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  #151  
Old 03-26-2012
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Re: Full or fin keel?

Quote:
Originally Posted by PCP View Post
That was a double hit, if the boat had a lot of inertia (a lot of weight for the same RM) and was still capsized when it was hit by the second wave, the story could be other. We can also see clearly the boat going sideways and rotating dissipating with movement the wave energy (look at the clouds) otherwise than with a rolling movement.

Regards

Paulo
If it was a properly designed rough water boat, it would have hardly heeled at all during either of the waves. Interestingly enough, rough water boats do not necessarily have high inertia. The predominant inertia from a sailboat invariably comes from its standing rigging. Not from its keel. We have to be careful not to confuse weight with inertia. Although no doubt, a light weight mast on a 10 Million dollar sail boat is bound to have less inertia than any cruising boat.

What you are seeing in the video is the racing boats return to vertical due to its incredibly high hull righting moment. Thats what you get with light weight and a wide beam. The way you pay for this incredible high RM is that the boat heels heavily in a wave as the video shows.

Here is a picture what a boat with only hull righting moment does on a wave. And then how a rough water boat deals with waves.

Your video shows exactly what I was describing earlier in this thread. That is a racing boat load of motivated sailors will hammer through rough seas to finish a race. A cruising couple will hunker down below till the storm is over.
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Attached Thumbnails
Full or fin keel?-racingboatonwave.jpg   Full or fin keel?-roughwaterboatonwave.jpg  

Last edited by BryceGTX; 03-26-2012 at 02:59 AM.
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  #152  
Old 03-26-2012
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Re: Full or fin keel?

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Originally Posted by PCP View Post
Outstanding the speed that this boats can return to its feet after being capsized, even with a lot of sail out That was a double hit, if the boat had a lot of inertia (a lot of weight for the same RM) and was still capsized when it was hit by the second wave, the story could be other. We can also see clearly the boat going sideways and rotating dissipating with movement the wave energy (look at the clouds) otherwise than with a rolling movement.

Regards

Paulo
Paulo, when I watch the video it appears there is a time edit between the first two waves, so there is no way to tell how long the interval is. Also, when I look at the clouds it looks like the boat is rounding up, I don't see the sideways motion.

Am I correct in thinking you mean that the narrow deep keel allows sideways slipping down the wave face, rather than tripping on a long keel with more surface area? I can see in my mind that if the wave stalls the boat, the deep fin will no longer be providing lift, and might allow side slipping, avoiding being as caught in the wave motion. I just don't see it in the video (where the heck is the camera? It seems to remain vertical when the boat is horizontal. Gimbaled?) I don't think that rounding up is a desirable trait.

This thread has been making me think! Comparing a "flat" bottom modern shape to a "round/V bottom" shape, the flat will tend to orient to the face of the wave it is on, thus quicker to roll and come back.

But that means greater rotational acceleration. That also means greater acceleration of the passenger's bodies and "guts", which means it's more tiring and less pleasant. Further, greater acceleration means that the tall rigging is under greater stress. Greater work hardening and metal fatigue means the continuously greater acceleration increases the risk of rigging failure. This could be another reason older style boats were round/V shaped, they had natural wood spars and natural fiber rigging, both heavier and weaker than today.

Of course, this is a racing boat with a large crew. If cruising one wouldn't have lots of sail up, and would possibly be riding out the storm rather than trying to make the most headway possible, unless near a lee shore.
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  #153  
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Re: Full or fin keel?

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Originally Posted by BryceGTX View Post
Here is a picture what a boat with only hull righting moment does on a wave. And then how a rough water boat deals with waves.
Bryce, I didn't see your last post, I'd just woken up (5:30 AM) and somehow missed that there was another page. You'd already discussed much of what I was thinking about when I woke up. Appreciate your insights.

Last edited by skygazer; 03-26-2012 at 07:33 PM.
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  #154  
Old 03-26-2012
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Re: Full or fin keel?

Jeff, I'd like to thank you for your reply, this is exactly what I wished for, reasoning based on physics and not on authority. I really appreciate you took the time.

Quote:
Originally Posted by Jeff_H View Post
the statement seems to imply that a boat with a full keel will out perform a fin keel boat in heavy conditions, and while that may be true for some fin keels vs. full keels, it is not a universally accurate statement.
I'm not a native English speaker and sometimes have trouble to clearly state what I want to tell. All I tried to say is that a fin keel may generate a lot more lift in calm water but that advantage is lost in heavy seas, making the full keel safer (not faster). I hope this at least partly is correct.
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  #155  
Old 03-26-2012
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Re: Full or fin keel?

Quote:
Originally Posted by BryceGTX View Post
You are correct that beam width is the best way to reduce heeling angle due to winds. Because it increases the hull righting moment. And I have said this earlier on this thread. However, excellent rough water boats are built with moderate beams with a more v-shaped hull design which actually reduces the effects of beam and such boats have significant mass in the keel.

....

Here is a blurb from a yacht design book from the 1800s that pretty much summarizes what we know even to this day. Vessels with wide beams are most affected by wave motion. I think this is pretty much common knowledge.
Bryce
It is really incredible that someone can believe that from 1800 to today nothing as been learned in what regards sailing boats , stability and particularly dynamic stability.

