Modular Cruising Catamaran

Discussion in 'Multihulls' started by ImaginaryNumber, Jul 19, 2009.

  1. groper
    Joined: Jun 2011
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    groper Senior Member

    What i should have done, was avoid the use of absolute values such as xx kN...

    The results/projections/whatever you like to call it, were never meant to be absolute numerical predictions of your hull resistance, or modified hull resistance. WHats more important, is the trends that can be shown using the software and how changing things effects wave making and other hull resistance components. Yes, you are correct - i should have made that clear from the outset.

    The trends show that;
    15kts is an inefficient speed for both configurations.
    The wave making drag is about 3 times greater than the viscous drag for both configurations in your desired speed bracket. - Which is why i got involved in this thread to begin with - you stated that the wetted area of a catamaran was the reason for poor light wind performance of a displacement sailing catamaran - which is clearly not the case.

    The other point i wanted to make, which may have got lost in the forest, is that there are clear contraints and reasons why efficient displacement catamarans of this size, can cruise efficiently above what is their calculated Fn based hull speed - which is above around 8-9knots in teh context of our discussion. You seemed to want this ability from your modification aswell...

    The reasons/constraints (i stated earlier) are;
    1. the hulls must be very slender - Wave making drag, is dominantly influenced by the waterline beam of the demihulls. If the wave system generated is kept very small, the hull doesnt experience an excessive "hump" to climb over.

    2. The hulls must be sufficiently spaced apart to avoid too much interference wave drag. - The larger the wave system generated by the individual hull, the greater the interference drag will be of course. The quantity of the effect, is shown accurately in the michlet predictions - its good for that.

    3.The displacement/length ratio must be good.

    You must have all of the above, in order to have an efficient displacement hull which can efficiently cruise above the calculated Fn based hull speed.

    Seems youll be happy with any improvement beyond what you already have, cruising with the configuration you have must be very frustrating... shame you didnt consider what you have recently, back when the boat was built...

    Fair winds to you...
     
  2. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Why did you equate a sailing cat with a powered cat? Since their SORs are totally different.
     
  3. groper
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    groper Senior Member

    Where and how did I "equate" ???

    To me a displacement catamaran is a displacement catamaran... what is it to you I wonder?
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    One is powered by the wind, the other by engines drinking diesel fuel.

    The one powered by the wind, which comes from its ability to counter a heeling force (from the wind) at low angles of heel. This is achieved by the sail area to weight ratio and the fact that wider spacing of hulls helps to create the righting arm to counter the wind heel force. If you make the hulls far apart it heels less and can carry lots of sail to go fast, like a hobbie cat. But cruising cats are not like that, they can’t afford the increase in structure weight of wider spacing and have hulls which are not “just immersed” owing to the fact that they little weight other than the mast hull and rigging. Cruising cats required buoyancy to carry things like galleys, bunks, air con, head, showers, generator, fuel etc etc..and the heavier structure to withstand ocean sailing, all of which means a hull which is not “just immersed” and not “widely spaced” ie wider and deep and not far apart, which is contrary to light fast sailing cat’s. All of which has nothing to do with how powered cats are driven fast, that’s an engine!

    I more comprehensive explanation can be found here:
     
  5. groper
    Joined: Jun 2011
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    groper Senior Member

    Im not sure I u derstand the point of your last post ad hoc? Besides stating some basic facts?

    Or perhaps your not aware of the typical well designed modern cruising sailing cats? Im talking about the ones that can sail faster than the wind speed despite having all the luxury cruising items aboard? They all share the same design constraints I outlined earlier.

    a powercat under 50ft lwl, designed to cruise in the 15kt range, should consider the wave interferance effect s when efficiency is weighted in the design SOR, yes or no? Stability and seakeeping is another matter worth weighting it against im sure...
     
