# What is the keel contribution in capsizing.



## shaile (Dec 29, 2009)

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


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## klem (Oct 16, 2009)

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.


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## shaile (Dec 29, 2009)

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 ?


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## SailorNate (Sep 19, 2009)

shaile said:


> 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


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## Dirtboy (Jul 13, 2009)

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


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## Jeff_H (Feb 26, 2000)

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 boat's 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.


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## klem (Oct 16, 2009)

Jeff_H, that was a very good explanation of the subject. Much clearer than mine.


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## Stillraining (Jan 11, 2008)

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?


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## Stillraining (Jan 11, 2008)

Goofy program wound not let me edit my own post to add the picture..


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## seabreeze_97 (Apr 30, 2006)

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.


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## Jeff_H (Feb 26, 2000)

Stillraining: 

Your question about using the centerboard is a good one. Its is not a cut and dry situation. In short chop with say a wave height less than your beam, I would imagine that your centerboard in the down position would provide quite a bit of roll dampening and so soften the motion by reducing both roll accellerations and roll angle. 

But at some point, as the waves start approaching 2 times your beam, surface sheer begins to become a significant factor and at that point, it would probably make sense to retract the board. I would add that I say this based on my assumption and recollection that the board on your boat is not all that heavy and so adds little to the overall stability of the boat and so the deeper ballasst position would not offset the impact of surface sheer on its projected greater depth. 

One of the reasons that I generally like keel centerboarders for offshore use is that you can use the board to balance the boat better when hove-to and so in really large waves and high winds, when hove to, you may want a small amount of CB to balance the boat and reduce leeway. 

Jeff


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## cormeum (Aug 17, 2009)

seabreeze_97 said:


> 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.


There are two issues here: resistance to capsize and capsize recovery.

As a first iteration, it is better to have a high roll mement of inertia as that will provide resistance to the wave impact ( which is a short term impulse) and help (maybe not enough)the boat to resist capsizing in the first place. The rolll moment of inertia is a measure of resistance to accelerations in the presence of a force acting on the moment arm.The farther this weight is from the roll axis the better. The two main suppliers are the keel ( lot of mass, but not that far from the roll axis) and the mast ( not so much mass but very far from the roll axis). Here, the mast usually dominates.

The second is recovery- for this a narrower beam and a deeper keel (We're fixing the mass here) reduces the angle where the boat will be "stable" while inverted. The trade off is that by losing form stability provided by a wider beam, you also lose the initial stability that lets you sail "flat" in normal conditions. Having a higher roll moment doesn't affect the angle at which the hull will go unstable and right itself, just the rate of acceleration of the roll.


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## johnshasteen (Aug 9, 2002)

Another way of looking at this is, if you didn't have a keel, you would capsize upon leaving the slip


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## Stillraining (Jan 11, 2008)

Jeff_H said:


> Stillraining:
> 
> Your question about using the centerboard is a good one. Its is not a cut and dry situation. In short chop with say a wave height less than your beam, I would imagine that your centerboard in the down position would provide quite a bit of roll dampening and so soften the motion by reducing both roll accellerations and roll angle.
> 
> ...


Thanks Jeff and you are correct ..about 600 lbs is all. so only about 8% of my total ballast..one reason not to worry if it ever gets stuck up , damaged or broke off completely..It was actually stuck up the day we were out in 47 knots..Im sure it would have helped with leeway but until I duplicate the event I wont know how much 

FWIW it does extend to vertical..I don't know why the drawing does not show that.


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## shaile (Dec 29, 2009)

Thank you, especially Jeff, for the comprehensive answer, it is real instructive.
Is there a quantitative measure of stability that takes these considerations into acount ? like the following measures: Estimating Stability
In the above article I havn't seen any reference to the keel's shape.


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## PCP (Dec 1, 2004)

seabreeze_97 said:


> 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.


Sometimes we tend to forget old empirical knowledge:On severe storms, Old sailors used to hoist weight to the top of the masts, to prevent dangerous rolling motion on their several mast sailing ship.



Stillraining said:


> 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?


