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Old 18-03-2008, 20:52   #106
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Originally Posted by masalai View Post
God you guys appear to be crazy... A design is paid for and you change it "willy-nilly" - are you better than the original designer - A boat is designed for a specific set of criteria - All this chopping and changing of a design (as implied in this recent thread posting is nothing short of stupid - 1) the designer will DIS-OWN your efforts, 2) Your build will become un-insurable. 3) no designer worth his salt will sign off on your project. 4) you run the risk of bearing the costs and everything in litigious action. 5) If it was my design I would sue for defaming my efforts or whatever bad rep your mis-taken idea in alluding it was someone else's work...

If you do not like a design get a design that meets your needs... Why buy a sports car and demand it carry a couple of tons of bricks to your house building project...?

Sorry for the blast but please remember what it is designed for and keep to those limitations... Then it will work as expected and give lots of pleasure with minimal trouble..

Fair winds and calm seas to you all...
Exactly right. Get a design which suits your needs. If the 44C doesn't have sufficient payload carrying ability, rather than asking the designer to corrupt his design by making it deeper, heavier and slower, build the 50C.
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Old 18-03-2008, 21:01   #107
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And your point would be?

Alas, poor 44'cruisingcat, this actually validates my points, which were that Oram's 44 is not poorly designed, but rather that it is easily overloaded in actual use by those who don't exercise stringent discipline. Maybe you should go ahead and ask your mum to read all this and explain it to you, because you are floundering about pretty desperately.
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Old 18-03-2008, 21:09   #108
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There really is no point discussing this any further with you. The boats are not overloaded - unless you claim to know more about this from a couple of photo's, than the actual designer can tell from seeing the boats on the water. They are loaded up to the bottom of the transom, but not beyond that. If the boats were overloaded Bob would not hesitate to say so.

One is a full time liveaboard, the other cruises for half a year at a time - they carry enough payload to suit those uses - enough for their purposes, and my purposes, maybe not enough for everyone.
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Old 19-03-2008, 00:52   #109
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Alas, poor 44'cruisingcat, this actually validates my points, which were that Oram's 44 is not poorly designed, but rather that it is easily overloaded in actual use by those who don't exercise stringent discipline. Maybe you should go ahead and ask your mum to read all this and explain it to you, because you are floundering about pretty desperately.
Its a performance cruiser not a barge.

It's made of Balsa and epoxy to keep it light

It has a conventional and fast sail plan because it is light fast and can sail.

Not everyone needs or wants to carry mountains of crap, some are happy with beanbags for furniture and a desal for water.

They even have outboards to save 400kg + in weight.

Most owners probably only stay away from supplies for a week at a time so feel no need to build an over high load carrying boat.

Most would prefer performance for the majority of their sailing and on the occasion when they MAY carry a bit of extra, they still sail fast and eventualy eat and drink the excess weight away.

A certain mentality is required to comfortably cruise a light displacement vessel and plenty cant do it.

Their lack of discipline should not be thought of as an issue with the boat or the design.

It is an issue with the owner.

Add: I also agree with 44ftCC, NONE, I repeat NONE of those pics look like an overloaded boat to me.

Dave
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Old 19-03-2008, 01:17   #110
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Acetic anchorites shall indeed savor an appropos milieu!

"Their lack of discipline should not be thought of as an issue with the boat or the design." Indeed, perfectly suited to acetic anchorites who eschew ethanol, comestibles, and accouterments. Allow the revels to commence, au naturale, confrere!
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Old 19-03-2008, 01:55   #111
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I got the idea from Derek Kelsall. I don't use his patented method of achieving compound curvature, however. I notice that Kurt Hughes is doing this now, as well, except for the underwater part of his hulls, and that he hasn't been gracious enough to credit Derek.

Rob
It is not about grace, it is about money. If a designer's idea is good enough to copy, it is good enough to pay for, whether you use the patent protected version or not, and doubly so if you are going to get sanctimonious about other designers, about whom you know absolutely nothing. Kurt may well be paying Derek, and I am sure Derek would rather be paid, than get meaningless credit from either of you.

Big cat
Rocker and Harryproas: Not only are Harryproas very light and double ended, but Rob has used a P.C. of .8, which is pretty close to being a rectangle to begin with, as a P.C. of 1.0 is a rectangle, so yes, they would be pretty rocker free.

