at the bow. My undersanding is that as the wavelength is shortened due to the shallow water effect the boats hull speed is now lower (it gets trapped in a now shorter slower moving wave). For a fast moving kayak you first feel this in four or five feet of water. Some Canoe racers can actually plane their canoe if they can keep their speed up (and break the wave barrier) on suddenly hitting an area of shallow water. > >Water flowing under the boat must be displaced and in turn displace other >water farther down and out. This chain of displacement eventually will push >out to the surface. Since water is incompressible the first effect of disturbing it can only be upwards, hence the bow wave an upward motion of the water. >But this is pushing fairly slowly, albiet a large mass. >When you get into shallow water, the water has fewer choices on where it >can go so it must move faster. This is made worse by the Bernoulli effect >which sucks the stern down even further making it even harder for the water >to get out of the way. Maybe but, once you are in water less than one foot deep the turbulence near the stern of the hull--the wake (water which the hull is dragging along a little in the direction the hull is moving is now also dragging against the fixed ocean floor and this really slows you down. At least that's how I understand it. There used to be a kayak race around Jetty Island which had extensive shallows around it. The trick was to get far enough away from the island to be in deep enough water to have enough extra speed to make up for the extra distance you had to travel by staying farther away from the island as you circled it. A real excercise in frustration if you got it wrong and those slower paddlers you had left behind earlier started passing you up. >Even though it may be pushing less water it is >pushing it out of the way at a higher velocity. Why would it be pushing less water? >The energy in the water due >to the motion is KE=1/2 m v^2. Moving a large mass slowly tends to be more >efficient than moving a small mass rapidly. > >The result is when the energy is released at the water surface, larger >waves are made. If the water was being pushed purely to the side, you would >not see such a dramatic change in shallow water. The typical wake of a >kayak is pretty small. Probably small enough to travel pretty easily in 1 >foot of water. A wave begins to "feel" the bottom in water about 1/2 as deep as the wavelength(the waves orbital motion below the surface is touching the bottom which slows it down--compressing its energy into a shorter space thereby making the bow wave higher, steeper and harder to climb). At 5.4 knots the wavelength created in deep water is about 16 feet long--which means it begins to feel bottom in 8 feet of water. [At 3 knots the wavelength is about 5 feet long so the wave-drag effect starts (but is probably not noticible yet in 2.5 foot deep water)]. In four feet of water the 16 foot wave is slowed a little to 5.2 knots. In 2 foot water that wave slows to about 4.4 knots and that sure slows your racing speed. In 1 foot deep water that waves speed is down to 3.3 knots and so is the kayaks hull speed unless it can break the wave barrier and start to plane. In 1/2 foot deep water wave speed is slowed to 2.37 knots. Remember that in water less than about a foot deep bottom drag due to turbulence drags one down even more than this. >If the water was just being pushed to the side, not much >would change in shallow water, but because the water must find a fast way >to get out of the way, shallow water slows you down. Sure it would. Water does speed up in a constriction--the Venturi effect (a special case of the Bernoulli effect)--which if under the boat would lower the boat further than normal and this would bring the stern turbulence even more into contact with the bottom. I should point out here that when two ships pass near each other or a ship passes near a wall the same effect moves them closer together (a real danger for ships). Two kayaks coasting along less than a foot apart will demonstrate this sucking together effect as well. This would seem to indicate that water is also moving around the hull (I'm applying the same arguments you are using to convince us that flow is going under a hull). It is easy to see in very shallow water why it would be impossible for much water to go down, it would just bump into the bottom. Due to the incomressability of water even in the deepest ocean no water can go down without moving some other water upwards and since the water can't go upwards though the hull it must go first to the side or raise up in front of the bow (due to the pressure in front of the moving hull). Here is another thought. In areas of lower pressure the flow will be moving back downward and in this area the flow would be diving under the hull as the flow past the hull moves parallel to the waters wave angle at the surface. It would seem the flow line on the hull would move back upward with the next wave crest but if that wave were well back on the hull the flow might keep moving down to fill in the area of low pressure behind the moving hull. I don't know if this is right just some thoughts I'm having in trying to apply basic principles to the flow question. Matt Broze www.marinerkayaks.com > >Nick > > > >Nick Schade >Guillemot Kayaks >10 Ash Swamp Rd >Glastonbury, CT 06033 >(860) 659-8847 > >Schade_at_guillemot-kayaks.com >http://www.guillemot-kayaks.com/ > >>>>>"It's not just Art, It's a Craft!"