Not wanting to take any of the credit of what has been said on the excellent post from Jeff and the one from Klem, I would like to contribute with two comments that come handy, one from one of the greatest sailors : Eric Tabarly (he had extensively sailed everything, from heavy to light monohulls, multihull and classic sailing boats, and he love them all); the other by Jacques Martens, a boat designer.

Tabarly says, with the assurance of heaving experienced extensively both kind of boats in any sea condition:

"The boats that have the worst behavior when lying ahull are the ones with heavy displacement and a long keel, especially when the keel is not heavily ballasted. The long keel and the deep of the underwater hull offer a big resistance to lateral displacement&#8230;then the waves that break against the boat push the superstructure and as the underwater offers a big resistance, capsizing can occur."
The ideal boat to lying ahull is a light displacement with a finn keel with a good form stability. This kind of boat offers little resistance to lateral displacement and moves fast sideways&#8230;protecting the boat from breaking waves&#8230;"

Jacques Martens says about centerboarders:

"The seaworthiness may come as a surprise to beginners or to those who have never sailed a shallow draft boat but experienced seamen know that the shallow draft boat is more seaworthy than a hull with a deep keel. 
Unlike the deep boat, a centerboarder will not resist the sea. Instead of tripping on it's keel, it will dodge the waves or lift over them. Properly designed, a keel centerboarder will have the same ultimate stability than a boat with a conventional keel. &#8230;"



SailorNate said:


> Sure there are lots of compromises. .... 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


Yes, a sailboat is always a lot of compromises, but I would not recommend anybody to the thread about "Blue water boats" unless that person is interested only in 30 or 20 year old boats. The best modern blue water middle sized production boat doesn't even appear on that list, boats like the x45; x42 cruising boats, or the Swan 46, not to mention the traditional French passagemaker that is typically an aluminum centerboarder. They include the Garcia, but forget about all the others.
A thread about Blue water boats should be about the compromises we are talking about, trying to understand them and what they mean in the boat behavior. Has it is it's (in my opinion) a very confusing thread.

Regards

Paulo


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## KeelHaulin (Mar 7, 2006)

Jeff_H said:


> 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.


Jeff; could you cite some references from which you arrived at the conclusions in your article? In the term "surface shear" are you referring to water completely below the waterline of the boat; or at the bluewater/whitewater interface where the breaking wave has two distinctly opposite flow directions?


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## deniseO30 (Nov 27, 2006)

Shailie, I can't help but wonder, do you have a boat that gets you concerned about a capsize?


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## shaile (Dec 29, 2009)

If I had a boat it would have been a bit too late to ask these questions.
I am planning on purchasing a boat for off shore cruising and my main concern (except that it would sail) is safety.


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## cormeum (Aug 17, 2009)

PCP said:


> Sometimes we tend to forget old empirical knowledge:On severe storms, Old sailors used to hoist weight to the top of the masts, to prevent dangerous rolling motion on their several mast sailing ship.
> 
> Not wanting to take any of the credit of what has been said on the excellent post from Jeff and the one from Klem, I would like to contribute with two comments that come handy, one from one of the greatest sailors : Eric Tabarly (he had extensively sailed everything, from heavy to light monohulls, multihull and classic sailing boats, and he love them all); the other by Jacques Martens, a boat designer.
> 
> ...


Regarding Tabarly and Martens please note that they are talking about lying ahull- Not the best tactic with a full keel boat. Storm tactics are driven, in part, by design. So not all tactics work well be all boats. 
The issue here is that for a boat to have sufficient stability to sail, while having minimal weight in a keel (e.g. a centerboarder) the physics of the situation necessitates a wide beam. So while the beamy centerboarder may be _more likely_ to side slip and not be capsized, if it is it will be less likely to recover. The Fastnet disaster highlighted the issues with excessive beam ( as well as a having a high metacenter)


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## AdamLein (Nov 6, 2007)

Jeff, as usual your post is fascinating and well-reasoned, but unless I'm missing something, I'm not convinced about one aspect here:



Jeff_H said:


> 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.


In the absence of external forces, the slowing of the boat's sideways motion would not cause the boat to stand back up for any reason I could see. The two external forces involved here are form stability and righting moment. Differences in form stability aside, since inertial mass = gravitational mass, a boat with greater roll inertia should have a proportionally greater righting moment, and therefore should take the same time to right itself.

