Narrow Beam Cat-ketch?

Discussion in 'Sailboats' started by mcm, Mar 17, 2011.

  1. mcm
    Joined: Sep 2006
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    mcm Senior Member

    Is there a formula to achieve both an easily driven narrow waterline beam and at the same time prevent excessive pitching in an ocean crossing cruising cat-ketch?

    Wylie-cat 48, single mast, blue-water cruising catboat claims an easily driven narrow waterline beam, but doesn't mention pitching.

    How could the Wylie-cat 48 press 1300 sq.ft. of sail, on a mast that looks to be barely 7 ft. from the bow, with out excessive pitching? Or does it?

    The only thing i could gleam from their sparse information is that they carry what little beam they have far forward (indicating a high prismatic coefficient) and that they combine that with a lot of top side flare.

    Is that enough to eliminate excessive pitch on a single mast catboat with a narrow beam?

    If so, then it should easily work to eliminate excessive pitch on a more balanced cat-ketch with a narrow water-line beam.

    Yes or No?
     
  2. Eric Sponberg
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    Eric Sponberg Senior Member

    Personally, I have never been on a Wylie Cat 48, so my direct experience with the boat is non-existent. But having designed a few cat-ketches, I can perhaps offer some comments. First, I don't know if the WC 48 has a pitching problem. If it does, then the shape of the bow and the weight of the mast forward most likely would contribute to that, based on the photos that Wylie Cat has on their website. The hull form looks very conventional, and to my eye seems rather narrow forward with a typical raked stem. In my cat-ketch designs, I tend to make my bows even fuller forward so that there is more buoyancy to support the weight of the forward mast. The best way to find out about pitching is to actually go out and sail one and see for yourself.

    You'll also note that all the Wylie boats are cat-rigged except for the 65 and 66, which are cat-ketches. If you would like a Wylie 48 cat ketch, you should talk directly with Tom Wylie and see if he would build one for you. To my mind, if you support the same amount of sail area from the cat rig on a cat-ketch rig, then yes, you will likely reduce pitching because you are spreading the rig weight aft.

    I hope that helps.

    Eric
     
  3. mcm
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    mcm Senior Member

    Thanks for your comment Eric.

    I've been reading Larrson and Eliasson, as well as Dave Gerr, and, while they both discuss the lateral heeling moments produced by a rig, nobody mentions any forward heeling moments produced by a rig's forward thrust.

    They only discuss pitch with regards to hull weight trim.

    Yet, if a rig's forward thrust is carried through the mast and backstay to the hull then i would think there would be a substantial forward heeling moment that would have to be compensated for with an (X) amount of increased forward buoyancy especially in a cat rig configuration.

    But i haven't seen any equations addressing this: What am i missing???
     
  4. Eric Sponberg
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    Eric Sponberg Senior Member

    MCM,

    What you are referring to is trimming moments, and these are addressed somewhat in boat design. But the effects underway are small, and the reason for this is because most of the load on a rig is actually athwartships, with very little drive by comparison pushing the boat forward. So, the boat's heeling moments are high in relation to their trimming moments. Also, a boat's heel righting moment at heel is relatively small by comparison to its trim righting moment which is huge, and this is why heeling under way commands so much more attention than trimming under way.

    Another factor which comes into play here is that if you load up a rig really high in the fore/aft direction causing the boat to trim, the boat is actually very unstable when trimming down, and it very quickly can broach. Heeling and timming motions are actually coupled to a certain degree in any given boat design, so as heeling happens, so does some trim, and as trimming happens so does some heel. This coupling tends to cause trim to induce heeling motion much more quickly and forcefully than does heel induce trim. And so we are back to the original problem--keeping the boat upright in heel commands more of our attention when sailing than does optimum trim.

    Back about 25 years ago, there was a tremendous amount of discussion in design circles and the yachting press about "gyradius", which is a measure of the boat's distribution of mass relative to its center of gravity. It was brought on by a focus in the sailing community with IOR measurement rule boats and how it affects boat pitching. About 10 years later, this also influenced the IMS measurement rule. Basically, you can think of the gyradius as the average radius of all the mass in a boat. And if you were to calculate the mass moment of inertia of a boat, which is the second moment of mass about the center of gravity, the gyradius is the average moment arm of all that mass. It is generally believed that the shorter the gyradius, the higher the natural pitching frequency of the boat, and then the less likely the boat will "hobby horse". Hobby horsing occurs when the period of wave encounter (excitation frequency) matches the natural pitching frequency of the boat, and you have pitching resononce. To reduce the gyradius, one concentrates as much weight as possible towards the center of the boat. A shorter gyradius generally means that it takes a higher frequency of wave encounter to cause resonant pitching--hobby horsing. With less likelihood of hobby horsing, boat speed is faster. Gyradius is discussed in Larsson/Eliasson in Chapter 5 on Hull Design.

    One has to be aware that every boat has a natural pitching frequency, and there will always be some wave encounter frequency somewhere that will cause any boat to hobby horse. The easiest and quickest way to eliminate hobby horsing is: change your heading. Designers vary on their thinking about gyradius, some thinking that it is very important, other giving it much less importance. In general, a boat with a shorter gyradius, will have lower amplitude pitching motion overall. With lower pitching motion, the aero and hyrdodynamics on the rig and hull are more stable, and the added mass of water entrained around the hull is less, making for less added mass drag. All of this translates to faster boat speed.

    However, I can give an exception to the rule. On Bagatelle, my lightweight 44' daysailer design, in order to get the boat to have proper static trim at the dock, we had to add about 400 lbs of weight up forward along the centerline keel timber. With that weight there, the boat powers through the waves that she typically encounters in Long Island Sound, which are short steep wind-blown waves. Without the added weight up forward, Bagatelle tends to get hit too hard, causing her to pitch upward and so slow down. That is, with the weight installed (and now bolted down in place), she had more resistance to wave impacts, smoother motion, and overall faster speed.

    So the watchword is, everything is relative. In the ovearall scheme of things, attention to heeling is more important than our attention to trimming, until trimming becomes a problem.

    I hope that helps.

    Eric
     
  5. mcm
    Joined: Sep 2006
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    mcm Senior Member

    Thanks Eric,
    You really helped to clear away my preconceived notions of forward motion.
     
  6. Hansen Aerosprt
    Joined: Mar 2007
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    Location: SF Bay

    Hansen Aerosprt Junior Member

    Just spent a couple days on a WylieCat 44. Pitching is almost non-existent, even upwind at hull speed on the ocean. I've also been on the Wyliecat 65 ketch on all points of sail and it was extremely comfortable. Best to talk directly with Tom but IMHO, an easily driven lightweight hull with drawn out rocker and plenty of ballast far below the surface combined with a flexible, unstayed rig does the trick.
     

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  7. mcm
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    mcm Senior Member

    Thanks H.A. for the first hand report.
    How important is that wide, flat transom to the overall motion of the boat?
     

  8. Hansen Aerosprt
    Joined: Mar 2007
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    Hansen Aerosprt Junior Member

    MCM:
    Wide, flat transom? To my eye, none of Tom's Wyliecats from the 17 to the 65 have a wide, flat transom.

     

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