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Old 17-11-2011, 04:53   #106
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Re: Understanding the Ratios

Quote:
Originally Posted by Adelie View Post
You keep relating these boats that are supposedly cruisers to racing boats. This just makes my point for me.

I am not saying these boats can't be cruised. What I am saying is that given their racing orientation they take a lot more attention and care to to so safely, and when attention or effort flags so does safety.

Looking at cars, there is a lot of technology that has filtered down form racing to street cars. But we don't all drive around in open wheeled, 500kw formula 1's with twitchy steering. Designers keep trying to filter down the whole boat from racing to cruising and in the long run it doesn't work very well.
I don't understand why you reply in red. That's consider bad manners and don't give you more reason.

It was you that have considered the Pogo a good example of a modern cruiser with good stability. It seems you didn't know that Pogo manufactured for many years racing boats (minis) before starting to make (at the demand of their clients) cruising boats with the same kind of hulls.

Regarding that "lot more attention and care" to to sail safely and the race car analogy, you seem to forget this are SOLO boats. That means that they will continue to go safely at speed in auto pilot with the skipper sleeping. This type of boats is very stable and very easy to sail.

For adding a lot more safety easiness and safety you have just to go a bit less fast. That's why while the hulls are essentially the same,the cruising rigs are less powerful. Like riding a Ferrari with super brakes and very precise direction but with a much less powerful and a fully controllable horsepower even by a medium experienced sailor. After all you don't need to cross the Atlantic in a 22ft boat with average speed over 7K


Quote:
Originally Posted by Adelie View Post
These boats are sailed over a specific course, with significant outside support in terms of available forecasting services and possibly weather routers by sailors who are well above average in skills and who are able to remain extremely focused because the race is of a fairly short duration.
A cruiser is meant to be sailed by people of modest skills for extended periods (meaning they aren't going focus that intently on the boat except for short, occasional periods.)
It seems you don't know nothing about the race as you don't know nothing about this type of boats, you can have some basic information here:

The race - La Charente-Maritime / Bahia - Transat 6.50

Routing is forbidden, they are there at their own, each one chosing its one course and you can only be kiddem about the short duration: From North the France to Rio de Janeiro, Brazil) on a 22ft boat, a "short duration course"? Probably you consider only a medium duration course a circumnavigation

As I have said this are solo boats and that means they are a lot of time under autopilot including when the skipper is sleeping.

Quote:
Originally Posted by Adelie View Post
Big waiting list means that more people want to buy the boat than there is a capacity to build. It doesn't mean that the boat is objectively safe (if that can ever be determined) or even that most people agree that it is relatively safer. Also the RM doesn't have nearly the draft that the POGO does, 5.25ft vs 9.2ft.
It seems you miss the point here. I have said that the RM is one of the most popular long range cruiser in France. The guys that are on that waiting list are not weekend sailors but long range cruisers and that means very experienced sailors, the kind of sailors that circumnavigate or do transats and as you know the French are buy far the ones that circumnavigate.

So this means that very experienced sailors make a long waiting list (more than a year) for an RM. Why do you think they all know less about a boat they now very well and that you practically don't know?

Regarding draft, the Pogo can also have as an option a bulbed keel with about the same draft as the RM. That only means more weight on the bulb for a similar RM. The ballast of the RM is, off course, more considerable than the one of the standard Pogo to compensate with weight the difference in draft.

Regarding to these :

What gives lever to the waves is not beam but freeboard and big cabin height.

you have said:

Quote:
Originally Posted by Adelie View Post

Not so. As the boat rides up the face of the wave it rolls with the face of the curling top has shifted the center of bouyancy towards the upwave side, the wider the beam the longer the lever arm it has to act on relative to the center of gravity. See fig.118A in 'Seaworthiness' by Marchaj. Fig 118B shows that as the roll developes further the downwave rail will dig in and start to trip the boat. The wider the beam the larger the lever arm that tripping mechanisms has to work on. Marchah indicates wider, lighter boats also have an increase tendancy to stay in synch or even roll ahead of the waves, while heavier, narrower boats tend to under roll and still be heeled a bit into the wave when the breaker hits. I don't recall that he differenciated which had the bigger effect, weigh or beam.
Well, that is just a 40 year's old book and even if it is a good one theories have to be proven true.

