Understanding the Ratios

[Polux]

Originally Posted by Adelie

Adelie

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....

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/...tml#post819095

Adelie, I am truly sorry. It seems you really had no intention. Happens that red is used normally to correct errors and also it is such a so strong color that it seems that someone is shouting when writing in red. on top of it around here it is considered bad education to write to other person in red. Red is just for correting things

You are right, probably a cultural thing.

But you are wrong regarding the Pogo 40 curve. It is a very good one and as I have said, it is just the same with the 3.0m draft or with 2.2m draft, but with more weight on the keel. I will post that curve and also one from the Vailant 40, a modern boat in its time and a boat considered rightly as a good blue-water boat. the Pogo curve is in m, the Vailant one in feet.





I also will post a link to a video with a internet pal going in his Wauquiez Opium 39, a boat that share similar characteristics with Pogo. Cruising with the family doing between 8 and 9K in a very stable and easy boat.

http://www.youtube.com/watch?v=SwukhneV6uc

[kaimusailing]

No mention of Colin Archer. His wave line theory was proved wrong, but his boats were the epitome of full keel seaworthiness. In the last century there were many reiterations of similar full keel heavy weather boats in thick fiberglass. The main problem is that they generally had rigs that were useless in light airs. They were double enders, short rigs, and many traditional circumnavigations were made with variations of that design.

In the 1970's I had an old boat that fell apart and I needed a boat for the rig. I was interested in some ads I saw in Popular Mechanics for a home built boat by James Wharram. His motto was "build it yourself for less" and I built one of his 23 footers. It was just a trial to see if his hulls would work, especially to windward. It turned out that the boat sailed fine, we never had any conditions to establish seaworthiness. It was heavily built for its size, the hulls were double ended, and then I got the idea, this is the Colin Archer of catamarans.

Later I found accounts of these boats sailing very rough passages, round the world, through hurricanes, and in some cases undercrewed. One sailor was Henk de Velde who went delerious and out of commission down below while his pregnant wife sailed the boat to Rapa Nui where the villagers carried the whole thing on shore. What a story. Rory MacDougall who circumnavigated in a 21 footer and is still sailing it. A 23 footer sailed from Delaware to Bermuda through a hurricane. A boat like that doesn't need a great sailor to survive at sea, it will help somebody like me, an accident prone good navigator (I always find land).

I picked up a larger model for my first big boat and found I could sail it singlehandedly with no problems. I could even sail it with help. Now I have one of similar size and as I age and question my own physical abilities, I can't question the ability of the boat. It rides smoothly over rough seas. I can put up 1200 sq ft in light airs and don't have to reef, in heavy weather I can reduce to a 300 sq ft staysail, or reef that to 175. I've carried 675 in 25 knots of wind with no problems. I like the possibility of carrying 1200 feet of sail on a 10,000 pound boat. 5 knots in 7 knots of wind.

I think the ratios have to be equated with the real world, what happens to a particular boat in certain conditions. When you are out there and experiencing it, you have a good idea what you are comfortable with. I've had some good sails with a friend on his Allied full keel ketch. A steady 9 knots close reaching in a 30 knot Chesapeake chop. Very strong boat.

I think anecdotal reports are highly valued and you can see what the hard core people are sailing offshore. I'm now "watching" the Caribbean 1500 online and seeing how the boats are doing. I'm at the dock.

[Polux]

Originally Posted by kaimusailing

...
I think anecdotal reports are highly valued and you can see what the hard core people are sailing offshore. I'm now "watching" the Caribbean 1500 online and seeing how the boats are doing. I'm at the dock.

The next boat to arrive will be a Benetau oceanis 46, a boat that many would say that is not an offshore boat, a marina condo and a lot of other nonsense.

Arriving before only a big 73 ft cat, 2 boats with more than 70ft a 60ft and a Moody 54. The Benetau 46 is almost there and the boats nearer are many hours away, more precisely about 120nm back. They are 3: A Tayana 64, a Halberg Rassy 53 and a Hylas 54.

On the tail of the rally, but far away from being the last, the smaller boat, a Bavaria 36 leaves behind a Cabo Rico 45 and other bigger boats. I know this is not properly a race but nobody likes to stay behind and it is always good to see how the boats perform with amateur crews

[Adelie]

Originally Posted by Polux

No problem on the color thing.

The Pogo10.50 was the boat previously mentioned but if you would rather compare to the Pogo 40 I can do that too.

