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The Physics of Listing

 
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C-Val



Joined: 15 Sep 2012
Posts: 243
City/Region: White Rock
State or Province: BC
C-Dory Year: 1988
C-Dory Model: 22 Angler
Vessel Name: Seaduced
Photos: C-Val
PostPosted: Mon Jan 28, 2019 2:26 pm    Post subject: The Physics of Listing Reply with quote

Hi Everyone

Strange thing happened on the Fraser river yesterday with my flat bottom classic in about a 3 knot current ( I don't have trim tabs -- yet)

We had just a slight list to port going down river with 4 people aboard.
As soon as I turned around to face the current on my starboard bow, the list to port suddenly became very dramatic. I had to suddenly stop the boat (in which case my wife was just standing up to move and fell over!)and rearrange my gear.

Does this make sense to anyone? This is the second time this has happened and both times were in the river when I changed direction.

Would trim tabs actually help with this or is it my driving that needs to improve?

If so I was reading here about Bennett Tabs M120. Would that be the correct ones for my '81 classic?

Still Learning (with a few bruises to go along with it!)
David and Val

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Writing a sea story with my C-Dory !
1982 22' Classic Popeye
1981 22' Classic Bad Boy Brutus
1988 22' Angler
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thataway



Joined: 02 Nov 2003
Posts: 16934
City/Region: Pensacola
State or Province: FL
C-Dory Year: 2007
C-Dory Model: 25 Cruiser
Vessel Name: thataway
Photos: Thataway
PostPosted: Mon Jan 28, 2019 2:52 pm    Post subject: Reply with quote

What speed were you running at, up and down river? Did the sudden list occur during turning or after turning. The difference is speed thru the water--and yes, increasing speed thru the water may increase listing in some circumstances.

Trim Tabs are only effective at speed--to some degree at displacement speeds, and more at planing speeds.

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Bob Austin
Thataway
Thataway (Ex Seaweed) 2007 25 C Dory May 2018
Thisaway 2006 22' CDory November 2011 to May 2018
Caracal 18 140 Suzuki 2007 to present
Thataway TomCat 255 150 Suzukis June 2006 thru August 2011
C Pelican; 1992, 22 Cruiser, 2002 thru 2006
Frequent Sea; 2003 C D 25, 2007 thru 2009
KA6PKB
Home port: Pensacola FL
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journey on



Joined: 03 Mar 2005
Posts: 3246
City/Region: Valley Centre
State or Province: CA
C-Dory Year: 2005
C-Dory Model: 25 Cruiser
Vessel Name: journey on
Photos: Journey On
PostPosted: Mon Jan 28, 2019 3:04 pm    Post subject: Reply with quote

If I understand you correctly (covering my ass) you had a list to port, turned so that the current hit your starboard bow and the list increased.

If I got it correct, that's what I would expect to happen. A force on the starboard side, caused by the current, would push and roll you to port, especially if you were already listing to port, increasing the starboard cross section.

Boris
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C-Val



Joined: 15 Sep 2012
Posts: 243
City/Region: White Rock
State or Province: BC
C-Dory Year: 1988
C-Dory Model: 22 Angler
Vessel Name: Seaduced
Photos: C-Val
PostPosted: Mon Jan 28, 2019 3:43 pm    Post subject: Reply with quote

Yes Boris that is exactly what happened

Dr Bob I was cruising down river at displacement speed and when I turned around I kicked it up to planing speed - about 20 knts
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thataway



Joined: 02 Nov 2003
Posts: 16934
City/Region: Pensacola
State or Province: FL
C-Dory Year: 2007
C-Dory Model: 25 Cruiser
Vessel Name: thataway
Photos: Thataway
PostPosted: Mon Jan 28, 2019 6:19 pm    Post subject: Reply with quote

At displacement speed, best to trim the boat by moving people and gear. If there is already a list--as Boris notes, either the current, or a sudden turn, especially to port, would increase the list. At the higher speed the list might be more.

If the C Dory goes over a wake, hits a current, and the bow is trimmed own, at times the list can be severe enough to throw people off balance. I have bubble levels for both fore and aft and lateral trim. I try and keep the boat trimmed level. In the 22 especially, if on plane, I would ask Marie to tell me if she were going to cross from one side to the other, so I could compensate with the trim tabs, and also not be taken by surprise by the change in steering force needed.
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Marco Flamingo



Joined: 09 Jul 2015
Posts: 821
City/Region: Seattle
State or Province: WA
C-Dory Year: 2004
C-Dory Model: 16 Cruiser
Vessel Name: Limpet
Photos: Limpet
PostPosted: Mon Jan 28, 2019 8:33 pm    Post subject: Reply with quote

The C Dory has a hard chine that can "catch an edge" crossing an eddy or entering current at an angle. Whitewater kayakers are familiar with this and when leaving the river current and entering an eddy, where the water is basically running upstream, it is necessary to anticipate, lean over, and brace with the paddle. There is no way to do this in a larger boat and, depending on your bottom configuration, can shift the boat sharply to one side for much longer than one expects. This may be what you experienced.

There is a YouTube video of a tug attempting to run up along the shore on one of the BC narrows. When they cut into the current attempting to shoot through the narrows, the current catches the hard chine on the steel tug. The difference in current is essentially a 20 knot current coming at 90 degrees. The 75 foot tug slowly leans over, causing the chine to dig deeper, and turns turtle.

Trim tabs can't really help on this because it is caused by water going sideways over the chine.

Mark
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ssobol



Joined: 27 Oct 2012
Posts: 1757
City/Region: Leesburg
State or Province: VA
C-Dory Year: 2008
C-Dory Model: 22 Cruiser
Vessel Name: SoBELLE
Photos: SoBelle
PostPosted: Mon Jan 28, 2019 10:02 pm    Post subject: Reply with quote

journey on wrote:
If I understand you correctly (covering my ass) you had a list to port, turned so that the current hit your starboard bow and the list increased.

If I got it correct, that's what I would expect to happen. A force on the starboard side, caused by the current, would push and roll you to port, especially if you were already listing to port, increasing the starboard cross section.

Boris


Isn't this basically "bow steer"? Can be a bit of a surprise if you are not expecting it.
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