Modern cats

[UpOnStands]

The only truly safe monohull - designwise.
notice that all seacocks and thru-hulls are safely above the WL.
The trees prevent rouge waves from flooding the companionway and sinking the yacht.
Should sailing be desired, small wheels can be added to the frame.
Note that MOB drills could prove fatal. A warning notice to this effect must be attached to the stern rail.

[arsenelupiga]

Originally Posted by UpOnStands

The only truly safe monohull - designwise.
notice that all seacocks and thru-hulls are safely above the WL.
The trees prevent rouge waves from flooding the companionway and sinking the yacht.
Should sailing be desired, small wheels can be added to the frame.
Note that MOB drills could prove fatal. A warning notice to this effect must be attached to the stern rail.

not really 100 % - if keel falls off, we have a problem in winds above 25 kn.

[UpOnStands]

Bother, forgot about the risk of keel failure. It does happen in the Atlantic all too often. Sadly no photos are possible given the boats are at the bottom of the ocean.
So you think the operating manual should stipulate 3x 10 foot sand screws P&S with heavy duty straps to restrain the boat if winds exceed 20 knots?
Amazing how much fun you can have knowing that you and your boat are completely safe from being lost at sea.
Now I think we can return to the OP's original comment about the height of modern catamarans.

[barnakiel]

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

"If you increase the beam you increase stability. yes /no"

Caeteris paribus, yes.

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

cp, No. It further depends on what part of the boat gets lighter.

"If you reef the sails you increase stability. yes/no"

cp, No. It depends on what reefing system you you have.

"If you replace a performance rig with a cruising rig you increase stability. yes/no"

cp, No. This further depends on the weight of the cruising rig and its height.

"If you increase displacement by adding toys you decrease your performance. yes/no"

cp. Qualified yes. If you = performance = speed.

Just my take. Many of the answers depend on further discussion of what you actually mean - e.g. by a 'cruising rig'. Cruising rigs tend heavier but also shorter. In effect there can be either no change in stability or else a gain or loss. Etc.

Cheers,
b.

[UpOnStands]

Originally Posted by barnakiel

Cheers,
b.

"If you decrease the displacement you decrease the stability. yes /no"
cp, No. It further depends on what part of the boat gets lighter."
OK, if you start with a 3 level flybridge condo, remove the top 2 levels, and shorten the mast accordingly you should improve stability - . But not according to Marchaj and Warram agrees only through a drop in COE.

"If you reef the sails you increase stability. yes/no"
cp, No. It depends on what reefing system you you have."
This needs elaboration.

"If you replace a performance rig with a cruising rig you increase stability. yes/no"
cp, No. This further depends on the weight of the cruising rig and its height."
Seems as if you are talking about static design stability.
IMHO - Cruising rig = shorter mast with smaller sails.
Not sure if you mean the cruising mast is made of lead and weighs a whole lot more than the performance mast.
mast boom sails fitting rigging, the works.

I find Marchaj to be wrong? (probably his paper states his assumptions)-- at least the RM = half beam*displacement*gravity referenced earlier and Warram's heuristic, like his cats, is incomplete.
Have at it.

[UpOnStands]

My opinion? For a catamaran the only stability that matters is dynamic stability under sail and the key factor is the rapidity with which wind pressure can be shed. Turning into the wind, reefing. But I'll sign off for some shut eye.

[barnakiel]

Expanding on our earlier exchange:

"If you reef the sails you increase stability. yes/no"
cp, No. It depends on what reefing system you you have."

This needs elaboration.

ELABORATION:

Sails reefed down (slabs) pull the weight down. I believe this could make the boat more statically stable (by this I understand more area under the curve, not necessarily more initial stability at 0 degrees). However, less weight aloft reduces moment of inertia. So the boat may end up more stable initially but at the same time it is easier to dynamically tip her.

Sails furled around stays and into the mast seem to make no difference to stability in any way. I think (???). I assume.