Scaling from the GZ curve the AVS for the Pogo is 127 and the Valiant lists the value at 125. Slight advantage to the Pogo, but essentially the same.

Scaling from the curve the peak lever arms for the Pogo40 are 3.90' and -1.75', multiplied by the published vessel displacement of 10,582lb gives peak righting moment of 41300ft-lb and a peak inverted moment of -18,400 ft-lb.

Scaling from the Valiant curve the peak lever arms are 2.34' and -1.15. With a displacement of 23520 lb, righting moments are 55,150 and -26,950ft-lb.

So the Valiant has 1/3 more peak righting moment than the Pogo and one presumes a similar amount of area under the curve despite the Pogo's 10' draft vs the Valiant's 6'.

Inverted the Valiant has at least 50% more negative righting moment when inverted so the Pogo has that advantage.

But none of those facts is important.

Testing has shown that boats are more llikely to roll over without a mast than with one despite the fact that the mast decreases the peak righting moment and the AVS.

What are the roll inertia's for each boat? That is a much more important indicator of capsize resistance.

Sorry for not doing this metric but I always screw up the kg to N transformation and I know kg-m is wrong, though most people would understand it.

[Polux]

Originally Posted by Adelie

No problem on the color thing.

The Pogo10.50 was the boat previously mentioned but if you would rather compare to the Pogo 40 I can do that too.

Scaling from the GZ curve the AVS for the Pogo is 127 and the Valiant lists the value at 125. Slight advantage to the Pogo, but essentially the same.

Scaling from the curve the peak lever arms for the Pogo40 are 3.90' and -1.75', multiplied by the published vessel displacement of 10,582lb gives peak righting moment of 41300ft-lb and a peak inverted moment of -18,400 ft-lb.

Scaling from the Valiant curve the peak lever arms are 2.34' and -1.15. With a displacement of 23520 lb, righting moments are 55,150 and -26,950ft-lb.

So the Valiant has 1/3 more peak righting moment than the Pogo and one presumes a similar amount of area under the curve despite the Pogo's 10' draft vs the Valiant's 6'.

Inverted the Valiant has at least 50% more negative righting moment when inverted so the Pogo has that advantage.

But none of those facts is important.

Testing has shown that boats are more llikely to roll over without a mast than with one despite the fact that the mast decreases the peak righting moment and the AVS.

What are the roll inertia's for each boat? That is a much more important indicator of capsize resistance.

Sorry for not doing this metric but I always screw up the kg to N transformation and I know kg-m is wrong, though most people would understand it.

Adelie,

I am not interested in a pointless direct comparison between the Pogo 40 and the Vailant 40 regarding stability or seaworthiness. It was not for that that I have posted those curves but just to show you that the Pogo has a very good stability curve even if compared with what is normally considered by people that like old boats ad a benchmark in seaworthiness.

As you know GZ curves (the ones that I have posted) are the curves thar should be used to see if a boat is well designed or not because they are independent of the weight of the boat and partially of the size of the boat.

The GZ curve from the Vailant is typical of boats from that era with a max GZ of about 0.7m at 62º. The Pogo curve is outstanding, even among mothern 40fts with a GZ of about 1.2m at 70º. Sometimes modern beamy boats have good GZ values but a lot of inverted stability and a poor AVS. You can see that is not clearly the case with the Pogo that has a better AVS than the one of the Vaillant and a small inverted stability.

I say that a comparison of the seaworthiness of the Pogo and the Vailant is pointless because while the Pogo has water proof compartments that make it unsinkable and a better dynamic stability, the Vailant, that is much more heavier, has more righting moment.

But of course that discussion about righting moment is flawed because to overcame all that weight, for the same sailing speed the Vaillant has to carry much more sail than the Pogo and is occupying to that effect a much bigger percentage of its RM than the Pogo, not to mention that the Pogo can escape bad wind doing 18/20K while the Vaillant would hardly make half of that speed. another positive point for the Pogo is that its stability curve is better than the one from the Vaillant in what concerns reserve stability. At 90º the Pogo will be making proportionally more force to right itself up than the Vailant.

If you look at the GZ stability curves you will see that while the one from the Vaillant starts to come down at 62º to an AVS of 125º, the one from the Pogo will only come down at 70º to a further 128º AVS. This makes that the GZ at 90º will be even proportionally better than the one between the two max GZ.

