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Old 26-04-2007, 04:34   #1
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Just another dumbarse question.

In theory,would a yacht with a 12ft keel be less likely to roll than one with a 6ft keel.Given that they are the same design and the conditions were also.Mudnut.
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Old 26-04-2007, 06:13   #2
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Assuming that you mean draft...

As I understand it the distance of the centre of gravities from the fore and aft roll axis is one of the components of stability.

Normally (and please correct me if I am wrong) stability in a boat is calculated by the multiplying the weight of each component by the distance from the roll centre to get a moment.

Moments can be positive or negative.

As a boat starts to roll the water displaced exerts a positive moment (tending to move the boat back towards the upright position). THe weight of the keel on a mono has little effect at this point.

In a catamaran the initial positive righting moment is large as the distance of the displaced water from the roll axis is large (compared to a mono).

However as a boat continues to tilt it reaches a point where the righting moment from displaced water starts to diminish.

For a catamaran the negative moment of the mast and rigging now tends towards being larger than the positive moment from the displaced water and the boat tends towards being negativly stable.

For a mono the moment due to the weighted keel should now be larger than the negative moment of the mast, rigging etc.

If a yacht should become perfectly inverted then there is no moment to return it to the upright position. Narrow monos need only a small variation from this inverted position to become positivly stable again.

If we consider more and more beamy vessels the variation from the upright inverted position needed to acheve a positive righting moment becomes larger and larger to a point where it would normally not happen on many catamarans.

You can find a link to the stability diagram for a typical modern catamaran/trimaran (John Shutleworth) and a mono (Dudley Dix - Didi 26).

Note that the catamaran/trimaran becomes negativly stable at 85 degrees while the mono becomes negativly stable at 120 degrees.
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Old 26-04-2007, 11:03   #3
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Yes, in theory.

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Old 26-04-2007, 11:14   #4
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I agree with the "in theory, yes" comment, but have an add on question/comment:

Isn't it true that a shallow draft vessel, in the condition of being hit broadside by a breaking wave, will be less likely than a deep draft vessel to capsize in the first place? (assuming a narrow beam?)

If I picture Mudnut's hypothetical boat in that situation, I see the 12ft draft as a liability, tipping the boat at a further angle than a boat with a 6ft draft as both slide down the imaginary face of the breaking wave (sideways).
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Old 26-04-2007, 12:29   #5
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Sean, that kinda depends as well. It depends on the way th wave hits you. Firstly you need to think about what the rudder is doing in regards to "sideway's" momentum. Now is the boat falling down the side of the wave or is the wave moving under the boat. This places energy against the side of the rudder and depending on which side that nergy is placed, will either help with stability or deter it. Then add into the design picture, the rigging. this is the other half of the pedulum. The Mass of the rigging can act against the movement or with it. Once again, it depends on wave, how it hits you and many other circumstances. The depth of keel is most often part and parcel to the design of the rigging.
The other factor in design is the weight of bulb and slenderness of keel. Slender "stilletto" keels are often long because they have a set of race design parameters they are trying to meet. So they often have taller Riggs for speed, sleek slender keels for less water friction and a bulb way down to make the boat stand upright with the big rig it has. these designs also tend to have "planning hull" type designs to make the best of speed.
IMO, they tend to be the lest comfortable for cruising, but thats my opinion.
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Old 26-04-2007, 12:53   #6
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Quote:
In theory,would a yacht with a 12ft keel be less likely to roll than one with a 6ft keel.Given that they are the same design and the conditions were also.
This is even less significant than capsize ratio (i'm not a big proponenet of it myself) and we have a long thread on that already that you could examine on your own. The truth is that deeper draft boats are designed with that as part of the design in mind just as shallow draft boats have that part of that design in mind. No design would stay the same with such a dramatic difference in draft so the two boats would not be the same. The idea that they could be the same is only half as much information as capsize screening ratio would accomodate in it's value.
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Old 26-04-2007, 17:51   #7
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Originally Posted by Pblais
The truth is that deeper draft boats are designed with that as part of the design in mind just as shallow draft boats have that part of that design in mind. No design would stay the same with such a dramatic difference in draft so the two boats would not be the same.
Paul makes a good point here. Of course nobody would design a deeper draft boat with the everything else identical ignoring that factor.

Such a comlicated subject. I read on in from time to time and find that each person seems to have their own answers based on their own hypothetical "thought experiments" (to steal an old Physics term)
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Old 26-04-2007, 19:54   #8
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I read on in from time to time and find that each person seems to have their own answers based on their own hypothetical "thought experiments"
I would agree. Each type of boat may have a specific problem in mind. Shoal draft boats have a specific issue - shallow water. A 12 ft draft is not about coastal waters and could be a PITA in many parts of the world (like where I live).
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Old 27-04-2007, 02:03   #9
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Quote:
Originally Posted by Pblais
This is even less significant than capsize ratio (i'm not a big proponenet of it myself) and we have a long thread on that already that you could examine on your own. The truth is that deeper draft boats are designed with that as part of the design in mind just as shallow draft boats have that part of that design in mind. No design would stay the same with such a dramatic difference in draft so the two boats would not be the same. The idea that they could be the same is only half as much information as capsize screening ratio would accomodate in it's value.
That sets me straight Paul.The two boats couldn't be identical.Hey,I said it was a dumbarse question.I might have went overboard on the 12'/6' comparison.I have seen the same boat's for sale with different draughts.Yes,overall dimensions exact.Thats what sparked my D/Arse question.Albeit they were a bit different on deck.Thanks Mudnut.
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Old 27-04-2007, 05:07   #10
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[FONT='Calibri','sans-serif']I have seen the same boat's for sale with different draughts.[/FONT]
This is a market driven factor and boat manufacturers have to sell to that issue. The deeper draft version will perform better in most all cases when this happens. What effect it has on the stability is more complex than just the difference in draft. Some boats do change the ballast and keel design when making the two versions.

Some others made only deep draft versions and just sawed off the bottom. Others just bolt on a different keel. Our CSY 33 is one. It is a deep draft version (5-3) but to make it into a shoal draft you cut along the dotted line on the encapsulated keel and glass over the bottom. You lose pointing ability but you get into waters that are shallow.
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