<<<< > > >*************************************************************************** >PaddleWise Paddling Mailing List >Submissions: paddlewise_at_lists.intelenet.net >Subscriptions: paddlewise-request_at_lists.intelenet.net >Website: http://www.gasp-seakayak.net/paddlewise/ >*************************************************************************** *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Matt Broze wrote: <snip A whole bunch of stuff> > A wave begins to "feel" the bottom in water about 1/2 as deep as the > wavelength(the waves orbital motion below the surface is touching the bottom > which slows it down--compressing its energy into a shorter space thereby > making the bow wave higher, steeper and harder to climb). At 5.4 knots the > wavelength created in deep water is about 16 feet long--which means it > begins to feel bottom in 8 feet of water. [At 3 knots the wavelength is > about 5 feet long so the wave-drag effect starts (but is probably not > noticible yet) in 2.5 foot deep water]. In four feet of water the 16 foot > wave is slowed a little to 5.2 knots. In 2 foot water that wave slows to > about 4.4 knots and that sure slows your racing speed. In 1 foot deep water > that wave's speed is down to 3.3 knots and so is the kayaks hull speed unless > it can break the wave barrier and start to plane. In 1/2 foot deep water > wave speed is slowed to 2.37 knots. Remember that in water less than about a > foot deep bottom drag due to turbulence drags one down even more than this. Thanks for this, Matt -- never had anything quantitative before, though I sure noticed the "shallow water" effect. Feels like paddling in mud. > Water does speed up in a constriction--the Venturi effect (a special case of > the Bernoulli effect)[snip] > I should point out here that when two ships pass near each > other or a ship passes near a wall the same effect moves them closer > together (a real danger for ships). Local examples: 1. There is a very narrow place in the shipping channel where it is jammed up against the WA side of the Columbia River -- about 3 to 4 miles downriver from Skamokawa. It is notorious for this "Venturi" effect on freighters passing in opposition, and the River pilots slow way down there. 2. There was a glancing bow-to-bow collision a year or so ago off Hammond, OR (close to the River mouth) in which a downbound freighter tagged an upriver-bound Coast Guard buoy tender (150 foot ship?) pretty good. No serious injuries, but probably a lot of soiled underwear <G>. Night time, but good visibility. Scuttlebutt is that neither skipper thought they were "too close." Guess they were. -- Dave Kruger Astoria, OR *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
At 12:53 AM -0700 5/26/99, Matt Broze wrote: <snip> >>Even though it may be pushing less water it is >>pushing it out of the way at a higher velocity. > >Why would it be pushing less water? In deep water the motion can be distributed over a larger volume. In shallow water it must displace the water available which is not as much as available in deep water. > >>The energy in the water due >>to the motion is KE=1/2 m v^2. Moving a large mass slowly tends to be more >>efficient than moving a small mass rapidly. >> >>The result is when the energy is released at the water surface, larger >>waves are made. If the water was being pushed purely to the side, you would >>not see such a dramatic change in shallow water. The typical wake of a >>kayak is pretty small. Probably small enough to travel pretty easily in 1 >>foot of water. > >A wave begins to "feel" the bottom in water about 1/2 as deep as the >wavelength(the waves orbital motion below the surface is touching the bottom >which slows it down--compressing its energy into a shorter space thereby >making the bow wave higher, steeper and harder to climb). At 5.4 knots the >wavelength created in deep water is about 16 feet long--which means it >begins to feel bottom in 8 feet of water. [At 3 knots the wavelength is >about 5 feet long so the wave-drag effect starts (but is probably not >noticible yet in 2.5 foot deep water)]. In four feet of water the 16 foot >wave is slowed a little to 5.2 knots. In 2 foot water that wave slows to >about 4.4 knots and that sure slows your racing speed. In 1 foot deep water >that waves speed is down to 3.3 knots and so is the kayaks hull speed unless >it can break the wave barrier and start to plane. In 1/2 foot deep water >wave speed is slowed to 2.37 knots. Remember that in