Understanding the Ratios

[Jim Cate]

Originally Posted by Adelie

The trick is to design a lightweight boat that has those qualities.

The only design I've seen that might even come close to that is the POGO 10.5, a 35' boat with a 9' draft. Even then I think the only place it will have the advantage is righting moment at 90º. Without a significant coachhouse, the AVS is just over 120*. As far as dynamic stability goes it's beam of almost 13' gives waves a lot of lever arm to act on, the fin is high aspect minimizing water entrained during rolling and decreasing damping and given that significant effort has been made to minimise mast weight so roll inertia is also minimised.

No, the formula is still pretty good, it has limitations, but nobody has started designing boats that seriously pushing those limitations.

The limitation that the formula has is not the disregard for CG, but for distribution of mass and that it completely disregards lateral area.

The fact is that since designers that make light boats have all, to the best of my knowledge, also gone to great efforts to minimise mast weight and wetted surface area. So weight has continued to be a good proxy for roll inertia and it turns out lateral area needed for roll damping.

If you want to build a boat that's a formula buster here's the way to start:

Take a Cal 40 mold, build a hull using all the tricks you can to minimise weight: Balsa core, Kevlar and carbon fiber, vacume bagging, nomex bulkheads or whatever is lightest, undersized engine. Modify the mold to raise the cabin aobut 3-4". Instead of molding the ballast into the hull, bolt is to the bottom of the keel in a bulb. If you want to really go overboard, use depleted uranium or better tungsten. Don't change the fin shape. Going with the bulb allows you to decrease the mass of the ballast without decreasing static stability and the change in roll inertia is probably a wash. The mast is slightly taller, 2-4', but section stays the same.

Now you have a boat that is 4000-6000lb lighter, roll inertia is about the same, and dynamic damping is a bit higher with the increased draft of the bulb.

Given the decreased weight, immersed shape is going to change, but this is a good starting point. You've drastically decreased weight, significantly increased sail area, without changing the things that contribute to capsize resistance. AVS may have increased too.

Now you have beat the formula. Nobody is doing that these days.

Wheee! What a fun project... when do you start??

But a question: why do you prefer Tungsten to D-38 for ballast?

Cheers,

Jim

[downunder]

Originally Posted by Adelie

I’m in the middle of reading Marchaj’s ‘Seaworthiness, the Forgotten Factor’, so I have all this trivia on tap.

Marchaj comments on how long overhangs and highly raked rudders have lived on long after the circumstances that led to their adoption.

Essential reading for all offshore sailers. or intending purchasers of vessels.

[Adelie]

Originally Posted by Jim Cate

Wheee! What a fun project... when do you start??

But a question: why do you prefer Tungsten to D-38 for ballast?

Cheers,

Jim

I would love to try it but, alas, I lack the means. I would be happy to acquire a standard model Cal 40. Probably I will get it's smaller brother the Cal 36

Tungsten has a very slight advantage over uranium in density.
Tungsten - Wikipedia, the free encyclopedia
Uranium - Wikipedia, the free encyclopedia

Then there is also the whole radiation thing and getting permission from various gov't agencies, all with their own agendas.

I know DU isn't that radioactive, but the the licensing thing has got to be murder for time and budget.

That said, casting tungsten to the shape you want has got to be pretty involved given the extremely high melting point (6200F) so that won't exactly be cheap either. Plus the metal itself is in the $25-60/kg range.

[Polux]

Originally Posted by Adelie

The trick is to design a lightweight boat that has those qualities.

The only design I've seen that might even come close to that is the POGO 10.5, a 35' boat with a 9' draft. Even then I think the only place it will have the advantage is righting moment at 90º. Without a significant coachhouse, the AVS is just over 120*. As far as dynamic stability goes it's beam of almost 13' gives waves a lot of lever arm to act on, the fin is high aspect minimizing water entrained during rolling and decreasing damping and given that significant effort has been made to minimise mast weight so roll inertia is also minimised.

No, the formula is still pretty good, it has limitations, but nobody has started designing boats that seriously pushing those limitations.

The limitation that the formula has is not the disregard for CG, but for distribution of mass and that it completely disregards lateral area.
....

There are many boats similar to Pogo, meaning, designed with the same characteristics. One of them are the RM that are one of the French prefered long range cruisers. They, like the Pogo have a big waiting list

But there are some confusion here, beam increases (and a lot) stability. that's why the multihulls have such a big righting moment, much bigger than monohulls.

What gives lever to the waves is not beam but freeboard and big cabin height. Pogos have a reduced freeboard and a very small cabin height.

Another advantage that boats like Pogo have, in what regards dynamic stability is the reduced dimension of its keels. I mean they have a considerable draft but have a narrow foil that ends up in a bulb. When hit by a wave these kind of boats, with little inertia (light), beamy and with a small underwater lateral resistance, just slide away dissipating in movement the wave energy, a common characteristic that they share with centerboarders like the OVNI (that have that characteristic in an even greater degree).

