mid level /detailed tide theory

Discussion in 'Boat Design' started by Tiny Turnip, Sep 10, 2021.

  1. Tiny Turnip
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    Tiny Turnip Senior Member

    Not quite sure where to put this.

    I've just had another wonderful trip in my wee sailing canoe with my mate, but there was precious little wind, so we were paddling a great deal. Its got me animated about tides, and their complexity, as with only human power, tides become particularly important.

    What I'm looking for is 'medium/detail' level references on tides. I understand the macro level principles, influence of sun and moon, ocean basins, coriolis etc, and I understand that there are many factors that influence the detail of tides - shape of the sea floor and coast, interraction of connected bodies of water etc. but I would welcome any sources for further detail in this area, without getting too mathematical.



    I'm trying to get at local detail. For instance, this screenshot from tidetimes.org.uk shows some up and coming tides for Craighouse on Jura, where we've just been. What local interractions might lead to this compexity, (not expecting anyone to have local knowledge, just factors that might cause this compexity) and the vast differences from one day to another? And, in particular, why are there sometimes lags of several hours between local high/low tides and the change of direction of tidal flows?

    craighouse tides.jpg



    For clarity, we use gps with navionics, savvy navvy, memory map with full charts, and have admiralty tidal streams, so not particularly looking for navigation information, just more understanding on the detail of why tides do what they do, The admiralty tide manual looks very heavy on maths, is very expensive and out of print. Any other suggestions?
     
  2. hoytedow
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    hoytedow Carbon Based Life Form

    You may want to factor in the shape of the seafloor in your area of interest. That can influence how the tide moves in and out, speed and direction of local tidal currents, etc..
     
  3. hoytedow
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    hoytedow Carbon Based Life Form

    Do a little rough hydrology experiment. Use a steel pan, some magnets of different sizes, some water and some food coloring and have fun placing the magnets on the bottom of the pan in various arrangements to mimic sandbars and shoals etc. and try to recreate in the lab what you observed in the field(sea). Have fun and report your findings.
     
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  4. philSweet
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    philSweet Senior Member

    That sort of period bifurcation is a fundamental property of a vast array of natural phenomena.

     
  5. Ike
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    Ike Senior Member

    You might want to look at this San Francisco Bay-Delta Model – Northern California Geological Society http://www.ncgeolsoc.org/past-field-trips/san-francisco-bay-delta-model/, in Sausilito CA (across the bay from San Francisco), The US Army Corps of Engineers has constructed a model of the bay and tidal tributaries such as the Sacramento River, To study how the geology of the bay affects tides and tidal currents. I visited it back in the 80's and it is very interesting. Tidal currents don't always work the way we think they should. Also https://www.usgs.gov/centers/pcmsc/...ce_center_objects=0#qt-science_center_objects
     
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  6. kapnD
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    kapnD Senior Member

    Your own observations are often more accurate than those hallowed numbers posted on tidal charts, for reasons many and varied.
    “She blinded me with SCIENCE!”
    Enjoy your trips without busting your brain (or your biceps) by allowing extra time for passages that may be difficult.
     
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  7. latestarter
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    latestarter Senior Member

    The factors you quote pretty much covers everything barring wind and atmospheric pressure which can not be predicted.

    My "local knowledge" consists of cruising in the area a couple of time 50 years ago.;)

    The tidal bulge approaching from the west hits the coast of Islay and Jura. If you look at the tide tables for Ardnave Point on the west side of Islay it is a smooth consistent form.
    Islay, Jura and the string of islands to the north act like a dam, the water is squeezed through the gaps which creates the complexities you see at Craighouse.
     
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  8. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I'm looking for similar on river flow speeds and depths over the seasons. If I'm wanting to float down the North Fork of the South Platt or the Rhine, how fast will I being going during a given week and how big and deep a boat?

    That got Classes for rapids rarely for seasons and what about bigger calmer more navigable rivers? I suggest about a 6 character rating system for speed, depth, temp, width of deepest section, rapids/boulders, clarity. 112114 might be Mississippi.
     
