What is stopping a cat being as wide as it is long?

Discussion in 'Multihulls' started by andycarr9999, Aug 21, 2011.

  1. andycarr9999
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    andycarr9999 Junior Member

    I know very little about boat design but do know about sailing fast boats. I am soon to commence the build of a 21 foot 'offshore' carbon beach cat. Storage room is not an issue so how wide should i make it? Speed is the sole aim but it has to be able to deal with a swell.
     
  2. Tim B
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    Tim B Senior Member

    The greater beam you have, the better the static stability, so in flat water you can carry more sail. However, this tends to result in harsher motions in a sea-way (though with a cat you've pretty much got that anyway).

    Unfortunately, increasing beam increases the bending moments at the cross-beam connections to the hull. Consequently the cross-beams need to get bigger to stop the whole boat flopping around.

    I would suggest that you have a play with some catamaran dinghies, and see what you like. Don't go anywhere near the Carbon until you've got a workable design! All the investigation you do now will pay massive dividends later.

    Good luck,

    Tim B.
     
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  3. yipster
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    yipster designer

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  4. Corley
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    Corley epoxy coated

    normal cat length to beam ratios are 2 to 1 or slightly wider you want the leeward hull to depress and hoist the windward hull out slightly to reduce wetted surface area a cat in the size range your proposing is liable to capsize and I'd like to be able to right it myself without external help. Curved lifting foils could be worth looking at to help with righting moment.

    A square cat would be very difficult to right and more likely to overcome its diagonal stability and pitchpole. There are challenges even with carbon achieving adequate stiffness some cats some address the issue with vestigal hulls above the water at rest like Decision 35 or VSD Catamaran or use a stressed central member like the SL33 by Hakes to achieve adequate stiffness I doubt that would scale well to a very small boat. There are reasons why catamarans stick to relatively modest length to beam ratios.
     
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  5. Doug Lord
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    Doug Lord Flight Ready

    Beam of Racing Cats

    =================
    Convention, mostly. There is a feeling among some that a wide cat won't tack as well as a narrow cat. In my experience with numerous test model RC cats that's not true. And, apparently, some full size racing cat designers tend to have found that the advantages of wide beam outweigh the disadvantages :
    1) Hydroptere.ch-
    a. LOA 10.85m ( 35.58' )
    b. Beam 10.4m (34.11 )
    Beam 96% of length
    -
    2) Decision 35
    a. LOA 10.81 m ( 35.46' )
    b. Beam 6.89 m ( 22.59' )
    Beam 64% of length
    -
    3) Alinghi AC cat
    a. LOA 33.5m ( 110' )
    b. Beam 25.3m (83' )
    Beam 75% of length
    -
    4) AC 72
    a. LOA 22m ( 72' )
    b. Beam 14m ( 46' )
    Beam 64% of length
    -
    In contrast, the new AC 45's are more conventional( beam wise)-
    5) AC 45
    a. LOA 13.45m ( 44' )
    b. Beam 6.9m ( 22.6' )
    Beam 51% of length
    -
    6) Extreme 40's
    a. LOA 12.19m ( 40' )
    b. Beam 7.92m (26' )
    Beam 65% of length
    -
    7) Whites Dragon(full flying foiler) http://www.boatdesign.net/forums/multihulls/whites-dragons-mini-hydroptere-esq-35062.html
    a. LOA 7m ( 22.96' )
    b. Beam 4m ( 13.12' )
    Beam is 57% of length
    -----
    Any new racing cat design should also consider the use of curved lifting foils(Nacra 20, A Class Cats and others) for better handling and increased speed thru "foil assist" or full flying lifting foils as on "Whites Dragon" and Hydroptere.ch .
    By all means read this interview with Martin Fischer-top beachcat designer: http://catsailingnews.blogspot.com/2010/05/cs-interview-martin-fischer.html
    Trailerability may be a consideration at the length you're designing. As a suggestion, there is lots and lots of video of the AC 45, Extreme 40 , Nacra 20 and others-it may pay to watch as much as possible and to sail a NACRA 20 or similar boats with and without foils to learn all you can before commiting to a design.
    Good Luck!
    PS-I strongly suggest you build an rc model(as large as you can) to test your design-you can learn about foil placement, rig placement, and tacking ability with such a model and you can easily experiment with greater or lesser beam.
    PS #2-all design involves compromise: while curved lifting foils are proven speed producers they also reduce righting moment and that needs to be understood and designed for. Same is true with the surface piercing foils on Whites Dragon and Hydroptere .ch.
    PS #3-thanks-and you know who you are-appreciate it!

    Pictures, L to R- AC 45, Alinghi AC cat, Hydroptere.ch, Whites Dragon, Decision 35(Seb Schmidt design), Extreme 40, NACRA 20 w/foil assist

    click on image-
     

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  6. Richard Woods
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    Richard Woods Woods Designs

    You may want to contact Dick Ogilvie at Torpoint as he has built a couple of boat similar to what you propose. One may still be at Southdown, Millbrook. See photo. I have sailed it, a very fast boat - great fun

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     

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  7. Ilan Voyager
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    Ilan Voyager Senior Member

    On a beach cat the max beam is dictated by light weather sailing and practical handling.
    A square cat means a lot of weight because of the size of the beams with a acceptable rigidity. A wobbling cat is not efficient. But a bit of flexibility is good for dissipating some of the stresses.
    In light weather, if the cat is too beamy, the 2 hulls will stay in the water and the wetted surface will "scotch" the cat on the water.
    There is also a practical max size of the sails to be handled by a hand without winches, even with the best ratchets. Too big and the sails are very hard to keep tuned.
    Also you're limited by the length of the hulls and their ability to withstand longitudinally the power of the sails. Sudden capsizing is the defect of the too wide beach cats, unless the hulls have a lot of volume (thus weight, and bad perfs in light and medium wind) and/or foils.
    Experience says that a 65-70% cat (21*14-15 feet) is a practical max for a good all around beach cat, with good offshore capability for raids. Lateral "wings" (like on the Hobie 18 or a lot of other sail boats) are a cheap and efficient way to get more stability, and it's comfortable for sailing long hours.
    Rather that a "very-very" wide cat is better to work on the tuning of the sails/mast ensemble (easy power/depower), plus eventually foils (that's another matter).
    As always design is matter of compromise.
     