Statistical evidence, the one that is been refereed on this thread as fundamental to access the seaworthiness of several old boats, has shown that this type of boats as well has the centerboarders have a superior dynamic stability. Do you know as many boats for the same model that each have crossed the Atlantic as the minis: 70 to 80 in each edition on a 34 year race? That means many hundreds of boats crossing without capsizing. Most of those old boats that have a very good reputation were not even built in that number.

And in what regards centerboarders, that are seawothhy using the same kind of dynamic stability you can take the words of Jimmy Cornell that after having circumnavigated with heavy old boats had changed for this type of boats and of course, circumnavigated again and going to high latitudes:

The question of stability generally (not just for the OVNIs) is a tricky one, and I must admit that I am not an expert on the theoretical aspects of it. Looking at it purely subjectively - and having sailed some 30,000 miles on the OVNI43, including crossing Drake Passage twice (once in winds over 50 knots) as well as being battered in Le Maire Strait - So if the boat is sailed properly, reefed early, etc you have absolutely no reason to be concerned...

As far as I know, of some 600 OVNIs built in the last dozen years (between 30 and 45 feet LOA) none has capsized or got into any serious trouble because of its design...

Yes, I did take my OVNI 43 to Antarctica and also sailed on a friend's OVNI 39 to Spitsbergen. I have confidence in the OVNI design BUT when it comes to stability.... it is not just the stability of the boat that counts if it may come to extreme weather and the possibility of capsizing, but also the experience of the crew and skipper. So while I was quite happy to take my own OVNI into the Southern Ocean (and had an experienced crew) I would be reluctant to recommend to anyone to take a light displacement centreboarder into that area. The OVNI is OK for ANY job, but a lot depends on who is in charge!

I just completed a circumnavigation on my OVNI 43 and I can assure you that the boat is as comfortable as a heavier boat. Your concern, about how an OVNI handles in heavy weather is understandable. I have been in winds 60+ knots (briefly) and sustained 40 knot winds, without any problem, but I continued sailing - because this is what you do in a light or medium/light displacement boat. ..



Regarding old boats, or other type of boats this kind of boats whem well designed are generally harder to capsize due to its better dynamic stability.

Sharing some common characteristics in what concerns dynamic stability they are however very different. In what regard the Pogo/RM type they nornally have an AVS between 120 and 130º and the kind of its righting curve will give them a superior righting moment at 90º, taking into consideration the weight of the boat, but as all modern boats, the design is pointed to prevent capsizing.

That is true that older narrow boats while having not so much static stability (for the weight) nor a so good dynamic stability are better when inverted in what regards to return to its feet again. Not a big difference in most of the cases, but anyway a better performance in what regards that particular case.

Bottom point. It seems that I am a fanatic of these kinds of boats. I am not, that is not the boat that I would chose as a cruising boat but that is also true that even if want to cross the Atlantic, I do no want to circumnavigate or being most of the time sailing offshore in the trade winds.
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Old 17-11-2011, 10:33   #107
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Re: Understanding the Ratios

Sorry about the bad English of the last post, but it seems that I am not allowed to correct it
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Old 17-11-2011, 10:39   #108
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Re: Understanding the Ratios

There is no need to apologize. Perfect English is not a requirement at CruisersForum. Just express yourself the best you can.
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Old 17-11-2011, 12:08   #109
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Re: Understanding the Ratios

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Originally Posted by Polux View Post
I don't understand why you reply in red. That's consider bad manners and don't give you more reason.
In almost 2yr of being on this forum, you are the first person to comment on my color choice when responding within someone else's quote. Perhaps it is a cultural thing, perhaps you have another motive. Unless you object, I will use green.

It was you that have considered the Pogo a good example of a modern cruiser with good stability.
No what I said was that it was 'The only design I've seen that MIGHT even come close ' to having good ultimate stability, and then only by having a draft of almost 3m. Inverted stability is still likely to be poor, I haven't been able to find the stability curve.
http://www.cruisersforum.com/forums/f47/understanding-the-ratios-71329-6.html#post819095

It seems you didn't know that Pogo manufactured for many years racing boats (minis) before starting to make (at the demand of their clients) cruising boats with the same kind of hulls.
Given that they are making 'cruising' boats that look like racing boats I guessed as much without bothering to research the issue or make the point explicitly.


