[Rhodes22-list] Fluid Dynamics
Bill Effros
bill at effros.com
Mon Jan 16 20:06:02 EST 2006
Peter,
Those who think I know nothing about anchoring will be pleased to learn
I know even less about fluid dynamics. As I understand this thread,
that is what you are asking about, and I hope some of the physics people
will jump back in, incoming flak notwithstanding.
First let me reiterate that I would never attempt to anchor in a 5 kt.
current. All boats are out of control at this speed--they can't stop;
if they are drifting they can't steer; and if they are coming at you and
you are stopped in the middle of the current they are going to hit
you--hard.
The current in the East River is not always 5 kts. 5 kts. is the peak,
and it doesn't hit that peak every time. I have gone down the East
River at 4-5 kts. and it is a thrill seeker's ride. At 4-5 kts. you are
drifting with the current. You can't steer because the water is
traveling at the same speed as the boat. This is true for all boats.
The big working boats -- the barges and tugs; tankers; etc. -- generally
travel with the current to save fuel. They have the right of way, and
there is absolutely no way for them to steer around you if you are in
trouble, right in front of them. They move very fast -- I would guess
10-15 kts. 5 kts. is the current, and then to maintain control they
must move faster than the current. So they travel at 5 - 10 kts. on top
of that current for a combined speed over ground in the 10-15 kt range.
You do the same. You fly down the river, and as I've mentioned before,
you often go down the river faster than the cars on the adjacent FDR
roadway. So, you're doing 10 kts. over ground, not 5.
Meanwhile, high powered ferries and hydroplanes are crossing back and
forth between Manhattan, Brooklyn and Queens, and you may encounter a
Circle Line Cruise going the wrong way, or turning around at the end of
its run.
The chop in the water is chaotic--these are big boats pushing a lot of
water around. You are basically a cork with an outboard. Sometimes
it's windy.
No matter what, throwing an anchor overboard is probably the worst thing
you can do in an emergency under these conditions. What will you do if
it catches?
If you threw an anchor overboard and it caught you would be stern
anchored. Now you would have water flooding over your stern into the
cockpit and cabin with no way to stop it. The amount of water that
would flood in will sink the boat in a blink of an eye. It would be
impossible to turn the boat into the current -- you would broach if you
tried -- but presumably you have no motor or rudder or you wouldn't have
thrown the anchor overboard in the first place. You would not be able
to free yourself from the anchor if you had attached the end to anything
substantial.
In short -- don't try to anchor in this situation. Know that if
something bad happens you've got to come up with some other solution.
Having said that, I will now tell you how we were forced to anchor in
the East River. I was going down the river with a friend in a 30 foot
Cal. Something wrapped around his propeller when we were near North
Brother Island, and the motor just stopped. The current was not 5 kts.,
and I'm not sure it ever reaches 5 kts. at that point.
We steered into the current while we still had some weigh on, and
dropped the Danforth style anchor into the river from the bow as we lost
forward momentum. The anchor immediately gripped. We let out a lot of
scope. Tied off the anchor. Removed the junk from the propeller.
Motored up to the anchor as we pulled in the line. And continued on our
way. That's all there was to it.
Here's the fluid dynamics part. As I understand eddies, they occur
because the water is not one solid mass flowing at a constant speed. I
think the water can move faster at the surface than the bottom. So if
you drop an anchor it encounters turbulence which may cause it to tumble
or spin on the way to the bottom, but the water may be traveling far
more slowly at the bottom allowing the anchor to set at a much slower
speed than the boat is moving.
I could be completely wrong about this.
In any case, we did set the anchor from the bow in a wide part of the
"river" by successfully bringing the boat to a controlled stop out of
the flow of traffic and anchoring with our "Go-To" anchor in the normal way.
Bill Effros
Peter Thorn wrote:
>Michael and Bill,
>
>Is it really the same problem? In current, the anchor and everything else
>is swept along the bottom with the current. I imagine it might be easier to
>set from a moving boat but in still water. The flukes, being dragged, might
>catch and "sail" through the still water, plunge down and set. Drifting
>with a current there would be no fluke steerage, just everything being swept
>along.
>
>--------------------
>
>I suppose that is a loose use of the term sailing. 7/2/2003, Mac26x, 50hp
>at 2,000 rpm ~ 2-3 knots, ballast tank full, one rudder down -- it's a Mac
>thing. Passed under Throgs Neck Bridge after flood tide. At one point
>GPS had us 8 knots over the bottom later than morning near Brooklyn Bridge.
