I guess I need to weight in here too.
My boot is twelve and a half feet long and the tonnage with me in it, is
about 260 pounds.
A kayak.
The main thing is I am more intimate with the water than large boats. I can
feel more of what is going on. Oh, I also kayak the Lower Russian River
almost everyday, all year long for the last eight years or so.
Now, it's interesting, this discussion. I've experience an effect that I
just couldn't quite figure out until you guys started talking about it.
I'd be going along the shore line parallel to it, maybe three to ten feet or
so from the shore. All of a sudden, my boat would turn away form the shore,
forcibly.
This happens mostly in shallow water, about a foot, but I have had it happen
in several feet of water with a steeper shore lines, but mostly happens on
gradual descending shore lines. It especially happens where there is an
irregularity in the shore line, like a little point coming out. I look at
the water as a huge laboratory, since I'm somewhat of a scientist type.
Anyway, I can repeat this in certain places. I thought about this a lot, but
just couldn't get a handle on it.
Here is what I think is happening, at least in my boat. Think of the kayak
as a diamond shaped thing. The front of the diamond, the bow causes a wake.
Normally this wake just hits the shore gently and I just keep moving along,
but some times this wake climbs up on the shore, it makes a lot of noise,
taking more water with it.
This is the part I think I just figured out. Because this wave climbs the
shore in certain spots, the back part of the diamond, which doesn't make a
wake now has less water acting on it, because of the wave piling up on shore
taking the water with it, it's just lower on that part of the boat. The wave
lowers the water level on the back side that is nearest shore. This makes
the boat turn hard away from the shoreline, with quite a force.
It's a hard force to overcome. Think of it this way for you down hill
skiers. To stop in skiing, you turn ninety degrees to the way you are
traveling and in doing so, all the forward motion has to be dissipated in
the stop. Snow flies. So the faster you go, the more energy has to be
dissipated and the more snow you throw. Now when the boat starts to turn,
I'd say the faster you are going the harder the turn would be, and it's hard
to stop it form turning because of the energy trying to dissipate, along
with the wave action.
Here's another example of this dumping of energy thing. Play boats or small
white water boats don't glide well, they are made to just float with the
water down stream.
While paddling one of these boats on open water, I noticed whenever I
stopped paddling, it would turn abruptly to the left and stop and it was
almost impossible to stop it from doing so, I think mostly because it was
dissipating the forward energy when it turned. I thought play boats sucked.
:O) I like to glide.
Anyway, I just thought I throw this out to add to what was being discussed
and also saying that this effect does apply to smaller ships. :O)
Most boats don't travel as close to shore as I do and likely would not
experience this effect much. Water is some real amazing stuff.
Bob Noble
http://www.sonic.net/bnoble
--------------------------------------------------
From: "Lonner" <lon4@yahoo.com>
Sent: Wednesday, August 18, 2010 7:15 PM
To: <electricboats@yahoogroups.com>
Subject: [Electric Boats] Re: Water movement around a hull
> Ron,
>
> Here is how I look at the effect you describe. I think if you take into
> account the "displacement" of a vessel, the movement of water around a
> hull and interactions with nearby vessels in a narrow channel may be more
> easily understood. That is, a ship rated at several thousand tons
> displaces that much water to all sides as it passes. In the ocean there is
> less resistance to the movement so the bow wave and wake are the major
> effects. In a narrower waterway there are forces from the displaced water
> meeting the confinement at the river banks that results in a height
> increase from the heavier displacement vessel. That, and possibly a
> reflecting bow wave, plus the water's habit of seeking a normal level help
> to move the two vessels together.
>
> I'm a design engineer, no naval architect, so my analysis may be flawed.
>
> --- In electricboats@yahoogroups.com, "Ron" <rlgravel@...> wrote:
>>
>> Wow!! so many different things and people that need a response. I have a
>> question about something that happened years ago, knowing what happened
>> and the outcome, I still am not sure what the really correct action
>> should have been. The answer might be a help to anyone active in boating
>> today.
>>
>> The situation took place on the Mississippi river just below New Orleans,
>> I was at the wheel (rudder stick) of a 65' pushboat going downstream
>> pushing two barges (end to end) the river bank on my right, the engines
>> were set at 3/4 throttle. From behind, a freighter (running lite) is
>> moving downstream at a much faster speed, his location seemed to be near
>> the center of the river and as everyone should know it is pretty wide.
>> Signals were made for the passing and as far as I knew everything was
>> fine.
>>
>> As the ship gets closer I notice the space between us starts to narrow,
>> but still no need for worry (so I thought), I give a little right rudder
>> thinking of moving closer to the bank, in a very short time it is obvious
>> the gap is closing (more right rudder). The bow of the ship now about
>> even with me, things are changing fast, so I go to full throttle and hard
>> right rudder, at just a few moments we are at mid ship and I know this is
>> kicking my boat harder towards the ship, but I'm trying to get enough
>> space to allow hard left rudder and not swing the barges into the side of
>> the ship. At what I thought was the right moment (and maybe was) I go to
>> full left rudder, at which point my boat is under the stern of the ship
>> and 10 to 15 feet away from the ships propeller, which is chopping water
>> at about 1/3 above surface. the last image I had of the ship pilot, was
>> him leaning over the bridge rail as far as he could and a real look of
>> concern was easy to see.
>>
>> It was a very close call and I survived as is obvious....the point in
>> relating this event is to draw attention to the massive amount of water
>> that moves, as Chris and a few others have commented, in relation to a
>> hull of any size.
>> The surface in some cases seems to move forward with the vessel while a
>> large amount under water and not visible is being pushed away, which in
>> my mind forms something like a vortice or contrail behind the wing of an
>> airplane. As it seems in my mind, my boat and barges started riding the
>> top of that swirling cone shaped body of water and at some point started
>> being floated or sucked toward the ship.
>>
>> Even in a river as large as the Mississippi, if you are close to the bank
>> you can see water fall and rise as a ship approaches and passes.
>>
>> Other than more space, does anyone have anything to suggest or make
>> comment on that would help someone else keep from falling into a possible
>> similar situation ?
>>
>> Ron
>>
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