Modular Cruising Catamaran

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

  1. apex1

    apex1 Guest

    You meant the other Richard I know. But let me say, if your hulls are narrower and longer (for the same displacement) they of course provide the same buoyancy. And the boat will be faster (much faster). It´ll be a topheavy boat, but that can be balanced with a appropriate design. It all depends on how sophisticated you can engineer the fixtures deck / hull.
    In principle it can be done, and is´nt a too great task.

    Regards
    Richard
     
  2. Richard Woods
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    Richard Woods Woods Designs

    Apex1

    That is only true to an extent (as you well know). Top speeds might be higher, but at low speeds a long thin hull tends to be proportionately slower.

    That is because of the extra wetted surface area. Imagine a coke can, unopened it holds, say, 500ml. Drink the contents and then flatten the can. You will get a long thin "hull" but it won't now hold as much coke. Or, putting it another way, for the same area of can (hull) a long thin shape cannot hold as much fluid (have as high a displacement) as a fatter one.

    So those sailing in lightwind areas are often better off having a slightly fatter hull.

    And a fine hull will sink quicker than a fatter hull as it is loaded. So that also means a cruising boat shouldn't have hulls that are too fine.

    All you have to do is decide what "too fine" means!

    Richard Woods of Woods Designs

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

    "...Top speeds might be higher, but at low speeds a long thin hull tends to be proportionately slower...."

    No true. The length displacement ratio is significantly better for long thin hulls compared to short fat ones and hence the wavemaking resistance is lower. This more than offsets the increase in frictional resistance at low speeds too.
     
  4. Richard Woods
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    Richard Woods Woods Designs

    Well all I can say is that isn't what I've found on the race course where, for example, my Strider design is faster than a Merlin in light winds but much slower in strong winds. The Strider has a fatter hull.

    Richard Woods of Woods Designs

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

    Richard

    I suspect that when you try and compare one against the other (Merlin v Strider), all the "facts" are not the same. So your observation whilst obvious when on board, the reasons for such, are not as clear cut as you image. Too many parameters affecting the observations.

    Hydrodynamically, and nothing else, no wind, no sail area, AoA etc etc...the above statement is correct regarding long slender hulls and their lower length displacement ratios for a hull.
     
  6. rattus
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    rattus Señor Member

    ...and hydrodynamically, in low speed conditions (i.e. boat speed << hull speed), friction resistance dominates over that of wavemaking resistance for any hull size; therefore a long slim hull of equal displacement, and thus of greater wetted surface area to that of a shorter fatter one, will be slower at these speeds.

    Taking that to an extreme, at infinitesimally slow speeds, a half-sphere would be the ideal hull shape.

    Ad Hoc, you are what is known in engineering circles as an *******. Richard(s), carry on with your conversation, most of us appreciate it!

    Mike
     
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  7. Guest625101138

    Guest625101138 Previous Member

    Richard
    Slow for a sailing cat for me would be 4kts.

    The lowest drag cat hull for this speed with a total ship displacement of 800kg will be 6.5m long, maximum beam of 700mm and draft of 280mm. How does your Strider hull compare with this. Length is the most important factor of course.

    Rick W
     
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  8. Ad Hoc
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    Ad Hoc Naval Architect

    "...Ad Hoc, you are what is known in engineering circles as an *******..

    Is that the engineering circle that prefers anecdotes to evidence?

    I thought that was the school playground....oh yeah, that's where name calling usually occurs too.....silly me!

    Such a mature non-engineering circle comment, thanks for the erudite contribution Mike :)
     
  9. bill broome
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    bill broome Senior Member

    ummm, btw: are you sure you don't want a trimaran?
     
  10. apex1

    apex1 Guest

    Can we please stop this crap! Apart from being offensive, that statement was wrong. In the circle of real pro´s around on this Forum, he´s known as a real pro.

    To topic:
    At speeds where a cat has it´s advantages, wavemaking is the main factor we have to fight, so there is no doubt: the longer the better. period

    Regards
    Richard
     
  11. catsketcher
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    catsketcher Senior Member

    top speed

    To topic:
    At speeds where a cat has it´s advantages, wavemaking is the main factor we have to fight, so there is no doubt: the longer the better. period


    I don't agree on this for a cruising cat. Most cruisers get sailed at far below top speed after they reach say 8 or 9 knots, depending on conditions. As Richard says there are many other important considerations such as immersion rate, pitch resistance, dryness on deck, the livability of the hulls, access into the hulls etc, stern steps and very importantly tacking ability. I would not care if my cat never went above 15 knots if it got to 5 knots quickly - so you need fatter hulls than top speed calcs would suggest. As an ex racer I can sail our cat hard. We have averaged 10 knots over 16 hours for a family cruiser (Coffs to Southport) but our average on the GPS is 7 or 7.2 knots. Too fast and we all get tired. Better to have hulls around the 10 - 12:1 ratio and not get too extreme.

    The advantages of a cruising cat are not always speed - increased livability, no roll at anchor in swell, no rolling on a square, real shoal draft, easy to set kites etc are also great advantages.

