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Old 12-05-2008, 13:56   #46
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As for the pitching motion it is fundamentally connected to rocker in the hulls.

I beg to differ. Pitching motion is caused by sea state. Weight in the ends can make this worse. Rocker is under the waterline mostly so does not come into play.

It is alot more to do with bouyancy distribution and Prismatic coefficient as far as I can see.

Too much bouyancy too far forward creates too much upward motion too fast. The same is valid if the transoms have too much bouyancy, because they will then lift the sterns and push the bows down.

Regardless, of how well designed a boat is, there will always be some sea states that are very uncomfortable at a given speed.

Around here we have mostly short choppy waves, so I feel that after 18 years of sailing different cats and looking at others, there aren't any that don't get uncomfortable at some stage. Some are better than others,bad weight distribution makes it worse, but generally bigger is more comfortable.

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Old 12-05-2008, 14:00   #47
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The key to multis, particularly cats, is getting the weight out of the first and last thirds of the boat. Get all the weight central and low.

I keep hearing this stuff and yet almost all the engines are "in the rear" and the tanks forward of center, I guess to balance it all.

Why is that?

Are most all of them wrong? Just trying to compromise?
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Old 12-05-2008, 14:48   #48
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I keep hearing this stuff and yet almost all the engines are "in the rear" and the tanks forward of center, I guess to balance it all.

Why is that?

Are most all of them wrong? Just trying to compromise?
Yes, if all the weight is concentrated in the centre, especially in a light boat accelerations when pitching will be increased - this is a physics fundamental, but, of course, other aspects of the design also have an effect.

I am still away on the boat so can't give any exact references for anyone wanting to follow up on that but I recall there is some coverage of this point in Marchaj's (spelling?) "Seaworthiness" where he talks about foredeck accelerations and crew comfort and in "Principles of Yacht Design" by Larsen & Eliassen (spelling?) - two readily available books and probably in many largish libraries. These use monos as examples but the laws of physics apply to both cats and monos (and everything else too ).

So, taking your comment regarding distribution of the engines and tanks away from the centre on sailing cats I wonder if that is done on purpose for the above reason?

Maybe someone with professional design experience of cats will risk sticking their head in and commenting?
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Old 12-05-2008, 18:45   #49
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First, the boat Sean pictured is a FP 42-43, the four opening ports on the port side give that away. The 38s and smaller have only two. Secondly, I agree with Alan above. I know that Fountaine designs their Cats with alot of weight carrying ability forward of the mast. I carry 300' of chain, 45lb Delta, 2 extra Fortress, windlass, and two built in 85 gallon water tanks just forward of the mast. A lot of ability to adjust the weight from the forward third+ of the boat. I suppose if the Cat Sully saw was all rope rode and fairly empty water tanks it's easy to see why it would be more apt to point skyward. But who knows where, why and how much weight any boat you see out there is carrying. Too many variables. My guess looking at the pic that it's just a weekender and probably carrying a lot lighter load than possible. Just my .04 cents worth (price adjusted for inflation)
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Old 12-05-2008, 23:30   #50
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I can tell ya, I noticed a profound difference in how my boat handled steep waves when I removed the 45 CQR and fitted my new lighter anchor.
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Old 13-05-2008, 03:09   #51
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centreing weights

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I can tell ya, I noticed a profound difference in how my boat handled steep waves when I removed the 45 CQR and fitted my new lighter anchor.

Yes Alan we drag the anchoring gear off the bow of our cat if we have a nasty windward passage to negotiate. I have never heard , read or seen any cat sailor do anything other than centre weights for best performance and comfort. The only reason we see sail-drives etc in the stern of cats is to free up the accommodation. Nothing to do with sailing performance. Pete Brady did lots of design work with motor sailor cats in Australia and centred his motors.
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Old 13-05-2008, 06:52   #52
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The only reason we see sail-drives etc in the stern of cats is to free up the accommodation.
.......and to permit isolated engine rooms.

Just another compromise. Pick yer poison.

Dave
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Old 13-05-2008, 12:47   #53
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Great thread. I suppose Maxing did the sensible thing and waited for the weather / wind to change. I'd try a bit of sail first if there was an option to tack into the wind. Any weight moved forward needs twice as much moved rearward to maintain the cg where it should be. Spare but empty 'long range' tanks could be useful if space permits but generally theres not much hand moveable stuff in the middle.
By the by, I'm surprised there is so much motoring going on but I haven't done much sailing yet, guess it goes with time, destination and fuel costs. Certainly the modern cruisers are not so weatherly or streamlined above decks and slamming is only a problem in rougher weather.
Broadblue tried bulbous noses but with nothing conclusive and a few lobster pots hung up decided they were not for coastal cruising. (Ron Underwood comments at Southampton 2007) Perhaps the real problem was the range of vertical motion they had to cope with. Would trim plates (power boat style) at the rear help?
The older Prout's, particularly the 37' snowgoose's, went for canoe hulls with no particular problems. Bows were slender and well flared, the flare inhibits buried bows and damps up/down motions. The french style for upright stems gives little of either. Some years ago we did a sailing day at Portsmouth with several boats available. One of our tests was to stand each on a bow, on the french boat we were there for half a second. Pitchpole stability test failed. Note also that the french introduced escape hatches because of the number of boats overturning. (No mono men listening, are there.) Fashion and Market are driving design the wrong way. maybe. Certainly cats will pitch whatever we do with them, it can be reduced but the designer has the biggest input, not the skipper. The skippers choice is to know his boat and adjust his voyage to suit the conditons. C'est la vie.
Centralised weight should improve sailing performance. Distributed weight will slow down the responsiveness, not a factor under moderate power.
Just a tuppence worth from sound engineering point of view.
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Old 23-05-2008, 09:59   #54
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Because we are light, our bows climb over short chop instead of punching thru it. When the wave period approaches our LWL, the motion gets really uncomfortable. That happens in shallow water. The solution is to quarter the waves, but in the normal confused state of the Chesapeake, thats difficult. When we are overloaded, there's too much mass in the fore and aft ends, so our pitch oscillations are amplified like a teeter-totter. Its not a unique problem of catamarans: I've been lifted three feet in the air standing at my post on the backstay of a Navy 44 racing on the Bay. But remember, the Mids get extra points if the coach wets his pants.
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Old 29-05-2008, 05:50   #55
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They ALL do it!

