What is the keel contribution in capsizing. - 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? 


Reply
 
LinkBack Thread Tools
  #1  
Old 12-29-2009
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
What is the keel contribution in capsizing.

Hi all,
I am a newbie here (in the world I am veteran),
I wonder whether the keel contributes to decelerating a potential capsize in breaking waves or contributes to the rolling and thus to the potential capsizing.
In other words, is the stream under the boat slower than the rolling velocity while capsizing (then the keel stops the rolling) or does the stream under the boat actually pushes the keel (in breaking waves) and contribute to the rolling ?

thanks.

Please note, English is not my native language so forgive me for my clumsy english
Reply With Quote Quick reply to this message Share with Facebook
  #2  
Old 12-29-2009
Senior Member
 
Join Date: Oct 2009
Location: Gloucester, MA
Posts: 586
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 6
klem is on a distinguished road
The surface area of the keel does in fact contribute to a capsize when hit on the beam by a breaking sea. The force applied by the keel helps to roll a boat down the face of a wave. The breaking wave is forcing the boat to travel with it while the water the keel is in is essentially stationary. If you watch a surfer, there is a reason that they have to keep that edge of the board up to prevent tripping them. Outward Bound had a policy at one point that their open boats had to pull up the centerboard when running before the wind due to a capsize that had happened. They must have felt that tripping was a big problem for their boats and that it outweighed other factors.

However, you do not want to get rid of the keel. It plays many important roles in terms of righting arm, lateral resistance, steering, and roll inertia. Without the keel, the boat would not have sufficient ballast low enough to carry any sail or be stable at all. In addition, the keel is required to prevent making excessive leeway. In addition, the keel aids in the steering of the boat, trying to control a boat without the keel would be extremely difficult. Finally, having your ballast low increases the roll inertia of the vessel which greatly reduces the chances of capsize. There was some interesting wave tank work after the famous fastnet race where they found that sailboats are less likely to capsize in breaking waves when they have greater inertia.
Reply With Quote Quick reply to this message Share with Facebook
  #3  
Old 12-30-2009
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
So I assume there has to be a compromise between those two conflicting requirements.
So for safty oriented sailing, what is the optimal keel size/length/depth/weight for blue water sailing that involves also rough weather ?
Reply With Quote Quick reply to this message Share with Facebook
  #4  
Old 12-30-2009
Junior Member
 
Join Date: Sep 2009
Posts: 25
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 0
SailorNate is on a distinguished road
Quote:
Originally Posted by shaile View Post
So I assume there has to be a compromise between those two conflicting requirements.
So for safty oriented sailing, what is the optimal keel size/length/depth/weight for blue water sailing that involves also rough weather ?
Sure there are lots of compromises. What exact risk are you trying to mitigate? If your only concern is capsize then a high aspect ratio with a torpedo at the bottom while making way is probably optimal. That is a long thin keel with a big chunk of dense material on the end, usually, but not always lead. The shape in the water as well as the long lever with weight on the bottom help keep the boat upright. This also happens to be about the fastest design as well. The problem with it is that it is not as robust as some other configurations.

My boat has a Peterson keel, that is it is shaped kinda like a 30 60 90 triangle (30 90 corners touching the hull) and the bottom angle, the 60 is cut off partway parallel with the bottom of the boat. This is a balance between performance, righting moment and strength. A full keel, one that runs the length of the hull is stronger still but sacrifices performance. There are many more configurations and there is more to consider than these simple tradeoffs I have listed here.

Perhaps a better place to start would be to do a search here for "blue water boats". There are some pretty good lists and then take a look at what kind of hull design and sailplans those boats have. I suspect you will notice many similarities.

Nate

Last edited by SailorNate; 12-30-2009 at 06:45 AM.
Reply With Quote Quick reply to this message Share with Facebook
  #5  
Old 12-30-2009
Dirtboy's Avatar
Senior Member
 
Join Date: Jul 2009
Location: Florida
Posts: 732
Thanks: 5
Thanked 1 Time in 1 Post
Rep Power: 6
Dirtboy is on a distinguished road
I wonder where bilge keels figure into this? Westerly's are known for their seaworthyness. Not the fastest set-up, specially to weather, but very stable and easy to live with.

DB
Reply With Quote Quick reply to this message Share with Facebook
  #6  
Old 12-30-2009
Jeff_H's Avatar
Moderator
 
Join Date: Feb 2000
Location: Annapolis, Md
Posts: 6,684
Thanks: 5
Thanked 105 Times in 81 Posts
Rep Power: 10
Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about
This is an excerpt from a draft of an earlier article that I wrote for a different purpose but which addresses the basic issues that you are asking about.

The impact of the keel and rudder design on the tendency of a particular boat to capsize has become a bit of a controversial topic. The testing that resulted in the wake of the Fastnet Disaster seemed to come up with what might appear to be contradictory information. I hope to explain some of those contradictions.