Rob
You don't know what you are talking about. Prismatic of 1 would be the same shape as the deepest/widest section all the way to the bow. (Llarsson and Eliasson, p 18). It would not be a rectangle, but a prism. It may have a rectangular plan or profile view, but these can have prismatics as low as 0.5.
.
Prismatic of 0.8 is nowhere near a box. Harrypros have fine entry/exits (see the video), although these are spoilt by an unnecessarily blunt fairing on this boat. Future ones are sharp, with less turbulence and spray. Midships is a semicircular with near paralell buttocks for a lot of the length. Godzilla (drag program) and common sense (water prefers to travel along straight lines than around curves) say this is the fastest shape, and the boat in the video supports this view.

You may be confusing the prismatic with the block coefficient which on harryproas is between 0.55-0.6 depending on the design. Confusion over the basic principles of design do not bode well for the success of your design, nor for the value of your comments on this group.

You might also like to tell us what difference the weight makes? I would use the same basic hull shapes for a 10 tonne version. It would be faster, track better, pitch less, carry more weight for a given beam and width and have a nicer motion than a hull of 0.6. Unfortunately such qualities come at a price. It would be hard to tack, so is only suitable for proas.

Can anyone tell me how to use the multi quote function to highlight sections of a post I am answering, please?

regards,
Rob
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Old 19-03-2008, 02:22   #112
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... Indeed, perfectly suited to acetic anchorites who eschew ...
Did you mean “Ascetic” ?
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Old 19-03-2008, 03:58   #113
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Big cat
I got the idea from Derek Kelsall. I don't use his patented method of achieving compound curvature, however. I notice that Kurt Hughes is doing this now, as well, except for the underwater part of his hulls, and that he hasn't been gracious enough to credit Derek.
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Can anyone tell me how to use the multi quote function to highlight sections of a post I am answering, please?

regards,
Rob
A bit messy but I hope the pic explains it a bit Rob


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Old 19-03-2008, 05:30   #114
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Hello BigCat,










I think Rob (harryproa) Denney addressed the first point -obviously a double-ender can be designed without rocker. As to the second point, I am well aware that this is the "multihull" section; I don't think that should preclude generic discussion about boat design or using monohull examples to illustrate a point. I have the impression that a lot of designers are fixed to rules and formulae that while valid for monohulls, may not be true for multihulls. My readings tend to support my gut instinct that rocker is not as important in multihull design.



I did the google search, but didn't find anything definitive. I believe it was Robert Cateran who replied in this thread about his experience with bow steering - thanks. If I read correctly, it didn't seem too onerous. I wonder if running in a steep sea where the rudders might come out of the water, if having a deep forefoot would maintain directional stability and reduce the tendency to broach?




Kevin
If the rudders leave the water, the bows will go where they want to, including a broach. It may give you a little more time as there is increased resistance to turning,
Robert
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Old 19-03-2008, 09:29   #115
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BigCat,
Thanks for the reply. I think you misinterpreted my comment on 'keel profile - by 'profile' I meant the side view, as in terholme's drawings. The profile of the 'no rocker' boat is a straight line; the others have a simple curve. By his definition then, if you wanted to keep 100% rocker fore and aft, and keep your LWL and BWL while increasing displacement, then you would increase draught.

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I keep hearing claims about rocker causing pitching, but I see no reason to believe it. Why would rocker increase pitching? Pitching is the vertical lifting of the bow about the axis of the CB (or perhaps the CLF) followed by its vertical plunging.
Or pitching is the vertical lifting of the stern, or the depression of the stern or bow. I don't think any definition is perfect, but I take pitch to be rotation about the lateral axis. As to the effect of rocker on pitch, let's take the two extremes, a log (no rocker) and a ball (all rocker). Cause the ball to pitch (if you can figure out which end is the bow) and it will continue to pitch in the same direction, only stopping due to friction; it has no longitudinal stability. If you lift or depress one end of the log, it will quickly return to the level - it has magnificent longitudinal stability. Rocker doesn't affect pitching, so much as it affects reaction to pitching. I think the true utility of rocker is comfort and to some degree seakindliness - a boat with little rocker might be too stiff, yet too much rocker may make the boat tender. As has been said before, hull form and Cp also play a part in controlling pitch.

Quote:
If your encounter with the waves is rhythmic enough, you can set up "pitching" which is essentially a pendulum effect. This pendulum effect requires that the weight distribution in your boat (pendulums have a natural period) coincides with your encounter of the waves. So, this will vary with wave length and your course and speed. Designers center weight in order to decrease the likelihood of these periods coinciding (the pendulum period and wave length.)
I wonder if the designers have noted that pendulums have all their weight concentrated in a small central area

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Multihulls are bound to have less rocker than ballasted monohulls because they weigh less, have higher P.C.s, and because their displacement is distributed over two hulls.
I don't think that has anything to do with it - a designer could ultimately put in as much, or as little rocker as he desires. Not having a large lump of lead at the bottom of the keel to pull a cat back to upright, limits the usefulness of rocker in a cat, IMHO.

