Heavy weather strategies

[billp]

I'm working on a project about power catamarans, and am curious how any of you handle rough weather under way.

Most of the cats I have input from are much larger than PDQs, more like 50 and 60 footers, but I would think some of you have experienced heavy weather as well. And I'd like to hear how you handle it. Changing course and speed are some obvious strategies, but perhaps there is much more to the subject.
Thanks.

[duetto]

well first, i've got to believe that a 41 is a whole other animal than a 34!

with that said if it's more than 3' and coming at us especially if the waves are close together, we don't go. we have the luxury of picking our time. we have ended up in those conditions numerous times and the slamming on the bridgedeck is wearing. waves on the beam can be a problem if the period is about 7' because then you get the snap roll as the wave passes under you. waves from behind are easier, our 34 tracks like a train.

when we have ended up in head seas, "tacking" helps a lot. i'd say a 30 degree angle eliminates a lot of the slamming. also we tend to drive from inside in these conditions.

your 41 has much fuller bow sections and i would expect would get up and over many waves.

[thinwater]

I'm a sailor (PDQ 32), but we certainly motor at times. Our definition of foul weather goes way beyond 3-foot waves--that's the beginning of good sailing--to include weather where you are looking up at waves.

* Tacking, of course. You need to play with the angles. The shortest course is not always the least tiring.
* Often it makes sense to chose the course the autopilot like best.
* Sometimes running the leeward engine at slightly higher RPM will make the helm more neutral. This can be VERY useful in very strong thunderstorms, when you wish to slow down but still have steerage to keep the bow into the wind.
* The lower on the boat, the less the motion.
* Downwind the autopilot will like straight down but not quartering waves, as a rule.
* Deeper water is smoother. Avoid river mouths with strong currents or crossing winds.
* Read sailing forums; sailors have to be more in touch with the weather and water.

[Stray Cat]

Of course, a catamaran hull displays just about the opposite relative motions in a seaway that a soft chined monohull will. The catamaran will be exceptionally resistant to rolling motion apart from each hull floating up and down as the wave passes beneath it. However with our slender catamaran hulls, entirely without the reserve buoyancy of a heavily flared bow and without significant lifting strakes forward, the pitching motion can become quite alarming.

This is a harmonic motion, meaning it is bouncing with a speed or frequency determined by the distribution of mass inside the boat, precisely as a pendulum moves. The closer the heavy stuff is to the center of gravity of the boat the faster the harmonic pitching, and as I said above, without aid of flared bows or lifting strakes that harmonic rise is the principle force keeping the bows from burying in an oncoming wave, and potentially tripping the boat in a disastrous pitchpole capsize.

The designer's ardent efforts to keep all the massive stuff as close as possible to the center of gravity are frequently blunted by the owner's desire to stash heavy stuff in the obviously open spaces in the bow compartments, slowing the harmonic pitching and increasing the risk of burying the bows in a steep wave.

These theoretical issues are born out by our experiences. On our maiden voyage delivering Stray Cat across Lake Erie we had eight to twelve foot steep seas (shallow water, 25 kt East winds for several days) dead astern. It defeated our autohelm, a separate issue, and induced a whopper of a baby buggy harmonic pitching action. I was too busy to think to time the harmonic pitching but my distant recollection puts it around one to two seconds. Changing speed did little to resolve the alarming boat motion but changing course (and destination) significantly allowed us to take much of that motion on the side where our roll resistance was useful. The most successful turns were around forty five degrees away from the direction of the waves.

The following spring we took Stray Cat up the wild Washington coast and down the Straits of Juan de Fuca into Puget Sound for a great summer of cruising. Here we were taking deep sea swells from all across the Pacific, and in the later half of the one day run from Astoria to Neah Bay at the tip of Cape Flattery we were dealing with very impressive twelve to fifteen footers, but as long period sea swells, periods around 12 to 14 seconds. An 'OMG' wall of water you couldn't see the top of would cruise up on the port beam, the boat would gently tilt at the inclination of the water surface as the wave passed beneath us, and then the wave would relentlessly move away to starboard. Cool. Grins all around. That worked because we were twenty or thirty miles offshore, in 100 fathoms or better of water, and the relentless Pacific swell, although it was pretty tall, was still long and the wave faces were not particularly steep. After we turn into the Straits we have the same swell systems funneling into shallower water and we are taking them dead astern again. Less fun, and 'tacking' is limited by the commercial Vessel Traffic Separation system down the middle of the Straits, the other strategy there is to hug the coast where the wind and waves moderate somewhat.

Crossing a wake can work out to be perfectly flat if you get the angle right, somewhere near 45 degrees usually.

So, long deep sea swells are no big deal, when they get steep we're in deep trouble. Short steep seas can be a real challenge, which it mitigated by turning and letting the roll characteristics manage much of the problematic boat motion.