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Old 11-05-2008, 07:11   #31
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I had a bow bulb on my 23'alloy tri going across Torres straight. 6 fathoms of water, tidal races and 25knots. The boat was comfortable. I have a Jarcat and in similar conditions it bounces around. I have worked on 60' tuna boats that bounced around , with me frequently in free fall when trying to get some shuteye. My bunk was in the foc'sle.
In small power cats, the Marlin Broad bills have bow bulbs to help damp the motion in chop and I have seen some big tourist cats with undrwater bow extensions. In fact, one of the local tour operators nick named 'Cat ba spew' changed its character with a couple of old oxy bottles welded to its underwater bow section.
Harryproas, with high prismatic coefficient, zero rocker, and two hulls of different length and therefore different pitching frequency are less susceptible.
I believe it is a combination of light weight ends with insufficient water plane damplng and having two hulls of the same pitching frequency that allows this to occur.
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Old 11-05-2008, 07:35   #32
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99.9% an FP Belize 43.

All boats, monos, multi's etc etc will pitch and behave very differently in different wave cycles depending on speed, direction, weight location etc, but I agree to a point that multi's will be more susceptible to choppier steep wave formations as they do not carry the weight like mono's.
I think that's it. I guess I misjudged the size of the boat. It was certainly a 40 footer though, not the size of my boat.
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Old 11-05-2008, 08:17   #33
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Too bad you don't have a photo from the other boat showing how much of YOUR hull was leaving the water without you knowing it.
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Old 11-05-2008, 12:51   #34
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Too bad you don't have a photo from the other boat showing how much of YOUR hull was leaving the water without you knowing it.
Without me knowing it?? ha ha ha

Are you kidding?

I think I had mentioned that my cabin sole in my forward stateroom flew up so hard that it missed its notch and landed in the bilge.

Also, my computer was damaged pretty severely, drawers flew out and dishes that were in the sink were on the floor.

I think I knew it.

It was like T-5, 4, 3, 2, 1 BLASTOFF!
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Old 11-05-2008, 13:21   #35
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yeah, but wouldnt you love the photo....
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Old 11-05-2008, 13:23   #36
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yeah, but wouldnt you love the photo....
I would.
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Old 11-05-2008, 14:28   #37
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Hey there Sean!,
I think I agree with Alan. I have a 40# bruce and 275 of chain up forward. I bounce, but I don't belieive my bows leave the water. Also if I'm motoring into the chop, I have found my genie eases the bounce a bit, however still expeimenting. This boat is definatly differnt to handle than my hobie. Then again, I never went out in the higher wind conditions like I have recently.
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Old 11-05-2008, 16:04   #38
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Bearing in mind that my experience is mainly with power cats which I have managed the design, build and commissioning of, my understanding is the following applies also to sailing cats.

Cats tend to pitch more than monos (there are extremes in both camps though) because of the need to carry hull buoyancy well forward, that for both seakeeping and load carrying reasons (cats go bow down with loads towards the bow). These sections are also normally, and properly, U shaped which also contributes to the problem. As mentioned by others there have been lots of "experiments" with power cats to reduce this tendency to pitch such as bow bulbs and other protrusions and attempts at wave piercing, but none wholly successful. For the cats I have been involved with the problem has been solved by designing the vessels so that they can maintain high planing speeds, foil assisted, in most anticipated sea conditions in the area of operation. But once they get outside of that envelope the motion in heavy seas will have even the professional crews green both from pitching/heave and from the firm and jerky roll due to cats' high GM and passengers will not be carried.

Pitching accelerations tend to be high in cats due to their low longitudinal moment of inertia - monos have either deep keels with lead on the bottom or if long keeled ballast distributed lengthwise and are of higher displacement. The need to keep cat displacement down for performance (even in big powerful power cats there is a big emphasis on keeping weight down) also contributes to low longitudinal moment of inertia.

So, as some have suggested pitching accelerations can be reduced by getting weight out to the ends of the boat. In a mono, transferring weight to the ends imposes a small speed reduction but for a cruising boat the magnitude of the loss is not big enough to be important (but if pitching is reduced, the loss may be made up for). I imagine, for a cat the same minor speed loss will apply plus the added problems of the low weight carrying ability of cats when placed in the bows and their perfomance sensitivity to added weight that would not normally be carried - so any weight put in the bows should be transferred weight, not weight added for the sake of reducing pitching.

Moving on, all vessels have a resonant pitching frequency and if this condition is driven by waves of appriate pitch and height then very violent pitching will result. It looks to me that the photo shown earlier of the mono pitching shows it pitching resonantly. Resonant pitching, when it occurs, can be controlled by changing the speed of encounter with the waves - that by changing speed or by bearing off or up.

The resonant pitching frequency of a boat is dependant, among other things (length, being an obvious one), on its longitudinal moment of inertia. The bigger the moment of inertia the slower the frequency of resonant pitching frequency and the smaller the moment of inertia the faster the frequency of resonant pitching. So one can change the pitching performance of the boat insofar as tendency to pitch by moving weight (and as pointed out before, one can change the accelerations by doing the same).