Regarding that "lot more attention and care" to to sail safely and the race car analogy, you seem to forget this are SOLO boats. That means that they will continue to go safely at speed in auto pilot with the skipper sleeping.
As solo boats in a long distance race most skippers will sleep 20-60 minutes at a time in order to keep an eye on the boat, some totaling less than 4hr in 24.

As I have said these are solo boats and that means they are a lot of time under autopilot including when the skipper is sleeping.
Actually they spend as little time as possible under autopilot, the winner, said he spent every waking moment at the helm. http://yachtpals.com/mini-transat-7052

This type of boats is very stable and very easy to sail.
If they were really that easy to sail there wouldn't be a rest stop part way thru the race where sailors get to sleep and fix their boats.



For adding a lot more safety easiness and safety you have just to go a bit less fast. That's why while the hulls are essentially the same,the cruising rigs are less powerful.
That's like saying it's safer to ride a smaller motorcycle than a bigger one. Doesn't make riding the smaller one safe, just less dangerous. I'd rather be in an accident in a Yugo than on a cycle. (Before you riders get your knickers in a twist I am a rider, I just accept the added safety problems.)

Like riding a Ferrari with super brakes and very precise direction but with a much less powerful and a fully controllable horsepower even by a medium experienced sailor. After all you don't need to cross the Atlantic in a 22ft boat with average speed over 7K


It seems you don't know nothing about the race as you don't know nothing about this type of boats, you can have some basic information here:

Or maybe I know a lot more than you think.

The race - La Charente-Maritime / Bahia - Transat 6.50

Routing is forbidden, they are there at their own, each one chosing its one course
I was unsure about routing and said as much, but you are correct on that point. However they still get forecasts from shortwave radio.

and you can only be kiddem about the short duration: From North the France to Rio de Janeiro, Brazil) on a 22ft boat, a "short duration course"? Probably you consider only a medium duration course a circumnavigation
In the US the 3 weeks of sailing would be considered a vacation, in Europe a short vacation. And 3 weeks with a rest break partway. In comparison to the length of most cruises, months or years, 3 weeks is a short duration and it is reasonable to stay very focused for that long. Distance is not the issue.





It seems you miss the point here. I have said that the RM is one of the most popular long range cruiser in France. The guys that are on that waiting list are not weekend sailors but long range cruisers and that means very experienced sailors, the kind of sailors that circumnavigate or do transats and as you know the French are buy far the ones that circumnavigate.

So this means that very experienced sailors make a long waiting list (more than a year) for an RM. Why do you think they all know less about a boat they now very well and that you practically don't know?
You are making my point. The boat is only appropriate for very skilled sailors like apparently everyone in France. I'm not saying it is a patently bad boat, just that it is not appropriate for the majority of sailors.

Regarding draft, the Pogo can also have as an option a bulbed keel with about the same draft as the RM. That only means more weight on the bulb for a similar RM. The ballast of the RM is, off course, more considerable than the one of the standard Pogo to compensate with weight the difference in draft.

Regarding to these :

What gives lever to the waves is not beam but freeboard and big cabin height.

you have said:



Well, that is just a 40 year's old book and even if it is a good one theories have to be proven true.
The book is less than 30yr old. Einstein's theories are 100yr old, should we toss them too? How about Archimedes theory of displacement (2000-3000yr)? No we need to keep that one or our boats will sink. Ignoring the lessons of the past because they are old is just a road to repeating the mistakes of the past rather than finding new and more interesting mistakes to make. I have seen no studies that overturn the science Marchaj presented in the book, only things that refine it and provide more nuance.

Statistical evidence, the one that is been refereed on this thread as fundamental to access the seaworthiness of several old boats, has shown that this type of boats as well has the centerboarders have a superior dynamic stability. Do you know as many boats for the same model that each have crossed the Atlantic as the minis: 70 to 80 in each edition on a 34 year race? That means many hundreds of boats crossing without capsizing. Most of those old boats that have a very good reputation were not even built in that number.

And in what regards centerboarders, that are seawothhy using the same kind of dynamic stability you can take the words of Jimmy Cornell that after having circumnavigated with heavy old boats had changed for this type of boats and of course, circumnavigated again and going to high latitudes:

The question of stability generally (not just for the OVNIs) is a tricky one, and I must admit that I am not an expert on the theoretical aspects of it. Looking at it purely subjectively - and having sailed some 30,000 miles on the OVNI43, including crossing Drake Passage twice (once in winds over 50 knots) as well as being battered in Le Maire Strait - So if the boat is sailed properly, reefed early, etc you have absolutely no reason to be concerned...

