Reverse bows on cruising cats pros and cons

Discussion in 'Multihulls' started by DennisRB, Mar 1, 2013.

  1. Moggy
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    Moggy Senior Member

    Bob appeared then disappeared.... hmmmm... we need a seance ---> R U still there? :) Don't go! We need some thing to drool over!!!! :)
     
  2. MikeJohns
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    MikeJohns Senior Member

    Actually the science is called Naval architecture. And each design compromise is carfully weighted against the statement of requirements for the vessel. Be careful about hinging a design paradigm on a generalization. Nothing in naval architecture can be generalized on it’s own. All aspects of the hullform must be assessed for the range of sea states, and headings and speeds.

    The NA study is called a series of RAO’s (or response amplitude operators) which describe vessel motion wrt heading speed and wave encounter. A craft that’s fast to windward in a chop in sheltered or semi sheltered waters is not necessarily fast at sea, either to windward or downwind. Or a feature that works on a huge ultralight craft may not work at all on a smaller sensible cruising craft.

    In a sea state that tends to induce pitching in a certain craft, speed suffers considerably as a result. By decreasing buoyancy fwd you can reduce pitching to windward in some sea states. But the danger is a boat that plunges in larger waves that can be slow and dangerous and very hard to control downwind. As the craft gets substantially larger the responses don't change linearly to features like low buoyancy bows.

    A pitching study of the design should be undertaken early and you should have some idea of the vulnerabilities of your design too. Unfortunately a lot of yacht designers tend to ignore and are often completely ignorant of dynamic longitudinal stability.

    If you start promoting novel hullforms because some exotic large high speed vessel uses them, then you should study the limitations of the feature not just it's imagined advantages. Plunging and pitchpoling are closely related on cats. Science doesn't favor one feature, it's a complex interactive system of features that make the whole.

     
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  3. DennisRB
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    DennisRB Senior Member

    Can you please explain this a little further?
     
  4. Gary Baigent
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    Gary Baigent Senior Member

    Well, these "imagined advantages" have turned out to be in reality (speaking of multihulls) ... advantages.
    Advantages in speed performance, less pitching (in all conditions), and a longer and more balanced waterplane area, producing "longitudinal stability" - that is, if you're concerned about catamaran pitchpoling, these reverse or plumb bow features surely are an important step to improving (lessening) this problem.
    Since the thread is related to (light) multihulls with the said bow designs, relating heavy and slow craft plunging antics in seas and downwind in waves, is a comparison that is of no relevance to the discussion.
     
  5. Richard Woods
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    Richard Woods Woods Designs

    Actually the thread title is

    " Reverse bows on cruising cats pros and cons " so it could/should be relevant to heavy and slow boats sailing in big waves offshore. And the OP wanted to know the cons as well as pros. There is nothing in life, whether boats, religion or women that has no cons

    Richard Woods
     
  6. Charly
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    Charly Senior Member

    Exactly. So we have PROgress and CONgress.

    sorry:D I just couldn't resist
     
  7. catsketcher
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    catsketcher Senior Member

    Design spiral

    On Kankama, my 38ft Chamberlin. If she were to be redrawn and rebuilt with reverse bows you would have to move the forebeam aft. It is about 200mm back from the stem and about as far forward as you can build it in and bolt it on a bulkhead still. So a reverse bow design just like Kankama with the same sail area and jib foot length would have to be longer. Or I get a shorter distance between forestay tang and mast. If she is longer then it is hard to say whether reverse bows or extra length is the cause of any improvement.

    I still do not think the case has been made by cruising cat designers that their designs operate in any of the same ways that trimaran floats, A class or AC cats operate or designs like Sid or Jim Brown's Windrider (main hull and float bows are reverse -I loved the Windrider 16). These hulls definitely are low drag and have usually none of the nets, beams, cleats, even deck paint that a cruising cat bow has. For their use the reverse bow is a great idea. To design a cruising cat bow and then do longitudinal stability analysis of the design without the significant drag of the deck gear and beams added is bad practice.

    I like a lot of what other designers have done on their boats but I can't do a cut and paste job with different features just because they look good. Anyone who has built a few boats will recall how integrated a boat design is and how one factor will affect others far removed from the design feature. Put reverse bows on your design if you need a wave piercing bow but do it properly. Remove all drag sources from the bow area (so no nets and beams up the front), do an analysis of the longitudinal stability to show that the bridgedeck won't be slamming, or that the deck hatches won't be filling up. You may find that you can't even get the bow under water before the boat lifts to an unacceptable degree rendering the whole concept futile. (If it takes a rotation of 20 degrees of bow down pitch to get your bow under water then your reverse bows are taking too long to do their thing - I pulled the 20 degree number out of my head but a good designer will be looking at whether the rudders are out, whether the drag goes up hugely anyway, etc) I don't mind reverse bows at all but do them for better reasons than fashion or because someone else did it on their boat. After you have done all of this design study the designer should then compare the drawn hulls with other bow profiles and select the best not the most fashionable.

