Why can't most catamarans get over the hump ?

Discussion in 'Multihulls' started by tommymonza, May 4, 2014.

  1. waikikin
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    waikikin Senior Member


    L:B:D?

    Jeff.

    Ha..... that should have been L : B : D ! but smilies turned on !!!!!!!!!!!!
     
  2. rwatson
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    rwatson Senior Member

    I dont think you its a 'few %' in real life - no matter what the little graphs say.

    Heres a case in point where power was applied to a unsuitable yacht hull

    http://www.boatdesign.net/forums/bo...rs-powered-oday-javelin-14-9-mph-48787-2.html

    With enough horsepower to do 30 mph with a properly designed hull of identical length and beam, this is a lot more than "a few %"

    The Owners SOR was "lots of spray", as Ad Hoc was happy to judge acceptable - but the hull shape produced a good 30% reduction in top speed, let alone fuel efficiency.

    Hull shape is very important to a successful planing craft, as real boats performance proves.
     

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  3. Ad Hoc
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    Ad Hoc Naval Architect

    And you can support this assertion with facts? By facts I don’t mean glossy sales pitch, I mean independently peer reviewed evidence. Since we can all blow our own horns.

    Ahh there’s the rub….a properly designed hull.
    Which begs the obvious question (which has still eluded you so far from the post made here)…what is a properly designed hull? I wonder whose text you’ll cut and paste for that one??



    Aahh..the logic of such simplicity..but why would you have a hull 15’ wide….aahhhh…that’s not the Q I hear you ask….well, if that 15’ wide hull displaces the same…do you think the WSA has changed a little or a lot, and then answer yourself whether the other parameters have remained the same.

    It seems the concept of designing a hull, that is to say a 3D shape that is to satisfy an objective , is wrapped up in black magic for some, over thinking everything. As it MUST be..surely it MUST BE XX or YY….simply because some sales pitch in a magazine says so. Despite there being endless actual factual evidence to the contrary….for some, it’s like religion nothing will deter one from accepting that there is no god. Sigh….:eek:

    It bothers me not if you accept it, as it is written by many others not just me. But I suspect it won't figure much into your next design...:rolleyes:
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Who said anything about WSA ? Your sermon was to the effect that L/D ratio was 90-odd % of the resistance story, let's not worry about whether it is form resistance or frictional drag, that wasn't specified. And bugger the 15 foot wide boat, I'd prefer a 25' square hull, with still the 2 ton displacement. How is my resistance curve looking ? This boat will be able to move sideways, backwards, and forwards, with equal aplomb, as the L/D ratio doesn't change from one direction to the next. And compared to the 25' x 6' boat I can play mini-golf on the 25 x 25. I just have this terrible mindset that the beam of the boat is somehow relevant to the resistance, that I can't rid myself of. Amen.
     
  5. tommymonza
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    tommymonza Junior Member

    So at what point does the length to beam ratio produce diminishing returns and in a very narrow hull what is there still an advantage to have a upward sweeping bilge if the hull form is similar to a Tornado?
     
  6. Richard Woods
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    Richard Woods Woods Designs

    Sorry, I am late to more on this discussion. I am travelling right now. Left home in the UK 4.30am, now in Vancouver airport (10.15pm UK time) and still 8 hours to go.

    Agreed, Adhoc means the slenderness ratio is the most important factor, as I said, length and weight.

    But wave making resistance is proportional to BWL cubed, so unless you plane you're better with a finer hull

    If you go too fine, say over 15:1 then the WSA will be a major factor at low speeds.

    As proof. My 24ft Strider has a L/B ratio of about 12:1. The Firebird and Gwahir about 16:1. The Strider is the fastest boat in winds/boatspeed under 10 knots. (In our first race in our Strider Turbo we beat all 6 racing Firebirds boat for boat) But in stronger winds the finer hulls are a lot faster, the Strider hull is not competitive. It seems to peak in flat water at about 18 knots, the others in the mid 20's

    I have also done model multihull testing (5ft models) in a test tank (at Southampton), and also watched models being tested at Haslar. I was surprised how in-accurate the testing was.

    See here for more of my thoughts.

    http://sailingcatamarans.com/index.php/articles/12-to-be-published-mainly-technical/51-hull-shapes

    and

    http://sailingcatamarans.com/index....ormance-questions/107-performance-predictions

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
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  7. Ad Hoc
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    Ad Hoc Naval Architect

    Now all you're doing is showing your ignorance on design. Since all you're focusing on is one attribute whilst ignoring how it affects others. So, have you designed a hull before?

    Since if you have designed a hull before you would understand that it is for a purpose. Unless of course you design a hull and then bin it.....design another...and bin it..oh and then design another..and bin it. All for amusement, hey. Unless of course you have an objective..an SOR. In which case you design the hull to suit your SOR.

    In the process of designing a hull, to suit your SOR, several parameters end up dictating the 3D shape. These are...oh hey...you alreday know about these, since you're suggesting WSA is not one of them. Great :p

    Hey, funny that, The resistance, oh..wait a min, is that all of the resistance or just part of it??..Hmm...let me think. Oh yeah, I remember now...the residuary resistance is proportional to the B^2. Ok, great.

