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Marc Hughston
Moderator
Username: hughston

Post Number: 813
Registered: 03-2003
Posted on Sunday, March 04, 2012 - 11:53 pm:   Edit Post Print Post

Re: Slack Before Flood or Ebb discussion

I can now say with first hand experience that the answer to the question is, Slack Before Ebb. That's the best time to enter when there is a question about the conditions. Slack before Ebb is the time of highest tide, and the waves will only get steeper and break more frequently as the tide goes out thru Slack Before Flood, the time of lowest tide.

Below are some photos I took of the entrance to the Santa Cruz Harbor on a day when the shoaling conditions at the harbor "bar," the sand bar that builds up between the break waters, made me decide not to go out. Sometimes the Santa Cruz Harbor entrance is closed, other times it is recommended that no one transit, and the rest of the time it's up to the boater. My concern this day was not just the breaking waves, but the depth. ProMotion draws 7 feet and I did not want to chance grounding near the rocks when a subsiding swell planted us on the hard. See the "Harbor Survey Depths" at the bottom.

4036
The dredge with a view of the harbor entrance. The shallowest part is near the breakwater on the right where the wave is breaking.

4043
You can see how the breaking wave continues left to the deeper portion of the channel

4049
Note the red dinghy next to the rocks.

4060
The breaker is across the entrance now.

4062
The FJs sailing in the distance never made the attempt to exit.

SC Harbor Depths
This chart is produced by the Harbor Department and posted online, but it is not updated every day.
 

Marc Hughston
Moderator
Username: hughston

Post Number: 798
Registered: 03-2003
Posted on Monday, December 26, 2011 - 12:12 pm:   Edit Post Print Post

Re: Slack before flood or ebb [Marc & Eric G.]

Mark, we were focusing on the dangerous river bars and harbor entrances up the west coast – that’s where the reasoning was applied. I think the Slack before flood argument had to do with the prevailing W or NW wind. Theoretically, the wind would be with the tidal current if you got there after slack tide and would make for a much smoother ride. For the slack before ebb argument, you rightly point out that slack before ebb is the time of greatest water depth, and the waves at the bar should be gentler. On the other hand, if you miss on the timing, the ebb current flowing out into the oncoming W or NW wind as the depth decreases could really make things nasty.
 

Mark Howe
Moderator
Username: unclemark

Post Number: 640
Registered: 08-2003
Posted on Thursday, December 22, 2011 - 09:03 am:   Edit Post Print Post

Re: Slack before flood or ebb [Marc & Eric G.]

I got sucked into this discussion by wondering about your reasoning for least hazardous time entering a harbor with a shallow entrance [if I understood correctly]. Since you never actually gave your reasons, I will throw some out here.

For entering a harbor on an incoming tide [flood] the current will be in your favor, the water will be getting deeper in case you run aground, but since it is shallower to begin with, the waves will be steeper.

Conversely, at slack before an outgoing tide, the deeper water will result in smaller or no waves, altho' if you are delayed the ebb current and risk of grounding are greater.

Is this your reasoning? and/or did I understand the discussion? It was somehow related to the rule of 12ths.
 

Eric Gritzmacher
Intermediate Member
Username: eneveaux

Post Number: 14
Registered: 08-2008
Posted on Friday, September 16, 2011 - 06:26 pm:   Edit Post Print Post

Re: Slack before ebb
I don't recall exactly what Jack said, but i do have my notes from the class somewhere and i will take a look.

My Manson anchor is a plow style, and looks pretty much like a CQR only cheaper by a few hundred dollars. So far, so good. At 40lbs with over 400ft of rode (65ft chain), i have no complaints. I considered a Manson Supreme but it would not have worked given the small area between the bow stanchions. Besides, it is quite expensive.
 

Marc Hughston
Moderator
Username: hughston

Post Number: 795
Registered: 03-2003
Posted on Friday, September 16, 2011 - 02:04 pm:   Edit Post Print Post

Re: Slack before flood or ebb

Now that I think more about it, I have to say I'm not sure if I remember correctly what Jack said. I can see making the argument for slack before ebb as the least hazardous time. Eric, do you remember for sure?

Any thoughts out there?
 

Marc Hughston
Moderator
Username: hughston

Post Number: 794
Registered: 03-2003
Posted on Friday, September 16, 2011 - 12:55 pm:   Edit Post Print Post

Re: Rule of Twelfths

You're right, it absolutely does apply. Knowing that maximum flow occurs in the 3rd and 4th hours after slack can help you time your entrance or departure. I can remember entering San Diego Harbor on an ebb tide, making around 4 kts through the water but barely advancing past one of the channel buoys. We must have had 3 kts against us. If I had know then what I know now, I could have enjoyed sailing outside for another couple of hours and made the entrance later in the day with little current to fight.

What Jack is concerned about is the condition of the bar at the harbor entrance - a real concern at river inlets and narrow harbor inlets up the west coast. You don't want to be there when the onshore breeze stacks the waves up against an ebb tide. What I remember Jack saying was that the slack before flood was the least hazardous time to make the transit. Makes sense when you think about it.

BTW, how do you like your Manson anchor?
 

Eric Gritzmacher
Intermediate Member
Username: eneveaux

Post Number: 13
Registered: 08-2008
Posted on Friday, September 16, 2011 - 12:14 pm:   Edit Post Print Post

Re: the rule of 12ths

When i learned about this rule from Jack Peterson at OCC years ago, if i recall, it was in regard to situations where you were entering a narrow channel when the tides when there was a large tidal effect. Jack spent a lot of time (still does?) in the Pacific Northwest around Vancouver Island where this was a big issue. He also focused on the concept of slack before ebb vs. slack before flood. Am i correct that the rule is also useful in these situations?

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