Modern cats

[sv Grateful]

Originally Posted by Seaslug Caravan

For all the capsizes, Wharram catamarans are the safest with the lowest capsize ratio by a country mile .

WTF? Would you care to quantify that incredibly broad statement?

[Polux]

Originally Posted by 44'cruisingcat

What we're disagreeing with is Pollux claiming that weight has double the effect increasing beam has. How you can even compare totally different properties measured in different units is baffling for a start. Does one kilogram of extra weight increase stability more than one metre of increased beam?

I didn't understand that you did not know the units that should be used: for displacement pounds for the beam feet. If you had looked to the calculations on this page you would saw what these the units used for the formula. (I have already posted this)
https://www.wharram.com/site/how-we-...aran-stability

This is a non sense discussion. It is in fact very easy to understand, knowing something about boat stability:

RM (righting moment)=GZ (arm)xD (displacement)

GZ (the arm) is the vertical distance between de CG (center of gravity) and the CB (center of buoyancy.

It happens that on a catamaran (contrary to a monohull) that distance is approximately constant since the cat heels very little and it is approximately half of the beam (measured at the center of the hulls - less than the max beam).

That is why displacement counts two times and beam counts one or if you want, weight counts 1 time, beam counts 1/2.

[44'cruisingcat]

So one pound of displacement is equal to two feet of beam? ********.

[StuM]

Originally Posted by Polux

That is why displacement counts two times and beam counts one or if you want, weight counts 1 time, beam counts 1/2.

So you still claim that a 10% increase in displacement and a 20% increase in beam will provide the same change in stability?

Did you read Post #102?

If so, please provide a numeric example that supports your claim using your own choice of stability formula.

[PhiSig1071]

Originally Posted by Polux

I didn't understand that you did not know the units that should be used: for displacement pounds for the beam feet. If you had looked to the calculations on this page you would saw what these the units used for the formula. (I have already posted this)
https://www.wharram.com/site/how-we-...aran-stability

This is a non sense discussion. It is in fact very easy to understand, knowing something about boat stability:

RM (righting moment)=GZ (arm)xD (displacement)

GZ (the arm) is the vertical distance between de CG (center of gravity) and the CB (center of buoyancy.

It happens that on a catamaran (contrary to a monohull) that distance is approximately constant since the cat heels very little and it is approximately half of the beam (measured at the center of the hulls - less than the max beam).

That is why displacement counts two times and beam counts one or if you want, weight counts 1 time, beam counts 1/2.

How can you believe that when only one hull is lifted? An increase in weight isn't only going into the windward hull. So any increase in weight is divided between the hulls.

The formula is used to determine static stability, you're trying to use the formula to determine what has a greater effect on static stability. Those aren't the same.

[StuM]

Originally Posted by Polux

It happens that on a catamaran (contrary to a monohull) that distance is approximately constant since the cat heels very little and it is approximately half of the beam (measured at the center of the hulls - less than the max beam).

That is why displacement counts two times and beam counts one or if you want, weight counts 1 time, beam counts 1/2.

Let's try to explain it to you another way.

It doesn't matter whether you double the displacement, the beam or the half beam,
Doubling the half beam" is exactly the same thing as "doubling the beam*

DOUBLING ANY OF THESE WILL DOUBLE THE RIGHTING MOMENT.

BTW, units are irrelevant to the discussion, when calculating righting moments, you can just as easily work in metric tons and metres - the comparative ratios will be the same. (per the example below 20 ton metres = 131,214 ft lbs). If you want to calculate a common "stability index", then you obviously need to apply an appropriate numeric multiplier i.e. in your Wharram formula (Post #100) , the "14 x" for the foot/lb based calculation would obviously be a different number for the metre/ton based calcuation)

Take a boat with a displacement of 10 metric tons.
Start with a beam of 4 metres. Half beam is 2 metres.
Righting moment is 10 x 2 = 20 ton metres (aka tm or tf.m)

Double the beam to 8 metres. Half beam is now 4 meters.
Righting moment is now 10 x 4 = 40 ton metres

Instead, double the displacement.
Displacement is now 20 tons.
Righting moment is no 20 x 2 = 40 ton metres.

So again doubling displacement or beam (or half beam) will have the same effect on righting moment. One doesn't "count double, or half, the other"

[PaulinOz]

So if you increase the beam you increase stability. yes /no

If you decrease the displacement you decrease the stability. yes /no

Not worried about the percentage change just that there is a change and in which direction.

Two simple questions, requiring two simple answers.

By the way this is rhetorical, most of us already know the answer.

[Polux]

Originally Posted by PhiSig1071

How can you believe that when only one hull is lifted? An increase in weight isn't only going into the windward hull. So any increase in weight is divided between the hulls.

....

You are not understanding the picture that comes from one of the references in what regards understanding sailboat's stability. That is not an increase of weight, that is the position of the CG that when the boat is heeled goes to the side of the heeling.