The Vailant GZ value at 90º is 0.40m, the one from the Pogo is 0.83m and that is more than the double. For doing the same righting moment the Vailat should have almost more than the double of the Pogo weight and it has: The Pogo weights 4500kg and the Vailant has 10 200kg.

With a diference, the Pogo would be making almost the same righting force to re-right 4500kg while the Vaillant would be making that force to re-right 10200kg.

Anyway both are good bluewater boats and that is not the point. I am not saying that the Vailant is not a good bluewater boat. It is you that are saying that the Pogo is not a good bluewater boat.

I will post a calculator to finish with those conversion problems

Moment of Force Conversion - FREE Unit Converter

[Adelie]

Originally Posted by Polux

Adelie,

I am not interested in a pointless direct comparison between the Pogo 40 and the Vailant 40 regarding stability or seaworthiness. It was not for that that I have posted those curves but just to show you that the Pogo has a very good stability curve even if compared with what is normally considered by people that like old boats ad a benchmark in seaworthiness.

As you know GZ curves (the ones that I have posted) are the curves thar should be used to see if a boat is well designed or not because they are independent of the weight of the boat and partially of the size of the boat.

The GZ curve from the Vailant is typical of boats from that era with a max GZ of about 0.7m at 62º. The Pogo curve is outstanding, even among mothern 40fts with a GZ of about 1.2m at 70º. Sometimes modern beamy boats have good GZ values but a lot of inverted stability and a poor AVS. You can see that is not clearly the case with the Pogo that has a better AVS than the one of the Vaillant and a small inverted stability.

I say that a comparison of the seaworthiness of the Pogo and the Vailant is pointless because while the Pogo has water proof compartments that make it unsinkable and a better dynamic stability, the Vailant, that is much more heavier, has more righting moment.

But of course that discussion about righting moment is flawed because to overcame all that weight, for the same sailing speed the Vaillant has to carry much more sail than the Pogo and is occupying to that effect a much bigger percentage of its RM than the Pogo, not to mention that the Pogo can escape bad wind doing 18/20K while the Vaillant would hardly make half of that speed. another positive point for the Pogo is that its stability curve is better than the one from the Vaillant in what concerns reserve stability. At 90º the Pogo will be making proportionally more force to right itself up than the Vailant.

If you look at the GZ stability curves you will see that while the one from the Vaillant starts to come down at 62º to an AVS of 125º, the one from the Pogo will only come down at 70º to a further 128º AVS. This makes that the GZ at 90º will be even proportionally better than the one between the two max GZ.

The Vailant GZ value at 90º is 0.40m, the one from the Pogo is 0.83m and that is more than the double. For doing the same righting moment the Vailat should have almost more than the double of the Pogo weight and it has: The Pogo weights 4500kg and the Vailant has 10 200kg.

With a diference, the Pogo would be making almost the same righting force to re-right 4500kg while the Vaillant would be making that force to re-right 10200kg.

Anyway both are good bluewater boats and that is not the point. I am not saying that the Vailant is not a good bluewater boat. It is you that are saying that the Pogo is not a good bluewater boat.

I say again, righting moment is unimportant to capsize resistance, beam and roll inertia are.

I have not said that Pogo could not be sailed offshore, I have said that they are not appropriate for most sailors.

Would you leave a 13yr old on watch for 4hr in 20kt of wind on the Pogo?

I would do so with my nieces on the Valiant once they'd been on the boat for several weeks. Neither has been sailing before to my knowlege.

[Polux]

Originally Posted by Adelie

I say again, righting moment is unimportant to capsize resistance, beam and roll inertia are.

Adelie that makes no sense! The energy (produced by a capsizing wave) required to capsize a boat is represented by the the area under the positive part of the righting moment curve. If in two boats that area is the same the inertia is completely irrelevant. What inertia does is that the boat with more inertia will roll more slowly that the lighter one, but inertia plays from both ways. The lighter boat will stop rolling and get in its feet faster while the boat with more inertia will continue to roll till eventually the righting moment is enough to stop that rolling and the boat will start to re-right as slowly has was brought to its side.

That could have a negative effect because, considering both boats hit by a wave at the same time, while the lighter boat is already on its feet, the heavier one is still deeply heeled and if a new wave hits both boats, one will have all the energy to fight back (it is already upside) while the other will only have left the energy that goes to that point of heel to the AVS point.

