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reading currrents

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9 years 4 months ago - 9 years 4 months ago #23312 by Teddy_the_grownup
I want some tips on how to read currents. More specifically, I want to be able to use currents around shoals to my advantage.
I have seen some videos of the Kayak Coach zipping near a reef to pass his pupils. Can I do the same thing with the underwater sand shoals ? For example, here in the Chesapeake Bay in the USA we don't have reefs, but we have hundreds of underwater shoals. How do you use shallow water to go faster ? My guess the currents in shallow water can seem unpredicable if you don't read the current charts before you go out. So, this is probably not an easy question.

S18S stellar ski since 12/14
Last edit: 9 years 4 months ago by Teddy_the_grownup.

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9 years 4 months ago - 9 years 4 months ago #23317 by Kocho
Replied by Kocho on topic reading currrents
The Chesapeake Bay is tidal, so start with the tide charts for your specific location. Under the Bay Bridge, for instance, you can get as much as 4mph currents on occasion. Add wind direction and you can get a good estimate of the current speed.

As for shallows, you need to take into consideration the depth and the current in the main channels. Sometimes you may get some back currents hear shore, while the main current is in the opposite direction in the deep.

Also, if it is too shallow, you may actually go slower - suck water effect.

I don't paddle the Bay enough to know many locations, but generally, on the bigger tributaries and channels off the main bay area, the fastest currents are in the deepest places. I'm sure local sand bars here and there might be the exception. Is there a specific place you have in mind? Sometimes you can guess the speed of the current from how the crab net floats are tilted, or how the boats that are anchored are oriented.

For races though, you are better off talking to the locals who know the course. The Bay and its tributaries are often like rivers - if you know how to read a river's current, you can read them too: for instance, the current is faster along the outside of an turn and in the middle of channels where it is deeper; and slower near shore around the inside of the turn in most cases. So you have to optimize - go with the fast current and travel a longer distance at a higher speed or go with the slower current and travel a shorter distance at a slower speed - not always clear which is better...

A shallow or an island (or even the bridge's pillars) in the middle of the channel can create a "shadow", calmer area with less current. If you are paddling with the current, you want to stay away from these, but if you are paddling against the current, you may want to paddle through that slack water rather than battle the faster main channel. Just like rocks in a swift river current...
Last edit: 9 years 4 months ago by Kocho.
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9 years 4 months ago - 9 years 4 months ago #23324 by red_pepper
Replied by red_pepper on topic reading currrents
As for currents, note that whenever water flows past/around/through something, it alters the flow. Going downstream, the current speed will increase if the flow is squeezed between a couple of items, like an island and the shore. You have a large volume of water trying to get through a reduced volume, and it will have to travel faster to compensate. Also, when water flows by an object, such as a rock, bridge piling, etc., it will swirl around it. If you watch the direction of the flow, you can use it to either give yourself some assistance moving upstream, or a little extra punch going downstream.

With regards to shallows, river racers know they can often "pop" the boat onto the wave generated by the wave off the bottom of the boat being reflected off the bottom of the river (or sand bar, etc.), picking up a considerable amount of speed. As the water gets shallower, the wave speed is reduced, and the ability to ride this wave is easier (you basically have to drive the boat over the wave to ride it - essentially surfing your own wave). It's often easiest to pop the boat in a foot or less of water. As the water gets deeper, the wave speed increases and you'll get to a point where you won't be able to drive your boat fast enough to get over the wave and onto the front of it (since you're essentially running the boat uphill onto the wave). This is suck water; it slows you down with little you can do about (except draw your competition in after you and then leave the suck water...). Then as the water continues to deepen your speed will increase as you move out of shallow water effects.

To pop your boat, lean forward in your boat and paddle at a very high cadence as you come over the sandbar or other shallow. You can feel it when you get on the wave, and your GPS will definitely tell you you're running faster than normal. In the 1980's a group of top level Midwestern canoe paddlers won an OC6 competition in Hawaii by heading over a shallow reef and popping the boat, picking up enough speed to pull ahead of the competition.
Last edit: 9 years 4 months ago by red_pepper.
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9 years 4 months ago #23352 by Spacehopper
Replied by Spacehopper on topic reading currrents
As said above, the simple answer is the current runs more slowly over shallow areas, and another effect is that the tide will in many places turn first in the shallows. Looking at a chart of any channels will give you an idea of how the water is likely to flow around bends.

As for spotting it - in my local estuaries with sand shoals it can manifest itself in three ways:

- a line of brown froth that forms the border between the flow and areas of slack/slower water. This is often obvious even when there's a lot of wind chop on the water.

- when conditions are calmer then actual eddies and swirls are visible.

- also in calm conditions, areas of 'jumping' waves (they just bounce up and down, even when there's no wind) often indicate changes in current.

There's also watching the boats and buoys (but a yacht with a keel will lie to the tide while a motorboat may lie to the wind instead).

