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  #581  
Old 01-09-2013
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Re: Full or fin keel?

Quote:
Originally Posted by blt2ski View Post
Do not remember which number, be it capsize or comfort ratio, but was not one of them "hood" a naval architect that came up with that ratio. Later said it was good, but at the end of the day with some of the newer designs not as useful as with past designs. Not saying that that ratio should be thrown out per say, as it favors longer/skinnier designs vs some of the fatter hulled designs of today. Rather apparent that hull design will potentially make or break a design depending upon useage as to if it will or will not work for the end user. Not just fin vs full vs bilge vs CB or some combo there of!

The more I type, I believe it is the motion comfort ratio number. Even short will come out on the lower side of things than width. As a 30'L 10' wide boat will come out with a worst number than a 60' x 20', even tho the length.width ratio is equal. I am also recalling disp being part, maybe that needs to be equally doubled to get the same ratio, where is going up double in length, usually (typically) quadruples or equal the disp of the boat. That would be an interesting plug in numbers to see what or if one can get different equal length and beam to equal....

Marty
Hi Marty,

Many years ago, having already an interest in boat design I bought a more complete and complicated performance boat calculator the kind wolfenzee used to compare those boats and also many years ago I trough it to garbage.

That type of calculators only works with old boats and even so they have to have a similar hull shape and type of keel. The Capsize ratio ratio is particularly misleading and the comfort ratio is pretty meaningless. Jeff posted a thread about motion comfort in modern boats and posted in it also a very good article about that.

Modern Hull forms and Motion Comfort

Regarding motion capsize ratio probably the best discussion about it is on other forum but the results from the discussion are quite clear. I do not even feel the necessity to discuss that on sailnet. I guess that almost all know that the ratio does not make any sense when applied to modern hulls and by modern I am talking about 35 year's old hulls, at least the good ones.

Capsize Ratio's - Cruisers & Sailing Forums

Regards

Paulo
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Last edited by PCP; 01-09-2013 at 02:14 PM.
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  #582  
Old 01-09-2013
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Re: Full or fin keel?

Quote:
Originally Posted by blt2ski View Post
Do not remember which number, be it capsize or comfort ratio, but was not one of them "hood" a naval architect that came up with that ratio. Later said it was good, but at the end of the day with some of the newer designs not as useful as with past designs. Not saying that that ratio should be thrown out per say, as it favors longer/skinnier designs vs some of the fatter hulled designs of today. Rather apparent that hull design will potentially make or break a design depending upon useage as to if it will or will not work for the end user. Not just fin vs full vs bilge vs CB or some combo there of!

The more I type, I believe it is the motion comfort ratio number. Even short will come out on the lower side of things than width. As a 30'L 10' wide boat will come out with a worst number than a 60' x 20', even tho the length.width ratio is equal. I am also recalling disp being part, maybe that needs to be equally doubled to get the same ratio, where is going up double in length, usually (typically) quadruples or equal the disp of the boat. That would be an interesting plug in numbers to see what or if one can get different equal length and beam to equal....

Marty
It was Ted Brewer that thought up the "comfort ratio" Ted Brewer Yacht Design this link actually explains what the numbers mean.
"COMFORT RATIO (CR): This is a ratio that I dreamed up, tongue-in-cheek, as a measure of motion comfort but it has been widely accepted and, indeed, does provide a reasonable comparison between yachts of similar type. It is based on the fact that the faster the motion the more upsetting it is to the average person....." Not all of the ratios in those calculators are actually valid....this one actually is.

A friend of mine with a nice wide Catalina figured because my boat was much narower and shallower it would be rougher....in actual comparison and calculator agreed with each other and proved him wrong (according to the calculator is is 46.7 vs 23.9).

Last edited by wolfenzee; 01-09-2013 at 02:05 PM.
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  #583  
Old 01-09-2013
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Re: Full or fin keel?

As I often point out when someone tries to cite the Capsize Screen Formula and the Motion Comfort Index in that vessel's defense or prosecution (and I am about to point out yet again and apologize yet again for cutting and pasting this from an earlier post of mine) these surrogate formulas tell almost nothing about how the reality of a boat's likelihood of capsize or its motion comfort. In fact they provide so little indication of a boat's behavior that to rely on them in any way borders on the dangerous. Due to their lack of reliability these formulas have fallen into almost total disuse and disrespect amoungst practicing yacht designers.

Both of these formulas were developed at a time when boats were a lot more similar to each other than they are today. As such these formulas have limited utility in comparing boats other than those which are very similar in weight and buoyancy distribution to each other. Neither formula contains almost any of the real factors that control motion comfort, the likelihood of capsize, or seaworthiness. Neither formula contains such factors as the vertical center of gravity or buoyancy, neither contains weight or buoyancy distribution (of the hull both below and above the waterline), the extent to which the beam of the boat is carried fore and aft, and neither contains any data on dampening, all of which really are the major factors that control motion comfort or the likelihood of capsize.