I have already told you that beam is not bad in what regards to capsizing. It can have some negative effects in some boats that are not well designed (in what regards ballast) and that will result in a bad reserve stability, a low AVS and a big inverse stability.

We have already saw that all those parameters are very good on the Pogo 40(better than in the Vailant).

Notwithstanding those problems there is nothing that contributes more to positive stability than beam. I have already give you the example of Multihulls, that don't have reserve stability but that in what regards to positive stability have a lot more than a monohull. For the same weight a modern multihull has a a Max GZ about 5 times bigger than a monohull.

There is a paper from the University of Southampton (the best NA school) that , based on tank testing, relates the size of the wave needed to capsize the boat with the bean of the boat. I think it was 3X the beam but I am not sure.

Why do you think that modern cruisers have so much beam? Do you think that the best yacht designers are all crazy?

Originally Posted by Adelie

I have not said that Pogo could not be sailed offshore, I have said that they are not appropriate for most sailors.

Could not be sailed? The Pogo was designed to be sailed offshore! For its size the Pogo 40 is one of the most seaworthy boats around. They even have circumnavigated by the roaring forties, I mean really high latitudes, big waves and huge winds.

Regarding not to be appropriated to most sailors I would agree to you but not probably by the same reason. The boat is rock steady stiff and very safe. If a sailor is not very experienced he can use the boat always on the first reef till he learn enough to enjoy the full speed potential of the boat. Unlike a Porche Panamera, that cannot de-tune the power of its engine a Pogo can. So it will be like driven a Porsche with incredible brakes, incredible passage speed in a turn, very precise steering without the problems that a lot of horse-power can bring to a a inexperienced driver.

On a Pogo 40 an inexperienced sailor would have, in a reefed boat, an huge stability an incredible steering a boat that could be safely let on autopilot and that will make, even reefed, more speed than most other sailboats.

It is not a cruising boat for all because this is a boat that cannot carry a big load and as all its speed potential has to do with light weight, the interior is quite spartan. Besides it is not a very comfortable boat upwind and speed on the water translates always by a harder movement. This does not mean that there are not sailors that would prefer to travel light for the fun of sailing. There are and that's why the cruising Pogos have a big waiting list. Comfort in a boat is a very subjective thing and some will trade some comfort for a superior sailing pleasure.

Originally Posted by Adelie

Would you leave a 13yr old on watch for 4hr in 20kt of wind on the Pogo?
...

Well, no they are already grow up, but when they had that age I have done that many times on a 36ft that is way less stable than the Pogo 40. And not only on 20K wind but at least one time in 30/35 winds with the boat doing 8 to 10K.

It was safe safe, if it was not I would not let my soon do that, but with a Pogo 40 I could just leave it on autopilot. On these conditions (35K downwind) a Pogo 40 can go well over 20K. For added safety or an inexperienced sailor (or on autopilot) it would only be necessary to reef that boat to make between 13 and 15K and the boat would go steady as a rock.

For what do you think is that huge positive stability for a such light boat?

Note: The cruising Pogo40 is called Pogo 12.50 even if it is actually a 40ft

[Adelie]

Originally Posted by Polux

Adelie that makes no sense! The energy (produced by a capsizing wave) required to capsize a boat is represented by the the area under the positive part of the righting moment curve. If in two boats that area is the same the inertia is completely irrelevant. What inertia does is that the boat with more inertia will roll more slowly that the lighter one, but inertia plays from both ways. The lighter boat will stop rolling and get in its feet faster while the boat with more inertia will continue to roll till eventually the righting moment is enough to stop that rolling and the boat will start to re-right as slowly has was brought to its side.

That could have a negative effect because, considering both boats hit by a wave at the same time, while the lighter boat is already on its feet, the heavier one is still deeply heeled and if a new wave hits both boats, one will have all the energy to fight back (it is already upside) while the other will only have left the energy that goes to that point of heel to the AVS point.

I have already told you that beam is not bad in what regards to capsizing. It can have some negative effects in some boats that are not well designed (in what regards ballast) and that will result in a bad reserve stability, a low AVS and a big inverse stability.

We have already saw that all those parameters are very good on the Pogo 40(better than in the Vailant).

Notwithstanding those problems there is nothing that contributes more to positive stability than beam. I have already give you the example of Multihulls, that don't have reserve stability but that in what regards to positive stability have a lot more than a monohull. For the same weight a modern multihull has a a Max GZ about 5 times bigger than a monohull.

