Angle of Vanishing Stability - SailNet Community

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > General Interest > General Discussion (sailing related)
 Not a Member? 


Like Tree7Likes
Reply
 
LinkBack Thread Tools
  #1  
Old 12-06-2011
shaile's Avatar
Junior Member
 
Join Date: Dec 2009
Posts: 20
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 0
shaile is on a distinguished road
Angle of Vanishing Stability

I am reading the book "Adlard Coles' Heavy Weather Sailing" by Peter Bruce. In the chapter "The stability of yachts in large breaking waves", it tells about an experiment done on boat models in a water tank, that measures the AVS of various boat shapes. One of the outcomes of this experiment is that adding a coachroof to a boat without one may move the AVS further towards the 180 degree, or alternately, two boats of the same height above water line, one with high freeboard and no coachroof and the other with coachroof and lower freeboard, the later shows much less inverted stability.
The reason for this phenomenon is that the coachroof, when submerged in the water, adds buoyancy and raises the center of buoyancy to higher point with regard to the vessel.
This may raise some thoughts:
1) An inflatable boat stowed on deck may cause the same effect and thus contribute to the stability of the boat.
2) We can take this idea a step further and install a balloon on deck. Such a balloon can be empty in normal sailing and automatically inflated when the boat is inverted. If this idea is right, I suppose I would have not been the first one to raise it. So why don't we see such systems on boats ?

Btw, I have a picture of the said page from the book but I don't know if I am allowed to display it here.
Reply With Quote Share with Facebook
  #2  
Old 12-06-2011
blt2ski's Avatar
Senior Member
 
Join Date: May 2005
Posts: 6,694
Thanks: 0
Thanked 21 Times in 20 Posts
Rep Power: 10
blt2ski will become famous soon enough
Probably because it is cheaper to built it with a roof, vs with out per say and an inflatable balloon/pfd if you will. The other you would have to deal with on the balloon, is where the lines etc are at the time of the balloon explosion! you may do more damage to the rigging etc with this option than designing a boat to roll easily when upside down.

Along with, most of us will probably never be in a rolled situation where and how we sail, so the balloon is a REALLY expensive solution. now maybe as an option for some one crossing and ocean, where the waves are big enough to roll a boat.........maybe......

my 02 not that .02 is worth much these days!

Marty
__________________
She drives me boat,
I drives me dinghy!
Reply With Quote Share with Facebook
  #3  
Old 12-06-2011
LandLocked66c's Avatar
Sunsets and Warm Beer....
 
Join Date: Dec 2009
Location: The Bottom of a Pint Glass...
Posts: 2,111
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 5
LandLocked66c is on a distinguished road
Off topic but I know that Donald Crowhurst was trying to develop a balloon at the top of the mast that was supposed to right his trimaran.
__________________
1971 23' Oday Pop Top
S/V Frida

You can't steer a boat that isn't moving? Just like a life - P. Lutus
Reply With Quote Share with Facebook
  #4  
Old 12-06-2011
PaulinVictoria's Avatar
Senior Member
 
Join Date: Aug 2009
Location: Sidney, BC
Posts: 1,875
Thanks: 7
Thanked 45 Times in 44 Posts
Rep Power: 6
PaulinVictoria is on a distinguished road
TIDEL : Self Righting Bags for RIBs and Inflatables : Dartmouth Brixham Salcombe Plymouth Devon Cornwall Torquay on a RIB, but you get the idea
__________________
Orange Crush
1974 C&C27 MkII

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Reply With Quote Share with Facebook
  #5  
Old 12-06-2011
Senior Member
 
Join Date: Sep 2011
Location: Liberty Landing
Posts: 665
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 4
peterchech is on a distinguished road
It seems to me (this is worth less than 2 cents btw) that few boats will stay turtled in the kinds of conditions that caused them to turn over. The question may be how watertight is the boat, how much water gets in before the next breaking wave flips you back upright.
Reply With Quote Share with Facebook
  #6  
Old 12-06-2011
kwaltersmi's Avatar
Broad Reachin'
 
Join Date: Aug 2006
Location: Grand Rapids, Michigan
Posts: 1,856
Thanks: 1
Thanked 12 Times in 12 Posts
Rep Power: 9
kwaltersmi is on a distinguished road
Quote:
Originally Posted by peterchech View Post
The question may be how watertight is the boat, how much water gets in before the next breaking wave flips you back upright.
I think Peter hit on a big issue: How watertight is the boat? Any water allowed to enter the cabin/hull will quickly change the AVS and righting ability. Properly secured hatchboards, strong ports, sealed deck hatches, watertight lazerettes, etc. are likely just as critical to surviving turtling/capsizing.
__________________
Catalina 34