>We really had no desire to anchor that day. New York's Finest boat cops
>probably wouldn't have allowed it anyway. They were in high security mode
>so close to the 4th.
>
>PT
>
>
>
>
>
>----- Original Message -----
>From: "Michel Meltzer" <mjm at michaelmeltzer.com>
>To: "'The Rhodes 22 mail list'" <rhodes22-list at rhodes22.org>
>Sent: Saturday, January 14, 2006 4:19 PM
>Subject: RE: [Rhodes22-list] anchoring obsession
>
>
>
>
>>Bill the current is an example of the general problem, which is, how do
>>you set anchor from a moving boat? Is could be from current, wind, wave
>>etc.. Extra points if you have no motor(motor failer) and have no time
>>to set sail, the east river is a fine example but any confined space is
>>saying "little time" to get stopped before something bad happens. -mjm
>>
>>
>>
>>>-----Original Message-----
>>>From: rhodes22-list-bounces at rhodes22.org [mailto:rhodes22-list-
>>>bounces at rhodes22.org] On Behalf Of Bill Effros
>>>Sent: Saturday, January 14, 2006 4:12 PM
>>>To: The Rhodes 22 mail list
>>>Subject: Re: [Rhodes22-list] anchoring obsession
>>>
>>>Peter,
>>>
>>>Since we have both sailed down the East River (did you actually sail
>>>
>>>
>>or
>>
>>
>>>did you motor? -- in an R-22 or a different boat?) I will confine my
>>>remarks to that body of water--as soon as I can get back to it--not
>>>now. I assume you were traveling in the direction of the flow?
>>>
>>>Bill Effros
>>>
>>>Peter Thorn wrote:
>>>
>>>
>>>
>>>>Bill,
>>>>
>>>>Two years ago, sailing down the East River, the current was about
>>>>
>>>>
>>that.
>>
>>
>>>In
>>>
>>>
>>>>Beaufort, NC (where Bill W anchored) there can be some pretty strong
>>>>currents as the Altlantic makes it's way through the inlets during
>>>>
>>>>
>>tidal
>>
>>
>>>>changes. We're planning a Cape Lokout trip later, so it's possible.
>>>>
>>>>
>>I
>>
>>
>>>don't
>>>
>>>
>>>>think anchoring in a 5 knot current is likely, but if it can be done
>>>>
>>>>
>>I
>>
>>
>>>sure
>>>
>>>
>>>>would like to know how.
>>>>
>>>>PT
>>>>
>>>>
>>>>
>>>>----- Original Message -----
>>>>From: "Bill Effros" <bill at effros.com>
>>>>To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
>>>>Sent: Saturday, January 14, 2006 2:37 PM
>>>>Subject: Re: [Rhodes22-list] anchoring obsession
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>>Peter,
>>>>>
>>>>>I'm back. Just like you, more interested in this than in what I'm
>>>>>supposed to be doing.
>>>>>
>>>>>I'm going to break my replies to your comments into subject headings
>>>>>
>>>>>
>>so
>>
>>
>>>>>anyone trying to follow doesn't have to read the whole thing.
>>>>>
>>>>>Do you actually sail in waters where there is a 5 kt. current? Have
>>>>>
>>>>>
>>you
>>
>>
>>>>>ever sailed in a 5 kt. current? Where?
>>>>>
>>>>>Bill Effros
>>>>>
>>>>>
>>>>>
>>>>>Peter Thorn wrote:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>Hi Mary Lou,
>>>>>>
>>>>>>It's quite windy and blowing in cold here and could get as low as
>>>>>>
>>>>>>
>>28
>>
>>
>>>>>>tonight! My skipper cancelled the NYRA winter series racing at
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>Blackbeard
>>>>
>>>>
>>>>
>>>>
>>>>>>SC on the Neuse River, so I spent most of the morning messing
>>>>>>
>>>>>>
>>around
>>
>>
>>>with
>>>
>>>
>>>>>>that Anchor Catenary program Ron attached.