It is also hard to believe that someone that is obviously not a professional in sail design can believe that the best professional in that area, that design blue-water boats, don't know what they are doing and are designing today worst bluewater boats than what they were doing 30 years ago, not to mention 200 years ago

One of the most trusted bluewater boats are the Halberg-Rassy. Back in 1982 they did not use a full keel anymore and their 42ft had 3.78m beam.







Today its new boat, the smaller 415 has 4.11m of beam and use a modern bulbed fin keel.







And this is not an isolated case but the norm in what refers to modern bluewater cruising boats. Sure, you can find narrower boats still made today but almost in all cases are old designs that are still produced today and that don't reflect the state of the Art neither the actual knowledge in hydrodynamics and boat design.

Happily there are very few like you, I mean that believe that know more than the best boat designers on the market, otherwise bluewater brands like Najad, Malo, HR, Moody or Oyster would still be making the same type of boats they have made 30 or 40 years ago

Regards

Paulo

Last edited by PCP; 03-26-2012 at 08:15 AM.
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  #156  
Old 03-26-2012
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Re: Full or fin keel?

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Originally Posted by skygazer View Post
Paulo, when I watch the video it appears there is a time edit between the first two waves, so there is no way to tell how long the interval is. Also, when I look at the clouds it looks like the boat is rounding up, I don't see the sideways motion.

Am I correct in thinking you mean that the narrow deep keel allows sideways slipping down the wave face, rather than tripping on a long keel with more surface area? .. I don't think that rounding up is a desirable trait.

This thread has been making me think! Comparing a "flat" bottom modern shape to a "round/V bottom" shape, the flat will tend to orient to the face of the wave it is on, thus quicker to roll and come back.

....
It is not me that thinks that a a narrow deep foil keel allows a boat to move sideways or to rotate much easily than a full keel. I have learned from others including some very experience sailors and boat designers

They say that the boat was hit by a double breaker. This is the worst situation in what regards capsizing and one that leads many times to it.

Well, I can see clearly the boat moving, sideways and rotating. Any kinetic movement that uses the wave energy in any other away than on a rolling movement (induced by a big immersed area of a full keel) is welcomed and that includes rounding.

Regards

Paulo

Last edited by PCP; 03-26-2012 at 08:28 PM.
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  #157  
Old 03-26-2012
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Re: Full or fin keel?

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Originally Posted by Jeff_H View Post
it is easy to see that a modern fin keel boat could easily develop much higher dampening moments and so have better dampening than a full keel boat, making Pvajko statement incorrect that "The most important reason is that a full keel with its bigger surface area damps the rolling motion better."
BTW Jeff, doesn't this imply that Paolo's argument with full keel boats "tripping over the keel" vs. fin keels moving easily sideways is incorrect?

It appears that the two statements contradict each other. I know you are talking about high dampening moments but doesn't that mean also higher resistance sideways?
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  #158  
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Re: Full or fin keel?

Bryce, thanks for your input on a very interesting subject. You too Pvajko.
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  #159  
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Re: Full or fin keel?

Quote:
Originally Posted by skygazer View Post
Am I correct in thinking you mean that the narrow deep keel allows sideways slipping down the wave face, rather than tripping on a long keel with more surface area? I can see in my mind that if the wave stalls the boat, the deep fin will no longer be providing lift, and might allow side slipping, avoiding being as caught in the wave motion.
Quote:
Originally Posted by PCP View Post
It is not me that thinks that a a narrow deep foil keel allows a boat to move sideways or to rotate much easily than a full keel. I have learned fro others including some very experience sailors and boat designers

Regards

Paulo
Paulo, I did not express myself clearly enough. Just be glad I'm not trying to express myself in Portuguese!

I was not questioning whether the idea of sideslip is true, I was trying to ask if it seemed I correctly understood the idea of sideslip. It seems quite possible that it could be a real benefit. By the way, I'm weak in higher mathematics, so the real life experience of others is exactly what I'm interested in.

I also enjoy your passion for the newer boats. I have zero experience with the modern designs, so I need to hear from people who do have experience.

I don't think anyone believes that they are designing worse boats today. They may however be designing with different goals, for a different market. Part of what's good about this discussion is it attempts to describe the attributes of the differing designs, it is then up to each of us to chose the attributes that best fit how we desire to sail.

While they did not have computers until recently, they did have many thousands of years of trial and error to refine their designs and gain understanding. The sea never changes, so many principals remain the same. Today we have newer materials, which allows further exploration. A big thank you to all racers and designers who push the limits. Now we have to struggle to understand what these new designs mean for us, and how each may find their own balance point.
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  #160  
Old 03-26-2012
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Re: Full or fin keel?

When caught beam on to a large sea, a boat will try and slip sideways, downhill. The important part is can it do so easily or does the hull want to but the keel not want to.

As far as design changes over many decades, I think there are many more boats designed today that are designed to sail well in different conditions without being tweaked and distorted to meet a racing rule. There were certainly many distortions during the IOR rule years but remember many popular long keel cruisers were designed to a rule as well - a better rule but a rule that defined design type all the same. Most of the long keel designs that are popular now for offshore use were designed to race under the CCA rule - Albergs being a good example. Many of them were not intended by their designers to be sailed offshore either.
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