As far as I know, of some 600 OVNIs built in the last dozen years (between 30 and 45 feet LOA) none has capsized or got into any serious trouble because of its design...

Yes, I did take my OVNI 43 to Antarctica and also sailed on a friend's OVNI 39 to Spitsbergen. I have confidence in the OVNI design BUT when it comes to stability.... it is not just the stability of the boat that counts if it may come to extreme weather and the possibility of capsizing, but also the experience of the crew and skipper. So while I was quite happy to take my own OVNI into the Southern Ocean (and had an experienced crew) I would be reluctant to recommend to anyone to take a light displacement centreboarder into that area. The OVNI is OK for ANY job, but a lot depends on who is in charge!

I just completed a circumnavigation on my OVNI 43 and I can assure you that the boat is as comfortable as a heavier boat. Your concern, about how an OVNI handles in heavy weather is understandable. I have been in winds 60+ knots (briefly) and sustained 40 knot winds, without any problem, but I continued sailing - because this is what you do in a light or medium/light displacement boat. ..


Regarding old boats, or other type of boats this kind of boats whem well designed are generally harder to capsize due to its better dynamic stability.

Sharing some common characteristics in what concerns dynamic stability they are however very different. In what regard the Pogo/RM type they nornally have an AVS between 120 and 130º and the kind of its righting curve will give them a superior righting moment at 90º, taking into consideration the weight of the boat, but as all modern boats, the design is pointed to prevent capsizing.

That is true that older narrow boats while having not so much static stability (for the weight) nor a so good dynamic stability are better when inverted in what regards to return to its feet again. Not a big difference in most of the cases, but anyway a better performance in what regards that particular case.

Bottom point. It seems that I am a fanatic of these kinds of boats. I am not, that is not the boat that I would chose as a cruising boat but that is also true that even if want to cross the Atlantic, I do no want to circumnavigate or being most of the time sailing offshore in the trade winds.
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Old 17-11-2011, 12:22   #110
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Re: Understanding the Ratios

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Extending that statement to cars, any normally competent driver should be able to finish the Paris-Dakar in a Mini, a Scion xB or any other stock street car.
A few years back one of my neighbours drove to Beijing (from Europe). It was an organised event (route and visas), but not to the extent of having any backup........the challenge was that car must not have cost more than £500 He made it not everyone did ......but a £500 car easy to walk away from.

Time was that trip would have involved several Landrovers and maybe even a truck. Before that it would have been a Camel train and probably a detachment of soldiers.

Whether it is progress of course is another thing........
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Old 17-11-2011, 13:06   #111
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Re: Understanding the Ratios

Well, once again I find that I'm needing to learn more about a sailing topic. What is a good source for information about ratios, sail plans and all those other trems you guys throw around that I don't understand.... ?
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Old 17-11-2011, 13:11   #112
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Re: Understanding the Ratios

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A few years back one of my neighbours drove to Beijing (from Europe). It was an organised event (route and visas), but not to the extent of having any backup........the challenge was that car must not have cost more than £500 He made it not everyone did ......but a £500 car easy to walk away from.

Time was that trip would have involved several Landrovers and maybe even a truck. Before that it would have been a Camel train and probably a detachment of soldiers.

Whether it is progress of course is another thing........
London to Beijing, roads must really have improved. That whole chunnel thing has got to be a real boon too. , fitting a snorkle to a £500 car would be a real PITA. Yeah, yeah, I know, chunnel only carries trains.
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Old 17-11-2011, 13:40   #113
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Re: Understanding the Ratios

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Originally Posted by Matt sachs View Post
Well, once again I find that I'm needing to learn more about a sailing topic. What is a good source for information about ratios, sail plans and all those other trems you guys throw around that I don't understand.... ?
What terms do you want to know about?
Large animal or small animal practice?
Which ratios do you want to know about, or all of them?
I assume your boat is named for your kids.
I don't really know where to get a good primer on terms, ratios , ect. Some of what I know I picked up ad hoc, some I picked up doing a year of naval architecture in college, ship not sailboat, though a lot of the concepts are common to both.
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Old 17-11-2011, 14:00   #114
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Re: Understanding the Ratios

