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  #521  
Old 10-03-2012
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Re: Full or fin keel?

Quote:
Originally Posted by PCP View Post
I hope you start reading more recent books, you need it. XXI century sailboats boats have little in common with XIX century boats

Regards

Paulo

In a "more recent' book, I read that fore and aft rigs were preferred for coastal sailing because they were more weatherly. But I was surprised to read that square sails were actually preferred for offshore work - faster and far more comfortable on long fair wind passages, without the danger of a gybe. Further, in light winds with large seas the fore and aft sails tended to slam from side to side. The schooner Kineo used up 14 sails on one long passage in 1905.

I hadn't realized that square sails were ever preferred, I thought fore and aft were far superior in all ways. That helps explain the mystery (to me) of ships with fore and aft mainsails and square topsails.
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  #522  
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Re: Full or fin keel?

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Originally Posted by PCP View Post
A sailboat is not a motorboat that bobs around in waves and in bad weather will be strongly “tied” to a side by the force of wind on its sails. You don’t need a book to know this; you have only to experience it.

Regards

Paulo
A typical powerboat is considerably stiffer than a sailboat of similar size. And that is exactly why they are worse on rough water than a good rough water sail boat. Again, common knowledge.

Just so everyone knows, the stiffness of a boat is invariably determined primarly by the hull. A wider hull invariably creates a stiffer boat. Adding weight adds some stiffness, but nothing to that which can be had by modifying the hull. A wide flat power boat creates exactly such a stiff boat. And given that it carries its width longer than a sailboat, it inherently has more stiffness. The power boat has the problem that its CG is invariably above the waterline. This causes the CG to actually create a heeling moment. Which explains why some power boats can appear less stiff.

The stiffest sailboats are catamarans. They carry the beam to extreme resulting in a huge hull moment. And as we know, cats do not heel like monohulls. As a result can be very fast.

You seem to be confusing performance with rough water capability and those with experience know that rough water boats are different from performance boats. As far as I am concerned, this is widely held common knowledge.

The force of the wind on the sail has nothing to do with stiffness of the hull. I suggest you pick up a book and convince yourself. That is not to say a sail does not help stabilize the boat.
Bryce

Last edited by BryceGTX; 10-03-2012 at 11:06 PM.
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  #523  
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Re: Full or fin keel?

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Originally Posted by PCP View Post
A stiffer boat will have more stability and a superior RM than a tender boat,
Paulo
I have also pointed out that the use of a stability diagram on a boat in a wave results in totally the wrong conclusion.

Keep in mind that the values in the stability diagram are the related to the bouyancy type force and the weight acting through the center of gravity. The force acting through the center of gravity always acts vertically. But the bouyancy force acts perpindicular to the water.

As long as the water is horizonal, the effective bouyancy force acts vertically and the weight vector acts vertically. In this case, the righting moment diagram is correct. However, when you place the boat on a wave, the effective bouyancy force acts at an angle equal to the wave angle. But the weight still acts vertically. This dramatically reduces the righting moment. There are additional secondary effects that further reduce the righting moment.

Furthermore, when we consider the dynamic effects, the axis of rotation is no longer at the metacenter. This has profound effects on how the boat heels. Some people want to analyze the dynamics as if the dynamic axis is at the metacenter. This is a severe error, particularly as the boat goes onto a wave. In the most extreme case, the dynamic center falls below the center of gravity.

So basically, we need to throw the stability diagram and metacenter out the window when we have dynamics or when we place the boat on waves. On the other hand that is why they call it the STATIC stability diagram.
Bryce

Last edited by BryceGTX; 10-03-2012 at 11:14 PM.
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  #524  
Old 10-04-2012
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Re: Full or fin keel?

Quote:
Originally Posted by BryceGTX View Post
I have also pointed out that the use of a stability diagram on a boat in a wave results in totally the wrong conclusion.

Keep in mind that the values in the stability diagram are the related to the bouyancy type force and the weight acting through the center of gravity. The force acting through the center of gravity always acts vertically. But the bouyancy force acts perpindicular to the water.

As long as the water is horizonal, the effective bouyancy force acts vertically and the weight vector acts vertically. In this case, the righting moment diagram is correct. However, when you place the boat on a wave, the effective bouyancy force acts at an angle equal to the wave angle. But the weight still acts vertically. This dramatically reduces the righting moment. There are additional secondary effects that further reduce the righting moment.

Furthermore, when we consider the dynamic effects, the axis of rotation is no longer at the metacenter. This has profound effects on how the boat heels. Some people want to analyze the dynamics as if the dynamic axis is at the metacenter. This is a severe error, particularly as the boat goes onto a wave. In the most extreme case, the dynamic center falls below the center of gravity.