There is a paper from the University of Southampton (the best NA school) that , based on tank testing, relates the size of the wave needed to capsize the boat with the bean of the boat. I think it was 3X the beam but I am not sure.

Why do you think that modern cruisers have so much beam? Do you think that the best yacht designers are all crazy?



Could not be sailed? The Pogo was designed to be sailed offshore! For its size the Pogo 40 is one of the most seaworthy boats around. They even have circumnavigated by the roaring forties, I mean really high latitudes, big waves and huge winds.

Regarding not to be appropriated to most sailors I would agree to you but not probably by the same reason. The boat is rock steady stiff and very safe. If a sailor is not very experienced he can use the boat always on the first reef till he learn enough to enjoy the full speed potential of the boat. Unlike a Porche Panamera, that cannot de-tune the power of its engine a Pogo can. So it will be like driven a Porsche with incredible brakes, incredible passage speed in a turn, very precise steering without the problems that a lot of horse-power can bring to a a inexperienced driver.

On a Pogo 40 an inexperienced sailor would have, in a reefed boat, an huge stability an incredible steering a boat that could be safely let on autopilot and that will make, even reefed, more speed than most other sailboats.

It is not a cruising boat for all because this is a boat that cannot carry a big load and as all its speed potential has to do with light weight, the interior is quite spartan. Besides it is not a very comfortable boat upwind and speed on the water translates always by a harder movement. This does not mean that there are not sailors that would prefer to travel light for the fun of sailing. There are and that's why the cruising Pogos have a big waiting list. Comfort in a boat is a very subjective thing and some will trade some comfort for a superior sailing pleasure.




Well, no they are already grow up, but when they had that age I have done that many times on a 36ft that is way less stable than the Pogo 40. And not only on 20K wind but at least one time in 30/35 winds with the boat doing 8 to 10K.

It was safe safe, if it was not I would not let my soon do that, but with a Pogo 40 I could just leave it on autopilot. On these conditions (35K downwind) a Pogo 40 can go well over 20K. For added safety or an inexperienced sailor (or on autopilot) it would only be necessary to reef that boat to make between 13 and 15K and the boat would go steady as a rock.

For what do you think is that huge positive stability for a such light boat?

Note: The cruising Pogo40 is called Pogo 12.50 even if it is actually a 40ft

P:
Please post a link to the paper so I can read it.
A

[Adelie]

Originally Posted by Adelie

P:
Please post a link to the paper so I can read it.
A

Never mind, I found it:
http://www.wumtia.soton.ac.uk/papers/STAB2000BD.pdf

MODEL TESTS TO STUDY CAPSIZE AND STABILITY OF SAILING MULTIHULLS

I assume multihulls behave exactly the same as monohulls, right?

[Polux]

Originally Posted by Adelie

Never mind, I found it:
http://www.wumtia.soton.ac.uk/papers/STAB2000BD.pdf

MODEL TESTS TO STUDY CAPSIZE AND STABILITY OF SAILING MULTIHULLS

I assume multihulls behave exactly the same as monohulls, right?

Why the hell would I be talking about multihulls? And why do you assume a multihull paper was what I was talking about? Because that's the only one you found in internet? You should think that anything can be find on internet or that I have an obligation to find anything for you, from the stability curves of the Pogo and Vailant to papers about stability?

Well, back on the thread subject:

The capsizing ratio of the Pogo 40 is a very bad 2.75 that of course has not any correspondence with the real stability of the boat neither with its stability curve.

As I have said, the Capsizing ratio does not work, specially with modern boats because it does not take into account the boat CG, a basic factor in what regards boat stability.

Regarding other ratio that are also used to access boat stability, the Ballast/displacement gives more meaningful information about the boat, not properly about the energy needed to capsize the boat, because does not take into account form stability, but it can give some indication about the boat reserve stability and about the boat AVS.

However it does not take into account neither draft neither keel design and its information is only meaningful when comparing two boats with the same draft and with the same type of keel.

To understand better what I mean look here, posts 1883 and 1884 of this thread that explains well the problem:

Interesting Sailboats - Page 189 - SailNet Community

But of course there is nothing better to access a boat stability than a Stability curve. That tells not the all story because does not take into account dynamic stability but it its the best tool, incomparably better than B/D and capsizing ratio put together.