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Apr 13, 2018, 08:14 AM
Latitudes vs Attitudes
Bob Gaito's Avatar
Discussion

Ground effect-Pre grounding effect?


Here I go again talking about my younger days in the 60s on my dads boat. It was a wood boat made at the end of the evolution of wood boats-so it was a well made boat (Mahogany and oak) and well designed (Dave Martin). I learned a lot during the 30 years we spent on that boat. We boated on the LI south shore in what is Nassau County and it is comprised of many wetlands with gently curving natural channels with mud bottoms leading to Jones Inlet.
A phenomenon that was noticed early on was that the boat, just prior to running aground, would speed up and the wake would flatten out as if it was running 250 rpm faster. Over the many years through trial and error we experimented with those gently curving channels which ran from two ft to 12 ft deep. We found that with the boat set up just past being on plane and with the rudders straight and aimed down the center of those underwater channels the boat would come to the sloping side of the deep water and (I guess thru ground effect)the boat would lift on the shallow side and actually turn back to the deep water and since it was set up to run straight it did just that until again it came to the another underwater slope . It would continue to follow the deep water long past the point of coincidence. This could be duplicated over and over as long as there was no wind and the tide was slack. I am sure that the fact that the 32ft boat only drew 27 inches and the props wouldn't hit until 24 inches kicking in a ground effect because it can get that much closer to the bottom?? I wonder if anyone has experienced this...
Latest blog entry: Billy Joel 101-at the Garden
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Apr 13, 2018, 09:13 AM
Mmm, tugs...
patmat2350's Avatar
(what did he say?)

Lots of model tugs here, and a handful of real boaters (not me for sure!)

I bet you'd get some lively discussion at http://forum.woodenboat.com/
Apr 13, 2018, 09:25 AM
Latitudes vs Attitudes
Bob Gaito's Avatar
In Summary : a well designed boat will, under certain conditions, follow a channel-hands off. A while back we had a discussion about walking a twin engine boat sideways- I'll check out the woodenboat forum-thanks.
Latest blog entry: Billy Joel 101-at the Garden
Apr 13, 2018, 09:28 AM
AndyKunz's Avatar
You'll probably get better answers in Modeling Science, Bob.

http://www.rcgroups.com/modeling-science-136/

Andy
Apr 13, 2018, 12:23 PM
Sea Dragon-Lover
Umi_Ryuzuki's Avatar
It is do to water displacement.
The wake of your boat on the deep side displaces into clear water.
The wake on the shallow side hits the bank, or bottom and some of that will push
back against the hull as it builds up and begins to travel up the bank or shore.

In model tug towing, the Spokane, WA, event pond was shallow, less than 18" in some areas.
When pushing a loaded barge, the displaced water would hit the bottom of the pond and
then build up in front of the tow. No matter how much power you put to the tug and it's tow, the
there was a maximum speed that could not be exceeded. This was because the model was
pushing up against all the water that was being displaced ahead of the barge.
Essentially the model was trying to push "up" a continious swell, or hill.

This is a similar issue in large scale.

Here is a combination of factors...
A container ship displacing water into the bank of a canal, and the
venturi affect of the two ships displaced water moving quickly between the ships.

Collision between Colombo Express and Maersk Tanjong off Suez Canal (0 min 26 sec)


Accident between two container ships through the Suez Canal. (2 min 27 sec)


.

Another example in model, 1/24 scale.
This is nice, because you can kind of see how the wake of the passing ship pushes
the stern of the slower ship out, and then draws the bow in.

Failed overtaking of another ship in a canal - Port Revel Shiphandling (0 min 58 sec)


.

And again in model form, displacment past an anchored ship.

Passing moored ship in a canal with manned ship models at Port Revel (1 min 0 sec)
Last edited by Umi_Ryuzuki; Apr 13, 2018 at 02:51 PM.
Apr 13, 2018, 12:43 PM
Just Plane Nutts
AirDOGGe's Avatar
I don't believe "ground effect" is the right term, though I could be wrong. It usually applies to aircraft when the air becomes trapped between the bottom of the wing and the ground and is momentarily compressed, providing more lift than when in flight.

But water is incompressible. I suspect the boat is being pushed against when you near the bank or running in shallows and the water cannot move out of the way quickly enough. And since it cannot be compressed nor get out of the way, it ends up pushing back in turn.


It might be the same thing. Interesting to learn about. Thanks for that!
Apr 13, 2018, 01:02 PM
Boats on the brain!!
green-boat's Avatar
The bow wave in shallow water on one side gets "bounced" back from the bottom because it has nowhere to go. In turn it pushes back up on the bow turning the boat back into deeper water.
Apr 13, 2018, 01:48 PM
Registered User
From my training in the past:
The effect is called "bank cushion" and "bank suction".

It works a Umi has described, but it isn't limited to shallow water. It is more prevalent there but not exclusive
Apr 13, 2018, 02:01 PM
Registered User
tghsmith's Avatar
things get even more interesting getting large ships in and out of locks...
Apr 13, 2018, 02:43 PM
Sea Dragon-Lover
Umi_Ryuzuki's Avatar
Quote:
Originally Posted by Tom_Tugboat
From my training in the past:
The effect is called "bank cushion" and "bank suction".

It works a Umi has described, but it isn't limited to shallow water. It is more prevalent there but not exclusive
Thanks Tom, nice to know the terminology.

Here is a deep water version, they may still be in a channel, but the
it's the bow wave from the larger freighter that "turns" the smaller ship.
You can hear the moment they actually make contact.

Amateur footage shows: Turkish bulk carrier collision to Vietnamese cargo vessel on Singapore (2 min 7 sec)
Apr 13, 2018, 03:07 PM
Latitudes vs Attitudes
Bob Gaito's Avatar
Thats sickening to watch...I guess in my case "bank cushion" was working to our advantage..
Latest blog entry: Billy Joel 101-at the Garden
Apr 13, 2018, 04:04 PM
Lot of water; some gets thin
chum444's Avatar
Bob,
You started a very interesting discussion. Never experienced the phenomena myself since I get nervous one a 1:1 with less than 6'of water under the keel. In my part of the ocean the water ain't soft-bottomed it comes at you in big lumps.
Apr 13, 2018, 04:45 PM
Latitudes vs Attitudes
Bob Gaito's Avatar
Yeh I know scary-even on our north shore...they do a big business in props and struts! With the little boat we started with if you ran aground, you got out and clammed until the tide came in.
Latest blog entry: Billy Joel 101-at the Garden
Apr 13, 2018, 06:23 PM
Lot of water; some gets thin
chum444's Avatar
BTW, Umi has the fluid dynamics correct.
Apr 13, 2018, 07:06 PM
Latitudes vs Attitudes
Bob Gaito's Avatar
Umi is very smart (don't tell her I said so)
Latest blog entry: Billy Joel 101-at the Garden


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