@Wilfred Darr I will do that soon. Very interesting. Thanks

@Captain John's Skipper Tips Maybe you can help me: I'm Canadian, but looking into a trip to the states, and as such I've been trying to figure out bridge heights, specifically on the James River, to see how far up I can go, and for the life of me I can't figure out how Americans correlate MHW to tidal heights (off of MLLW). For a bridge near a harmonic station this is relatively easy as NOAA publishes the tide tables at all relevant datums, including both MLLW and MHW, but at a subordinate station it appears to be a completely different matter...The short version is that I've gone to every length I can think of and am stumped. The long version is this:

I realize I could cross in the Newport News Channel at the tunnel area so this first bridge is something of an exercise, but, I've been here and the traffic is very big, so I'd stay out of the main Channel area if I could, and further, if I can't figure out the very first bridge height, what hope do I have making it up the river unscathed?! Any example would do, but the example I was specifically looking at was the MONITOR-MERRIMAC MEMORIAL BRIDGE on the James River VA, 30' MHW vertical clearance, located at bottom right of Chart 12248, using the Newport News (or Pig Point) Subordinate Tidal Station, Reference Station Sewells Point (All my RYA courses have included difference tables, but for the life of me I can't find difference tables from NOAA anywhere?). Thanks. No rush, I can wait for it to be the topic of one of your videos, but would much appreciate understanding this.

Wilfred, thanks for your comment. Glad it was helpful.

@Hawaiian Pride If you're really interested in this stuff, NauticEd has a really great "Coastal Navigation" course that gets into this stuff; feel free to use my promotion code 'wilfsnauticed' to get a discount (I've already completed all their courses so I don't get anything from you using it, but it still works for you to get a discount so, please do!).
*note: be aware that the navigation methods in Module 4 and 5 methods don't jive with the method that's taught in module 3. This is the single frustration I've had after completing every one of Grant's courses. The method in module 5 is the one taught by the merchant marine academy, so it's hard to argue that it's wrong, but it isn't as accurate as the method in module 3: many students have complained but Grant hasn't fixed it (being that so many professional American sailors are taught this method, I'm not sure fix is the right word, but it can be very confusing and frustrating to learn two different conflicting methods in the same course). It's my only complaint with any of the NauticEd courses, which is why I still recommend it. If something is confusing you, click to the 'sea talks' chat page where most of the questions are answered.
Take care
Wilf

@Hawaiian Pride Like a clock: the same tide has happened for millions of years, exactly the same, every 18.6 years. Only times it differs is from extreme weather like a hurricane (the extreme low pressure on the center of a hurricane lifts the water higher, like what happened in New Orleans, but generally, yes you can predict them with near perfect accuracy: in 18.6 years the EXACT tide that happened in Thailand today will happen again in Thailand, and the same tide that happened anywhere else in the world today will happen again.

Really, you can predict tide height. That's very interesting.

Thanks for your note!

@Captain John's Skipper Tips Thank you. I looked and noticed some have two floods before an ebb. I think that is interesting.

@Wilfred Darr: Yep, rotary currents are present off the coast of New England too.

@Hawaiian Pride: Be sure you are looking at the tidal current tables to determine slack water time. https://tidesandcurrents.noaa.gov/noaacurrents/Regions
Click on one of the states. Then, click on one of the current stations in the list. A graph will appear to the left and a table to the right. Note the slack water (horizontal blue line in the graph) or, in the table, the notation "slack". Next, note the times and speed of ebb and flood current. Most tables now assign a (-) to ebb current speed. Hope this helps.

@Wilfred Darr That's really interesting because I heard strange sailing stories about San Francisco. I will share with my capn friend. Thanks

@Captain John's Skipper Tips Tide tables show the slack time because that's the time the tide stops and then starts again. Right?

There are some places where the tide circles around like a washing machine (off the coast of San Francisco IIRC, something about coriolis effect acting on the water as it exits the bay); some places only have one high and one low tide a day (Gulf of Thailand) but these aren't the rule. Usually, yes, the tides are roughly the same direction for 6 hours, then turn around for 6 hours.

You're welcome!

@Captain John's Skipper Tips That is very useful. Thank you Captain John.

Yes and the time from slack water to slack water lasts about six hours. Maximum current strength peaks at about three hours into the cycle. The velocity is not linear, thus the 50-90-100 rule. Good to know if you need to shoot through a pass, transit beneath a bridge, or navigate through an inlet. Check the tidal current tables for the predicted times of slack water, and direction and speed of current.

With 4 tides (2 highs and 2 lows) in a day that's what it approximates to.