Modern cats

[UpOnStands]

Originally Posted by StuM

And if you had an experience with catamarans or were at least able to understand the graph you reproduced earlier, you would realise that the "area under a righting moments curve" is largely irrelevant to stability in catamarans.

This is good. Area under the righting curve is a crock of black tank scrapings.
Marchaj develops his "righting curve" by raising one hull to rotate the catamaran in degrees and calculates the LOAD to raise the hull as a function of ANGLE. He focuses on the effect that the center of mass moves closer the axis of rotation as the angle of inclination from the horizontal increases. This is a theoretical truism but is it not what we are interested in.

To correct Marchaj. The maximum righting moment of a catamaran is calculated by multiplying
MASS LIFTED (weight of hull being lifted plus bridge deck etc. etc.)
by
LEVER ARM (horizontal offset of center of MASS LIFTED from center of floating hull (rough approximation its the beam minus hull width)

[Polux]

Originally Posted by StuM

Again you reveal your lack of understanding of the issue.
Stability is not the same as righting moment.

Of course a boat has different righting moments depending on heel.
Stability equals the total area under the Righting moment curve and that is not the same thing.

Originally Posted by StuM

Righting moment is only one side of stablity. Sails are the primary component of the other side of stability.
...

And you say that I lack understanding about the issue LOL
"Sails are the primary component of the other side of stability", that is a good one..

Originally Posted by StuM

And if you had an experience with catamarans or were at least able to understand the graph you reproduced earlier, you would realise that the "area under a righting moments curve" is largely irrelevant to stability in catamarans.

That is even better!!!!!

Originally Posted by StuM

Did you not understand the relevance of "Sail Area and CofE" in the Wharram Stability Index you reproduced earlier?...
The critical issues are the peak righting moment and the angle of maximum righting moment.

Yes, the Stability index regards not catamaran stability but the relation between a catamaran stability, the total sail area of the boat and the wind able to capsize while it sails without reefing (full sail).

But you know, a catamaran can be reffed and reefed or not its stability is the same even if the rotational moment created by the wind on the sails is different. When that rotational moment is bigger than the boat stability the boat capsizes.

On two catamarans with the same beam one having the double of displacement the heavier boat will only capsize with the double of the sail area and wind (same wind) that is able to capsize the first. That is pretty simple. If you think this is irrelevant regarding catamaran stability I find it really odd.

Boat stability is calculated the same way being it a monohull or multihull and it is calculated multiplying GZ (the arm between the horizontal distance between CB and CG by the displacement. That gives you RM. The positive area below a RM curve equals total boat stability, the energy needed to capsize the boat, being it a cat or a monohull, even a ship.



"Stability is evaluated both in relation to wind and wave action. In particular reference will be made to Prof. Marchaj's recent work - ' Seaworthiness the Forgotten Factor.' Multihulls are studied under the same criteria as monohulls are evaluated in the book, giving a clear comparison between old and new multihull designs, and monohulls, particularly with regard to capsize in wind and waves. Other factors affecting seaworthiness, such as pitching, surfing, rolling, yawing etc, are discussed......

shows the same righting moment curve for the cat versus a typical modern cruiser/racer monohull. The energy required to be input into the yacht in order to roll it from 0 degrees to the point of capsize (90 degrees in the cat and 135 degrees in the Mono) is given by the area under the curve. ... "

However in all cases, in order to a capsise to occur, the energy from the wind and the waves (equal the area under the righting moment curve) has to be transferred to the vessel in the form of rotational energy. "
Considerations for Seaworthiness

[Polux]

Originally Posted by UpOnStands

...

To correct Marchaj. The maximum righting moment of a catamaran is calculated by multiplying
MASS LIFTED (weight of hull being lifted plus bridge deck etc. etc.)
by
LEVER ARM (horizontal offset of center of MASS LIFTED from center of floating hull (rough approximation its the beam minus hull width)

That is even a better one!

[UpOnStands]

Originally Posted by Polux

That is even a better one!

Its better because it is more accurate.

Oh, and just to let you know
"When that rotational moment is bigger than the boat stability the boat capsizes."
should be
When that rotational moment is bigger than the boat's righting moment the boat capsizes."
of course the wording is now more accurate but your conclusion may be right, may be wrong.
see this Non-capsizing catamaran.

https://www.youtube.com/watch?v=NbzvmH0NKWo

[rallyman1122]

How did we get here, so far off the original topic. Everyone there is a post about catamarans, it seems some comes in and starts bashing away. How about we just agree to disagree, accept that some people like monohull's, some like catamarans and some prefer tri's.

[daletournier]

Originally Posted by rallyman1122

How did we get here, so far off the original topic. Everyone there is a post about catamarans, it seems some comes in and starts bashing away. How about we just agree to disagree, accept that some people like monohull's, some like catamarans and some prefer tri's.

