Slack Tide?

[BlueDevil]

Where does it do that? I thought according to the "rule of twelfths", tidal flow was highest smack between tides.

Port Phillip Bay (Melbourne, Australia) would be an example. Slack water is approx 2.5 to 3 hours after high tide. In a large enclosed bay such as this, with a relatively small entrance, slack water occurs when the water level inside the bay is exactly equal to the water level outside the bay, hence there is no net water flow. This will occur at some point between high and low water. You would not want to be diving around Port Phillip Bay entrance at high water...or hang on for the ride!!

So there is no simple answer to the question "When does slack water occur?" since it will depend on the location and what type environment is encountered there.

 

[Zippsy]

Thanks Blue. I learned something new again today; well, at least a better explanation that I had before. BTW, did you know that the Duke Blue Devils are just about to win yet another NCAA basketball championship?

 

[Bretagne]

Where does it do that? I thought according to the "rule of twelfths", tidal flow was highest smack between tides.

Rule of twelfths is to compute tide heigth. For tidal flows, even if they are usually at the slowest at high and low tide, local configuration can create shifts. Better to learn about local conditions to prepare a dive.

 

[Michael Schlink]

Gary D. we don't have much in the way of tidal change but have you noticed the puddle we dive in next to Houston and right below Louisiana, Miss. and Alabama.LOL

 

[String]

Slack water times arent that easy to predict and can vary a lot over small distances due to difference in the sea bed, the effect of land to funnel the water and so on. Some sites can be high water slack, others can be low water slack. Best way to find slack is local knowledge. It isnt always directly calculatable from the high/low water times as a result.

Im off diving this weekend and we're pretty much on a spring tide. 6.4m high water, 1.2m low water so a 5.2m (about 17ft) tidal range. Given we have zero chance of organising ourselves to catch slack anywhere i suspect some fierce currents and possibly end up doing 2 drift dives.

 

[SFScuba]

I always thought slack tide was the time when there is no flow which would be either at high or low tide. When reading tide charts, one has to take in mind that "high tide" is really only for the pinpoint location that chart is specified for.

I used to fish a lot on Cape Cod, there was "high tide" but then there were time corrections +/- to that tide time across the Cape depending on which harbor or how far along you were.

Slack tide is the actual high/low tide for that location, but as mentioned in previous posts, the time and duration can vary depending on local geographic conditions.

 

[quimby]

Then there is places like the Chesapeak and Delaware Canal.
The wind plays a bigger part than the tides, meaning on a east wind the "tide" may run the same way for days. The difference tween the two bays determines the direction and the normal tide times just change how high the water is but not if or when there is a slack.

 

[Zippsy]

SF, I think what several of the previous posts are saying is that slack tide is not what we learned in OW, i.e., it is definitely NOT necessarily at high and low tide for that spot. I believe it is the case in all the normal sites that I dive on weekends which are offshore pinnacles or small round islands but if you are diving in bays, lagoons, atolls and other similar places, slack tide may well be no where near the high or low water times.

 

[tonaskai]

tonaskai, what is your location?

Tacoma Washington where the currents seem to be a real concern.

 

[archman]

SF, I think what several of the previous posts are saying is that slack tide is not what we learned in OW, i.e., it is definitely NOT necessarily at high and low tide for that spot.

What you see in your open water manuals is a condensed version of the same thing in oceanographic texts, and IS essentially correct. Slack water occurs at the end of tidal extremes, and the rule of twelfths dictates the strength of tidal flow. It's a general rule of thumb and applied model that works for most parts of the world, especially as you close with the equator. There are very few radical exceptions to this, but those exceptions are over-publicized... 'cuz they're er, exceptions. Accuracy of tide charts bears direct relation to the proximity of the forecast area, as all skippers know. Tides often are a lot more persnickety than local weather patterns. One side of an island a few miles away from your tide station likely could be offset by several minutes. But the principles and theory of slack water still apply.

Polymictic (wind driven) tides are a separate category altogether and are regulated by seasonal meteorological events. They cannot be reliably predicted in most cases.