Looked at another way, a boat's keel is essentially a pendulum, and we all know that the period of oscillation of a pendulum is independent of the mass (and mass distribution).

Your discussion of keel stalling however is really interesting, and while I had a vague intuition about that effect I had never thought about it in such precise terms before. I will have to read a lot more about surface shear and the tendency of different keels to stall... one thing that seems missing to me is that even assuming a high aspect ration keel is indeed less likely to stall, a lateral force deep on the keel has a proportionally larger effect than the same lateral force on a shallower keel.

Thanks again!


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## PCP (Dec 1, 2004)

AdamLein said:


> The two external forces involved here are form stability and righting moment. Differences in form stability aside, since inertial mass = gravitational mass, a boat with greater roll inertia should have a proportionally greater righting moment, and therefore should take the same time to right itself.


My phisics are little rusty but I think RM = GZ x Boat Mass

Beamier boats have not only a bigger form stability but also a bigger GZ. That bigger GZ is also much increased by the use of a deep keel with a bulb.

For having the same RM than the beamier boat with a deep fin with a bulb (much bigger GZ), a long keeler with moderate beam and draft would have to compensate with a much bigger Mass. Therefore it would have a lot more inertia.

Regards

Paulo


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## PCP (Dec 1, 2004)

cormeum said:


> Regarding Tabarly and Martens please note that they are talking about lying ahull- Not the best tactic with a full keel boat. Storm tactics are driven, in part, by design. So not all tactics work well be all boats.
> 
> The issue here is that for a boat to have sufficient stability to sail, while having minimal weight in a keel (e.g. a centerboarder) the physics of the situation necessitates a wide beam. *So while the beamy centerboarder may be more likely to side slip and not be capsized, if it is it will be less likely to recover*.


We are just talking about keel contribution in capsizing. Not about a specific boat design or seaworthiness. The two comments that I had quoted seemed to address the problem (contributions of different keel designs to a capsize).

I am not defending centerboarders over other kind of boats, but I believe that you have not fully understood what Martens is saying. He says:
Properly designed, *a keel centerboarder will have the same ultimate stability than a boat with a conventional keel*" and "Unlike the deep boat, a centerboarder will not resist the sea, Instead of tripping on it's keel, it will dodge the waves or lift over them" therefore "the shallow draft boat is more seaworthy than a hull with a deep keel".

For ultimate stability he means final stability, the one that is useless to sail and has to do only with safety. What he is saying is that a centerboarder with the same stability (RM curve) of a long keeler is a safer boat.

If you take a look at the RM curve of a Southerly you will understand what Mertens mean: You will see a centerboarder with a good AVS and a relatively small inverted stability.

s/y Troldand - Stability

You say that a centerboarder has to be beamier to compensate having minimal height in a keel, but that is not what happens on the boats that are on the market.

Modern centerboarders are not beamier than other modern cruisers, but because they have almost all the ballast on the bottom of the boat, for having the same RM (to compensate), they have to carry more ballast. They are heavier than other modern cruisers and as they have the same RM they cannot carry more sail and so, they are slower boats.

Regards

Paulo


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## AdamLein (Nov 6, 2007)

PCP: I guess I was talking specifically about righting moment due to ballast, not righting moment due to buoyancy. I'm not exactly sure what GZ refers to, however... something about center of mass perhaps.

You're right of course that a deep keel would require less mass for the same righting moment, and that a long shallow keel would require larger mass. But for exactly the same reasons, a deep keel with a given ballast M concentrated at the bottom will have a larger rotational inertia than a boat with the same ballast M at a shallower depth; that was my point. 

Algebraically, rotational acceleration is righting moment divided by rotational inertia. If we assume the ballast is concentrated in a point at depth D, Righting moment and rotational inertia are M*g*D*sin(heel) and M*D, respectively, so rotational acceleration due to ballast is dependent only on gravity and the angle of heel and the force of gravity, not the amount or depth of ballast. If we drop the assumption of single-point ballast the argument is the same, but with integrals.


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## dillybar (Nov 10, 2009)

Interesting topic. I suppose this is why I've read that it is preferable to hove to with a para anchor at 45 deg to weather? Presumably this helps balance the lateral forces above and below the waterline?