Adelie, All of them. About the time I think I know a little something about sailing, I find I know almost nothing.
I have a mixed practice, mostly small animals.
Yes, its named after the kids - that are now off in college now.
I bought and read the current Chapmans. Is there something better for sailing ?
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Old 17-11-2011, 16:55   #115
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That is true that older narrow boats while having not so much static stability (for the weight) nor a so good dynamic stability are better when inverted in what regards to return to its feet again. Not a big difference in most of the cases, but anyway a better performance in what regards that particular case.
While the smaller area under the stability curve is useful and a feature of older designs. In reality this feature is rarely useful. Boats that invert or do 360 rolls firstly tend to be in seaward that can easily right side them again.

In addition cabintops significantly contribute to lower negative area under the stability curve ( see RNLI lifeboats and moody DS) once you keep them watertight. Since cabintops are not taken into account in official stability curves and raised cabintops are a feature of modern boats the actual inverted performance is much better then the AVS suggests.

It is simply ridiculous to suggest that boat design peaked into the 1920s. So much of yachting design at the times was influenced by racing rules. Whereas today this is not the case as much.

In addition computerised design has changed things enormously.

The fact is that really nobody is building these old slow designs anymore. This debate is beginning to resemble arguing the advantages of the dinosaur. They may have been great but they are extinct.

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Old 18-11-2011, 00:19   #116
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Re: Understanding the Ratios

Quote:
Originally Posted by Matt sachs View Post

Adelie, All of them. About the time I think I know a little something about sailing, I find I know almost nothing.
I have a mixed practice, mostly small animals.
Yes, its named after the kids - that are now off in college now.
I bought and read the current Chapman’s. Is there something better for sailing ?
Last to first:

Chapman's is OK, but is geared towards powerboating much more than sailing, though there are a lot of common areas like navigation, anchoring, rules of the road, ... Annapolis Book of Seamanship would probably be a better reference for sailors.

Numbers and Definitions-most used in bold

Displacement (D)- What the boat and everything on weighs in air. Happens to be exactly equal to the amount of water displaced by the hull and appendages, which is where the name comes form.

Ballast (B) - The weight placed in the boat, as low as possible, in order to help the boat stay upright.

Sail Area (SA)- Several ways to define this. Generally only 2 are used which can be deduced from context. Most calculations use the sail area of the main calculated from luff and foot lengths with no accounting for roach (area beyond straight line between clew and head) plus the area of the foretriangle in front of the mast with no accounting for headsails overlapping the mast or staysail area. Accounting for mizzen area is hit and miss. The foretriangle is used for calcs for consistency, deciding to use a lapping jib or genoa would invite bending the definition of how much overlap to use.
The other use is total sail area hoisted or hoistable in any combination, or max sail actual sail area including multiple jibs, main roach, mizzen sails, spinnakers, etc. This is the 'Oh Wow' number that you pull out to impress people with how big a boat you have or how god Awful much sail you can put on an undersized boat.

Beam (Bm) - Generally taken to be the maximum width of the hull anywhere. Some esoteric formulas use beam at the waterline.
I, J, P, E, and several variations - Names for the particular dimensions of the main and foretriangle. See this rig KEY for a visual description.

Draft - Depth of the deepest part of the boat below the waterline. Keep in mind the draft increases as weigh is added to the boat.

Lightship - the boat with only basic sails and motor aboard. Fuel, water, crew, provisions, personal gear, etc is extra. Generally the safe thing to assume is that any displacement figure is for the boat in lightship condition.

LOD, Length on Deck - Length of hull excluding all appendages like transom hung rudder, bow sprit or bumpkin.

LOA, Length over All – Total length including appendages

LWL, Length of Waterline – Just like it sounds. On most boats the hull forward forward and aft from where it enters the water.

Righting moment – The amount of force a boat exerts to right itself, expressed in ft-lb or N-m. At a 10* angle of heel some random boat will have a righting righting moment of 1500ft-lb, equivalent to supporting a weight of 100lb 15’ from centerline, or 150lb 10’ from center line. Actually overturning moment is provided by wind pressure on the sails or movement of crew weight to one side or the other.

Heel – The angle of roll in either direction from a boat’s normal at rest position.