[UpOnStands]

Lets make the list a bit more complete

So if you increase the beam you increase stability. yes /no
If you decrease the displacement you decrease the stability. yes /no
If you reef the sails you increase stability. yes/no
If you replace a performance rig with a cruising rig you increase stability. yes/no
If you increase displacement by adding toys you decrease your performance. yes/no

[Polux]

Originally Posted by PaulinOz

So if you increase the beam you increase stability. yes /no

If you decrease the displacement you decrease the stability. yes /no

Not worried about the percentage change just that there is a change and in which direction.

Two simple questions, requiring two simple answers.

By the way this is rhetorical, most of us already know the answer.

I give up. How can you be asking that after all those explanations and links regarding catamaran stability?

If you decrease the displacement you decrease stability YES

If you increase beam you increase stability YES

Today performance cats and condo cats have about the same beam. The Lagoon 450 has even more than an Outremer 45 and it is not efficient to increase more the beam.

After a given beam ratio, on a given cat, the only way to increase stability is to add displacement.

The Lagoon 450 has almost the double of the displacement than the Outremer 45 with a smaller beam therefore the Outremer 45 has about half the stability of a Lagoon 450.

On a cat RM (righting moment) = D (Displacement)X B/2 being the beam measured at the hull's center line.

Stability index that take into consideration the stability in proportion with sail area don't take into account that the boats can be reefed or that in extreme situations can be sailed in bare poles.

Even if considering the stability index, considering full sail, performance cats are easier to capsize due to a bigger SA/D. As I have showed the beams of modern performance cats are not bigger than the ones of condocats.

That is the reason why a same sized condo cat is much more dificult to capsize than a performance cat cruiser.

That is the reason why on the last year all big cruising cats capsized by gusts are performance cats and also the reason why during the same period o condo cats where capsized by gusts.

Why is this so dificult to understand for some?

[PhiSig1071]

Originally Posted by Polux

You are not understanding the picture that comes from one of the references in what regards understanding sailboat's stability. That is not an increase of weight, that is the position of the CG that when the boat is heeled goes to the side of the heeling.

You said that weight counts double for beam, I am referring to that, not to the picture.

[UpOnStands]

Big Wave Rider has been recovered and your best friends want you to help them sail it with the home port being the Derwent River Tasmania (where it originally overturned).
You know that Big Wave Rider is an ex-speedster and holds the around Australia record with a crew of one.
Your friends are newbies but are keen to learn sailing.
Do you
1. fill the bilges with lead shot in epoxy OR
2. reduce the sail area significantly.

[Polux]

Originally Posted by UpOnStands

Lets make the list a bit more complete

So if you increase the beam you increase stability. yes /no
If you decrease the displacement you decrease the stability. yes /no
If you reef the sails you increase stability. yes/no
If you replace a performance rig with a cruising rig you increase stability. yes/no
If you increase displacement by adding toys you decrease your performance. yes/no

No, the boat stability is not dependent on sails, it is equivalent to the amount of energy needed to capsize it.

That energy can be created by the impact of a huge wave or can be created by a gust on the sails or a combination of the two things.

Of course if you have a reefed boat the energy of the same wind will be less and lesser the rotational moment created by it. The boat stability is what allows the boat to sail with the righting moment created by the displacement and beam opposing the moment created by the wind force on the sails and the boat windage. When that force is bigger than the boat stability the boat will capsize.

If you put a smaller mast you increase slightly stability lowering the CG and creating less windage and as with a smaller mast you will carry less sail with the same wind it will create a lesser rotational moment. But you can do the same reefing a bigger sail on a bigger mast.

Regarding increasing stability with "toys" it all depends where you have the toys. They can lower the CG or rising it. If the toys are carried down below it will have a positive effect on stability (more weight) providing they don't rise the CG significantly.

Regarding weight it is also important to know if it affects the boat balance or not so the location is important for the boat stability.

[UpOnStands]

Originally Posted by Polux

No, the boat stability is not dependent on sails, it is equivalent to the amount of energy needed to capsize it.

Why then do you introduce the Warram formula? It specifically indicates the reefing wind speed and includes the factors of center of effort (sails) and sail area (sails)

BTW. The understanding is that heavy cats are more prone to capsize due to wave action as the bottom hull digs into the face of the wave and the boat is unable to slide quickly.

[arsenelupiga]

Originally Posted by UpOnStands

Why then do you introduce the Warram formula? It specifically indicates the reefing wind speed and includes the factors of center of effort (sails) and sail area (sails)

BTW. The understanding is that heavy cats are more prone to capsize due to wave action as the bottom hull digs into the face of the wave and the boat is unable to slide quickly.

hum.... there is around 3000 lagoons out there, probably half not in charter, many circling globe, and I heard of 2 capsizes when sailing/motoring. One delivery of L 380 in winter and the other with only mainsail up and motoring when caught in large gust between islands.

And then unknown number at moorings/berths in cyclones when gusts > 150 kn. That is when houses go flying...