A full keeler or a boat with a large surface keel when hit by a wave cannot dissipate the energy that way. The big surface of the keel prevent a lateral movement and all the wave energy is transformed in a rotational movement that can or not capsize the boat, depending on the wave and boat size.

[callmecrazy]

Originally Posted by Polux

There are many boats similar to Pogo, meaning, designed with the same characteristics. One of them are the RM that are one of the French prefered long range cruisers. They, like the Pogo have a big waiting list

But there are some confusion here, beam increases (and a lot) stability. that's why the multihulls have such a big righting moment, much bigger than monohulls.

What gives lever to the waves is not beam but freeboard and big cabin height. Pogos have a reduced freeboard and a very small cabin height.

Another advantage that boats like Pogo have, in what regards dynamic stability is the reduced dimension of its keels. I mean they have a considerable draft but have a narrow foil that ends up in a bulb. When hit by a wave these kind of boats, with little inertia (light), beamy and with a small underwater lateral resistance, just slide away dissipating in movement the wave energy, a common characteristic that they share with centerboarders like the OVNI (that have that characteristic in an even greater degree).

A full keeler or a boat with a large surface keel when hit by a wave cannot dissipate the energy that way. The big surface of the keel prevent a lateral movement and all the wave energy is transformed in a rotational movement that can or not capsize the boat, depending on the wave and boat size.

It would seem to me that the force hitting above the waterline would want to cause the boat to roll, but any force hitting below the waterline, would cause the boat to roll the opposite direction, or not roll..

[mitiempo]

A boat with a centerboard like an Ovni, with the board up will slip sideways with the force. A fuller keeled boat will not slip sideways and can be tripped by the keel's resistance to the sideways motion.

[Jim Cate]

Originally Posted by Adelie

I would love to try it but, alas, I lack the means. I would be happy to acquire a standard model Cal 40. Probably I will get it's smaller brother the Cal 36

Tungsten has a very slight advantage over uranium in density.
Tungsten - Wikipedia, the free encyclopedia
Uranium - Wikipedia, the free encyclopedia

Then there is also the whole radiation thing and getting permission from various gov't agencies, all with their own agendas.

I know DU isn't that radioactive, but the the licensing thing has got to be murder for time and budget.

That said, casting tungsten to the shape you want has got to be pretty involved given the extremely high melting point (6200F) so that won't exactly be cheap either. Plus the metal itself is in the $25-60/kg range.

OK, roger that, mate!

The density is essentially the same, and as you note, Tungsten is both expensive and intractable. In some areas of the world there is a lot of scrap D-38 lying around the desert, neatly cast into bullet-like shapes. I reckon that if one really wanted to use some, there would not be any gov red tape if you scavenged it (just joking, before anyone craps on me). In fact, you might get paid for ridding the environment of this deadly material... some sort of gov cleanup contract!!

But, for optimum keel material, one could consider Osmium or Rhenium, both with densities over 20 gm/cc. The bill for a few tons would stretch the budget a bit, but in the world of superyachts who cares?

Still sounds like a fun project, at least in the planning stages. Meanwhile I guess I'll continue to carry 4 tonnes of highly toxic lead around with me, just in case I need to poison someone some day!

Cheers,

Jim

[Adelie]

Opps, so sorry, not the way it works.


[QUOTE=Polux;819682]
There are many boats similar to Pogo, meaning, designed with the same characteristics. One of them are the RM that are one of the French prefered long range cruisers. They, like the Pogo have a big waiting list
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.

But there are some confusion here, beam increases (and a lot) stability. that's why the multihulls have such a big righting moment, much bigger than monohulls.
No confusion. Beam significantly improves INITIAL stability; in and of itself beam is irrelavant to ULTIMATE stability (in practice it usually has a negative effect on ultimate stability since it is usually the means to decrease the keel weight and the CG rises which is detrimental to ultimate stability); and it significantly increases INVERTED stability.

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

Pogos have a reduced freeboard and a very small cabin height.
The better to remain inverted.

Another advantage that boats like Pogo have, in what regards dynamic stability is the reduced dimension of its keels. I mean they have a considerable draft but have a narrow foil that ends up in a bulb. When hit by a wave these kind of boats, with little inertia (light), beamy and with a small underwater lateral resistance, just slide away dissipating in movement the wave energy, a common characteristic that they share with centerboarders like the OVNI (that have that characteristic in an even greater degree).
When the boat is hit by the curl, it will not slide away, the downwave rail digs in and provides even more lever for continued roll towards capsize. Damping has little effect during the wave impact, the need for keel damping is well before the wave hits. When waves are passing the boat near its natural roll frequency a boat will roll farther and farther with each wave unless the rolling is damped. Low aspect and high area fins provide better damping than otherwise. (fig 153) Having the boat rolling in synch with the waves just makes it that much easier for a wave to capsize the boat.