To begin with, studies of the contribution of waves on capsize have shown that it requires a breaking wave that is twice the beam of the boat for the wave to be a major contributing factor in causing a capsize. On most ballasted keel sailing cruisers of a reasonably modern design (i.e. mid-20th century onward) a true capsize is not possible without a wave of that description. This would suggest that greater beam would be better in resisting waves, but as the studies showed that as boats became beamier and the beam of the boat was carried further into the ends of the boat at some point the rail of the boat would dip into the water and its suddenly increasing resistance could contribute to a capsize.

Of course greater beam or carrying the beam into the ends of the boat also adds to form stability, and it was shown that boasts with a lot of form stability have an increased tendency to stay perpendicular to face of the wave and so are more likely to be pushed to a greater heel angle by the wave.

Breaking wave studies showed that there was a difference in speed between the water at the surface of the wave and water deeper in the wave. This difference in speed results in what is referred to as surface sheer. In theory the deeper the keel, the greater the difference in surface sheer induced speeds experienced by between the hull at the surface and the bottom of the keel. In theory, since the hull experiences faster moving water than the tip of the keel, the boat is pushed over to a steeper heel angle by the rotational difference between the two speeds. This was thought to be especially true in smaller breaking waves, waves that were just large enough to contribute to a capsize because in really big waves the surface layer is so much deeper that for all practical purposes the entire boat and keel experiences water speeds that are essentially the same.

The current thinking is that any detrimental effect from surface sheer of deep draft is easily offset by having the greater stability that can be achieved by a deeper keel. Another aspect that determined the impact of surface sheer is the aspect ratio of the keel as it pertains to the likelihood of a keel stalling at deep angles of attack.

To explain, when you have a deep draft keel that is short fore and aft, there is a tendency of that keel to stall as the water is moving closer to perpendicular to the keel. When the keel stalls, it generates smaller sideward resistance relative to its area. A shallow draft keel that is longer in length, (such as full keel, or even the Peterson style IOR keel mentioned above) has less of a tendency to stall. Normally a stalled keel would be a negative producing a lot of leeway, but in this case the tendency of modern deep fin keels to stall reduces the impact of surface sheer and so reduces the tendency for surface sheer to rapidly heel the boat. As a result, even if shallower than a modern fin with bulb, a full keel or lower aspect ratio keel could actually have greater tendency towards a surface sheer induced capsize.

Then there is the issue of roll moment of inertia. This is another one of those seemingly contradictory items. In theory, a boat with a higher roll moment of inertia is less prone to capsize due to wave action. Roll moment of inertia is the resistance due to the weight of the boat to accelerating the speed of speed of the boatís roll for any given roll impact. It is not the same as stability. Roll moment of Inertia is calculated as the amount of weight multiplied by its distance from the instantaneous Roll Axis (the imaginary axis about which the boat rolls at any angle of heel). In other words, a small weight that is a long distance from the roll axis (say at the top of the mast) would have the same impact on Roll Moment of Inertia as a very large weight that was closer to the roll axis (say in the keel). This would suggest that a heavy mast could reduce the chances of a boat capsizing. But this is not really the case.

To explain this it is important to understand the relationship of roll inertia to the motion of the boat on a wave. If you visualize a boat starting down the face of a wave from the crest, the force of gravity tries to pull the boat sideways and the keel tries to resist this sideward motion. The difference between these two forces, creates a rotational force (a moment) trying to heel the boat over. If we compare two boats, with equal rotational forces but one has much greater roll moment of inertia, the boat with a lot of rotational inertia will resist that rotation and so initially will not heel as fast as a boat with minimal roll inertia relative to the forces that are being imparted into the boat. This makes a big difference in short close waves, but in waves big enough to capsize a boat, as the two boats slide down the wave, at some point the heel angle of the two boats becomes very similar, and the boat with the larger roll moment of inertia has stored more kinetic energy, and that stored energy will become significant as the boats reach the trough of the wave.

As the boat reaches the trough, the angle of the wave face flattens out, and so the force of gravity lessens and so does the acceleration of the sideward speed of the boat. That slowing of the boats sideward speeds causes the boat to want to stand back up. A boat with a minimal roll inertia will stand up more quickly than a boat with a high roll moment of inertia. Here the high inertia of the boat causes it to continue to roll further past the point at which the roll moment forces are reversing. This can actually result in the boom dipping into the bottom of the trough, or worse yet, the mast dipping into the back of the next wave either of which are the equivalent of applying the brakes and forcing the boat over further, perhaps exceeding its limit of positive stability.

And here is where the location of the weight becomes critical. In the case of a boat that has a high roll moment of inertia that is the result of weight carried high in the boat (say a heavy mast or teak decks) the position of that weight, will be such that it is helping to lever the boat over further and therefore contributes to a capsize, even though its high roll moment of inertia may actually seem to reduce the likelihood of a capsize. On the other hand, if the high roll moment of inertia was the result of something carried low in the boat, say a heavy bulb on the end of a fin keel, then that weight would be attempting to right the boat and as such would somewhat mitigate the tendency to continue the roll.