"If you replace a performance rig with a cruising rig you increase stability. yes/no"
cp, No. This further depends on the weight of the cruising rig and its height."

Seems as if you are talking about static design stability.

IMHO - Cruising rig = shorter mast with smaller sails.

Not sure if you mean the cruising mast is made of lead and weighs a whole lot more than the performance mast.
mast boom sails fitting rigging, the works.

ELABORATION: It is not just weight. It is also the arm. The center of the rigging happens to be quite far away from the center of the (total) weight. Although in case of a cat, due to the distances between the hulls, the difference may/will be smaller than in case of a mono.


I find Marchaj to be wrong? (probably his paper states his assumptions)-- at least the RM = half beam*displacement*gravity referenced earlier and Warram's heuristic, like his cats, is incomplete.
Have at it.

---

>>>

Doh. I think we must see the static (initial), the static (the area under the curve) and the dynamic (the inertia) and then also all the rest of the factors that make up the stability. It is not up to me to weigh them. It is up to the owner of that cat.

But I am not versed. I only make experiments and break things. And I sail boats. Overall, to me light and strong is great and heavy is nearly always bad. I could actually opt for a less stable but lighter boat. Pigs do not fly and heavy boats tend to get terrible bashing from the sea. They simply pretend to be like that rock in France. Except the French tend to sail the lightest boats of us all and if I were to judge them by their success I would be converted.

Cheers,
b.

[Polux]

Originally Posted by UpOnStands

How about a little rewrite to make understanding easier.
"Assuming that the beam of a built catamaran cannot be increased, the only way to increase its design stability after launch is to increase its displacement."

You missed the point. 40 years ago many non performance cats had less beam than performance cats.

Today that is not true. Performance catamarans had extended their Length/beam ratio to the max sailing efficiency and stability (you cannot increase the beam after a certain point without having more problems than benefits) and the condocats have done the same, for having the bigger possible interior space.

That results evident on a comparison between the Outremer 45 and the Lagoon 450, being the Lagoon considerably beamier.

So it is not about the design, both type of boats are already designed with the maximum beam they can have without losing efficiency.

Simply to make a design with more stability without increasing the length, the only option is to increase displacement. Off course you can also lower the center of gravity using ballast but that is normally an option on cats.

I don't know if you noticed that most condo cats start their line with 40ft, some with 36/37ft but almost all performance cruising cats start their line with 42/45ft.

The reason is simple, a performance cat with lesser length (has necessarily a smaller beam) will present more capsizing risks. Capsizes are bad publicity to any company and they are not willing to take the aggravated risk proposing boats with a smaller stability.

Condocats manufacturers can propose boats with a smaller length because the much superior displacement gives them much more stability than the one a much lighter performance cat with the same size will have, being today the length/beam used on modern cats about the same on both types.

[Polux]

Originally Posted by barnakiel

...
"If you reef the sails you increase stability. yes/no"

cp, No. It depends on what reefing system you you have.

...Cheers,b.

Stability has nothing to do with sails. Stability of a boat, cat or monohull, is given by the area under a righting moment curve and it is the energy the boat opposes to a capsize being it by a wave or by the pressure of the wind on the sails (or both things). If the stability is exceeded by any force creating a rotational movement the boat will capsize.

Regarding the furling system you may be right if you refer to a main furler system and a slab furling system but again it has not to do with the pressure on the sails but because on a furler system the CG of the sail will be higher with the sail furled on the mast (when reefed) but except if we are talking about some big reefing, the diference will not be big.

A boat stability counteracts the force that the wind makes on the sails and therefore the amount of sail that is used should be way less than the available stability, for a bigger margin (due to gusts), but that is a all different story even if related with boat stability.

[44'cruisingcat]

Originally Posted by Polux

You missed the point. 40 years ago many non performance cats had less beam than performance cats.

