Modern cats

[fxykty]

Remember that the various indexes are static - they don’t take into account the actual performance characteristics of the boats. So while the Kelsall index shows a slightly lower gust could capsize the O45 vs the L450, the behaviour of the boats when hit by the gust is vastly different. Acceleration in a cat is akin to heeling in a monohull - it’s a way of absorbing the wind energy.

Finally, how many capsizes have there been? We know of two Atlantic’s recently, but only one for sure was due to wind. Do we know of others? Based on my shakey memory of statistics, a sample size of two is anecdote and not statistics.

[44'cruisingcat]

Originally Posted by Polux

Posted by Polux: [COLOR="Blue"][B]..and as weight counts two times for stability .

Because you say so?

[Polux]

Originally Posted by 44'cruisingcat

Because you say so?

Again? Even if you don't know It is so difficult to check facts before taking embarrassing attitudes:

"The ratio of the stabilizing moment of the boat (displacement multiplied by the half-beam or D*Bcl/2) to the overturning moment (sail force times the height of the center of effort, Hce, of the sail above the center of lateral resistance of the hulls and boards or keels) provides the Stability Index, a useful comparison factor."

If you don't understand the above, D*Bcl/2 it refers to the boat stability, that is the way to in a simplified way find it, the "overturning moment" refers to the energy needed to capsize the boat trough the force of wind applied on the sails and windage.

D is displacement, Bcl is the beam measured at the center line of each hull and /2 means that the beam counts only half for stability compared to displacement, or said in another way, displacement counts two times, beam one.

http://www.multihulldynamics.com/ima...THULLS2013.pdf

[44'cruisingcat]

In what units?

[StuM]

Originally Posted by Polux

....and as weight counts two times for stability you can see that the Lagoon is way more than two times harder to capsize than the Outremer 45.

That's the second time you've stated that. Would you care to explain what you mean?

[StuM]

Originally Posted by Polux

If you don't understand the above, D*Bcl/2 it refers to the boat stability, that is the way to in a simplified way find it, the "overturning moment" refers to the energy needed to capsize the boat trough the force of wind applied on the sails and windage.

D is displacement, Bcl is the beam measured at the center line of each hull and /2 means that the beam counts only half for stability compared to displacement, or said in another way, displacement counts two times, beam one.
f

OK, now I've read that and see where the problem lies. You simply don't understand basic mathematics.

D*Bcl/2 can be re-written as Bcl*D/2 and is still the exact same formula and gives exactly the same result with the same numbers values.

So by your logic, simply re-arranging the formula makes beam count twice for stability compared to displacement.

[44'cruisingcat]

Originally Posted by StuM

OK, now I've read that and see where the problem lies. You simply don't understand basic mathematics.

D*Bcl/2 can be re-written as Bcl*D/2 and is still the exact same formula and gives exactly the same result with the same numbers values.

So by your logic, simply re-arranging the formula makes beam count twice for stability compared to displacement.

[Go Wildcats]

Originally Posted by StuM

.....

D*Bcl/2 can be re-written as Bcl*D/2 and is still the exact same formula and gives exactly the same result with the same numbers values.

.....

TRUTH

[rallyman1122]

Originally Posted by Polux

Wrong, that is cruising. Sailing is about the performance you can have while sailing and the fun you can have with that. You mistake sailing with cruising.

It is while cruising that a lot of other things can be done, not while sailing.

There are many types of cats, the name condo cats has to do with the ones that focus in maximizing interior space at the cost of performance, special upwind and light wind one. Nothing wrong with that and those are by far the dominant trend by a huge margin.

Most of them motor upwind anyway so not a problem there because they have good sized engines and a good performance while motoring. They are cruising boats. Nothing wrong in motoring a cruising boat.

Sailing is sailing, it can be used while cruising or just for the fun of it.

Sailing is not the same as cruising. There are boats that cruise full time motoring, there are others that motor most of the time upwind and there are others that are designed to sail very effectively upwind and even on those some sailors motor upwind.

There are sailing boats that are designed to sail most of the time, they can sail with very light winds, others are designed to sail only when the true wind reaches 8 or 10K and therefore will have to use much more the engine.

A condo cat is not among the first, it is the type that maximizes the interior (at the costs of sailing performance) so it seems to me evident that the ones that chose a condocat chose it mostly for its bigger interior otherwise would be choosing another type of cat or boat. No sneer in that, just logic.

Nothing wrong in wanting the bigger interior one can have for a given size, a cruising boat is also a house and the name condocat has nothing depreciative since it is from all the sailingboats the one that offers the best house, therefore the name.

Sailing is different for each one of us. Your definition is not the same as mine nor as the next person. Yes, there is an art to maximizing the performance of a given boat. And each design is different, thank goodness. But to say cruisers are not sailors is somewhat denigrating to cruisers.

