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  #231  
Old 03-31-2012
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Re: Full or fin keel?

Quote:
Originally Posted by mitiempo View Post
Wind alone will not capsize a boat. It will knock it down but when the boat is at around 90 degrees the keel will produce a large righting force and the boat will not go over any farther. It takes waves to pass that point.
And the effective sail area is zero.
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  #232  
Old 03-31-2012
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Re: Full or fin keel?

Quote:
Originally Posted by mitiempo View Post
Wind alone will not capsize a boat. It will knock it down but when the boat is at around 90 degrees the keel will produce a large righting force and the boat will not go over any farther. It takes waves to pass that point.
Quote:
Originally Posted by jackdale View Post
And the effective sail area is zero.
Totally agree with both of you, poor choice of words on my part. As you heel the wind has less and less to grab, thankfully.

I meant that if the waves alone did not do it, and a gust of wind (in combination with the wrong wave) was a factor, than it might take longer for a lucky wave to right the boat.

But sooner or later the right, or wrong, wave will come along. Nature rarely has pure waves. Wave motion seems simple, but wave interaction is wonderfully complex.
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  #233  
Old 03-31-2012
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Re: Full or fin keel?

As I will try to look for a boat that will fit my needs first I need to figure out what that need is right now the 13 ft flying junior fits me. I was going to buy from a 16 to 26 foot looking at what could be pulled down the road. already looking at dreaming of the next boat. Do others do this? This kind of leads away from a fin I am in KY no huge waves here. I grew up on Lake Eire feel just fine on that water any summer wind or wave in my sunfish. Still this is not blue water. I plan to move but for now I am here. Next boat for here sailing in the lakes should be? I Like to scuba dive play on the beach . I would like to get on and in the water easy from the boat. From what I am learning about fin vs full a full keel shoal draft might fit my needs better? The next boat should be? Also I could get a slip an go up to say 36 foot but this would be a boat that would have to make the move with me and the move is 8 years min time frame away
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  #234  
Old 03-31-2012
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Re: Full or fin keel?

Lou,

From what you have described, it would be neither fin or full, but a "centerboard!" IE, this can get very close, if not beached, if it has a swim platform, you can get in and out easy from the water.........

The question on which keel to get, is not as simple as this or that. Reality is, one needs to know also where they will sail, use the boat etc. From what you described, it would be a centerboard, or possibly a wing/shoal fin or the scheel fin that was mentioned too. The Centerboard is the only one that is beachable or able to go into really shallow water.

It should also be pointed out, all of these models have potential stability issues on par to somewhere in the middle, or completely different than already mentioned in this debate.

Marty
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  #235  
Old 03-31-2012
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Re: Full or fin keel?

Quote:
Originally Posted by mitiempo View Post
Wind alone will not capsize a boat. It will knock it down but when the boat is at around 90 degrees the keel will produce a large righting force and the boat will not go over any farther. It takes waves to pass that point.
Waves and not any wave but a breaking wave

Regards

Paulo
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  #236  
Old 03-31-2012
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Re: Full or fin keel?

Quote:
Originally Posted by skygazer View Post
Unfortunately I was referring to University researchers. I was a science "nut" until I got to university and found out that 90% of people in science are just doing a job. The research is under a professor, but carried out by graduate students - using undergrads for help. They know the results desired by the organization providing the funding. There is no "pure science" any more, don't believe all that you hear. We are in many ways in the dark ages, due to funding and politics.

Do I have a good scientific mind? Compare what I decided by viewing the physical problem of the "inverted boat with mast" in my mind. It matches exactly what your paper reported, except I go further because I've seen wave tanks and know they can make identical pure waves and control the orientation of the boat, unlike in real life.
You cannot know about the seriousness of that tank test without reading the paper do you?

I found out a free version of that paper posted when it was presented at the Chesapeack symposium in 2005. It is a reduced version but I think it is enough for you to evaluate the seriousness of the work and the validity of the results:

http://eprints.utas.edu.au/6670/1/17...er_2005_F2.pdf

To be trustful I have to say that when I heard about this results showing that an inverted boat with a mast will re-right itself faster than without a mast I had a similar reaction, I mean disbelieve but after reading the paper and looking at the methodology that they implemented in testing I was forced to change my opinion....and it was not only me that had a hard time in believing the results, the research nautical community had the same concerns. The tests were repeated in other places and the results validated.

