Flipping Cats - More 'Flippable' than Monos ?

[niel12]

As boat builders improve on boat building so does Marine architects and with so many cats and mono's on the water surely there must be some sort of do's and dont's than could help avoid a flip. Naturally there is those waves that is unpredictable and a strong engine might even the odds somewhat in control.

Downside could be weight and fuel consumption but i believe that one can matmatically determine what size engines is suitable. As a 40' cat with a 18 Hp engine seems to be way out in power to weight ratio.

Is there any one with ideas on this matter?

[robertsapp]

Are you asking for opinions on the relative stability of both designs, or specifically about whether catamarans would benefit from more powerful engines? Regarding the general issue of stability, it is important to remember that catamarans derive theirs from their greater width relative to their weight, while monohulls derive theirs from the righting moment generated by their suspended keel. Since all vessels continually seek their position of maximum stability, this means a catamaran is most stable when sitting upside down on the surface. A monohull is most stable sitting right side up on the bottom of the ocean.

[multihullsailor6]

Some years ago an approx. 45' Privileage cat called "Blue Ship" did a circumnavigation. One of their storm tactics, when there was searoom, was to run downwind with full mainsail bouyed by the fact that they had two big and heavy engines at the very back to keep the bows up.
Your 18hp seems a bit small for a 38' cat!

[niel12]

I was wondering about that!

[sailingaway221]

I had said before that no cat had EVER flipped with all its sails down.... but I was proven wrong.. apparently a 44 footer managed to pitchpole in a severe storm on a sea anchor ...but i have been assured this was the only one.....

[niel12]

About 3 years ago i designed a unit i called catbrake. I employed the services of a Architect/ Engineer who assisted me with the design and since i have been playing with a database of the various parameters of wind strength, wind direction, wave height, time of day, longatude/latetude and the various posistions of the catbrake when employed. The objective of this was to build a database of relative conditions so that when certain conditions are experieced catbrake will be automatically employed. The catbrake design will bring a catamaran to almost standstill on top of a wave without the use of a drogue or a parachute or ropes. To employ the catbrake you would need to upgrade your hydraulic capacity. This is not a selling stint as i have not yet produced it comercially but would like any input.

[S/V Illusion]

The practical righting moment of any cat is zero. Any monohull is better than 0 and therefore an improvement. No gadget can change the above.

[niel12]

Without a doubt!

[sandy daugherty]

This thread is doomed.

[capngeo]

Originally Posted by sandy daugherty

This thread is doomed.

Very likely!

[Factor]

Cats are far far less "flippable" than monos - Far less. CJ MArchaj indicates that the comparative roll moments of inertia

If we call the average mono as a value of 1 (.65 is the rig, .15 the hull, .15 Ballast and .05 other)
then the "average" cat has a value of 2.03 rotating on centre point, and once rotating on hull a value of 3.69.

As well as demonstrating that, the work also was interesting in that it showed that the rig contributed so much to roll moment. Indeed the work demonstrated why once a mono rolls, if it self rights before it sinks, if it loses a rig in the process then it is likely to roll much easier , i.e. using the 1 as the mean a mono without rig drops to .35 - or put another way, if mono without rig is 1, then mono with rig is 3ish, cat on centre line is 6.5 and cat on one hull is 10.5.


Please dont attack me on this work, if you disagree take it up with Marchaj or the report of the US Yacht racing union and the US society of naval architects and marine engineers.

I am not saying that all monos will flip sooner than all cats, what I am saying is that the research shows a particular position and illustrates interesting data about the effect of rigging on rolling over.

[rallyman1122]

Here we go again. What? The 333 previous threads on this subject were not enough?

[niel12]

Originally Posted by rallyman1122

Here we go again. What? The 333 previous threads on this subject were not enough?

Regret to have bothered you, skip the tread if its boring or a repeat!

[cat man do]

Read the first part of this page from Lock Crowther's design notes



from this thread http://www.cruisersforum.com/forums/...ised-5910.html

[Amgine]

Donald Street said on his first sail aboard a catamaran the designer let him take the helm, but kept nagging him about not burying the lee hull, something a bit foreign to his plank-on-edge monohull background...