    So, lets make a very narrow hull, surely by the same logic, it must be very slippery and have little resistance. Hmm..how narrow...well, let's say 500mm wide, great. Oh...hang on, where is the buoyancy then...er...um...oh yeah..let's put it into a tube like shape. Ahh...you mean like a swath...yeah, that's it. A swath..it has a narrow beam....so it must be super fast then??

    Oh wait...no, they are very draggy....oh bummer...why is that....er...um...damn those pesky parameters like WSA. How does that affect resistance, I thought it was just the beam that had an influence:confused:. That's the problem with myopia, gets in the way of a good opine!

    Hey hull design is easy, just press buttons, cut and paste text, and just ignore all the parameters required to satisfy an SOR. :eek:
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Stop talking in riddles and rubbish, you are the one hawking the magic formula, length/displacement=all you need to know about hull resistance, not me. Too late to start bringing in the "conditions apply" fine print for you, you were adamant anything outside length/displacement ratio was basically fluff.
     
  9. tommymonza
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    tommymonza Junior Member

    Very informative reading Richard.

    I especially liked your take on the need for a raked stem and built in flare of the topsides when going offshore as opposed to plumb bows and vertical sides of the racing daysailors.

    Also was a question of mine when going offshore is the need for some bow rocker so when going downwind the bow is not driven down into the wave.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    There are no conditions. It's simple. It's simple to those that actually design and not just read and cut and paste text without comprehension. I gave you some hints above, but you have elected to ignore them as it doesn't fit into your one size fits all mantra.

    Since according to your logic, just fit a powerful engine into any boat it'll go faster.

    Ok, put a 10hp onto a 4m RIB...no wait...put a 1000Hp engine into a 4m RIB....let's see what difference one requirement to satisfy in an SOR does? Hey the 1000Hp goes a heck of a lot faster using than same 4m hull with with just the 10Hp engine :eek:
     
  11. hump101
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    hump101 Senior Member

    If you have a hull that is a rectangular solid, density 0.5xrho_water, length L, depth 0.5L, width 0.1L, then when floating it will have a draft of 0.25L, and a beam of 0.1L, so a slenderness ratio (L/B) of 10.

    Will the resistance of this hull be the same if it is rotated 90 degrees about its longitudinal axis? Beam is now 0.5L, draft is now 0.05L. The WSA remains the same, as does the length/displacement ratio, as does Cb, Cm, Cw and Cp, but L/B is now 2 instead of 10.

    No, the resistance will not be the same due to increased wave-making resistance. Froude proved this in some of his earliest experiments, and Richard confirms this above. Slenderness ratio (L/B) is not the same as length/displacement ratio, and slenderness matters for resistance.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Dodge, duck, weave, all I asked of you was to clarify in the discussion of the length/displacement ratio whether that was assuming the same beam, I never got an answer then, or since. Others can form their own opinions, but it seems you have now drifted away from that mantra into talking about SOR, when I thought it was about hull resistance, nothing else. I don't want hints, mate, just a straight answer.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    No, of course not. No one said it would be the same. Only those with fancy colour plots and a series of output numbers appear to be suggesting its a major difference, as its their only frame of reference, as noted at the beginning of this thread.

    What you're all grasping at is the notion that it will be a SIGNIFICANT change. The amount of text i read when some says that they changed the hull shape and gained 20% fuel efficiency, or an extra 10 knots etc...all hype and sales rubbish.

    Changes to B/T have a minor effect as already noted by research by Molland

    Molland LD varying BT ratio 1995.jpg

    and many others noted, just look at the Classic series by Yeh and the experiments by Warren et al.

    Each has their own little sweet spot too over a range, as one would expect, as one size does not fit all. But does changing the B/T of a hull have a major effect, no. A simple interrogation of the graph shows the difference between a B/T of 1.5 to 2.5 has approximately 2.5% reduction...Unless one is trying for a world record, I would hardly say that is a major effect or influence as many are suggesting. That's the premise which is being overlooked by many in their zeal to find text that suits their argument and cut and paste. With increasing L/D ratio shape plays an ever decreasing role in the total resistance.
     
  14. hump101
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    hump101 Senior Member

    This is where we have a difference of opinion, because for my clients 2.5% is very significant.

    Firstly, 2.5% reduction in resistance corresponds to a 5% reduction in installed power and hence fuel consumption, and that is a huge saving through the life of a vessel, worth millions of dollars for a typical ship. We would consider 0.25% to be significant in this context.

    Secondly, for a racing yacht, reducing the resistance by 2.5% gives increased speed, so increased apparent wind, increased drive, increased lift, and will thus give more than a 2.5% increase in performance. In the last of the AC's in mono's in Valencia, top teams were spending several million dollars to realise 0.05kt gains, about 0.5%.

    I don't know anywhere outside of the leisure boating industry where 2.5% would be considered insignificant, and as Fn increases above unity, the difference becomes even more significant.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    B/T....is what, exactly ? We have Richard Woods saying wave making resistance is proportional to BWL cubed, and with wave making resistance the major factor in some boats, hardly a minor difference in resistance with changes in beam.
     
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