Debate is healthy, you learn stuff..... the attitudes are the problem.

[tuskie]

Originally Posted by rallyman1122

How did we get here, so far off the original topic. Everyone there is a post about catamarans, it seems some comes in and starts bashing away. How about we just agree to disagree, accept that some people like monohull's, some like catamarans and some prefer tri's.

It's not a cat vs mono thing.

It's just that some of our catamaran brethren have great trouble accepting and acknowledging that all other aspects being similar, adding weight to a catamaran increases stability. Modern catamarans have largely shifted to heavier, MORE STABLE designs. I'm very aware that increasing weight has other, adverse consequenses such as loss of performance, costlier rigs, etc but there are advantages too. That's what's being discussed.

More weight = more stability.

They are arguing about formulae, areas under curves, units of measurement and lots of other distractions in the hope that this basic fact will be hidden. I know its hard acknowledge any benefits of increased weight as their's is a religious belief that "light is good" and any statements to the contrary is heresy.

[PhiSig1071]

Originally Posted by tuskie

It's not a cat vs mono thing.

It's just that some of our catamaran brethren have great trouble accepting and acknowledging that all other aspects being similar, adding weight to a catamaran increases stability. Modern catamarans have largely shifted to heavier, MORE STABLE designs. I'm very aware that increasing weight has other, adverse consequenses such as loss of performance, costlier rigs, etc but there are advantages too. That's what's being discussed.

More weight = more stability.

They are arguing about formulae, areas under curves, units of measurement and lots of other distractions in the hope that this basic fact will be hidden. I know its hard acknowledge any benefits of increased weight as their's is a religious belief that "light is good" and any statements to the contrary is heresy.

The biggest issue isn't that people are refusing to accept that "adding weight to a catamaran increases stability", they're calling BS on Polux's remark that weight increases stability by a factor of 2 when compared to increasing beam.

[daletournier]

You can debate and educate if you disagree without being aggressive, that's the problem here, not the fact you disagree . It's easy to be rude and aggressive behind a keyboard.

[UpOnStands]

Originally Posted by tuskie

Modern catamarans have largely shifted to heavier, MORE STABLE designs. I'm very aware that increasing weight has other, adverse consequenses such as loss of performance, costlier rigs, etc but there are advantages too.
More weight = more stability.

BS. This argument would have some weight (pun) if absent the additional mass the design was so unstable as to be dangerous. So therefore the designer/builder adds double refrigerators, large freezers, washing machines, dedicated gensets, A/C to all cabins, TV media centers in all cabins, fancy wood finishes etc etc in an effort to raise the stability to an acceptable level.

Are you stating that if all the toys etc are removed the boat is unstable?

Certainly the righting moment is lower (if the toys are removed)but the question is, is the reduced righting moment dangerous in service?
Most monohulls have lower freeboard than comparable catamarans. Do you deny that this is a serious design flaw and constitutes an operating hazard?? Good lord, fail to secure the hatch properly and rain will sink the boat.
Should we jump into the monohull fora and raise a stink?

[StuM]

OK, final one on this from me.

I've just realised that we've ended up confusing two issues.

Initially the discussion was about the likelihood of a catamaran overturning in windy conditions - This got into talking about catamaran stability via Polux's quoting of Wharram's Stability Index and his misconception about the scale of effect of displacement v beam.

But he then shifted to concentrate solely on inherent static lateral stability which as he correctly states is a matter of righting moment or CoG and CoB.

A number of us missed this change of tack and have been continuing to talk about stability in terms of the likelihood of capsize from wind effects.

[44'cruisingcat]

Originally Posted by tuskie

More weight = more stability.

You may think it's that simple, but it's not.

That's what we've been discussing. You could have a light cat with wide beam, vs a heavy narrow one.

Then there's the rig height and sail area. A heavy cat is likely to need to carry more sail. It's sail area that capsizes boats.

[44'cruisingcat]

Originally Posted by tuskie

It's not a cat vs mono thing.

It's just that some of our catamaran brethren have great trouble accepting and acknowledging that all other aspects being similar, adding weight to a catamaran increases stability.

But you keep insisting that if I add weight to my boat it will be the end of life on earth....

[44'cruisingcat]

Originally Posted by StuM

Did you not understand the relevance of "Sail Area and CofE" in the Wharram Stability Index you reproduced earlier?

.

Obviously not, and it would seem he's not alone.

[UpOnStands]

It would be possible to extend Wharram's stability index to account for coach houses fly bridges etc., but if you buy a modern catamaran the naval architect will specify, the recommended reefing points, which have been calculated far more accurately.

If Polux has no interest in sailing performance, only "stability", he should immediately add 1000 kg to his keel weight. His IMS racer/cruiser will immediately gain a fantastic amount of "stability".