  8. andycarr9999
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    andycarr9999 Junior Member

    Thank you all for response to be quite honest i am overwealmed.

    Richard i actually live very near Dick and was lucky enough to go out on concept 25 when he still had it. I will be speaking to all my local experts before going ahead with anything. Just want to get as much advice as possible. I have some c-class hull moulds (25 foot) that i may use. Have been out on a friends Nacra today and desperately need to go faster. Any further ideas are very welcome. Thanks again all.
     
  9. Doug Lord
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    Doug Lord Flight Ready

    ====================
    Andy, why are you, apparently, limiting yourself to a cat?
     
  10. Corley
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    Corley epoxy coated

    Alinghi 5 is the closest to a square cat listed and was a failure relative to USA 15's trimaran format. They both racked of course but on the trimaran it was controllable but on the cat the windward hull kept dropping into the water when they tried to fly a hull in the very moderate swell in Valencia. Alinghi 5 used a Y frame that had been used successfully on Le Black years before it didnt seem to scale well to a monster multihull. The wing that USA15 used certainly had a part in the boats success but the platform was so far superior it probably would have won with a soft sail as well.

    Doug is right lifting foils will reduce righting moment as the fulcrum of rotation moves inboard from the hull they are mounted in but they will give you more speed. Here again the trimaran format wins because you can move the beam out to the extreme width without compromising overall dynamics, Hydroptere is wider than it is long.
     
  11. Doug Lord
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    Doug Lord Flight Ready

    Beam-cat vs tri

    ==========
    I kinda disagree: I think what we saw was a lesson in the effectiveness of a solid wing vs an almost perfect square top "conventional" rig. I don't think it was so much tri vs cat as it was wing vs sail.
    But Alinghi was a winner in at least one respect: it showed conclusively that the old wives tale about wide cats not being able to tack was just that......
    I agree-a trimaran is the way to go for top speed. The Gougeon brothers proved that in the 60's when their "Victor T'' beat all current C- class cats at that time. Trimarans can also allow beachcat killing speeds from a center cockpit where the crew doesn't have to run side to side-ideal for old farts and others who love speed with a bit less work. The problem with high speed tri's utilizing the maximum beam that is possible is that the RM(righting moment) can become so great that the thing won't fly the main hull until its real windy. A solution to that is a bi-foil configuration on the main hull that lifts the main hull in very light air and then unloads the foils as speed increases. I think tri's designed like this with "foil assist" in very small amas are the future of small beachcat killing trimarans. Flys the main hull in light air-uses the max beam for RM in heavy air....

    picture-"Victor T" C Class trimaran:

    click on image for best detail----
     

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  12. andycarr9999
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    andycarr9999 Junior Member

    I want to stick to the catamaran idea as it is what i know and it is what i originally wanted, i am very aware of a problem i have which is 'lots of talking not a lot of doing' and hence want to stick to by original plan. I am very interested in the curved foils though, just need to learn more about them and try to get a prototype to test out positions etc. Most of my budget will be going on the rig as i have lots of boat builder friends that are going to help me with the hulls. Any tips on rigging, basic plan is a pretty conventional big square top main, fractional jib and big spinny. I also want to be able to reef it. Something along these lines...

    http://www.hobiecat.com/forums/viewtopic.php?f=4&t=31377
     
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  13. Corley
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    Corley epoxy coated

    It would be interesting to know if the nacra 20 curved foils and trunks will be available as a seperate part it would save you a lot of building if you could integrate what they are already making on a production line they sound like a good match to the kind of boat your proposing. If your not building to a rule and space isnt an issue then a little more length might not go astray maybe consider making your boat 25' rather than 21' the extra waterline length is always helpful.

    Unfortunately there will never be a test as to how the alinghi and usa15 matchup would have played out with a soft rig interesting to ponder though VPLP have gone on record in saying that they think the boat would have won with soft rig we did not see any alarming racking from the platform when it had its soft rig it certainly twisted but not badly. I suppose when you get to the extreme like they did with those boats every subtle decision makes a large difference to the outcome.
     
  14. Samnz
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    Samnz Senior Member

    so none of the cats you listed are square. why not?
     

  15. catsketcher
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    catsketcher Senior Member

    It's not hip to be square

    One big problem with being square is that your boat gets a very strange righting profile. We tend to think of righting being only sideways but it is also diagonal (over one bow) and longitudinal (over both bows). There have even been cases of multis capsizing diagonally backwards (over the leeward stern)

    What you never want is a boat that is much more stable one sideways than fore and aft. Then you head upwind - the load is mostly sideways and you have some stability in reserve. Then you bear away and - oops - the boat with less fore and aft stability then wants to cartwheel. In fact it is very hard to depower a boat downwind so fore and aft stability should be quite a bit greater than sideways. Its always easy to ease a bit of sheet but hard to do anything on a square run but pray.

    Then you add on the increase in weight, complexity and such and the overly square boat can be problematic. Richard has done some testing on his Strider (I think) and if I recall correctly he liked the wider version. So there may be a sweet spot that is wider in smaller cats as these can be proportionally lighter due to the square cube rule. Then again the stability issues don't really arise unless you sail square and push the boat.

    cheers

    Phil
     
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