what is that "bar" term they kept mentioning? - The Finest Hours (2016) Discussion

[–] jchallgren 8 years ago

I'm NOT a boater but live in S Chatham seasonally so it's my hometown story so I'll try and help out as best I can, ok?

Chatham's waters are ever changing in where the channels and passages and shoals are located due to shifting sands so the path where a boater needs to go for a few weeks/months to get from harbor to sea may not be usable later...part of this is the 'bar' which is a part of a barrier beach system but underwater so boats can pass over it but it causes the waves to break as if they were hitting the shoreline, ok?

So -- in the current beach layout now, the main channel that boats use to get from fish pier to Atlantic is thru a gap in the barrier beach which formed in a 1987 storm when it was so low that it got washed over in one spot and then widened and deepened, etc...however, even though it's deep enough at all times to pass over, at low tides it's still shallow enough so that the incoming waves will break at that point esp in heavy surf so when you look at it from light house overlook, you see a narrow strip of land on your left and right just offshore (maybe 1/3 mile or so? never measured it), then a gap in between where channel is but you see a wave/surf line going across the gap...

In the 1950's, this type of situation occurred in a different place further south but was similar in that they had to break thru waves over a major undersea sand bar that was there....and in a major storm like in this event, the breaking waves could easily be 30+ ft high at that spot...you've seen pix/video of surfers riding huge monster waves, right? Imagine trying to go to sea in a small boat facing waves like that...

Does that help any? Sorry if i'm not 100% correct on how this but by not being a boater/geologist/etc, some terms I use may not be quite perfect...though should be enough to hopefully explain it.

I found this video from Chatham Coast Guard from a few yrs ago that may help:
https://www.youtube.com/watch?v=gFncHcOhHEQ
That was with newer larger boats in much calmer conditions than CG36500 faced.

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[–] 8 years ago

[deleted]

[–] raf-33 8 years ago

Bar is short for barrier, they are either artificially created to close up a harbour to choppy seas or they can be natural and caused by reefs, sand or sunken vessels. Either way they are hazards because in low tide they are effectively just walls that gate you in and at high tide you might not notice them and rip the bottom of your boat open trying to pass them.

In The Finest Hours they could either spend a while navigating around the bar or just jump over it when a wave strikes it, hoping the wave will carry them over and not smash them in to it. You can clearly see the bar as he is counting the waves hitting it, in one scene the wave subsides and you see a very tall wall, he has to time when to accelerate the engine with when a wave was about to break against the wall so that it would carry them over.

Properly read, the Bible is the most potent force for atheism ever conceived. -Isaac Asimov

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[–] bhbmaster-175-791977 8 years ago

I hope I got this right:

So a harbor encompassed by a bar essentially has lower water depth (making it dangerous to hit boat against bottom rocks / seafloor) and when the small waves come in from the ocean (which has much deeper water depth) they grow really big at the border of the bar. It was that dangerous in that situation because the waves grew really big. A bar is kind of like a watery coast line, you still get waves there.

Also a low tide makes the waves worse at the bar -> lower water at the bar, ocean after the bar is still deep so the waves are stronger.

stronger waves when there is a bigger gradient (faster change) from deep to shallow water.

Does that all sound correct?

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[–] puirt-a-beul 8 years ago

A harbour bar (which actually isn't short for "barrier" but comes from the same word root) makes the water shallower, which makes the waves rise in height, crest and break. That's how waves work: they break at a beach because the water shallows, pushing the amplitude (vertical height) of the wave above the water level and causing the wave to topple forward and break.

So the waves tend to break over the bar, diminishing their force and making the seas less turbulent inside the bar. But at the bar itself, the waves can be violent and dangerous, as shown in this film.

The waves don't actually get stronger when there is a steeper gradient; it means their change in amplitude is forced into a shorter period of time, making their response more sudden and violent.

You might very well think that. I couldn't possibly comment.

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[–] CGSailor 8 years ago

So a harbor encompassed by a bar essentially has lower water depth

No. The Harbor doesn't.
The water is shallower AT the bar, not within the harbor.
The water in the Harbor would be calmer, as the oceanic waves would break at the bar because the bar is shallow.
The bar does not lower the water level in the harbor.

Think sand bar, or gravel bar.

Term: bar (n)
Definition: 1) A shoal usually composed of sand or sediment transported by currents causing an obstruction to navigation, usually a shoal lying just offshore the entrance to a harbour making passage possible only when there is sufficient tide. (from bar; to restrict or prohibit.) 2) A shifting sedimentary shoal found in navigable rivers such as the Mississippi; as in: “Only a fool would try to cross the bar without knowing the tides.”
See Also: shoal

I joined the Navy to see the world, only to discover the world is 2/3 water!