water less than about a >foot deep bottom drag due to turbulence drags one down even more than this. The reason the wave "feels" the bottom is due to water moving down. If there was no downward motion the wave would never care about the bottom. The fact that a boat feels a lot of drag when moving in shallow water is well estabilished. This drag is due to the difficulty of a wave of a given wavelength to propogate in shallow water. But why can't the wave propogate? Because the water is shallow there is not as much mass available to deal with the energy in the wave so the water has to move faster. The wave propogation speed can't increase because the water is not deep enough to sustain a the required longer wavelength. Instead the waves increase in height and eventually break, thus disipating the energy in friction. However, once the wave is generated it does not have much further effect on the boat. The wave can go its seperate way and disipate its energy anyway it finds convenient without further effect on the boat. Waves are just a symptom of drag. The bow wave and resulting trough at the stern when paddling in shallow water are a by-product of the interaction of the bottom of the boat and the bottom of the sea through the medium of the water. There would be no interaction without the water being moved downward under the boat. > >>If the water was just being pushed to the side, not much >>would change in shallow water, but because the water must find a fast way >>to get out of the way, shallow water slows you down. > >Sure it would. > >Water does speed up in a constriction--the Venturi effect (a special case of >the Bernoulli effect)--which if under the boat would lower the boat further >than normal and this would bring the stern turbulence even more into contact >with the bottom. I should point out here that when two ships pass near each >other or a ship passes near a wall the same effect moves them closer >together (a real danger for ships). Two kayaks coasting along less than a >foot apart will demonstrate this sucking together effect as well. This would >seem to indicate that water is also moving around the hull (I'm applying the >same arguments you are using to convince us that flow is going under a >hull). Kayaks generally have a greater beam than draft. I will not try to argue that no water moves sideways. John says empirical data shows that it tends to follow buttocks lines and I am prepared, to a certain extent, to accept that. Imagine a perfectly flat bottomed boat with perfectly vertical sides and a nice waterline shape. You would think the water hitting the sides and moving back along the length of the boat would move straight out and then straight back in. However, the bottom gets larger towards the middle. This larger area needs to get water to cover it from somewhere, so it draws down water from the sides. Obviously, there is some friction involved so the water does not move perfectly along the buttocks lines, but it does its best. So, in deep water, some water does get pushed out to the side, but alot of it goes down to the bottom to fill the large bottom area. The water near the surface which gets pushed out to the side creates enough Bernoulli effect to draw together two kayaks paddled side-by-side. But the effect between the boat and a shallow bottom is much greater. > >It is easy to see in very shallow water why it would be impossible for much >water to go down, it would just bump into the bottom. Due to the >incomressability of water even in the deepest ocean no water can go down >without moving some other water upwards and since the water can't go upwards >though the hull it must go first to the side or raise up in front of the bow >(due to the pressure in front of the moving hull). Yes, in shallow water, the water can not move down, but in deep water it can. In shallows the water below the boat must be squeezed outward as you would think. So yes, the water is being pushed sideways. But this causes increased drag, because it needs to displace the small amount of water _rapidly_ out of the way, instead of distribuiting this displacement over a larger mass of water which it could move slowly by initially pushing it downward. The problem in shallow water is not so much that the water is being moved sideways, which I will admit it has to do eventually in all cases, but that it must move that water much quicker than if it could move a larger mass of water downward before that motion is dissipated out to the side. Moving the small amount of water fast manifests itself at the surface as higher energy waves. Again, it is not the waves that are the problem, it is what causes them that matters. Nick Nick Schade Guillemot Kayaks 10 Ash Swamp Rd Glastonbury, CT 06033 (860) 659-8847 Schade_at_guillemot-kayaks.com http://www.guillemot-kayaks.com/ >>>>"It's not just Art, It's a Craft!"<<<< *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
This archive was generated by hypermail 2.4.0 : Thu Aug 21 2025 - 16:32:59 PDT