So basically, we need to throw the stability diagram and metacenter out the window when we have dynamics or when we place the boat on waves. On the other hand that is why they call it the STATIC stability diagram.
Bryce
Well, you are wrong and it is not needed any complicated explanation to show that. Your explanations should adapt to reality and not the other way around.

Just have a look at two boats, with similar types of hulls, about the same size, one with 4.5T and a substantial part of its weight down on a bulb at the end of a 3m draft, the other one with 9T with much more proportion of weight in its hull and 1.55m of draft. Both are well designed boats the total RM would not be very different (due to the much bigger mass of the heavier boat) and the lighter boat will be massively stiffer.

We are talking here about a 40 class racer and about an Oceanis 41. Go to an experienced sailor that know both boats and tell him that the Oceanis is more seaworthy because it is a tender boat with a bigger mass and the guy will start to laugh. When he understands that you are not kidding he can go away shaking his head or if he his a patient guy will try to explain to you that the 40 class racer was designed to race solo on the Ocean on the worst conditions, that several, while circumnavigating higher than the furious 40ís were caught in big storms and that all survived with flying colors. He may also say to you that the Oceanis is not designed to be sailed on high latitudes and that taking one there would not be a very smart thing to do.

He could also point out that even if the 40class boat is a racing boat, because the boat is fast and has a huge safety margin, older boats that are not competitive anymore are being bought by cruisers to circumnavigate with their families and that in this moment there are several doing that without any problem.

Regards

Paulo
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  #525  
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Re: Full or fin keel?

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Originally Posted by PCP View Post
We are talking here about a 40 class racer and about an Oceanis 41. Go to an experienced sailor that know both boats and tell him that the Oceanis is more seaworthy because it is a tender boat with a bigger mass and the guy will start to laugh. Regards

Paulo
LOL.. I will laugh also. I never said a tender boat is more seaworthy. You inferred it. I said that a stiff boat was not Steady. These are radically different concepts.

A steady boat has a large mass moment and less stiffness than a stiff race boat. It is exactly the huge mass and lower stiffness that makes it steady. On the other hand the simple example you pointed out shows that a more tender boat is not necessarily seaworthy nor steady. On the other hand, a steady boat is more tender than a stiff boat. You confused that all tender boats are also steady.

On the other hand, not sure I would call the Oceanis tender. It just is not as stiff as a racer. And It is not a rough water boat as I have pointed out in the past. And in no shape or form is a stiff racer a rough water boat.

But its good that you pointed this out. Others may have also missed the point.
Bryce

Last edited by BryceGTX; 10-04-2012 at 09:20 PM.
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  #526  
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Re: Full or fin keel?

Quote:
Originally Posted by BryceGTX View Post
LOL.. I will laugh also. I never said a tender boat is more seaworthy. You inferred it. I said that a stiff boat was not Steady. These are radically different concepts.

....
But its good that you pointed this out. Others may have also missed the point.
Bryce

Well your logic seems not right to me. You said:



Quote:
Originally Posted by BryceGTX View Post
...

"... A steady ship on the contrary when exposed to the action of waves keeps nearly upright. the stiffest ships are the least steady."

.... the characteristics of a rough water boat is it reamains vertical in the presence of waves. This is the characteristic of the IP.
Bryce
You have said that a rough water boat should remains vertical in the presence of waves, that a steady ship when exposed to the action of waves keeps nearly upright and that a stiffest ship are the least steady.

So by your words stiffness is prejudicial to a boat regarding to remain vertical when exposed to the action of the waves (steady). You say also that the characteristics of a rough water boat is to remain vertical in the presence of waves. Therefore, according to you, as the stiffness of a boat increases it decreases its capacity to stay vertical in the presence of waves and that is the characteristics that you say that make the boat a rough water boat. So a stiffer boat will be a worse rough water boat.

It seems clear that the main quality of a rough water boat is its seaworthiness.

I never said that the Oceanis is a tender boat I said that the 40class racer is massively more stiff and by your reasoning, since both boats share the same kind of hull, the Class40 should be less of a rough water boat (less seaworthy) than the Oceanis since the" stiffest ships are the least steady" and are, according to you ,the worst in what regards to stay vertical (steady) under the action of waves, the condition that you find more important to a rough water boat.

Acording with what you say, the Oceanis, that is a lot less stiff, should be a better rough water boat than the 40class racer. It is not, not even close, there is a huge difference. The Class40 is a true bluewater boat capable of sailing in high latitudes and really heavy weather and that is not the case with the Oceanis. Therefore all your reasoning is wrong.

Regards

Paulo

Last edited by PCP; 10-04-2012 at 11:00 PM.
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  #527  
Old 10-05-2012
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Re: Full or fin keel?