And my desired boat, the St. Francis 50,could be termed a condomaran. 4 staterooms, 4 heads, 2 powerful engines, the ability to carry large amounts of water and fuel. Even has a deep freezer and a 4 burner stove should you want it. But also capable of 10 to 12 knots when sailed properly.

My point being, sailing is defined by the people and the boats, not by any one person's specific definition.

[Polux]

Originally Posted by StuM

OK, now I've read that and see where the problem lies. You simply don't understand basic mathematics.

D*Bcl/2 can be re-written as Bcl*D/2 and is still the exact same formula and gives exactly the same result with the same numbers values.

So by your logic, simply re-arranging the formula makes beam count twice for stability compared to displacement.

Very funny, but the formula it is not Bcl*D/2 but D*Bcl/2 and it is not only on this one but on all, it is the beam that is half the value, not the displacement.

You can see that on the formula that is posted on James Wharran site (that regards not only static stability but static stability in relation to the wind force).


Again it is not (weight)/2 but B(beam)/2.

By the way, it is an interesting article with a comparison of the stability between two existing cats, one lighter, other heavier. The article is 25 years old and at that time many heavier cats had more beam than more sportive lighter performance cats.

Today the beam of both types tend to be very similar so the diference in stability towards the heavier boat would be even bigger (if the beam was the same on the two boats)
https://www.wharram.com/site/how-we-...aran-stability

Anyway you cannot increase beam (on a light or heavy cat) more than to a given ratio so you cannot increase the stability that way but you can increase weight and add stability that way.

[Factor]

Originally Posted by Polux

\ but you can increase weight and add stability that way.

Patently absurd statement. Patently. Absurd. Please stop.

[StuM]

Originally Posted by Polux

Very funny, but the formula it is not Bcl*D/2 but D*Bcl/2 and it is not only on this one but on all, it is the beam that is half the value, not the displacement.

Again it is not (weight)/2 but B(beam)/2.

The only thing that would be funny, if it were not so sad, is your total lack of comprehension about basic arithmetic.

Let's put some sample numbers in and see the result:

Start condition:
Displacement: 20,000 lbs
Bcl: 20 foot
Overturning Moment = D x Bcl / 2 = 20,000 x 20 / 2(= 20,000 x 10) = 200,000 ft/lbs

Now add 10% more Displacement:
Displacement: 22,000 lbs
Bcl: 20 foot.
Overturning Moment = D x Bcl / 2 =22,000 * 20 / 2 (=22,000 x 10) = 220,000 ft lbs

Alternatively, add 10% more Bcl:

Displacement 20,000 lbs
Bcl = 22 ft
Overturning Moment = D x Bcl / 2 = 20,000 x 22 / 2 (=20,000 x 11)= 220,000 ft lbs

So it doesn't matter whether you increase Displacement by 10% or Bcl by 10%, you get the same overturning moment of 220,000 ft lbs (i.e. a 10% increase).

[44'cruisingcat]

Originally Posted by Polux

Very funny, but the formula it is not Bcl*D/2 but D*Bcl/2 and it is not only on this one but on all, it is the beam that is half the value, not the displacement.

You can see that on the formula that is posted on James Wharran site (that regards not only static stability but static stability in relation to the wind force).


Again it is not (weight)/2 but B(beam)/2.

ay.

You conveniently ignore the fact that, in this example, the lighter boat has significantly greater sail area.

That would be appropriate for a racing boat. In a cruising application the lighter boat would be more likely to have LESS sail area than the heavier one.

It's one of the benefits of lightness. The boat is more easily driven, so can reduce sail area, so loads are much lower, which means many items such as the standing rigging can be lighter, winches can be smaller, which saves even more weight....

[tuskie]

Originally Posted by Factor

Patently absurd statement. Patently. Absurd. Please stop.

No, not absurd. In line with Archimedes' lever principle, where beam is the lever and the boat is the mass.

Using James Wharram's formula: in catamarans with identical beam and sail area, adding 10% weight will increase static stability and hence dynamic stability by 10%.

More weight equals more stability. Which is Polux's point.

Archimedes said about mass and leverage (no doubt in greek), “Give me a place to stand on, and I can move the earth.”

Would you have said "Patently absurd. Please stop?"

[Pizzazz]

I think tuskie described it very well. It is a simple force x arm length formula, on one hand you have the sail x center of effort, on the other hand you have the weight x (half the distance between hull). More weight, more spread between the hulls, less SA and lower center of effort all increase stability. You need to take the square root because the force of the wind on a sail is a square function of the speed.

Thus, if you load up a light catamaran it will be more stable (at least initially, until it flips, then it would be even more stable .