To touch on a couple more points contained this discussion, while deep fins with bulbs may require more careful engineering than other forms of keels they do offer some major advantages from a stability, capsize resistance, and motion comfort standpoint relative to full keel or longer shoal keel designs. While some of the longer keel designs are easier to bolt to a boat, bear in mind that a boat is a system and that one of the key findings of the testing was that IOR era boats (which includes boats with the Peterson style keels described above), tended to have too high a vertical center of gravity and too much form stability and hull forms that promoted a rather uncomfortable motion. These boats also had a high roll moment of inertia but one that came with a high vertical center of gravity.

The other question was about the bilge keels used on Westerlys. I would first off disagree completely that Westerlys are seen as seaworthy designs. While they were seen at one time as reasonably good cruising boats, similar to CCA and IOR era boats which were also considered good cruising boats at one time, that time has long past.

As far as bilge keels are concerned, whether they contribute to a capsize or not depends very heavily on their design and execution as well as the design of the boat they are attached to.. As done on the Westerly I would think that they would tend to fall in the category of a low aspect ratio foil with a high roll moment inertia and a high vertical center of gravity, making them somewhere between neutral to perhaps being a liability in terms of contributing to a capsize.
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay

Last edited by Jeff_H; 01-03-2010 at 12:23 PM.
Reply With Quote Quick reply to this message Share with Facebook
  #7  
Old 12-30-2009
Senior Member
 
Join Date: Oct 2009
Location: Gloucester, MA
Posts: 586
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 6
klem is on a distinguished road
Jeff_H, that was a very good explanation of the subject. Much clearer than mine.
Reply With Quote Quick reply to this message Share with Facebook
  #8  
Old 12-30-2009
Stillraining's Avatar
Handsome devil
 
Join Date: Jan 2008
Location: LaConner,Washington
Posts: 3,477
Thanks: 0
Thanked 1 Time in 1 Post
Rep Power: 9
Stillraining is a jewel in the rough Stillraining is a jewel in the rough Stillraining is a jewel in the rough
You never cease to impress me Jeff..

Now lets through a monkey wrench into the equation...Lets take boats like mine which is a beamy to the end, modified shoal drafted center board..Is the board helping much at all with all of this?
__________________
"Go Simple...Go Large"

Relationships are everything to me..everything else in life are just tools to enhance them.


The purchase price of a boat is just the admittance fee to the dance...you still have to spend money on the girl...so court one with something going for her with pleasing and desirable character traits others desire as well... or you could find yourself in a disillusioned relationship contemplating an expensive divorce.
Reply With Quote Quick reply to this message Share with Facebook
  #9  
Old 12-30-2009
Stillraining's Avatar
Handsome devil
 
Join Date: Jan 2008
Location: LaConner,Washington
Posts: 3,477
Thanks: 0
Thanked 1 Time in 1 Post
Rep Power: 9
Stillraining is a jewel in the rough Stillraining is a jewel in the rough Stillraining is a jewel in the rough
Goofy program wound not let me edit my own post to add the picture..
Attached Thumbnails
What is the keel contribution in capsizing.-line-drawing-irwin-41.jpg  
__________________
"Go Simple...Go Large"

Relationships are everything to me..everything else in life are just tools to enhance them.


The purchase price of a boat is just the admittance fee to the dance...you still have to spend money on the girl...so court one with something going for her with pleasing and desirable character traits others desire as well... or you could find yourself in a disillusioned relationship contemplating an expensive divorce.
Reply With Quote Quick reply to this message Share with Facebook
  #10  
Old 12-31-2009
Banned
 
Join Date: Apr 2006
Location: MS Gulf Coast
Posts: 711
Thanks: 0
Thanked 3 Times in 3 Posts
Rep Power: 0
seabreeze_97 is on a distinguished road
One key discovery that wasn't mentioned in all this was that during the tank work after the Fastnet disaster, it was found that, contrary to opinions of the testers involved, the dismasted models were much easier, and more likely to capsize than models with intact rigs. Repeated testing revealed the importance of the counterbalancing effect of the mast. Seems like an obvious thing, but apparantly the dramatic loss in stability (much more than expected) came as a bit of a surprise during the tests.
Reply With Quote Quick reply to this message Share with Facebook
Reply

Quick Reply
Message:
Options

By choosing to post the reply above you agree to the rules you agreed to when joining Sailnet.
Click Here to view those rules.

Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




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
My First Boat...Boat Term Question... Kacper General Discussion (sailing related) 38 09-23-2008 07:52 PM
Wing, fin, or bulb...what are the trade offs? langousta Gear & Maintenance 17 09-17-2008 05:54 PM
reducing keel/adding fin? abacosol Gear & Maintenance 9 07-01-2007 10:32 PM
Keel Bolt Repairs Don Casey Gear and Maintenance Articles 0 11-10-2003 08:00 PM
Keel Bolt Concerns Don Casey Gear and Maintenance Articles 0 10-12-2003 09:00 PM


All times are GMT -4. The time now is 07:22 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.