It has all to do with tripping, in this case the yacht will not trip on the keel bu on the mast

"That is, if the roll moment experienced by a yacht when undergoing sway velocity is a tripping moment (tending to turn the higher portions of the yacht into the direction of travel), then the chance of rerighting can be seen to be increased. This correlation exists because when hit by a breaking wave, a yacht will experience significant surfing even whilst inverted. The surfing will continue long enough after the wave has passed, due to the yacht’s inertia, and so the probability of re-righting will be significantly increased as the tripping moment is increased. "


Regards

Paulo

Last edited by PCP; 03-31-2012 at 02:38 PM.
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  #237  
Old 03-31-2012
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Re: Full or fin keel?

Quote:
Originally Posted by PCP View Post
That is a theoretical study of the capsizing possibilities based on the maximum angle of roll with and without a mast regarding roll moment of inertia. What that graphic shows is that according with his calculations the boat without a mast can roll to the point of capsizing while the boat with a mast remains safe. That was not tank tested or tested in any other way
No, Paolo, Marchaj writes on page 224 that Fig. 111. refers to capsizing tests made in the USA (honestly, I'm getting tired of scanning the book every time you tell me it's not true what I'm writing, I guess you have the book yourself, or you can download it from the link you posted earlier here)
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  #238  
Old 03-31-2012
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Re: Full or fin keel?

Quote:
Originally Posted by Jeff_H View Post
I am not 100% certain that Paulo and I are in agreement on all of this, in fact I am certain we are not, but that is okay. Through discussion I would expect that we will end up perhaps a little closer in our views. This is my understanding of some of the issues being discussed above. ….
Ok, lets see what we can do about this.

Quote:
Originally Posted by Jeff_H View Post
While these Open Class based designs have demonstrated an enviable record as race boats, and may even be a reasonable basis for coastal cruisers, I personally prefer more moderately narrow beam boats for distance cruising containing significant amounts of offshore use. ….
Me to, but this has to do with personal taste and nothing to do with the suitability of those designs (based on Open boats) for offshore use in what regards seaworthiness.

Most boats based on Open boats are not fast light boats and the ones that are light are for more frugal cruisers than me, or are richer and have money for a bigger boat.

Cruising in fast performance boats of this type imply to live in a very frugal way due to the light load capacity and to the simple interior. For living with the same comfort and carrying load of a more traditional cruiser you will need a bigger boat, and that costs more money.

But aside that, Open boats were design not only for offshore use but for solo offshore use. They are very forgiving boats and very safe boats.

I have already show to you that a 40 class racer has a better stability curve than a Vaillant 40, in what regards AVS and in the proportion between the energy needed to capsize the boat and the energy needed to re-right the boat when inverted, or putting in another way, in the proportion of the size of the wave capable to capsize the boat compared with the size of the wave that will re-right the boat. These boats can be not only easy but very seaworthy.

But of course, in what regards cruising boats based on these designs each case is a case and as in any other sailboat it is better to have a good look at the stability curve. That is not different than with any other sailboat, beamy or not. I know of some good narrow boats with a stability curve not as good as some of these boats in what regards AVS and in the proportion between the positive and negative part of the curve, not to mention the righting force at 90º.

Quote:
Originally Posted by Jeff_H View Post
As a word about my personal opinion in these discussions, and in an effort to provide a filter which would help provide a fair minded sense of where my personal prejudices tend, when it comes to offshore cruising boats, I am not a fan of the Open Class style boats that Paulo likes. They are clearly faster than more moderate designs, but that speed comes at a price in terms of motion comfort and self-righting. ….
Regarding this some confusion and some disagreement.

As I have said before many times for my personal use as a cruiser I prefer more classical boats and not boats derived from Open boats. If I could chose a cruising boat I would chose a XP 38 that is not a beamy boat neither a boat based on Open boats.

That does not mean that I don’t like Open class boats as I like many other types of boats. I can appreciate and understand their advantages and weaknesses and I have posted already about it.

