San Francisco Entrance Chart
The purpose of this article is to familiarize the average boater/fisherman with the often- treacherous bar outside the Golden Gate – the second most dangerous major port entrance on the west coast behind the Columbia River bar. It is not intended to be all-inclusive and is not able to replace experience and caution. I’ve seen many posts asking what the bar is. It is not wise to venture out without a good working knowledge of it.
The San Francisco Bar is a horseshoe shaped area of shallow water that begins in the north off Tennessee Cove to Pt. Bonita runs out about five miles and curves back to shore just south of the Cliff House to the SF Zoo along Ocean Beach. To best understand the reason that the bar is there and why it is shaped the way it is we have to first understand the dynamics of our entire watershed from the Sierras to the ocean.
As the rivers scour sediment from the mountains the high velocity of the water keeps the sediment suspended in the water column. As it hits the bay it often is dropped here. In fact, before hydraulic mining in the 1800’s San Pablo bay was 3-9 feet deeper. As larger tides or a big spring runoff occurs some of this sediment is again carried through the Gate and since the Gate is very narrow the water moves fast and the sediment cannot settle. This is why, as you can notice on your charts, just under the bridge is the deepest spot anywhere in the region until you pass the Farallones, 27 miles to the west. As the water rushes past Pt. Bonita and Pt. Lobos it hits open water and spreads or fans out. As it does this it slows down and drops its load. After thousands of years this process builds up a bar that may be hundreds of feet deep with sand made up from the erosion of the Sierras. As a side note, in the 1880’s a con man convinced investors that tons of gold that washed down from the Sierras settled in a hole right off Lands End and took in money to build a pier for the mining operation. Before the pier was finished he skipped town and a few parts of the pier still remain at low tide. It wasn’t a bad idea until you think about the weight of gold.
The Shipping Channel
The bar has a channel running down the middle of it and this is the main channel for shipping. This channel is partially caused by the slightly faster water going directly out the gate and is aided by dredging. However, the channel veers slightly to the south due to the water being skewed by the north to south flow of the California current. This channel is kept to a control depth of 55 feet and is approximately 20 to 30 feet deeper than the rest of the bar. Shipping traffic is very heavy here and the channel is very narrow – a mere 2000 feet. The channel is marked by eight buoys (4 red, 4 green) and splits the bar in two sections, the North and the South Bar.
Main Ship Channel
The North Bar, also called the Four Fathom Bank, stretches from the shipping channel to land on the MarinCoast at Tennessee Cove. This section of the bar contains the shallowest area of the bar called the Potato Patch. There is a little debate about the origin of the name. Either it is from loads of potatoes coming fromIdaho being washed off ships and found floating in the area by fishermen or for the white, foamy look on rough days that looks like mashed potatoes – you pick which one you like. Being very shallow here (23 feet) and situated to the northwest where the predominate swell comes from make this area the first to break. Between the bar and the immediate coast is the Bonita Channel, an area of deeper water that can offer an opening on moderate days. More on this later
The South Bar
This bar runs from the south side of the ship channel and runs to ocean beach. On the charts there is an area marked called the “South Channel”. This area has claimed many boats and many lives due to the fact that there is no discernable channel here. Also, many boats run out to the east side of the channel then run south to avoid the traffic in the ship channel, and they end up crossing the south bar on its outside edge. This area is very dangerous since in contains “first generation breakers” that seem to pop out of no where since it the first spot the wave hits the bar. This is what happened to the former bass player from the band “Loverboy” a couple of years ago. They never did find his body or the steering wheel of his boat that he was hanging on to when the breaker hit. This is just one boat of many that has met its fate here.
South Channel on the South Bar
We must think of waves not as what we see on the surface but rather as rolling circles of energy whose deep water shape is a perfect circle. As the wave approaches a depth of water that is shallower than this circle the bottom edge drags on the bar causing an oblong shape as the back and top of the wave energy is moving faster than the front. This oblong shape builds straight up until the shape is not sharp enough (120 degrees) to hold itself up and the top and back of the wave, going faster, tries to pass the wave and collapses on itself. The breaker is born.