Well, as others have said, ALL catamarans do this bouncing bows.

Today, I saw a Chris White (55??) motoring into 1 foot, smooth rollers and having its bows pop out a couple feet on each one. The boat was sighted off Fisher's Island, off Mystic and called "(something)bella." I couldn't make out the first few letters of the boat's name.
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Old 29-05-2008, 06:41   #56
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Two observations (Catalac 10M & Chris White 55) hardly constitutes proof positive that all catamarans do it (pitch, hobbyhorse, bounce).

Pitching is generally caused by too much weight in the ends of the boat. Accommodations and storage areas that extend much forward of the mast, solid fiberglass decking forward, engines and storage too far aft, not enough “empty” stern or bow hull area extending aft and forward accommodations & storage all contribute to increased pitching.
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Old 29-05-2008, 07:07   #57
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I have been offshore in both a Lagoon and a Fontaine-Pajot (in Thailand) and they both hobbyhorsed upwind. I didn't find the pitching especially uncomfortable, but it was a bit surprising at first. I was out on the Chessie last weekend and every big cat out there was doing the same thing to a degree (in a one-foot chop).

(disclaimer: I have sailed on several cruising cats and I think they have their advantages. The views expressed are solely those of the author and do not necessarily represent "the management" of cruisersforum.com )
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Old 29-05-2008, 07:47   #58
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Pitching is generally caused by too much weight in the ends of the boat. Accommodations and storage areas that extend much forward of the mast, solid fiberglass decking forward, engines and storage too far aft, not enough “empty” stern or bow hull area extending aft and forward accommodations & storage all contribute to increased pitching.
With respect Gord, I disagree. I know it's the common mantra of the multihuller that "all weight must be kept central", but I submit that has nothing to do with reducing pitching motion. With all the weight concentrated around the longitudinal centre, the bows (and by extension, the sterns) will be easily moved by any waves/swells. Weight in the ends would give them greater inertia, and they would be less apt to pitch. Of course, once the pitching started it would oscillate more and take longer to stop, thus necessitating fuller bow sections - more buoyancy results in a quicker return to equilibrium. Just my hypothesis.

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Old 29-05-2008, 09:01   #59
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It's not really pitching or hobbyhorsing per se, it is simply that catamarans ride over the waves and do not plow through them. It is a lively motion, so might feel like hobbyhorsing, but hobbyhorsing is when the boat continues to rock back and forth after the wave has passed.

If a catamaran is going into a 2' wave train, then the bows will look like they come 2' out of the water as the wave passes under them. Longer waterlines will make this motion relatively much less than it is for shorter waterlines (2' incline on 55' is much less movement than 2' on 34').

All catamarans, except grossly overweighted ones, do "it", but "it" is intrinsic to being a catamaran. Hobbyhorsing and pitching are different beasts that do result from hull shapes, weight distribution, etc.

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Old 29-05-2008, 11:15   #60
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Wave period is a function of water depth, becoming shorter and sharper in shallower water. Non ballasted boats have sharper motions because they weigh less, and have less inertia to overcome. Most objects tend to move around their center of gravity when their equilibrium is disturbed, unless there are other limiting factors*. Objects with the weight maximized in the center and kept light at the ends are more easily moved out of equilibrium, but restabilize quicker.

If you had an analytical bent as a child you might recall that it was easier to ride a teeter-totter with just two kids, and it was a chore pushing back up when ther was a lot more weight on it.

Designers strike a balance between pitching motion and recovery from deep bow penetration by changing the fineness of entry and motion damping aft bouyancy, and current thinking says that a flat section aft is faster because of some wave generating dynamics. These are things Naval Architects argue about.

That all means that us catamaran sailors need to get used to the fact that our boats sit on top of the water, and have sharper motions becasue we more closely follow the top surface of the water than boats with ballast because they get a smoother ride from the pendulum effect of the keel (even in pitch) and the inertia of greater mass. Chris White likes a fine entry and long bows for less disruption in waves, while other designers like fat forward sections to prevent submarining. Your catalac was designed more for interior volume at a certain length than for rocket ship performance, which would not have sold well in that era. You have a lot of boat for a small sacrifice in top speed.

The worst place for a 34' Cat is in 16 to 30' of water depth in wind driven waves, because the wave period almost matches the boat's waterline. A much longer cat bridges these waves, but at 45 to 65' finds itself suspended amidships in the same water. If it's heavy in the ends, it will start hobby horsing too, and can have a pretty wild ride.

The solution for the Chesapeake Bay (where more than half of it is between 16 and 30', with confused waves) is 1. a longer boat, but not too much longer, 2. turn 30 degrees to the principal wave direction, and 3. slow down. If you had really been in a hurry you would have taken highway I-95!

Someone else is sure to point out that this delimma applies (to a lesser degree) to shoal draft monohulls in the same hull length.

*Designers can move the dynamic axis of pitch forward or aft by changing the finess of entry, and the the sharpness of reaction can be changed by the relative positions of the dynamic pitch axis to the center of gravity. Nobody wants you to put a big weight in the bows!
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