Stability Curve – A graph showing the righting moment at every angle of heel from 0 to 180. Where the boat stops trying to right itself and begins to try to finish inverting itself righting moment is considered to have a negative value. See attached graph.

AVS, Angle of Vanishing Stability, LPS, Limit of Positive Stability, and several other variations The angle of heel between 0 and 180 at which the boat stops trying to right itself, and more heel and it actively wants to capsize. Righting moment is zero at this point. Statistically the higher the AVS the quicker a capsized boat will right itself. AVS may also play a role in capsize resistance but only a minor one.

Capsize – For a boat to roll into an inverted position, boat does not necessarily sink.

Knock down – When a boat rolls until the mast or spreaders touch or are somewhat immersed in the water.

Founder – Boat sinks.

Ratios-most used in bold

D/L, DLR, Displacement to Length - is not really D/L, it is (D/2240)/[(.01*LWL)^3]. Displacement is converted to a volume then is divided by the length cubed which becomes a volume. The resulting number is dimensionless. The ratio is an indicator of how heavy the boat is for its length. Below 200 is light, below 100 ultra-light. Above 400 is heavy. For most boats with moderate overhangs the ratio is fairly descriptive. For boats with exaggerated overhangs or no overhangs the number is skewed one way or the other. Use of LWL rather than LOD goes back to old handicapping rules that used LWL in the handicapping equations rather than LOD

SA/D, Sail Area to Displacement - Again not really SA/D. It is SA/(D/64)^666. Displacement is divided by 64 (density of seawater) to get a volume. The 2/3 power of a volume is an area, thus you are dividing an area by an area so the number that remains is dimensionless. The upshot is that it makes it easy to compare boats of different sizes. Doesn't work for boats of vastly different sizes. At the short end crew weight alone makes up a bigger percentage of actual displacement underway so sail areas tend bigger to make up for that. For boats 25-45' SA/D of 15-16 or so is normal. Racers tend higher at 17-19 into mid 20's. Really extreme boats will be high 20's into 30's or higher. Undercanvassed boats and motorsailors run 10-13. A boat with a high SA/D will get to its hull speed in lower wind strength and start surfing sooner if it can.
Percent Ballast, %Bal, %B, Ballast Displacement ratio - Oddly enough just like it sounds, Ballast divided by Displacement. 40% is pretty typical. Racers trend up if they are narrow and down if they are wide for their length.

L/Bm, Length to Beam Ratio.- Generally taken to be LOD/Bm. The water sees LWL/BM at waterline but beam at waterline is not generally published and since LOD/Bm tends to have a similar value that is the one that gets used. Boats around 30’ are about 3 or 3.5 to 1. The ratio trends up with length. Beam is a factor in initial stability. As length increases, overturning moment scales as the square of length, and righting moment scales to the 4th power. Rather than have a very stiff boat, which is uncomfortable, designers tend to have beam grow more slowly than length so this ratio tends to go up with length.

Hull speed - Generally hull speed is taken to be 1.34*LWL^.5. Hull speed increases slowly with increasing length. Physically this is the speed at which drag starts increasing dramatically for every extra knot of boat speed. Friction increases as the square of the speed of the boat which accounts for part of this phenomenon. The bigger component is wave creation. As the boat moves forward it creates a wave moving at the same speed as the boat. Wave length is related to speed, so when the speed reaches the point that the wave length is the same length as the LWL of the boat then there is a spike in drag related to this wave. The height of the wave is related to the relative width of the boat (L/Bm). Skinny hulls make a shorter (height-wise) wave so the drag spike isn't as bad. Catamarans make use of this, their hulls tend to be very narrow for their length. With the proper hull shape and lighter displacement will plane, getting dynamic lift from the water passage and breaking out of the wavemaking regime. Boats that have the proper shape but are overweight may plane in spurts when aided by waves, this is called surfing.

