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afraidofsharks 12-04-2018 01:37 AM

Golden Gate entrance
 
Hey all...
so yesterday (sunday), I took two friends out on my big 23' oDay. Sails were just flapping 75% of the time. Boring (but lots of seals and dolphins).

I was between the gate, the city, and sausalito. Decided to peek out the gate. It was at ~20 mins before slack tide. Water was almost glassy inside the bay some places - the calmest I've ever seen it. Got inspected by the coast guard. Fun times.

Upon reaching the gate, I see a wall-like swell coming at us, the first excitement of the day - as if it was the spearheading first swell of the incoming tide. More swells happen, then they get a little bigger, and more confused. When the confused waves/swells combined, they got big enough to be a concern, with breaking small waves at the top (maybe a foot). Since there was still just about zero wind, I rolled in the genoa and motored my ass back in the bay. At one point, I saw a seal surfing the top of a breaking swell for about 50 feet.

--> I am guessing that this due to the incoming tide plus the pinching under the gate, and waves bouncing off both sides of the shore? And is slack tide always like this? I've been through swells like this under the gate, but this was the most confused I've seen it plus with small bits breaking. Wondering how common it is. Thanks!

jephotog 12-04-2018 03:45 AM

Re: Golden Gate entrance
 
Quote:

Originally Posted by afraidofsharks (Post 2051567616)
--> I am guessing that this due to the incoming tide plus the pinching under the gate, and waves bouncing off both sides of the shore? And is slack tide always like this? I've been through swells like this under the gate, but this was the most confused I've seen it plus with small bits breaking. Wondering how common it is. Thanks!

I can't tell when you saw the swells compared to slack tide. But if it is any time close to slack there should only be a small tide for the first hour. My guess is you were looking at the result of ocean swells. Imagine this ocean swell combined with wind and an incoming tide. Not a place for the faint hearted.

capta 12-04-2018 08:56 AM

Re: Golden Gate entrance
 
When I lived and spent a lot of time on the Bay, I occasionally saw what you describe. It is very rare, so highlight it in your log book. I saw it most often between Angel Island and Belvedere.
What you saw was a tidal bore. This occurs when the leading edge of the incoming tide forms a wave (or waves) of water that travels up a river or narrow bay against the direction of the river or bay's current.

afraidofsharks 12-04-2018 10:44 AM

Quote:

Originally Posted by jephotog (Post 2051567620)
Quote:

Originally Posted by afraidofsharks (Post 2051567616)
--> I am guessing that this due to the incoming tide plus the pinching under the gate, and waves bouncing off both sides of the shore? And is slack tide always like this? I've been through swells like this under the gate, but this was the most confused I've seen it plus with small bits breaking. Wondering how common it is. Thanks!

I can't tell when you saw the swells compared to slack tide. But if it is any time close to slack there should only be a small tide for the first hour. My guess is you were looking at the result of ocean swells. Imagine this ocean swell combined with wind and an incoming tide. Not a place for the faint hearted.

It was 15 or 20 minutes before slack tide and I was there until slack tide or just after

midwesterner 12-04-2018 11:30 AM

Re: Golden Gate entrance
 
Hmmm, I found this discussion of common misconceptions about slack tides, on Wikipedia. Very interesting.

Misconceptions About Slack Water:
Slack water is a much misused term, often used to describe a period of equilibrium between two opposing streams when the water is anything but slack, but highly stressed. Although there may be no flow in either direction there may be many eddies, and since this so-called slack water occurs before high water while the tide is still rising, the tide may continue to rise even after the direction of the stream has reversed. Conversely, since it occurs after low water while the tide is rising, the tide may also continue to rise during this so-called low water slack period. Such conditions typically occur at river mouths, or in straits open at both ends where their entrances have markedly different physical characteristics. Examples include The Rip between Point Nepean and Point Lonsdale at the entrance to Port Philip Bay, Victoria, Australia; the Menai Strait between Anglesey and Wales; or the Strait of Gibraltar at the entrance to the Mediterranean Sea.

Capt Len 12-04-2018 02:13 PM

Re: Golden Gate entrance
 
In a perfect world water would be a viscus coherant liquid answering only to gravity (like higher to lower) .Unfortunatly the enertia of moving may not be so simple.

afraidofsharks 12-05-2018 02:12 AM

Re: Golden Gate entrance
 
Ok I checked the tables again and I must have used a different tide marker. I was there i think 45 minutes or an hour after max ebb, and 2+ hours before slack. That explains it more.

capta 12-05-2018 10:27 AM

Re: Golden Gate entrance
 
For any of you who live in the Bay Area, there is a scale model of the Bay in a shed in Gate 5 or 6 in Sausalito. It used to be free to visit and just hang out while they were doing their tests and I spent quite a bit of time there, absolutely fascinated.
They probably have regular visiting hours now and charge an entrance fee, but IMO it's well worth it, especially for you racers.

roverhi 12-05-2018 01:42 PM

Re: Golden Gate entrance
 
Recently sailed out of the Bay and headed south on a reach in the prevailing force 4 NW winds. Sea conditions were long period swells and I'd made the turn south what I thought was quite ways out of the Gate. My crew suddenly yelled in a panic to head up. I looked up to see a vertical breaking 10' plus sea that would have rolled us. Fortunately was able to get the bow pointed into it and just got soaked. For the next 1/2 hour or so had to regularly head up into very steep breaking seas that were big enough to roll the boat and had one poop us. The breaking waves soon settled down to normal long period ocean waves though the wind force increased to force 5 plus. From then on it was screaming hull speed surfing in typical ocean conditions. Really weird that there was this small patch of steep breaking dangerous waves that were well offshore.

Be careful out the Gate as wave conditions can get weird right quick. There have been a number of boats sunk and live lost in the freak waves that often develop.

Scotty C-M 12-05-2018 04:07 PM

Re: Golden Gate entrance
 
1 Attachment(s)
Ah yes, San Francisco entrance. Sounds like Rover has met the famous "South Channel". This is the southern brother of the famous Potato Patch. Shallow water and currents and wind and tide all working together to make a confusing maelstrom. Let's see if I can attach the chart ...

Like a big horseshoe surrounding the entrance. So the deal is to go WAY out. Like to channel marker R4. Then turn left. The South Channel is a good distance offshore - over a mile - and seems like a good idea. But on a falling tide, with a large swell over about 6 feet, the channel closes out into breaking waves, with no way out. A lot of boats have gone aground along that section of coast. I've used the south channel on calm days, with a rising tide, if there is good visibility. If not, then going out to the (almost) end of the shipping channel is just fine, thank you.

Closer to the Golden Gate, there are so many currents and counter currents. Add a bit of swell, mix in a handfull of tidal currents, a dash of wind, and voila, you get some pretty interesting water movements. I have to be honest, one of the worst (mistakes) I ever incountered was the wake from a large container ship. I was close because I was tucked in behind the south tower of the bridge. The wake looked so small as I headed into it. It hit the boat and shook it like a terrier with a rat in it's teeth. Opps. Let's not make that mistake again!


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