The wave angle stays at 120 degrees until the breaker dies out. This angle is far too steep for a boat to rise through it without taking a hard hit of water over the bow. This coupled with the white water that can not as readily provide floatation and the boat can not rise as well or as high make encountering breakers an often fatal meeting for small vessels not designed for such conditions (for example, the Coast Guard motor life boats).
The Effects of Current
Current has a profound effect on the wave steepness and the breaking ability. Since the swells are coming from the west or northwest, a tidal current flowing in the opposite direction (out the Gate, or and “ebb”) will cause the waves to become steeper as the front side of the wave is slowed down by the opposing current and the rear end tries to pass the front going the only way it can – up.
This is also the same for the area just outside the gate and to the points (Lobos and Bonita). The center part of this area has faster water here and then has more effect on the waves coming in making some very steep and uncomfortable conditions.
A flood tide has the opposite effect as the current moves with the wave flattening it out on the bar itself and making it less likely or less prone to break. This does not mean, however, that it can not break – itsjust less likely.
However, on a flood current it can be very rough at the points as the flood allows the waves to come closer as an ebb tide will push them back and away from the points.
The Bonita Channel and Pt. Bonita
This channel runs right off the Marin Coast and is naturally made by the water rushing around Pt. Bonita. The depths here are about 50-60 feet. Often the breakers are rolling across the bar and will dissipate when they hit the channel in moderate conditions. However, if the swells get large this channel can close up with breakers sweeping right across and straight to the rocks.
Pt. Bonita itself can be quite a nightmare due to the water and wind turning the corner here and colliding with the waves setting up its famous “washing machine” effect. This washing machine effect can occur during both the ebb and the flood with it closer to the point during the flood. This can be discussed for hours but it is best to keep a good lookout when approaching this area and look for the smoothest water ahead.
Point Lobos (Seal Rock area) can be quite sloppy during the flood stage of the tide and smooth out during the ebb (opposite of what you would think). During the flood it is best to pass outside of Mile Rock and run a mile or so off the point before making a turn. On the other hand, during the ebb running through the center will put you in a rather heavy lump caused by the swells running into the bay meeting with the water trying to get out of the bay.
The South Channel
Not there. Doesn’t exist. Cross it off your charts
Crossing the Bar Tactics
As a rule stay in the channel when running unless the swells are less than six feet. The bar doesn’t normally break until about a 12 foot well but you don’t want to be there when that one rogue wave comes through. Also, never cross when a large ebb tide is running and the swells are up – this just makes things worse. Wait a bit until slack water.
When coming back in the ship channel try to square your transom to the waves and don’t let the boat “surf” down the wave. If too much speed is built up the bow will not have the ability to rise as it hits the bottom and you can easily broach and capsize. When the boat begins to surf let off the throttle some and let the wave pass, then throttle up and try to stay on the back of the wave. If you’re on the way out and you approach a steep wave keep good throttle on until just short of the top and cut the throttle. Try to get enough momentum to get over the wave but not enough to fly off the back – there may not be much of a back there. And you don’t want to punch the bow into the wave. Get that bow to rise.
Go Easy but Keep Powering Up the Wave
If not, this may happen
Also, try not to share the ship channel with ships if you can avoid it. Besides their wakes and the chance for collision these large ships actually constrict the channel and can make breakers in the channel often stemming off their port quarter on incoming vessels and extending several hundred feet out. Not a good place to be. In low visibility days don’t be hesitant to call vessel traffic service (channel 13 to contact the ships directly and channel 12 and 14 to listen for traffic or to contact vessel traffic service) before entering the channel to see what ships are around. That’s what they are there for.
Overall however, the ship channel is the safest entrance as long as you take the shipping traffic into consideration and the Bonita channel is good as long as the swells are not above 10 feet. The ship channel can be clear of breakers in swells above 20 feet – but its no guarantee that it will be a safe ride.
The best place to see the dynamics in action I recommend that everyone drive out to Pt. Bonita during a heavy swell day (especially a no wind day) and watch the waves and how they react.
One last topic. If you are running the bar and you lose power, anchor immediately. The north to south current will quickly take you across the south bar.
Be safe out there and keep an eye on the bar!