>>>>>>
>>>>>>Using my imagined problem of being stuck out in the Pamlico, out of
>>>>>>
>>>>>>
>>>sight
>>>
>>>
>>>>>>
>>>>>>
>>>>of
>>>>
>>>>
>>>>
>>>>
>>>>>>land, in 20' of water over a muck bottom, using Raven's storm
>>>>>>
>>>>>>
>>anchor
>>
>>
>>>>>>
>>>>>>
>>>>(FX11,
>>>>
>>>>
>>>>
>>>>
>>>>>>16' chain, 150' 3/8" three stand nylon all out), and setting the
>>>>>>
>>>>>>
>>Kellet
>>
>>
>>>>>>
>>>>>>
>>>>at
>>>>
>>>>
>>>>
>>>>
>>>>>>75', here are the results:
>>>>>>
>>>>>> Kellet = 0# = 95.2#
>>>>>> Kellet = 10 = 131.5
>>>>>> Kellet = 20 = 168.0
>>>>>>
>>>>>>Now it's just a matter of trying to figure out what these results
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>actually
>>>>
>>>>
>>>>
>>>>
>>>>>>mean. I'm a builder, not a genius. My guess is that, with the
>>>>>>
>>>>>>
>>>catenary
>>>
>>>
>>>>>>shape described by the graph shown on the catenary analyzer
>>>>>>
>>>>>>
>>program,
>>
>>
>>>>>>
>>>>>>
>>>>those
>>>>
>>>>
>>>>
>>>>
>>>>>>are the various horizontal thrust forces holding each particular
>>>>>>
>>>>>>
>>>catenary
>>>
>>>
>>>>>>profile shape in equilibrium against an infinitely strong anchor.
>>>>>>
>>>>>>
>>So,
>>
>>
>>>a
>>>
>>>
>>>>>>
>>>>>>
>>>>10#
>>>>
>>>>
>>>>
>>>>
>>>>>>Kellet provides a 38% increase and a 20# Kellet provides a 76%
>>>>>>
>>>>>>
>>increase
>>
>>
>>>>>>
>>>>>>
>>>>in
>>>>
>>>>
>>>>
>>>>
>>>>>>the force needed to hold each's catenary shape versus no Kellet at
>>>>>>
>>>>>>
>>all.
>>
>>
>>>>>>
>>>>>>
>>>>If
>>>>
>>>>
>>>>
>>>>
>>>>>>my guess is right, then a Kellet does help quite a bit, adding
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>significantly
>>>>
>>>>
>>>>
>>>>
>>>>>>to the "shock absorber effect" Rik wrote about.
>>>>>>
>>>>>>Yes, we use a 10# steel mushroom anchor on 3/8 nylon as a lunch
>>>>>>
>>>>>>
>>hook.
>>
>>
>>>>>>
>>>>>>
>>>>Works
>>>>
>>>>
>>>>
>>>>
>>>>>>well in any mud bottom and doesn't seem to snag on the stumps in
>>>>>>
>>>>>>
>>Kerr
>>
>>
>>>>>>
>>>>>>
>>>>Lake
>>>>
>>>>
>>>>
>>>>
>>>>>>the Army Corp of Engineers forgot to grub out before they filled
>>>>>>
>>>>>>
>>the
>>
>>
>>>lake
>>>
>>>
>>>>>>
>>>>>>
>>>>in
>>>>
>>>>
>>>>
>>>>
>>>>>>the '60s.
>>>>>>
>>>>>>So... I'm thinking about modifying the 10# mushroom anchor with
>>>>>>
>>>>>>
>>another
>>
>>
>>>>>>quick link (through its nylon rode eye) and a big S hook to ride
>>>>>>
>>>>>>
>>down
>>
>>
>>>the
>>>
>>>
>>>>>>storm anchor rode as a make-shift Kellet. This capital investment
>>>>>>
>>>>>>
>>>might
>>>
>>>
>>>>>>
>>>>>>
>>>>be
>>>>
>>>>
>>>>
>>>>
>>>>>>a whopping $5, so there's not much to loose by trying. A possible
>>>>>>enhancement is to melt down some lead tire weights on a Coleman
>>>>>>
>>>>>>
>>stove
>>
>>
>>>>>>outside and pour 2 or 3 cups of hot lead weight into the mushroom
>>>>>>
>>>>>>
>>bell.
>>
>>
>>>>>>Maybe that's too obsessive-- even for me.
>>>>>>
>>>>>>The big worry is getting caught halfway to Ocracoke in a summer
>>>>>>
>>>>>>
>>storm.