If you know your rough average GPS speed (and maybe also how much it slows by in shallow water) then you may be able to pick out when the tide is against or with you (or if you've picked up weed).

red_pepper - I might be misunderstanding you, but surely a boat or kayak that could override it's own bow wave and then surf it would be some sort of perpetual motion machine...? :blink: :)
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9 years 4 months ago - 9 years 4 months ago #23353 by red_pepper
Replied by red_pepper on topic reading currrents
Spacehopper: No perpetual motion machines involved. :) The energy imparted by the reflected wave is less than the energy required to drive the boat, so you'll still have to put some power into it, just not as much (or, you'll go faster with the same power input). It's an assist, but it's real and it's very noticeable. Somewhat like drafting; you get an assist, but you can't stop paddling. :)

Maybe a better way to explain it is as follows: as the water depth becomes shallower, your bow wave is reflected off the bottom, generating a wave that your boat is now trying to climb; hence the reason you're slowing. As the water gets shallower, the wave slows (and, I believe, increases in size), creating increasingly difficult paddling conditions. If, however, you sprint your boat as you come into the shallow area, you can generate enough momentum to get up and over the wave. Now you're able to get an assist off the wave rather than being slowed. In some cases paddlers have been able to pop the boat and ride across areas too shallow for their boats otherwise. If you're a strong paddler in a light, fast boat you may be able to drive your boat over the wave after you've entered a shallow area.
Last edit: 9 years 4 months ago by red_pepper.

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9 years 4 months ago #23358 by Spacehopper
Replied by Spacehopper on topic reading currrents
Thanks for that - provided a nerdy evening looking up random scientific papers which I can only half understand.

I think the best explanation is here by John Winters:

www.myccr.com/phpbbforum/viewtopic.php?f=49&t=32683

"Increased resistance as a boat enters shallow water can reach 150% of deep water resistance at the same speed. As the boat approaches this maximum the created waves change shape and at some point (depending on speed, hull length, and water depth) the bow wave system takes on a concave shape (radiating from the bow and when viewed from above) roughly at right angles to the direction of travel. The speed at this point is aproximately Froude Number 0.99 with the wave traveling at the same speed as the boat and the wave-making effect concentrated in one wave. With additional power (creating a speed of aproximately Froude Number 1.4), the wave pattern changes to one of diagonal waves only (convex when viewed from above radiating from the bow) and the resistance falls to levels below that in deep water. The difference seems so dramatic that marathon paddlers believe that the boats jump over their bow waves and begin traveling downhill. In reality, the wave-making phenomenon has altered and the production of energy-draining transverse waves ceases and is replaced by a diagonal wave system that drains away less energy. This lacks the drama of boats leaping over their bow waves but it avoids the difficulty of having to explain how the boat travels down the front of a wave it has yet to create."

So it's not really that you're overriding the bow wave, but a change in how the wave system operates. Food for thought for some of our flatwater races here.
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9 years 4 months ago #23359 by red_pepper
Replied by red_pepper on topic reading currrents
I've attached another good explanation of shallow-water wave phenomenon; I don't pretend to have a full grasp of all that happens with shallow water waves and such; it's complex (the "ride the wave" concept is a greatly simplified means of explaining what happens). But I will tell you by experience that as you enter shallow water, depending on your speed, the bow will tend to rise and you'll slow, or you may be able to power through it and the boat feels like it lifts a bit and your speed increased notably. Also, in very shallow water, if you've "popped" the boat you may be able to get through shallower than normally allowable water while leaving too little water behind you for your competition to follow. :)
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9 years 4 months ago - 9 years 4 months ago #23360 by red_pepper
Replied by red_pepper on topic reading currrents
And by the way - thanks for posting that info by John Winters; it's some good information on a strange phenomenon! :)
Last edit: 9 years 4 months ago by red_pepper.

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  • MCImes
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9 years 4 months ago - 9 years 4 months ago #23361 by MCImes
Replied by MCImes on topic reading currrents
I come from canoe racing, and 'poping' the boat in shallow water wins or looses many races. The shallow water 'pop' or 'suck water' effect is amplified many times by a 27-34" wide boat.

If you get left behind the wave going into shallow water, you dont have a shot at getting over it, and the guy in front of you is riding the 'pop' leaving you in the dust.

Skis are less affected by this, but there is still an advantage to be gained by timing a sprint into shallow water. The article above is gold if you race shallow water.
Last edit: 9 years 4 months ago by MCImes.

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9 years 4 months ago #23362 by red_pepper
Replied by red_pepper on topic reading currrents
Hi MClmes: you have a note on your reply that you've moved to the Midwest USA and you're looking for paddling partners and races. Where in the Midwest are you located? We have lots of excellent paddlers throughout the region and quite a few races. The USCA hosts quite a number of canoe & kayak races on various rivers and lakes; there are also a number of surfski races taking place on the Great Lakes.

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