The disagreement :

I don’t consider that boats based in Open class style boats are faster than other modern boats based on more traditional rule like IRC or ORC. Actually only downwind in very heavy conditions a racing Class 40 will be more fast than a racer like a Ker 40. Even in what regards fast performance cruising boats a ker 39 will beat any fast cruising boat derived from the Open classes, the same way a Volvo60 will beat an Open 60.

So what is the advantage?: The easiness of use. You need a crew and a good one to race a ker 40 (or a Volvo), the boat is very nervous and need a constant trim especially downwind while an Open boat is designed to go downwind on autopilot. Upwind, the Ker 40 will slam less and will be more faster but it will be a more nervous boat with less form stability and a less stable platform.

It will be more easy and safe to leave the boat on autopilot and go forward in a class 40 then in a ker 40 type of boat (more narrow and with a huge ballast/displacement ratio).

The ease of use is the main reason that motivates the designers to base their cruising boats on the type of hulls of the Open boats. After all most cruising boats are sailed solo or with the little help of a wife that is not normally much, so what better model could they have than the type of hull of a boat that is designed just to be sailed offshore solo and have been improved through the years?

And I don’t mean only performance cruisers like the Pogo, but almost all modern designs by all the main Na and main boatbuilders: Beneteau (Sense and Oceanis), Hanse, Jeanneau, Bavaria, Dufour, just to name a few.

I agree that advantages (not the speed) comes with a price in what regards motion comfort upwind with waves but I completely disagree in what regards a lesser ability to self-righting.

That it will depend on each boat but as a rule I don’t think that is true. Generally the AVS of these boats is not lower than the previous boats from the same brand with a narrower hull, but the overall stability is bigger.

Putting in another words, the wave energy needed to capsize these boats is bigger than on previous less beamy models and the proportion between the wave capable of capsizing the boat and the wave capable or re-righting is about the same. As an important bonus these boats have more RM at 90º.

How was this possible? Look at the keels and the drafts. The B/D ratio is about the same but now almost all the keels have more draft and more weight on the bulb, some fast cruising boats have fin torpedo keels and their number is increasing.

You pointed to me once a Claughton paper that related capsizing resistance with beam and that is logical in fact. A wide cat is much more difficult to capsize (with no sails) that any monohull with the same size. So these boats have advantages in what regards positive stability and also in what regards dynamic stability : their keels have much less area than the previous ones and that and beam will make these boats harder to trip on the keel and will make them more able to dissipate the wave energy with any other movement than not a rolling one.

There is a good reason for all main boat builders and main yacht designers going on this direction. I still prefer a boat with better tracking ability upwind and with a better motion in waves upwind, but understand the reasons that makes most sailors prefer other type of boats. Many don't sail much upwind or with waves

As I have explained elsewhere I think these boats based on Open boats make a lot of sense if you will sail extensively upwind and want a forgiving boat that don't heel much. These boats will be very adequate for voyaging with the trade winds that is the way most of the cruisers sail when crossing oceans.

About the rest, I will try to reply in another day, with more time.

Regards

Paulo
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Last edited by PCP; 03-31-2012 at 08:47 PM.
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  #239  
Old 04-01-2012
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Re: Full or fin keel?

Note to self: Never, ever get in a discussion with Paulo ever again.

Its like listening to an interview with Macho Man about why he is better than Hulk Hogan
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  #240  
Old 04-01-2012
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Re: Full or fin keel?

Quote:
Originally Posted by jackdale View Post
So far so good.

A full keel will generally track better dead down wind. Full keel boats tend to have a smaller draft allowing them to get into shallower water. But as you inferred, they do not tend to point as well as fin-keels. They also need more speed to get steerage way.
I don't known what you mean by that my boat will point 35degrees off the wind with main and working jib {better with cutter rig). It tracks well on any tack and doesn't require as much attention to the helm to keep on course as a fin.




This boat was designed by William Atkin in 1936...before the marketing industry tried to convince people that boats based on racing boats make good cruising boats...presumably because they are faster. BTW this boat is fast, sea kindly with a pleasant motion.
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Last edited by wolfenzee; 04-01-2012 at 04:15 AM.
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