>>
>>
>>>>>>
>>>>>>
>>>>Out
>>>>
>>>>
>>>>
>>>>
>>>>>>in the middle you lose sight of land for about an hour and there's
>>>>>>
>>>>>>
>>no
>>
>>
>>>>>>
>>>>>>
>>>>place
>>>>
>>>>
>>>>
>>>>
>>>>>>to hide. The wind driven waves are short and square. They come up
>>>>>>
>>>>>>
>>>fast,
>>>
>>>
>>>>>>
>>>>>>
>>>>so
>>>>
>>>>
>>>>
>>>>
>>>>>>having a method ready to keep the bow to the wind would be very
>>>>>>
>>>>>>
>>good
>>
>>
>>>>>>
>>>>>>
>>>>indeed.
>>>>
>>>>
>>>>
>>>>
>>>>>>That Boy Scout training never goes away.
>>>>>>
>>>>>>Late last October at Silver Lake on Ocracoke at 5:30am a local
>>>>>>
>>>>>>
>>storm
>>
>>
>>>hit
>>>
>>>
>>>>>>
>>>>>>
>>>>our
>>>>
>>>>
>>>>
>>>>
>>>>>>trailer sailor group. 55 Knot winds were reported on WX. Raven,
>>>>>>
>>>>>>
>>with
>>
>>
>>>>>>
>>>>>>
>>>>the
>>>>
>>>>
>>>>
>>>>
>>>>>>pop top cover up, heeled 30 degrees against her docklines. It was
>>>>>>
>>>>>>
>>gone
>>
>>
>>>>>>
>>>>>>
>>>>in
>>>>
>>>>
>>>>
>>>>
>>>>>>30 minutes, but imagine being out in that!
>>>>>>
>>>>>>PT
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>----- Original Message -----
>>>>>>From: "Mary Lou Troy" <mltroy at verizon.net>
>>>>>>To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
>>>>>>Sent: Saturday, January 14, 2006 11:41 AM
>>>>>>Subject: Re: [Rhodes22-list] anchoring obsession
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>>Peter,
>>>>>>>My purely theoretical take on this - having never used a kellet,
>>>>>>>
>>>>>>>
>>is
>>
>>
>>>that
>>>
>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>it
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>>might help - particularly if the winds were not very steady or if
>>>>>>>
>>>>>>>
>>>there
>>>
>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>was
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>>wave action.
>>>>>>>
>>>>>>>Have you used the mushroom much as a lunch hook?
>>>>>>>
>>>>>>>Mary Lou
>>>>>>>1991 R22 Fretless
>>>>>>>Ft. Washington, PA / Swan Creek, MD
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>At 04:29 PM 1/13/2006 -0500, you wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>GlacierRon, Dave, Bill or anybody:
>>>>>>>>
>>>>>>>>You have me thinking about anchors and not getting any work done
>>>>>>>>
>>>>>>>>
>>:)
>>
>>
>>>>>>>>Suppose I'm out in the Pamlico Sound, with it's mucky bottom in
>>>>>>>>
>>>>>>>>
>>20'
>>
>>
>>>of
>>>
>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>water
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>>>and a squall comes up. Would it help to use the lunch hook, a
>>>>>>>>
>>>>>>>>
>>10#
>>
>>
>>>>>>>>
>>>>>>>>
>>>>steel
>>>>
>>>>
>>>>
>>>>
>>>>>>>>mushroom anchor with a 3/8" nylon rode, as a Kellet shackled to
>>>>>>>>
>>>>>>>>
>>the
>>
>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>storm
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>>>anchor rode? The storm anchor is FX11 Fortress with 16' of 1/4"
>>>>>>>>
>>>>>>>>
>>>proof
>>>
>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>coil
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>>>chain plus 3/8" three strand nylon rode? Or, could it just make
>>>>>>>>
>>>>>>>>
>>>things
>>>
>>>
>>>>>>>>worse?
>>>>>>>>
>>>>>>>>Any thoughts welcome.
>>>>>>>>
>>>>>>>>PT
>>>>>>>>
>>>>>>>>__________________________________________________
>>>>>>>>Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>__________________________________________________
>>>>>>>Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>__________________________________________________
>>>>>>Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>__________________________________________________
>>>>>Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>>>>>
>>>>>
>>>>>
>>>>>
>>>>__________________________________________________
>>>>Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>>>>
>>>>
>>>>
>>>>
>>>>
>>>__________________________________________________
>>>Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>>>
>>>
>>__________________________________________________
>>Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>>
>>
>
>__________________________________________________
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>
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>
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