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Mono hull to Cat

3306 Views 20 Replies 13 Participants Last post by  Rockter
I am going to Key West at the end of this month and want to rent a sailboat for a day: 35-40 ft. I have looked at few places renting monohulls but there seems to be many more catamarans for rent. Probably better for the shallower waters around the keys.

I am a competent/experienced monohull sailor, but the only catamarans I have sailed are hobie cats. Would my monohull skills allow me to handle a catamaran? Or would that be reckless on my part. :confused:
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BTW, is this your quote:
"It has been suggested on other sites that if a 40 odd foot boat is pitching in an anchorage, this might be reduced by hanging a bag of rocks off the stern and possibly the bow also.
Just off the top of my head, I would guess that this bag would need to weigh a thousand pounds or more to have much effect, if any.
Can one of you really, really smart engineering types on here chime in with a bit of fact based information on this, please. Thank you."
If you want an engineering answer. you don't need to hang rocks. You can just redistribute the weight, either concentrating it more towards the center or spreading it more towards the end.

The tendency of a boat to pitch excessively in certain seas is due to resonance. The driving frequency of the waves happens to match closely to the boat's natural pitching frequency. When these two frequencies get very close to each other, you end up with resonance, which will exaggerate the resulting motions. Kinda like if you slosh water back and forth in the bathtub at the right frequency, you can create really big waves.
Resonance - Wikipedia, the free encyclopedia

I won't get into the math, just say that it has to do with the moment of inertia (distribution of mass fore and aft relative to the c.g.) and dampening. Dampening is why monohulls tend to have less of a pitch problem - their bow and stern tend to flare out a lot, so as they pitch they displace a lot more water, and that tends to dampen out the pitching motion. Catamarans tend to be designed with more vertical sidewalls at the bows and sterns, so there's less dampening. (A SWATH ship is a catamaran designed the opposite way - the majority of the displacement hull volume always remains underwater. So surface waves impart little pitching or rolling moment, and the ship is rock-steady regardless of surface waves.) If it got pretty bad on an anchored catamaran, you could dampen the motion by tying buoys to the bows so that they're dragged underwater as the boat pitches forward. That's increasing the dampening.

But dampening is not the only way to overcome resonance. You can also do it by changing the driving frequency and/or natural frequency so they're no longer a close match. The easiest way is to turn the boat slightly so the waves/swells are hitting it at a slightly different frequency, and thus are no longer a match for the boat's natural pitch frequency. But if you insist that the boat must point in that particular direction, you can change the boat's natural pitch frequency by redistributing mass away from or towards the c.g. You could do it by hanging rocks too, but why take on extra weight when you can just move some water and fuel jerry cans fore and aft?

As for amount of mass, the inertia goes as mass times distance from c.g. squared. So how far you can get the mass away from the c.g. (or move a mass from a distant point to the c.g.) matters a lot more than the amount of mass. e.g. If a jerry can was originally at the c.g., moving it to the end of the bow pulpit will have 4x the effect of moving it halfway to the bow pulpit.

There's also a second order effect on catamarans because their natural pitch frequency tends to be close to their natural roll frequency (or at least, closer than on a monohull). This is what causes the hobby-horse motion. The math for this gets a lot more complicated (in a nutshell, when the boat's pitch and roll inertia are not minimum and maximum moments of inertia, the resulting motion oscillates between the two). This can be combated the same way - moving mass away from the c.g. along one axis while moving it towards the c.g. along the other. But eliminating it requires careful mass distribution so that the boat's inertia tensor is diagonal (mass is distributed fore/aft and port/starboard symmetrically).

tl;dr - there are lots of ways to combat this without resorting to bags of rocks.
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