tag:blogger.com,1999:blog-3580987459140037495.post970231433346680987..comments2024-03-18T10:29:20.125-08:00Comments on TriloBoat Talk: Time, Tide and the Rule of Twelfths (or Tenths)Dave Zhttp://www.blogger.com/profile/13241033623115158564noreply@blogger.comBlogger4125tag:blogger.com,1999:blog-3580987459140037495.post-29225062711152355112012-02-22T13:21:00.901-09:002012-02-22T13:21:00.901-09:00Hi Shemaya,
That vertical stratification of curre...Hi Shemaya,<br /><br />That vertical stratification of current happens a lot up here in the long Straits and Passages, as well as tidal estuaries. <br /><br />I've been chafing to try a technique I read as used by the Hubbards (drifting the Mississippi in their barge riverboat... book is SHANTYBOAT by Harlan Hubbard). <br /><br />He called it a MUDSAIL... it's a square sail fixed at the bow and let down into the water(!), weighted along its foot with pipe or similar, control lines led aft. Use it to run in the current, even into the eye of a foul wind! <br /><br />Should be able to lower it into deeper strata of fair current when available to overrun a foul surface current.<br /><br />BRILLIANT!!!<br /><br />DaveDave Zhttps://www.blogger.com/profile/13241033623115158564noreply@blogger.comtag:blogger.com,1999:blog-3580987459140037495.post-10093582858596780742012-02-22T11:46:06.997-09:002012-02-22T11:46:06.997-09:00Hi Dave,
Yeah, I love Chart #1 -- what a discover...Hi Dave,<br /><br />Yeah, I love Chart #1 -- what a discovery that was! And it's those green spaces on the chart, between low water and high water lines, that always seemed so undefined. I'm really delighted with this tool you've shared for calculating those heights from soundings. It looks so simple AFTER it's been explained! Thank you for taking the time to write it out and post it.<br /><br />As for currents and their timing, what an interesting bunch of material they present. And then there are rivers -- here, where the Connecticut River nears the ocean, the flood current runs upstream for about four hours each cycle, and the ebb current runs downstream for about eight hours each cycle -- even though the tide height rises and falls every six hours like one would expect. When the river is really running after major rain or snowmelt, the flood current (on the surface) never happens -- but the ebb-direction flow gets milder for a few hours. I knew all about this theory for ages, but didn't really wrap my mind around it until kayaking downstream for a couple of hours, with an expectation of coming back up on the flood current. Ha! Ever since THAT experience, the whole thing has been clearer :-)<br /><br />Cheers,<br />ShemayaAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-3580987459140037495.post-61608427750023661312012-02-21T13:37:32.386-09:002012-02-21T13:37:32.386-09:00Hi Shemaya,
Excellent points... Sorry about the c...Hi Shemaya,<br /><br />Excellent points... Sorry about the confusions. <br /><br />I'll amend the post to include height ABOVE ZTD. Normally, that would be thought of as 'charted depth', but charts seldom concern themselves with the dryout shoals or even many of the edges. Lots of white space or shoreline apparently drawn from impressions made on the cartographer, rather than actual surveys!<br /><br />A complication is that tidal heights above ZTD are given as positive numbers and visa versa. DEPTHS are charted below ZTD, also as positive numbers. Worse, heights of land features are not given relative to ZTD at all, but to another datum, altogether. Sigh. NOAA CHART #1 explains all the fine print. Well worth going over with a fine tooth comb!<br /><br />My sample tide numbers aren't unusual in the Pacific NW, and I've not really sailed much elsewhere... didn't even think that the sample numbers might be extreme.<br /><br />Right, too about the currents. Things get complex, and simple assumptions are only a starting point! Narrows, too, develop differential based flow that may be well offset from mid-tides on either side.<br /><br />I should probably be more explicit that most of these posts are highly generalized, and are only starting points in a lifelong curriculum. It can be a school of hard knocks, too! Nor is what I'm writing necessarily gospel... I try to be accurate, but it all represents the best of my knowledge on the day I write! <br /><br />Thanks for helping keep that in sight!<br /><br />DaveDave Zhttps://www.blogger.com/profile/13241033623115158564noreply@blogger.comtag:blogger.com,1999:blog-3580987459140037495.post-53183341258939612262012-02-21T12:35:43.988-09:002012-02-21T12:35:43.988-09:00Hi Dave,
I had some fun puzzling over bits of thi...Hi Dave,<br /><br />I had some fun puzzling over bits of this one. Thought I'd share, in case it's useful to anybody...<br /><br />The rule of twelfths is quite familiar to me -- sailing a keelboat in Maine, with an inclination to anchor where there would be just enough water, provided lots of practice. So that was easy, and the conversion to tenths also followed easily. Though, maybe because it's what I learned first, so far I still like twelfths -- I like the way the number of feet of tidal range translates into number of inches for each 12th, which seems to suit my capacities for mental arithmetic.<br /><br />I spent more time puzzling over "height of bottom." I understand now, and it's probably obvious to everybody else, but it took me a while to figure out that this was "height of bottom over ZTD (zero tide datum)." A handy calculation, once one has wrapped ones mind around it!<br /><br />Another bit that I think made the example a little trickier was the part that the height of low tide in the example was 10 feet. I'm taking this to mean that it's a substantial version of "higher low" in a widely varying higher and lower low tide situation, but it did throw me off for a bit, in getting a good mental image of what I was thinking about, and the meaning of each term in the example. In eastern Long Island sound (Connecticut/New York) the higher high doesn't even reach 5 feet above ZTD, so I had some confusion remembering that 10 feet could be the low end of the tide range above ZTD. Though I can see the reasoning for setting up the example that way, because there is no missing that the sounding doesn't match up with the lowest amount of water to be expected!<br /><br />If you were inclined to add another practice problem or two, with answers, some of us would be entertained :-)<br /><br />The other thing that caught my attention was the discussion of current, and I wanted to point out for folks who might be unfamiliar, that the timing of slack and maximum tidal current does not necessarily coincide with high and low tide, and halfway in between. I'm quite sure you know all about this... but it seemed worth mentioning for folks who might not be familiar with it. We have so many areas around the northeast coast where those timings are vastly different, as I'm sure you do in Alaska. It's such a great mind bender, to watch the current flowing out of a bay with a narrow entrance, at the same time as watching the waterline on the shore rise! Explained by the water flowing in at deeper levels, while it rushes out on the surface, but still!<br /><br />So that's what came to mind -- thanks so much for the discussion. The concept of "height of bottom" has its own shiny new pocket in my collection of anchoring calculations!<br />-- ShemayaAnonymousnoreply@blogger.com