So if the boat pitches a lot one can look to increasing or reducing the resonant pitching frequency. If you want to reduce pitching in long wavelength sea conditions concentrating weight in the middle of the boat will bring the boats resonant frequency up higher than the frequency of encounter with the waves - but in such seas resonant pitching is not often a great problem to the crew as accelerations are lower. In steep short seas it is likely not possible to get the resonant pitching frequency of the boat higher than that driven by the speed of encounter with the waves so one may find it best to reduce that frequency by getting weight out to the ends so increasing the longitudinal moment of inertia.

As an example of this applied to a mono we sail mainly in an area where very (and I mean "very") steep seas are encountered - we went for a fine bowed boat, alot of buoyancy aft, and carry alot of weight in the forepeak in way of chain, anchors and warps. Being fine in the bow reserve buoyancy is gained from freeboard and our weight carrying ability forward is compromised (ie readily goes bows down with weight added forward) and in this I think is probably similar to, but not so exaggerated as a cat - with tanks full and the weight mentioned above, the boat trims statically about 50mm bows down from the design waterline. But it goes well into the seas we encounter and does not seem to have any problem digging the bow in when the same seas are following due to the reserve buoyancy from high freeboard forward.

Hope that is of some useful contribution and that the "wolves" stay away. Time for a beer .

John
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Old 11-05-2008, 16:28   #39
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I am far less knowledgable than most folks on this site, and I have stifled myself from replying on this thread several times because of that.

But I feel compelled to add that from what I have been taught, putting weight forward in a sailing cat is not a good idea. If you have significant winds, and are sailing other than downwind, putting weight forward would decrease the margin of buoyancy that prevents the leeward bow from burying itself, and the resultant risk of pitchpolling. I hasten to add that in tiny seas, or moderate winds, this would not seem to be an important factor, and if you are motoring, not a factor at all. But most folks don't shift weight around based on whether sailing or motoring, and if following the discussion on this thread, might put lots of weight forward when going sailing.

There. I feel better for having brought it up at least.

Kevin
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Old 11-05-2008, 17:24   #40
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It depends on the circumstances. You don't just put weight forward and leave it there. It is about balancing the boat for the situation. And that would be a one off. It is one of the negatives with a cat. When I was sailing and racing smaller cats, it was about where you placed you body weight to the conditions and wind and speed. Got it wrong and you got wet and on the odd occasion, it hurt.
In Sean's case, he could do two things. Take the chop at a slight angle till the affect of both hulls was now one, which would mean tacking up the bay instead of straight lining, or he needs to get weight forward to slow the action. Weight aft will do the same thing as far as slowing the action, but it allows the bows to stay skyward too long. Maybe the effect Sean was seeing in the other boat.
If you take the design of a cat and meld it to a mono hull, you would have a vastly over bouyant bow multiplied by a very light weight. A fact in some pure bread racing mono's. The result is the flat pounding they take and the fact you can't slow that things down. The cat has the same affect of that, but spread over two hulls far apart. So motion is slightly different. Plus add in all the other comments about keel weight and mast and not being able to dissipate energy. All are partly right, but all add together to get the closer result.
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Old 11-05-2008, 19:12   #41
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The worst boat I was ever on for this was a sail training ship ' the Alma Doepel" Please forgive the spelling. It was made overly stiff to satisfy the regulation righting moment. I reckon the people working on the bow sprit were having 2g driving them up and having to hold on against 1g down again. This was in Twofold Bay with about a 2-3m swell wrapping round the headlands giving a bit of interference patterns.
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Old 11-05-2008, 20:30   #42
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The solution to poor behavior when motoring into headseas with a mono is almost always to put up some sail, and motorsail. It doesn't take much sail, and you can point almost straight into the wind while powering along. The ride is enormously more comfortable than just a motor ride alone.

Pardon my ignorance, doesn't this work for multis?
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Old 12-05-2008, 01:55   #43
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The solution to poor behavior when motoring into headseas with a mono is almost always to put up some sail, and motorsail. It doesn't take much sail, and you can point almost straight into the wind while powering along. The ride is enormously more comfortable than just a motor ride alone.

Pardon my ignorance, doesn't this work for multis?

Yes , you are dead right. Get some main up and filling with the motor ticking over and life improves immeasurably.
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Old 12-05-2008, 03:24   #44
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Get some weight up forward Sean. How much chain do you have? If it is short duration, then you grin and bear it. If it is for awhile, you have to shift some weight around. It's all about balancing out your boat.

Never never never do that

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.

As for the pitching motion it is fundamentally connected to rocker in the hulls.
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Old 12-05-2008, 13:33   #45
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Never never never do that
OK, that's your opinion and as I assume you have had a great deal of Multi experience, I respect that. But what I have said has come from experience as well. So I am not sure saying "never never never" is quite so cut a dry. It is still about balancing your boat. Now remember we are talking about Sean's situation here. Not general big sea's handling. So can you tell me why getting some weight forward will not work in this situation??
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