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## PCP (Dec 1, 2004)

AdamLein said:


> .. I'm not exactly sure what GZ refers to, however... something about center of mass perhaps.


Boat stability curves normally are GZ curves or RM curves, GZ is the lenght of the arm, RM are the rightening moments (GZ * Mass = RM).



AdamLein said:


> ...Algebraically, rotational acceleration is righting moment divided by rotational inertia. If we assume the ballast is concentrated in a point at depth D, Righting moment and rotational inertia are M*g*D*sin(heel) and M*D, respectively, so rotational acceleration due to ballast is dependent only on gravity and the angle of heel and the force of gravity, not the amount or depth of ballast.


You mean that if two boats have the same RM at all points of heel but one of them has 4 times more mass, they will have the same rotational acceleration?

Regards

Paulo


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## Stillraining (Jan 11, 2008)

PCP said:


> Sometimes we tend to forget old empirical knowledge:On severe storms, Old sailors used to hoist weight to the top of the masts, to prevent dangerous rolling motion on their several mast sailing ship.
> 
> Not wanting to take any of the credit of what has been said on the excellent post from Jeff and the one from Klem, I would like to contribute with two comments that come handy, one from one of the greatest sailors : Eric Tabarly (he had extensively sailed everything, from heavy to light monohulls, multihull and classic sailing boats, and he love them all); the other by Jacques Martens, a boat designer.
> 
> ...


Makes a lot of common sense to me...but my common sense has been proved wrong once or twice..


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## deniseO30 (Nov 27, 2006)

geeze, I hope einstein doesn't ck in here.


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## shaile (Dec 29, 2009)

To summarize of what we had until now, there are some requirements of the keel and ballast:
1) increase moment of inertia of the boat 
2) provide a righting moment when heeling
3) provide lateral resistance when sailing close hauled
4) cause small lateral resistance when pushed sidewards by a breaking wave.

The solution for 1 and 2 is long arm with high ballast
Requirements 3 and 4 are contradictious and I think their solution is high aspect ratio keel.
So the solution (with standard keel !) for all four requirements is long high aspect ratio fin keel with high ballast at the tip.
Is this conclusion right ?
And there may be some non conventional configurations.

By the way, AdamLein, the moment of inertia I = M*D^2.


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## shaile (Dec 29, 2009)

Sorry, I wrote 'long' instead of 'deep'.
So the solution (with standard keel !) for all four requirements is deep high aspect ratio fin keel with high ballast at the tip.


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## AdamLein (Nov 6, 2007)

shaile said:


> By the way, AdamLein, the moment of inertia I = M*D^2.


Yeah I just double-checked that... I always forget


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## PCP (Dec 1, 2004)

shaile said:


> To summarize of what we had until now, there are some requirements of the keel and ballast:
> 1) increase moment of inertia of the boat
> 2) provide a righting moment when heeling
> 3) provide lateral resistance when sailing close hauled
> ...


That's why racing boats have that keel configuration.

But for cruising boats there are more things to consider:

A deep keel is unpractical for cruising.

A thine deep keel with a bulb generates a lot of stress on the hull and on the superior part of the keel. That means expensive hight tech materials and expensive structural reinforcements on the hull, or even an expensive steel supporting structure. This means an expensive boat.

That kind of keel allow a light fast and seaworthy boat, but the kind of sea motion that comes with it is not comfortable to all. People that suffers from seasickness don't appreciate it.


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## seabreeze_97 (Apr 30, 2006)

Originally Posted by Jeff_H View Post
As the boat reaches the trough, the angle of the wave face flattens out, and so the force of gravity lessens.......

Anti-gravity? That must be some keel!


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## Jeff_H (Feb 26, 2000)

_*"As the boat reaches the trough, the angle of the wave face flattens out, and so the force of gravity lessens" *_

That sentence has drawn a bit of fire. I apologize, it's a case of type in haste, repent at leisure.

That sentence should read _"As the boat reaches the trough, the angle of the wave face flattens out, and so the *side* force of gravity lessens and so does the sideward acceleration of the boat." _

Jeff


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