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Reply With Quote Share with Facebook
  #7  
Old 12-06-2011
Faster's Avatar
Just another Moderator
 
Join Date: Sep 2005
Location: New Westminster, BC
Posts: 14,672
Thanks: 68
Thanked 193 Times in 185 Posts
Rep Power: 10
Faster has a spectacular aura about Faster has a spectacular aura about Faster has a spectacular aura about
The current breed of beamy ocean racers often do sit happily upside down, most especially if they happen to lose the keel bulb... There are some images around, Tony Bullimore for one, where he's clinging to the keel fin on a seemingly very stable upside down boat. Multis, too, are happiest that way.

But then again not many of us sail those types of boats....
Agri likes this.
__________________
Ron

1984 Fast/Nicholson 345 "FastForward"

".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
Reply With Quote Share with Facebook
  #8  
Old 12-06-2011
Senior Member
 
Join Date: Dec 2003
Posts: 1,368
Thanks: 1
Thanked 15 Times in 14 Posts
Rep Power: 11
capttb is on a distinguished road
Airbags for keelboats ?
BTW I've seen tri's with some kinda giant floatation device at the masthead, usually pretty "unique" looking older tri's.
__________________
"Just call me TB"

Last edited by capttb; 12-06-2011 at 01:02 PM.
Reply With Quote Share with Facebook
  #9  
Old 12-06-2011
Member
 
Join Date: May 2009
Location: Mansfield TX
Posts: 66
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 6
FlyNavy is on a distinguished road
Hobie Cat makes a "baby bob" and a "momma bob" to be attached to the top of their masts specifically to prevent turtling. They won't bring the boats back upright but that's multi-hill bit of physics anyway.
__________________
'79 San Juan 7.7
Joe Pool Lake
DFW area TX
Sailing year 'round
(Not up North)
Reply With Quote Share with Facebook
  #10  
Old 12-06-2011
Senior Member
 
Join Date: Sep 2011
Location: Liberty Landing
Posts: 665
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 4
peterchech is on a distinguished road
Quote:
Originally Posted by Faster View Post
The current breed of beamy ocean racers often do sit happily upside down, most especially if they happen to lose the keel bulb... There are some images around, Tony Bullimore for one, where he's clinging to the keel fin on a seemingly very stable upside down boat. Multis, too, are happiest that way.

But then again not many of us sail those types of boats....
Rambler is a good example of this from the recent Fastnet.

If the keel falls off I think you have bigger problems than just remaining turtled. However, the types of racing boats which are most likely to have keel issues, flush decks and super-wide planing arses are also built very light, usually foam cored. So without their keels they are likely to remain afloat like a liferaft. Kind of like a multi.

If something like a canting keeled mini 650 turtles, if the keel is canted this may help get the boat back on its side at least. However the flush deck, and relatively low ballast to displacement ratio might make recovery from turtle much less likely, even with wave action, than recovery on your standard cruiser.

I often wonder about what you do if you are in the cockpit when the boat rolls. If you are tied in (you should be), you may be held under for some time before the boat comes back up especially if sheets are not released in time or if wave/wind action coincidentally holds the boat inverted longer than would be expected. If you cut away from the tether to get to the other side of the boat, you risk being separated and lost at sea. Webb Chiles talks about being rolled twice in the southern ocean on one of the "furled sails" podcasts, but he was in the cabin both times and the boat came right back up. I think I would be tempted to throw out a sea anchor and hide below if the going got really rough, but that isn't always an option and often it is a rogue wave that breaks unexpectedly which rolls the boat.
Reply With Quote Share with Facebook
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


Similar Threads
Thread Thread Starter Forum Replies Last Post
tacking angle mr31 Racing 22 05-23-2012 12:32 PM
Best angle to windward? malyea Racing 9 12-07-2010 02:46 PM
Angle of Vanishing Stability Grampian 30' 1974 akin_alan General Discussion (sailing related) 9 09-07-2010 04:42 PM
Bayfield 29, angle of vanishing stability celiro Sailboat Design and Construction 2 11-29-2007 05:19 PM
AVS - angle of vanishing stability stravaigin General Discussion (sailing related) 7 06-20-2007 12:55 PM


All times are GMT -4. The time now is 03:05 PM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012

The SailNet.com store is owned and operated by a company independent of the SailNet.com forum. You are now leaving the SailNet forum. Click OK to continue or Cancel to return to the SailNet forum.