    I am not a luddite - I have designed and built 2 trailerable cats, Kankama has a composite wishbone I love and adore, she has had 3 different cabins and now has a cabin I can see out of when sailing and easily see through when sitting in the cockpit steering, she has had whipstaff steering, tiller steering and now has wheel steering. She had no aft deck and swinging davits (Oh no!). She had huge aft locker and no cockpit (not good) and now she has a big normal cockpit. She has had three different pods for her single outboard (latest one pivots), her mast has no spreaders, she has a staysail on an inner forestay (uncommon on cats) and many of her deck fitting are stitched on with uni strands. I love new design but having been a victim of my own demand for the novel I am more than twice shy of bunging on new stuff.

    There have been many times when I have wished that I could grab a time machine and go back and give myself a severe talking to just when I was about to do something novel. That being said Kankama really suits me because she is different but about half the novel ideas were stinkers I had to redo. One quarter were good and the other quarter (the wishbone and the cabin I can see through) were gold.

    SO

    Before you go an put reverse bows on your cat think of all the reasons why it a bad idea as well. In the end reality will give you a good whacking about the head anyway. It is cheaper to do this metaphorically first.

    cheers

    Phil
     
  8. Moggy
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    Moggy Senior Member

    Heavy and slow utterly defeats the main reason to design a multi, even a cruising cat IMO. Give me longer, leaner, faster and IMO generally better behaved any day.

    2c
     
  9. Gary Baigent
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    Gary Baigent Senior Member

    Speaking relatively "light" as applied to most cruising cats compared to heavy other craft (carrying lead); perhaps not light compared to racing multihulls ... but still light ... repeat, comparatively speaking.
    Anyway, load a cruising cat with too much junk - you might as well have a monohull, because the latter will be a better load carrier, go to windward better, manoeuvre more quickly and so on.
    In this day and age of astonishing multihull platform and rig developments, building a plodding cat seems beyond ridiculous. Not talking about expensive and time consuming alteration of the bows of a conventional cruising cat, but starting afresh, designing a fast CRUISING multihull incorporating developments that have been proven excellent. Which means, clean hydrodynamics, long and balanced water planes, reduced windage and other above decks junk (as catsketcher Phil says).
     
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  10. Alik
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    Alik Senior Member

    What does it mean?
    :confused:
     
  11. Gary Baigent
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    Gary Baigent Senior Member

    Meaning not deep rocker, the minimum of wetted surface (to do the job required) and excellent foils.
     
  12. dstgean
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    dstgean Senior Member

    Phil, do you have photos online somewhere of your boat?
     
  13. catsketcher
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    catsketcher Senior Member

    Gday Dgstean - I put a video up here - http://www.youtube.com/watch?v=XEiIQn6Ikls

    Kankama has been suffering from my building of other boats so she needs deck paint in this vid. Also I have changed my mind about tacking the reacher out to the bow and am building a composite prodder.

    cheers

    Phil
     
  14. Gary Baigent
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    Gary Baigent Senior Member

    So neat and cleverly thought out, catsketcher Phil - you hopeless sailing/sketching nutter. Just joking, mate.
     

  15. MikeJohns
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    MikeJohns Senior Member


    Reduced buoyancy bows are a current fad amongst designers, whether plumb or even reverse. But no single feature is significant on it's own. People are prone to buying into a design trend because some current design they perceive as exciting has that feature. The advantages are then rationalised into the chosen design feature without any real study. Then it becomes a pervasive urban myth until somebody kills it with a detailed study or too many boats come unstuck in bad conditions.

    It's very important to quantify statements such as "faster " or "reduced pitching". How much faster ? What was the reduction in pitching, on what heading at what speed and what wave spectra ?

    The danger lies in applying significance to relatively minor design features that may or may not be significant on a race course. Then you have to identify whether the imagined advantage of a particular feature is really the advantage in itself or whether it simply ameliorates another problem. The reverse bow certainly does reduce the diving plane area of a low buoyancy low restoring force design. But does that feature it in itself make the craft significantly faster ? Weight reduction is really a paramount drive in the bow shape when you are looking for a win maybe only seconds over your competitor. So it makes sense to shave bows away as much as possible so a reversed stem that wraps right around onto the deck is both light, strong and reduces the deck diving plane area. But in doing so you run a much higher overall risk of pitchpoling running into a wave, since there is a significant loss of a restoring force from lack of reserve buoyancy that can lead to a very dangerous plunging characteristic.

    As for pitching; again it needs quantifying. Low buoyancy reduces the pitching frequency in some wave spectra but in others it has little effect. The reduction in longitudinal gyradius can also make pitching worse in some wave spectra. In resonant pitching conditions the bow may not even get wet much above the DWL. It's in the short chop to windward that the ULDB may even benefit from weight in the fore end along with a knife bow.

    In resonant pitching conditions the bow may not even get wet much above the DWL. It's in the short chop to windward that the ULDB may even benefit from weight in the fore end along with a knife bow.
    I'd argue that it's not the bow that makes the boat fast, its the D/L ratio the hull/appendage drag and the power of the rig.

    There is a significant risk with slender, low buoyancy bows of the craft tripping over its bows after plunging. Once the bow is submerged it's very draggy and that results in a high overturning moment.
     
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