A full keeler or a boat with a large surface keel when hit by a wave cannot dissipate the energy that way. The big surface of the keel prevent a lateral movement and all the wave energy is transformed in a rotational movement that can or not capsize the boat, depending on the wave and boat size.
With a full keel boat the damping of the large lateral area has done it's job of minimising the amount of roll the boat has prior to being hit by the breaking wave.
[/QUOTE]

Once again, there are ways to make cruising boats faster without degrading capsize resistance and other aspects of safety. Building them like race boats and saying we have better technology now is not it.

[Adelie]

Originally Posted by mitiempo

A boat with a centerboard like an Ovni, with the board up will slip sideways with the force. A fuller keeled boat will not slip sideways and can be tripped by the keel's resistance to the sideways motion.

The Ovni is not a centerboarder, it has a drop keel. Retracting the keel during a storm creates serious problem with the CG being too high unless your have the mast in a tabernacle to drop it.

Once you drop the mast roll inertia is seriously compromised, in addition to which retracting the keel eliminates most of your roll damping. You may not need a breaking wave to capsize the boat in this condition.

[Polux]

Originally Posted by callmecrazy

It would seem to me that the force hitting above the waterline would want to cause the boat to roll, but any force hitting below the waterline, would cause the boat to roll the opposite direction, or not roll..

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.

[Adelie]

Originally Posted by Polux

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.

The boat has rolled so far with the wave that the breaker doesn't hit from the side but from below, not directly below but at some angle about 45* or lower.

[goboatingnow]

Originally Posted by Adelie

The Ovni is not a centerboarder, it has a drop keel. Retracting the keel during a storm creates serious problem with the CG being too high unless your have the mast in a tabernacle to drop it.

Once you drop the mast roll inertia is seriously compromised, in addition to which retracting the keel eliminates most of your roll damping. You may not need a breaking wave to capsize the boat in this condition.

not so OVNIs have no ballast in the keel and are rightly called centreboarders. The boat has esssentially the same stability with the "keel" up or down. The ballast is in the hull bottom. This is what leads to the poor AVS figures.

Southerlys on the other hand are as you describe, a retractable ballast keel boat. The deep keel resulting is very superior AVS figures.

Dave

[Adelie]

Originally Posted by goboatingnow

not so OVNIs have no ballast in the keel and are rightly called centreboarders. The boat has esssentially the same stability with the "keel" up or down. The ballast is in the hull bottom. This is what leads to the poor AVS figures.

Southerlys on the other hand are as you describe, a retractable ballast keel boat. The deep keel resulting is very superior AVS figures.

Dave

The ballast arrangement is not as I understood for the OVNI, but the poor stability is worse than I thought.

[Polux]

Originally Posted by Adelie

The boat has rolled so far with the wave that the breaker doesn't hit from the side but from below, not directly below but at some angle about 45* or lower.

As I have said this kind of boats don't tend to roll but to be pushed by the waves. The very good dynamic stability of this type of boat is the only thing that explains the absence of capsizes on the several mini transats from France to Brasil (on 22ft boats). 70/80 compete in each transat and the boats only weight a bit more than 2000 pounds.

In some of the races the boats got really big winds and stormy seas and have managed to continue racing without capsizes. Most of the problems they have had to do with broken masts (they have big sails), not capsizes.

The Pogos and that type of boats have hulls that come from the "family", that was developed by the designers for the ocean solo racers and produced seaworthy, fast, easily handed solo boats. Those boats were first developed on the mini class and later on the 40class. Actually the bigger sister of the Pogo 10.5 shares the hull with the Pogo 40class racer.

[Adelie]

Originally Posted by Polux

As I have said this kind of boats don't tend to roll but to be pushed by the waves.
You find video of these boats, sitting beam on to waves, sails down, not rolling and I'll believe it.

The very good dynamic stability of this type of boat is the only thing that explains the absence of capsizes on the several mini transats from France to Brasil (on 22ft boats). 70/80 compete in each transat and the boats only weight a bit more than 2000 pounds.
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.)

In some of the races the boats got really big winds and stormy seas and have managed to continue racing without capsizes. Most of the problems they have had to do with broken masts (they have big sails), not capsizes.

The Pogos and that type of boats have hulls that come from the "family", that was developed by the designers for the ocean solo racers and produced seaworthy, fast, easily handed solo boats. Those boats were first developed on the mini class and later on the 40class. Actually the bigger sister of the Pogo 10.5 shares the hull with the Pogo 40class racer.

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.

32 yr ago we had the Fastnet debacle. There is a new generation of designers and salemen now for whom that is a bit of history with no emotional punch. It even predates my involvment in sailing by a few years. Sometime something similar will happen that will galvanize this generation or the next.