Quote:
Originally Posted by PCP View Post
Just have a look at two boats, with similar types of hulls, about the same size, one with 4.5T and a substantial part of its weight down on a bulb at the end of a 3m draft, the other one with 9T with much more proportion of weight in its hull and 1.55m of draft. Both are well designed boats the total RM would not be very different (due to the much bigger mass of the heavier boat) and the lighter boat will be massively stiffer.
Quote:
Originally Posted by PCP View Post
40class racer is massively more stiff

Acording with what you say, the Oceanis, that is a lot less stiff, should be a better rough water boat than the 40class racer. It is not, not even close, there is a huge difference. The Class40 is a true bluewater boat capable of sailing in high latitudes and really heavy weather and that is not the case with the Oceanis. Therefore all your reasoning is wrong.

Paulo
Ok.. lets see if you can back up your statements. The first place to start is to look at the stiffness. Since you say one is more stiff than the other.

What is the stiffness of each boat?
Bryce

Last edited by BryceGTX; 10-05-2012 at 05:22 AM.
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  #528  
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Re: Full or fin keel?

Quote:
Originally Posted by BryceGTX View Post
Ok.. lets see if you can back up your statements. The first place to start is to look at the stiffness. Since you say one is more stiff than the other.

What is the stiffness of each boat?
Bryce
I say that one is more stiff than the other? You have any doubt?

The stifness of a boat is directly related with the boat displacement and the sail area the boat can carry (SA/D). You can see that big difference yourself. The data on both boats is on the net.

I appreciate your effort to understand and relate seaworthiness and stability with mathematic formulas but on your efforts you start from the wrong basis:

Instead of assuming that boat design did not evolve in the last 50 years, as well as the way seaworthiness is obtained in new designs, and try to prove that old designs are the only way to obtain a seaworthy boat you should try to understand how modern NA have obtained seaworthy boats with different design criteria than the one that was used 50 years ago.

There are several ways of obtaining a seaworthy boat and mass and a relatively high CG is one of them, unfortunately one that gives slow sailboats. There are other ways, like the ones (based on static and dynamic stability) that permits a beamy low mass boat with a very deep CG, like a class40 racer, to be not only fast but very seaworthy and that's a fact.

You should try to understand how that massif seaworthiness (that is a fact) is obtained in a boat that contradicts the ways it was obtained 50 years ago instead of, against the reality, assuming that boat cannot be seaworthy because it is not designed accordingly with the principles that 50 years ago were used to design seaworthy boats.

Regards

Paulo
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  #529  
Old 10-05-2012
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Re: Full or fin keel?

Paulo you are right designers have long since abandoned full keel designs when it comes to production boats that require cheap quick building. They also abandoned the stay sail and bow sprite. I don't know how many of you have been paying attention but they both are back as high performance upgrades. It would appear the older designers got it right the first time. I have owned many fin and full keel boats, after 25 years of sailing I find I sleep better in a full keel than a fin keel so that's my preference. A good friend just lost his half million dollar fin keel to a submerged deadhead, his boat went down in under 3 minutes. If you want to know the difference in strength between full and fin keels, don't ask the designer, don't ask the builder, don.t ask the sailor. Ask the boat yard manager who has to do all the repairs on the boats, or at least the ones that didn't sink.
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  #530  
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Re: Full or fin keel?

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Originally Posted by barefootnavigator View Post
Paulo you are right designers have long since abandoned full keel designs when it comes to production boats that require cheap quick building. ... It would appear the older designers got it right the first time. .. A good friend just lost his half million dollar fin keel to a submerged deadhead, his boat went down in under 3 minutes. If you want to know the difference in strength between full and fin keels, don't ask the designer, don't ask the builder, don.t ask the sailor. Ask the boat yard manager who has to do all the repairs on the boats, or at least the ones that didn't sink.
I agree with you that is possible to build a full keel boat stronger than a fin keel boat the same way that is not possible to build a full keel boat that is so efficient sailing as a fin keel boat (both boats being well designed).

The question here is if the strength we can achieve with a fin keel boat is sufficient to make a safe sailboat (and if so we can have better sailing boats) or if fin keel boats are dangerous.

The huge number of fin keel boats without problems and the very low percentage of problems show that modern materials and building techniques can provide safe fin keel boats.

Regarding being cheaper and quicker to build, as Jeff has pointed already here you got it wrong, it is more complicated and expensive to build a fin keel boat than a full keel boat. The structure that has to be built to transmit the efforts to the hull is very expensive to built.

Saying this I can understand that a very small minority prefers to have (new) a full keel boat instead of a fin keel boat the same way that I accept that a guy prefer to drive a big truck instead of a saloon because it is safer.

Regarding the used market I can easily understand that some full keel boats are so strong that can, after some tens of years, offer a better warranty of solidity than a fin keel boat with the same age, but each case is a case.

Regards

Paulo
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Last edited by PCP; 10-05-2012 at 06:43 PM.
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