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Flam versus flare: You can't find it? I googled flam + flare and found it on the first entry, at flare: Definition, Synonyms and Much More from Answers.com - complete with drawings illustrating the difference. As the drawing shows, you can find flam, flare, and tumblehome on the same boat.
Ah! The trick is to put 'flare' into the google search, too. I just searched 'flam' and found plenty of references to drum-beats, a place in Norway and this:

flam

Origin: Cf. AS. Fleam, flm, floght.
A freak or whim; also, a falsehood; a lie; an illusory pretext; deception; delusion. "A perpetual abuse and flam upon posterity." (South)
Source: Websters Dictionary
(01 Mar 1998)

From whence we obviously can deduce the etymology of the term 'flim-flam'.

Even where 'flam' is related to boat design, it seems there is no consensus on the meaning -

Quote:
The side planking of a boat between the waterline and deck or rail is called the topsides. If they are drawn in toward the centerline away from a perpendicular, as they often do at the stern of a boat, they are said to tumble home. Forward they are more likely to incline outward to make the bow more buoyant and keep the hull dry by throwing spray aside. This is flare. Flam is that part of the flare just below the deck.
found here: Names And Terms; Types And Rigs | www.boatinginstruction.net

I concede that you are not alone in using the term 'flam'. I can't help but believe that it is either archaic in the extreme, or an American term.

Flare, on the other hand is well-defined: 4 a: a spreading outward; also : a place or part that spreads (from Merriam-Webster online).
In boat design terms, I have always understood flare to be an increase in breadth with height, usually described at the cross-section of a particular frame; tumblehome is a decrease in breadth with height. The topsides could additionally be described as being flat, convex or concave. The topsides of your design seem to be flat panels, inclined outwards - by your definition, it has neither flam, nor flare.


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Rudder location: Yacht design manuals encourage placing rudders as far aft as possible (though under the hull, to prevent ventilation,) essentially due to the leverage of the distance between the rudder center and the CLR. Another thought experiment should make it clear--if you placed your rudder just aft of the middle of your boat, wouldn't you expect less effect from it?
Doesn't 'ventilation' happen when the rudder breaks the water surface? Certainly you would have plenty of leverage at the stern, but in a 65' foot boat, you would still have a lot of leverage a few feet in from the stern.

Quote:
Another issue is draft. The farther aft you go, the deeper your rudder can be for a given draft, at least if you have rocker.
Well, your rudder would be larger, but not deeper. A smaller rudder located at a deeper point of the hull would have more bite, due to the pressure increase with depth; it would rarely ventilate; and the smaller size would have less form drag presumedly.

Quote:
As for med mooring, I have done that exactly once, in Papeete, Tahiti. I don't plan on doing it again, but if I did, I'd put the bows towards the quay and use a stern anchor. It's much more private to do that, too. Otherwise, you are cheek and jowl with passers-by in your cockpit.
That's a really good idea. How easy is it to get ashore?

Well, best of luck in your dealings with the USCG - I don't envy you having to deal with the bureaucrats.
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Old 19-03-2008, 11:20   #116
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If the rudders leave the water, the bows will go where they want to, including a broach. It may give you a little more time as there is increased resistance to turning,
That's the point, Robert. The rudders won't stay out of the water forever, so a little more directional stability (resistance to turning) during the time the boat is surfing down the wave-face, would be good thing, no?

I don't have experience sailing(properly) offshore; most of my 'sailing offshore' was in warships, which for the uninitiated, are long and narrow, have deep forefeet (and usually a sonar dome or two at or near the forefoot), no rocker, and usually a whole lot of weight up high. Yet I don't recall ever feeling that the bow was doing the steering, nor having been broached. Trust me when I tell you that the rudders can spend a lot of time out of the water - there's a lot of noise and vibration when screws driven with 50,000 SHP start churning air.

Now obviously having more lateral stability (resistance to heeling), a catamaran will be less inclined to broach anyway, but it seems to me that yawing while bow down on the front of wave could precipitate a pitchpole by further depressing the leeward bow. So resistance to turning should be a good thing. Opinions?