Sail Area to Wetted Surface, SA/WA – Just like it sounds. It is used to indicate relative performance in very light winds. In very low winds at low boat speeds wave making drag does not come into play, only skin friction, for the same sail area the boat with the lower wetted surface area will be faster than the boat with greater surface area, even if the first boat is significantly heavier. This ratio is not much used as wetted surface area is hard to measure, and naval architects don’t go out of their way to publish that value.
Horsepower per Long Ton of Displacement – An indication of how big a engine the boat has relative to how heavy the boat is. Big source of arguments on the Forum about how big you engine should really be.
Polar diagrams – A radial graph indicating expected boat speed for a given wind strength and true or apparent wind direction. Useful for determining the best point of sail to make the best progress directly upwind or directly downwind. Downwind, aiming for the goal is not always fastest, or safest. See attached.
Capsize Screening Formula, CSF -Beam / (Displacement / 64)^.333. Numbers below 2 are generally considered better for going offshore, greater than 2 requires more care. Greater beam provides breaking waves with a longer lever arm to roll a boat over. Displacement in an of itself is only moderately good at providing capsize resistance, however it is a good proxy for other boat properties like roll inertia and lateral area of the boat underbody which provides roll damping. Designers that lighten a boat usually decrease the roll moment of inertia and choose foils with smaller areas and higher aspect fins chasing lower drag which also decrease the damping. Arguments about the continuing validity of this equation can be rancorous. Ultralight boats have a hard time getting below a value of 2 until the boat reaches a length around 50’.
STIX, STability IndeX – A European ISO standard that tries to meld a number of ideas about boat stability and safety to present 1 number that is indicative of a boat’s seaworthiness. Higher numbers are better, numbers above 32 I believe are considered safe for offshore. The ISO is copyrighted. I have a spreadsheet that a European Naval Architect came up with that does all the math. It uses a fair bit of esoteric values for input. My personal feeling is that trying to get one number to represent safety when so many complex interactions are part of the mix is folly. Also the math behind it seems to have been developed by committee, everybody seems to have thrown in their pet theories and some clerk came up with a formula to weight everything. The process is actually pretty opaque and I haven’t seen that anyone has done any tank testing to compare how well STIX predicts capsize resistance compared to the CSF. To a certain extent I am withholding final judgment but am very dubious.

Esoteric Stability Terms
CG, Center of Gravity – the point in the boat all the mass balances around. For stability considerations it is usually considered to be on the centerline and its height is taken from the at rest waterline level. Fore and aft location is generally of less interest except to naval architects deciding where to place the ballast so that the boat is correctly trimmed to its intended waterlines, at least until some god damned boat owner decides to put a water tank in the bow and it trims down.
GZ – The horizontal distance between the center of gravity and the center of bouyancy, also known as a moment arm.
Center of buoyancy , CoB – The point at which all of the boats floatation balances. Transverse location is most important at low angles of heel with height above at rest waterline becoming more important at or near 90* heel. When the boat is at rest there are equal amounts of floatation or buoyancy on each side. As the boat heel there is less floatation on one side and more on the other, the center of buoyancy moves towards the more immersed side.
Metacenter – The point on centerline directly above the CoB at any given instant. For small changes in heel the metacenter does not move very much. The metacenter does not directly represent any physical physical property.
GM, Metacentric Height – The distance along centerline from the CoG to the Metacenter. This distance turns out to be an abstraction that is good in discussing the initial stability of a boat.
Initial stability – The rate of increase of righting moment for small angles of heel, 0*-15* or so. The slope of the righting moment curve at or near 0*. A stiff boat has a high rate of increase or a high slope. A tender boat is otherwise.
Ultimate stability – The highest righting moment the boat achieves, the peak of the stability curve.

This is should be enough to boggle your head. Feel free to ask for clarification on anything.
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Old 18-11-2011, 00:27   #117
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Re: Understanding the Ratios

Can't seem to upload graphs. Will try again tomorrow AM.
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Old 18-11-2011, 06:53   #118
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Re: Understanding the Ratios

Quote:
Arguments about the continuing validity of this equation can be rancorous
No Kidding!
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Old 18-11-2011, 09:45   #119
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Re: Understanding the Ratios

Thanks Adelie. No need to clarify, I did have to read it twice though. Thanks again. Now back to the regullarly scheduled refit, already in progress.....
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Old 18-11-2011, 09:57   #120
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Re: Understanding the Ratios

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Originally Posted by Polux View Post
Hum, I don't understand what you mean by hitting below the waterline. The problem in what regards waves are the breakers in 3m waves or bigger. Only the top breaks and that is well over the boat so will hit always above the waterline.
Not if the boat is heeled away from the waves, which is usually the case in a sailboat on a beam reach because the wave direction usually matches the wind direction.
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