  9. Tiny Turnip
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    Tiny Turnip Senior Member

    Many thanks all for your responses. I'm getting much happier now with understanding the huge tidal variations from one location to another, even places quite close together, and variations over time. This part of the coast is pretty complex in coastline and sea bed, and the funnelling effect between Islay and Kintyre (thanks @latestarter ) running North on the flood tide results in some quite ferocious tides further North. @philSweet - a really interesting and helpful film - thankyou. It makes sense of how the many factors interacting on tide mean that the simple cyclical model we get in school breaks down into the chaotic one I sampled on the tide graph for Craighouse, and then returns to the cyclical pattern. I did have to let some of the maths wash over me - 3d Mandelbrot patterns! @hoytedow @Ike the modeling is a good idea. I've found a few images really useful in visualising some of the stuff going on. Youtube films of models of a Tesla valve show how quickly a laminar flow can be broken down into chaotic turbulence by the shape of the coast. I also found the image of a ball (representing the tidal bulge) rolling in a gutter (a narrow sound) helped in understanding how you can get fast tidal flow with little tidal range. In the Sound of Gigha, the maximum springs tidal range is only a couple of feet, and it can be as little as a few inches at neaps, yet the flow is commonly 3 knots plus. We waited at the basin at the head of inner Loch Tarbert on Jura for the tide to turn before attempting to paddle South West down the narrow channel. The flow remained really strong to the North East for two hours after high tide - strong enough to knock us back a couple of hundred metres if we nosed out into it. If anyone's interested I made a youtube playlist of tide related stuff. The 'Earth Explained' lectures I found quite helpful and quite simply explained, particularly the one on dynamic tide theory. There's some videos of some of the flows further North too, just for fun (we stayed well clear of these!) Thunderchild 2 in the Corryvreckan is great - a spectacular boat (50 knots offshore, holder of the record for the circumnavigation of Ireland) in a spectacular sea.

    Here's the playlist:

    https://youtube.com/playlist?list=PLTjtuuHh9n47Sdnfm2w21oMjMrHtQg8lg

    Thanks again!
     
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  10. Tiny Turnip
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    Tiny Turnip Senior Member

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  11. bajansailor
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    bajansailor Marine Surveyor

    Thank you for that interesting play list - the fast interceptor boat Thunder Child II from Safehaven Marine is very impressive!
    They sent a drone up, and then later recovered it with a big scoop net, rtaher than trying to land it on the deck.

     
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  12. Tiny Turnip
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    Tiny Turnip Senior Member

    Glad you enjoyed it @bajansailor - Safehaven Marine have some great videos, and although Thunderchild is not really my thing, I still find it highly impressive. IIRC, there was a lot of work gone into the suspension systems for the seats to allow high speeds in big seas without pulping the passengers.
     

  13. bajansailor
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    bajansailor Marine Surveyor

    In a previous life 30 years ago I was working at the RNLI HQ in Poole, and I was privileged to be able to go out on sea trials one day on the FAB 3 lifeboat - this was the prototype for the 17 metre Severn class.
    It was blowing about a Force 6 - 7 from the south west, and the Trials Coxswain brief was basically to give her as much grief as possible.
    So he put the throttles down, and we were doing almost 25 knots into big head seas - twin 1,200 hp Caterpillar diesels delivered a lot of torque, and the boat was immensely strong. One of my colleagues at the RNLI described her as a 'blood and thunder' machine, and that was an apt description.
    They quickly found out with the new Fast Afloat Boats that the crews were now the limiting factors (rather than the boats), re how much abuse the boats could take.
    On the trial we were strapped into our seats with full harnesses - if we didn't have them on we would have been hitting the roof as a result of the G forces generated.
    The Trials Coxswain was a big lad, well over 200 lbs, and I heard that he was notorious for breaking suspension seats - he probably contributed to the research that went into the seats for Thunderchild II.
     
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