Today that is not true. Performance catamarans had extended their Length/beam ratio to the max sailing efficiency and stability (you cannot increase the beam after a certain point without having more problems than benefits) and the condocats have done the same, for having the bigger possible interior space.

That results evident on a comparison between the Outremer 45 and the Lagoon 450, being the Lagoon considerably beamier.

So it is not about the design, both type of boats are already designed with the maximum beam they can have without losing efficiency.

Simply to make a design with more stability without increasing the length, the only option is to increase displacement. Off course you can also lower the center of gravity using ballast but that is normally an option on cats.

I don't know if you noticed that most condo cats start their line with 40ft, some with 36/37ft but almost all performance cruising cats start their line with 42/45ft.

The reason is simple, a performance cat with lesser length (has necessarily a smaller beam) will present more capsizing risks. Capsizes are bad publicity to any company and they are not willing to take the aggravated risk proposing boats with a smaller stability.

Condocats manufacturers can propose boats with a smaller length because the much superior displacement gives them much more stability than the one a much lighter performance cat with the same size will have, being today the length/beam used on modern cats about the same on both types.

This is such a load of rubbish it's hard to know where to start.....

There are MANY performance cats under 40 feet! You can buy RACING CATS less than 20 feet.

The reason many performance cruising cats are longer is because it costs very little to extend hulls, and making them longer means you can have useable space in them while maintaining high LWL:BWL ratios.

Unlike some people on this forum who have never even sailed on a cat, catamaran designers are not fixated on capsizes.

[Factor]

Originally Posted by Polux

Stability has nothing to do with sails. .

Seriously. I mean seriously. Okay next time I am in 75 knots I will leave full sail up.

This is just getting ridiculous, two people with no understanding of multihulls (you and Seaslug) are stridently prosecuting ill informed ideas.

[sailjumanji]

Originally Posted by Polux

Stability has nothing to do with sails.

Then all of this is a bunch of dribble (putting it nicely). So the stability of a catamaran is the same whether sails are up or down? And the stability wind speeds you presented a couple of pages back are the same for no sails vs with sails? Does that make sense to you?

Frankly, the whole concept of "static" stability is a crock. All of this analysis for a boat that is not moving? Are we calculating wind speed to blow over a boat that is tied to a dock? Geeze, we just got thru Hurricane Harvey here about a month ago. I guess those guys that stripped all of their canvas and sails off the boat were morons, because according to your calculations that didn't increase the stability of the boat. And its even static conditions, since it was tied to the dock.

So we've already dribbled on to the point of someone pointing out that catamarans are more unstable than monohulls. (Again, I would rather be floating in an inverted stable cat, than sitting on the sea floor in a rolled monohull.) Its not like we haven't heard that before. But the more we learn about your static stability calculations, the more it appears to be an invalid metric.

[StuM]

Originally Posted by sailjumanji

But the more we learn about your static stability calculations, the more it appears to be an invalid metric.

Especially when our two inexperienced cat critics focus solely on static lateral stability and totally ignore fore/aft and diagonal stability. If they had ever sailed a cat they would appreciate that these are equally relevant design factors.

[UpOnStands]

Originally Posted by Polux

That results evident on a comparison between the Outremer 45 and the Lagoon 450, being the Lagoon considerably beamier.

So it is not about the design, both type of boats are already designed with the maximum beam they can have without losing efficiency.

So the Outremer is not as beamy as the Lagoon but it is impossible to redesign the Outremer to increase its beam to match that of the Lagoon because it "would lose efficiency"?
What ever do you mean by efficiency?

[StuM]

Originally Posted by Polux

Stability has nothing to do with sails. Stability of a boat, cat or monohull, is given by the area under a righting moment curve

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

Righting moment is only one side of stablity. Sails are the primary component of the other side of stability.
Did you not understand the relevance of "Sail Area and CofE" in the Wharram Stability Index you reproduced earlier?


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.

The critical issues are the peak righting moment and the angle of maximum righting moment.