I typically give this example to explain just how useless and dangerously misleading these formulas can be. If we had two boats that were virtually identical except that one had a 1000 pound weight at the top of the mast. (Yes, I know that no one would install a 1000 lb weight at the top of the mast.) The boat with the weight up its mast would appear to be less prone to capsize under the capsize screen formula, and would appear to be more comfortable under the Motion Comfort ratio. Nothing would be further than the truth.

And while this example would clearly appear to be so extreme as to be worthy of dismissal, in reality, if you had two boats, one with a very heavy interior, shoal draft, its beam carried towards the ends of the boat near the deck line, a heavy deck and cabin, perhaps with traditional teak decks and bulwarks, a very heavy rig, heavy deck hardware, a hard bottomed dingy stored on its cabin top, and the resultant comparatively small ballast ratio made up of low density ballast. And if we compare that to a boat that is lighter overall, but it has a deep draft keel, with a higher ballast ratio, the bulk of the ballast carried in a bulb, its maximum beam carried to a single point in the deck so that there was less deck area near the maximum beam, a lighter weight hull, deck and interior as well as a lighter, but taller rig, it would be easy to see that the second boat would potentially have less of a likelihood of being capsized, and it is likely that the second boat would roll and pitch through a smaller angle, and would probably have better dampening and so roll and pitch at a similar rate to the heavier boat, in other words offer a better motion comfort....And yet, under the Capsize Screen Formula and the Motion Comfort Index it would appear that the first boat would be less prone to capsize and have a better motion when obviously this would not be the case.

There are some better indicators of a vessel’s likelihood of capsize. The EU developed their own stability index called STIX, a series of formulas which considered a wide range of factors and provides a reasonable sense of how a boat might perform in extreme conditions. Unfortunately meaningful results require a lot more information than most folks have access to for any specific design. The Offshore Committee of US Sailing developed the following simplified formula for estimating the Angle of Vanishing Stability (Sometimes referred to as the ‘AVS’, ‘limit of positive stability’, ‘LPS’, or ‘Latent Stability Angle’ ):
Screening Stability Value ( SSV ) = ( Beam 2 ) / ( BR * HD * DV 1/3 )
Where;
BR: Ballast Ratio ( Keel Weight / Total Weight )
HD: Hull Draft
DV: The Displacement Volume in cubic meters. DV is entered as pounds of displacement on the webpage and converted to cubic meters by the formula:
Displacement Volume in Cubic Meters = ( Weight in Pounds / 64 )*0.0283168
The Beam and Hull Draft in this formula are in meters. These values are entered in feet on the webpage and are converted to meters before SSV calculation.
Angle of Vanishing Stability approximately equals 110 + ( 400 / (SSV-10) )

It should be noted that the AVS is only one indicator in evaluating the likelihood of capsize, meaning it only predicts the point at which the vessel wants to turn turtle. It does not predict the amount of force that would be required to heel the vessel to that limit, nor does it predict how the shape of the boat might encourage wave action to roll the boat closer to the angle at which it no longer wants to return.
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  #584  
Old 01-09-2013
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Re: Full or fin keel?

Jeff, be that as it may be...I think you forgot the newly-devised bouyancy/draft and light displacement cross-sectional quotient theorem emanating from some of the design-boards of Europe....
....here in the states naval architects have long had a simpler term for it...wait for it...




**** ....Pounding! ****


Last edited by souljour2000; 01-09-2013 at 09:09 PM.
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  #585  
Old 01-09-2013
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Re: Full or fin keel?

A friend of mine with a Catalina 30 regularly points out the good points of his boat or the bad point of mine, how some particular thing just does't work or is wrong....he does this with tools and all sort sorts things...and in every case he is right....my boat does not fit HIS application it is completely wrong for HIM.....my boat is completely right for ME though. (same goes for the tools too). A fin keel boat is without a doubt the best design...for some people, while a full keel is the best design for others (and a horse and buggy or motor home or house in the suburbs are best for someone else). Because different people have different needs and wants you can't say which is best there. Also because of the huge variety of boats that fall under each category you can not compare performance (it's sort of like comparing US designed cars to European designed cars, taking the Chevette as an example of US and the Lotus as an example of European).;
To sum it all up the ideal boat for one person might be totally wrong for another.
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Last edited by wolfenzee; 01-09-2013 at 11:15 PM.
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  #586  
Old 01-10-2013
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Re: Full or fin keel?

**To sum it all up the ideal boat for one person might be totally wrong for another**


Couldn't agree more Wolf...and it's been said before on this lengthy thread..but bears repeating...but this IS a captivating thread for the ages and though me and others may tease him... I like all the technical angles that Jeff and other keen and knowledgeable proponents bring to the table as much they offer is very educational. and illustrative of many principles in play in a moving sailboat while in my case not always quickly understood...and as I learn more...and who knows..I may indeed someday understand that Jeff or others are right about the superiority of the newer wider lighter boats in most conditions...as compared to older designs...but would still not feel I have lost the debate...perhaps to some degree because demanding shorter-term racing applications seem to be the yardstick for many who espouse the newer thinking as regards boat performance...or not..
Either way I will likely remain skeptical of higher-tech modern theory ...despite the improvements and new thinking it has brought ...especially in materials... and will do so mostly while in the support of that block of sailors who must sail older boats out of necessity in light of the economy or other reasons many in the sport face, and must thus retain a certain real and extant confidence in these earlier craft they sail ...and for good reason...as many are indeed fine boats.
So "better" is, as often is the case, a very subjective word and I expect to remain sure that the truth about superior boats lies somewhere closer to Wolfenzee's above summation...or to the effect of "...different boats for different folks..."