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Old 19-03-2008, 11:29   #117
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Gord's "acid" wit

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Did you mean “Ascetic” ?
Just a typo, no need to wax acerbic-
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Old 19-03-2008, 13:07   #118
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Reply to Rob and Kevin,about catamaran design issues

Rob - I'm sorry to see you taking such a harsh tone with me, as I have admired your creativity and daring. If you are thinking that I was criticizing your work, there has been a misunderstanding. Yes, you're right, that is a definition of the block coeffecient. However, while we are on the subject, I should point out that I have been much less daring than you-my PC of .65 has precedents, but your use of a PC of.8 is without precedent in yacht design. As far as straight portions of buttocks go, it is usually said that the shape of the hull as seen by a drop of water is that of a diagonal rather than that of a buttock or waterline.


As far as Kelsall goes, I think it is probably his method of using darts and curvature in the profile of the flat sheet to obtain compound curvature that is patented. Now, of course, Derek has moved on, and his latest thinking doesn't use darts any more to obtain compound curvature in sheet material, as he is now relying on compacting the foam core to attain compound curvature. The part that Hughes and I are using, making sheet material and bending it to fit a 'developed' form is surely 'prior art.' Hughes is continuing to use foam planks for the hull bottoms, and I am using a combination of traditional fiberglass wet molding for the curved part and bending flat material for the developed part of my hull bottoms. I am too conservative to use coring in the underwater portion of my hulls, as do Oram and Kelsall, because I want more 'bulletproof' hulls in case of underwater damage. I am willing to accept the 3.3% increase in total weight and 7% in structure weight (over using cored bottoms) that this entails. My use of skegs falls into the same category of trade-off, ie. that of choosing 'bulletproofness' over the ultimate in theoretical sailing efficiency.


Kevin, the distinction between flam and flare was used by Howard Chapelle, who has written books about the history of the design of sailing craft, and Ted Brewer (a fellow Canadian, yacht designer, and author of "Understanding Yacht Design.") Thinking that I originated it is giving me way too much credit. Given the limitations of the terminology, I think that flam is a better description of my topsides than flare. The problem with flare in the forward sections of the hull is that it leads to a sudden increase in buoyancy as the boat pitches. The increase in buoyancy of flam is gradual and continuous, and thus it avoids a sudden checking of the boat's motion as the boat pitches.


I am not following your comments about pitching in a sphere. I really don't believe that it would be possible to induce pitching in a sphere.


My point about ventilation of a rudder was that there are limits to the encouragement of design texts to placing a rudder farther aft. If I moved my rudder farther forward, it would have to have a lower aspect ratio or my draft would have to be deeper. With skegs, I feel the need of big rudders, as they will resist the boat's turning. Also, lower aspect ratio rudders need more area to obtain the same effect as higher aspect ratio rudders, and my rudders' aspect ratio is only about one to one. Low AR rudders have worked for Kelsall designed cats for decades, however, so I am not concerned. To understand about rudder placement, you should look at the history of yacht design. Rudders used to be at the aft end of the keels, which were quite long. As designers reduced the length of the keel, they kept the rudders attached to the keel. Boats got harder and harder to control, until designers removed the rudder from the aft end of the shortened keels, and placed them right aft where they had once been.

Boats with outboards sometimes have them pitch their props out of the water. I have never heard of that happening with inboards. Our little boats just aren't large enough to span large waves that way.


Keeping multihulls from slewing and yawing as they surf has never been a problem, as having two long skinny hulls in the water keeps them pointing straight. Monohulls tend to slew to windward for 2 reasons-the unevenness of the port and starboard sides when the boat heels, and the lever arm of the CE being off to one the lee side torquing the boat to windward. I have certainly experienced that, surfing down huge waves in the N. Pacific in the fall. Since multhulls don't heel much, they don't get these effects. Cruising catamarans, of course, hardly heel at all. 5 degrees is considered a lot of heel in a cruising catamaran.
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Old 19-03-2008, 13:19   #119
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Kevin, another issue you are overlooking when considering rudder placement is the need to keep the props in front of the rudder, for reliable maneuvering under power. To move the props forward, you would have to either move the engines forward, thus encroaching on the accommodations, or use a sail drive. My engines are too large for sail drives, and I prefer the traditional shaft arrangement anyway, feeling that is less prone to corrosion, can be made stronger, lends itself to the use of larger diameter feathering propellers, has less potential for flooding the vessel if there is damage to the assembly, etc.
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Old 19-03-2008, 13:27   #120
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Would you consider me a “neutral” commentator, were I to counterpoint my acerbic (acid) wit, with caustic comments?
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