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[–] deborahjwood 8 years ago

So a harbor encompassed by a bar essentially has lower water depth (making it dangerous to hit boat against bottom rocks / seafloor) and when the small waves come in from the ocean (which has much deeper water depth)

Not exactly - the water is generally more shallow inside the bar/shoal/reef NOT because of the obstruction but simply because the water inside just naturally becomes shallow closer to the shoreline - were it not for the bar those huge waves would crash onto the shoreline -- the bar causes them to crash (gain in amplitude or height) before they can actually reach the shoreline - which is why some are man made - dredged up - in order to protect the shoreline from erosion - I read on the USCG website that this specific area (where the lighthouses were originally built) was eroding at 30 feet per year - that sounds impossible to me and I'm a native Floridian - but that is what I read. The erosion was so bad they had to move the location of the two lighthouses (I'm not sure when one was either destroyed or decommissioned)... but now there is just one at Chatham.

But you do have the general idea - the Massachusetts has always been one of the more dangerous shorelines on the east coast which is why there are specially built boats to protect them - the boats in the movie were replaced with self-righting 44-foot steel hulled lifeboats in the 60s and have since been replaced again by 42' Hamilton jet driven Near Shore Lifeboats. All the boats used have shallow drafts to navigate the ever changing off-shore waters.

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[–] jchallgren 8 years ago

Re Chatham Light: The current structure was built in the last 1800's (1877-ish) and was the third one on the basic site as the first two fell over the edge before that... the second one was where Lighthouse Beach is now and not sure how far out the first one would have been.

Now when the current light was built, it was one of two twins so folks could better identify which one it was by nbr and pattern but in 1929 (I think), the northern/right side one was moved up the coast to become Nauset Light...which, BTW, is now well known as the logo for Cape Cod Potato Chips!

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[–] abark 8 years ago

Aside from a hurricane every few years, Florida does not have the extreme weather and storms that hit the MA coast.

Also Cape Cod, especially that part of it, is made up of mostly sand. It's basically where a glacier took a big dump.

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[–] JGAS 8 years ago

You see a wall in the movie - the line of sand bars is a wall you can see in the movie? Clueless.

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[–] abark 8 years ago

In the waters around Cape Cod sandbars are pretty common, even pretty far off the shore.

You can see the Chatham sandbar in the satellite image of Chatham.
https://www.google.com/maps/place/Chatham,+MA/@41.667587,-69.9910093,18781m/data=!3m1!1e3!4m5!3m4!1s0x89fb142168afbe53:0x714436ec7d485a53!8m2!3d41.6820897!4d-69.9597664

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[–] Rat_27 8 years ago

We call them sandbars.

https://en.wikipedia.org/wiki/Shoal

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[–] mark6785 8 years ago

we call them sandbanks in the UK, and are usually naturally occurring and tend to move around a bit

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[–] lwi101 8 years ago

Frequently, an harbor entrance is narrow, which tends to increase current effects and wave speed due to venturi effects. Current effects tend to build up the bottom, building the "sand" bar. Waves tend to break when they hit shallow water. So you have a narrow channel with fast current and breaking waves with shallow water due to the bar.Driving a small boat (under 65 feet in CG terms) through breaking waves is tricky, as the breaking waves can broach the boat (spin it sideways, which could lead to capsize). Also rough water sometimes stirs up sediment in fuel tanks, which can lead to engine cutting out.Even if it doesn't happen, you need to plan for it. Breaking waves mean the stern may be out of the water part of the time, Since the rudder and the prop are at the stern, you may not have steering or propulsion when you need it. All in all every thing bad is in front of you, with almost no margin for error and very little in your favor. Typically you will try to time crossing the bar for slack tide, but in a rescue situation that is not an option. In good weather crossing a bar is challenging. You can find youtube videos of various types, including Coastie, boats crossing bars.

L

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[–] johnnxxx 7 years ago

Bar is short for Sand Bar.. its what helps create the waves they had to overcome to get out to sea and it's what gives good waves to surf.. a storm could break a sand bar rendering a beach/shore etc wave less for decades or eternity..

Happened to Coogee beach in Sydney Australia that I frequented as a kid in the 70's.. it had good waves but now has just a flat shore dump and people find it hard to believe it had waves when I tell them..

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[–] action_on_deck 7 years ago

I didn't hear any talk of a bar, but did talk a lot about something called a bah.

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