Last edited by souljour2000; 01-11-2013 at 09:35 AM.
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  #587  
Old 01-10-2013
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Re: Full or fin keel?

Someone mentioned pounding...it's one of those features that I missed out on with my heavy disp full keel
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Old 01-10-2013
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Re: Full or fin keel?

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Originally Posted by Jeff_H View Post
And if we compare that to a boat that is lighter overall, but it has a deep draft keel, with a higher ballast ratio, the bulk of the ballast carried in a bulb, its maximum beam carried to a single point in the deck so that there was less deck area near the maximum beam, a lighter weight hull, deck and interior as well as a lighter, but taller rig, it would be easy to see that the second boat would potentially have less of a likelihood of being capsized, and it is likely that the second boat would roll and pitch through a smaller angle, and would probably have better dampening and so roll and pitch at a similar rate to the heavier boat, in other words offer a better motion comfort....
That is your opinion.. there are many of us that would argue forever against this because we have experience with light boats and heavy boats. I guarantee, if you do not pick the heavy boat over the light boat in rough water.. ask your wife.. she will set you straight in a heart beat.

You guys that think the bulb keel is the savior of all RM clearly seem to forget that the RM is determined by the TOTAL BOAT DISPLACEMENT acting at CG of the boat through the moment arm, not the bulb hanging off the end of the stalk.

The keel mass is picked to create a particular righting moment to counteract a heeling moment. This can be achived with any keel. And it just so happens, if you put more mass on the keel at a shallower draft, you get the added weight you need to create a more comfortable boat.
Bryce

Last edited by BryceGTX; 01-10-2013 at 03:11 AM.
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  #589  
Old 01-18-2013
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Re: Full or fin keel?

Yesterday I was watching some Fox sports video coverage of the 2012 trans-atlantic race.. quebec to St. Malo (France).

A really wide Class 40 beat out the trimaran ...makes sense since the whole race was run in a tropical depression ...their route and the depressions route were the same..so they all ran in 40-knots plus the whole way across...great video of boats um...haulin some serious azz....for thousands of miles...
...crews were wasted by end of trip as you can imagine

The "monos" did not seem to be pounding much..just surfin hard..reallly hard..I was admittedly impressed at the speeds and stability....horses for courses...but the new designs are indeed quite impressive...
The Class 40 " Campagne de France" boat that won the race set a record when it had a um... 360 mile day...while averaging over 15 knots...also seems stability and pounding seems to be getting alot better as the designs are refined...

photo below taken by Pierre Bouras
Attached Thumbnails
Full or fin keel?-m378_mabire_finish_620.jpg  

Last edited by souljour2000; 01-18-2013 at 09:03 AM.
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  #590  
Old 02-27-2013
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Re: Full or fin keel?

This has been an amazing thread, especially for a newbie such as myself (reading its entirely over the last few days). Many modern refinements to sailboat design appear to have changed significantly the playing field for ocean-going craft in recent decades. I have the following takeaways:

1 - fin keels are faster and more maneuverable than full keels, and that makes them desirable for many types of sailing and sailors
2 - modified fin/modified full keels and keel/centerboards are legitimate way-stations along the trail of compromise endemic to boat design
3 - newer high-performance, high-aspect fin keels and associated hull designs may be as safe as traditional low-aspect full keel boats even in rough seas, but the full keel designs may still provide better options (heaving to, laying ahull, drogues) for the non-professional sailor (e.g., a cruising couple) to handle and withstand rough seas and heavy weather
4 - the science and mathematics of the dynamic stability of boats in rough seas is complex and probably still not fully understood
5 - sailors have different risk tolerances, reasons for sailing, and personal preferences that strongly affect their choice of craft, and to a great degree these subjective choices are irreconcilable should be respected as such

There is something that Jeff_H wrote fairly early in this thread that struck a chord with me and provokes a question. He wrote:

"When it comes to cruising on the cheap, there are few decent choices left out there. Many of the boats which I might have recommended 10-15 years ago were rare enough even then that the few examples available have become worn out and so are no longer good candidates.

And I find it disconcerting when I see people advocating old, short waterline, short keel, attached rudder, cruiser- racers as being good offshore capable cruisers. One of the strengths of the type of boat that you advocate is that they have very long water lines relative to their lengths on deck. This helps with motion comfort and carrying capacity. Such is not the case with the CCA and IOR based cruiser-racers of the 1960's and 1970's that I often see advocated as offshore cruisers."

So - (A) what boats are there now available for offshore "cruising on the cheap," if any, and (B) what "CCA and IOR based racer-cruisers" should be avoided for that purpose?
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