  6. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    Groper, if I was designing a new boat for my purposes I no doubt would make the hull spacing somewhat wider and the hulls somewhat skinnier. But as we have established above, reality has an annoying habit of intruding into our idealized computer modeled world. If I made the boat 20 feet wide and 60 feet long to satisfy the hull spacing and beam standards you're demanding, I would have no place to berth it. Nor could I have afforded to buy it in the first place.

    I have the boat that I have and I'm stuck with trying to improve it. Did the builder make some mistakes on this boat? Yes. Did I? Yes. I don't need some sanctimonious computer jockey to point that out to me - I'm fully aware of it. Thank you very much. I come here seeking solutions.

    Regarding your modeling 'trends', you continued to provide me model predictions of what my current hull would do...oblivious to the fact that I actually OWN the hull and know exactly what the hull does in the real world, not in your computer model world...which bore scant resemblance to reality.

    I also did actual real-world model performance testing which confirmed my calculated predictions that the hull's performance would meet my needs up to and beyond 15 kts. Is 15 kts a particularly efficient speed to run that hull? Probably not. But who gives a s___? My calculations, my real-world modeling, and even your fantasy computer modeling, predict that I should be able to achieve something close to my target speed for the same fuel efficiency that I'm currently getting at 8 kts. If hull interference effects mean that I only achieve 12-13 kts...great! That's still a speed improvement of over 50%. And of course a huge increase in range at slower speeds.

    I have consulted Ad Hoc on this project and found his advice to be practical, pragmatic, and grounded in extensive real-world technical experience.

    And finally, let's be clear about your engagement here. You hijacked IM's thread because you wished to carry on with bashing my hull modification efforts. I was responding to IM on his questions - I never brought up my project here - you did. How about we give this thread back to its owner?
     
  7. groper
    Joined: Jun 2011
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    groper Senior Member

    I wasnt trying to bash your hull modifications Deering - i was only trying to point out the limitations with it. I always said - the potential benefits were upto you to decide their value. And this right here again is telling me you still dont understand it fully -

    Im not saying this isnt true, but i doubt youll end up running it @ 12-13kts... it will still trim and sink heavily at this speed and my prediction is you will learn to live with running it at 9knots via the same thought process that has you running at 8kts now. Youll ask yourself "is it worth doubling my fuel bill for an extra 3kts boat speed trying to push it over 9knots"

    Maybe im wrong, but ill leave this thread alone now...

    Lastly, Adhoc is very competent person in these matters - i cannot hope to compete with his knowledge and experience - youd be clearly better off following his advice and id be interested in hearing what his thoughts are regarding the cruising efficiency regime of the new hull to know whether im really off base on this...
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    The efficiency regime is irrelevant, a total red herring. Why??..what was Deering’s objective?... "..can he improve his existing hull with minimal effort..". Simple answer yes. The SOR was simple (listen to the client), the solution also simple, anything else is just unnecessary “noise”.

    Deering’s quote which you cite above is a clear indication that you don’t get it at all.
     

  9. Deering
    Joined: Feb 2005
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    Deering Senior Member

    Semantics. And I'm only "pointing out the limitations" of your opinions, which are based on a foundation of sand.

    If your pronouncement (yet ANOTHER one from you!) about what my future thought process will be regarding fuel savings were valid, why wouldn't I be running my current boat at 5 kts now, where my fuel economy is double what it is at 8 kts? Your same 'logic' would seem to apply in that situation. Because, as Ad Hoc has stated, you keep throwing out the same tired red herring of efficiency.

    My objective is to have a boat that's 'efficient enough' to meet my goals, and if the boat can run at 12 kts or 14 kts or 17 kts at anywhere around the same fuel efficiency that I currently get at 8 kts, that's where I'm likely to run it much of the time, because that's an affordable speed for me. And time has value. I feel that I have been very clear in this matter. I'm sorry you don't approve of those goals...check that, I couldn't care, actually.

    Now, as you obviously have nothing new to add to this subject, please sign off of this thread and let IM have it back.
     
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