    I would not like to cruise on a narrow hull cat unless it was very long and I don't like large cats for cruising (too big for fun) I would envisage this modular cat being max 42ft long with a displacement of 4000kg (pretty light) A 42footer with the room of your average 38 footer. You get headroom and still stay below some very scary loads that cost heaps in terms of rig size, winches, anchors and motors.

    I understand Richard's point about going it alone and not being able to sell the boat. I have built 4 boats and I do see the incredible time that goes into the curved bits of interior. Hulls are easy to build - it is the interior that makes me cry. I have designed and built computer cut small cats that are fun but every time I build a boat I think about going square - not the hulls, but the superstructure. My friend spent about 3 months fairing the interior of his lovely Schionning. This is normal now. The level of fitout and work required has increased greatly just to be average. I would like someone (not me) to burst the bubble of increasing time needed to complete the interior of a cat. Compound curved roofs, curved dinettes, coves everywhere and two pack interiors are a recipe for less time cruising. A simple, flatter painted ply or similar interior can save many hundreds or thousands of hours and still get you to the same cruising grounds. How to achieve this whilst not compromising the rest of the structure is the issue.

    cheers

    Phil
     
  12. Richard Woods
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    Richard Woods Woods Designs

    Once again I agree with Phil!

    To Rick. The Strider WL beam is about 550mm, depth about 260mm. Disp in racing trim 800kgs, so 400kgs per hull. Merlin has approx 70mm narrower hulls.

    700mm wide sounds like a trimaran main hull??

    I would say 4 knots was the slow end of slow, 6 knots the fast end. Striders expect to sail in the 8-11 knot range much of the time. Easy to do high teens. Several people have done over 20 knots in flat water. I believe the max is 24 knots.

    Merlin is quicker in more wind than a Strider.

    To Ad Hoc.

    I have sailed small multihulls for over 25 years and have owned one Gwahir, 7 (yes 7!) Striders, a Wizard and for the last 4 years sailed a Merlin. All these boats have been raced with some success. Eg I was European and UK National Micromultihull champion in 1987, 4th in the Europeans in 1994 and UK Champion in 1994.

    So I reckon that if I say the Merlin is a faster boat than a Strider in more wind, and vice versa in less, than I am right.

    So maybe you could outline your multihull sailing experience???

    Personally since, in another thread, you implied that you would like to see ALL multihulls under 12m long banned, I suspect it isn't very much, if any.

    Richard Woods of Woods Desiogns

    www.sailingcatamarans.com
     
  13. Chris Ostlind

    Chris Ostlind Previous Member

    Richard Woods,

    Never fear... Our boy, Ad Hoc-inem will be along shortly to launch one of his famous, misdirection attacks on you personally, while completely ignoring the substance of the post you have so wonderfully phrased.

    You'll have to pardon me while I am swept away by the vision of the festive Mexican Pinata with one wildly swinging party-goer well in over his head. There he is, flailing away, while the prize remains some distance from his grasp due to a non-existent set of on-the-water skills, in an otherwise featureless sea of cubicles. Don't you just love it when a "scientist" expounds on that with which he has little real life depth?

    Of course, the fellow can always step out of the shadows and publicly proclaim his worldly sailing experience as accumulated under his real name...?

    No, what's that... another pass at redirecting the issue...? You’re right, I really didn't think the function of honesty would be forthcoming.

    Apex Richard... Just where did you get this character, anyway? Hey, you vouched for him, right? Why don't you provide the substantive info that brings his tired, mechanical diatribes into something that resembles a person we should respect? (as in: he has extensive sailing experience in a broad representation of craft of all sizes to back up this routine he chuffs on a regular basis) Until then, this guy is a wannabe sailor sitting on some store bought chair with too much time on his hands, pontificating about what it's like sailing boats that are appropriate for the typical reader on these pages.

    Please guys, can’t you do better than this?
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Richard

    The discuss on one hull form or another has become blurred. Simply because your making reference to 'real world' attributes, such as "increased livability" or "dryness of deck". These decisions are all part of the "design spiral", ie a compromise. The debate, as i saw it, was purely one hull form against another. In which case one must compare apples with apples. The various parameters in the other boats are not all the same ergo one cannot make such a generalised statements about them 'hydrodynamically'. If the frame of reference is other than hydrodynamically then of course it is a different matter all together and when looking at "design" per se, is highly subjective. One just needs to rationalise what the 'debate is'....not what one 'thinks it is'.

    "..Personally since, in another thread, you implied that you would like to see ALL multihulls under 12m long banned..."

    I think you have me mistaken with someone else. Or, you have failed to understand what it was i was discussing.

    "..So I reckon that if I say the Merlin is a faster boat than a Strider in more wind, and vice versa in less, than I am right...".
    I cannot dispute your own observations, i don't doubt them, since that is your experience/opinion. However, i fail to see how this makes the observation "proof"... Unless of course you took detailed measurements of each hull you ever sailed against whilst competing and compared the L/D's, calculated the driving force, non dimensionalised the results and Rw's etc etc...so when you finished the race you could claim, eureka!

    "...So maybe you could outline your multihull sailing experience???..
    Shockingly poor, haven't done so for many years, not since a student. I'd be lucky to come in last..!!!

    But since you claim that naval architects don't design boats only yacht designers do...why would you be surprised by this?!
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Ahh..the metaphysical has popped up :)
     
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