# Modular Cruising Catamaran

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

1. Joined: Oct 2008
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Oh dear Mr buttons is really now showing his ignorance of the subject by his cutting and pasting and not understanding them…oh wait…that is the same as KAPER, he is not aware and chooses to remain blissfully unaware of how it works.

So, finally for those who are being mislead by a non-naval architects who’s only source of learning is a limited program called KAPER, which he says is all anyone ever needs and cutting and pasting quotes without comprehending them…read on to understand more than just how to press buttons.

So, the quote is:

“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.”

So, what does that mean….well can’t ask KAPER I needs buttons pressed…oh how do we try to understand this then children?? Ok, lets explore what this means…..so taking a boat:

Well, lets take a boat say 10m long, beam 1m and draft 0.5m
So
L=10
B=1
T=0.5
Which has a displacement of around 2.82 tonnes

So the length displacement ratio is….?? Yup 7.07

So, lets tow this, and see what happens….we get a total resistance of X

Now, lets rotate the hull 90dgerees, so the B= the T and the T= B
Ie
B= 0.5
T=1

So what is the length displacement ratio…oh it is the same, no change, as is the length.

So, now lets tow this what happens??...oh…look a slightly different resistance, no, really, don’t believe it….yes children, around upto 10% different is the effects of normal B/T ratios. The resistance is now Y.

Ok, lets now rotate the hull about the midships, since according to Mr Buttons, the resistance will be much better, why, because it is shorter…ok children lets watch…

So
L=1
B=10
T=0.5
Displacement the same (same boat)

The displacement is still the same, so the length displacement ratio is now…yes children it is 0.707….!!...my my what a difference…even though nothing has changed all we have done is made it shorter on the waterline. (Mr Buttons says this is better..)

So, lets tow the boat now….what happens….gosh, look at those waves and disturbance….the resistance is very very different from the first two…really...hands up children who thinks this last run with the L=1 is much better than the first either “X” or “Y”,...oh ok... and hands up who thinks it is worse???

why is that children?? …What is different?....

yes Wiggins, at the back……”Length displacement ratio Sir”….well done, take a house point.

So what was the quote again Sir:
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…”

Oh, so is that what Tupper means when he said:

Frictional resistance is directly related to the wetted surface area and any reduction in this will reduce skin friction resistance. This is not, however, a parameter that can be changed in isolation from others. Other form changes are likely to have most affect on wave-making resistance but may also affect frictional resistance because of consequential changes in surface area and flow velocities around the hull."

Yes Wiggens, well done boy!….changing one parameter cannot be changed in isolation.

So, how many of you see VLCC’s which are slow, with long lengths are moving beam on to the sea…that is to say, the length is now the beam, and their beam is now the length?? Hands up…oh, no takers…what about a Cross Channel ferries have the length short…any takers, oh ok, what about yachts, surely there are yachts with very short lengths and wide beams because Mr Buttons and KAPER says this is better?...no takers??

Oh ok, well children, clearly you have an acute understanding of basic principals of hydrodynamics that you can understand something as simple as length displacement ratio and its affect on resistance.

For those wishing to explore further in more detail, try “Hydrodynamics in Ship Design” Volume 1, SNAME publication.

2. Joined: Mar 2006
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### catsketcherSenior Member

here goes

I can't follow Ad Hoc here. Maybe because what he and I say is fat, or slow are actually not quantified. Take one example

In dinghy sailing it is quite common for fast cats to absolutely blast the opposition in normal winds. In drifter conditions the cats can become "stuck" and fat boats like 470s and 505s of similar weight can go faster with the same sail and weight.

This is an empirical demonstration of the fallibilities of cats. Most dinghy designers (Frank Bethwaite is one) put cat's very poor drifting speed down to the increase in wetted surface they have with two long thin hulls (and rudders and boards although induced drag is debatable here). At least with boats of the same weight, with similar rigs, cats don't do well until wavemaking seems to be the issue. This tends to happen at about 5-7 knots wind speed although modern cats get going sooner.

In very light winds (low speeds) many dinghy sailers lean their boats over to get the more rounded (less wetted surface) shapes. Racing yachts do the same in light winds - getting crew forward in a drifter to raise the stern out of the water. In fact many of these techniques reduce the length of the boat - thereby increasing the displacement to length ratio - but it is a technique that works well and is proven every time that a group of dinghy sailors run our of wind.

The IOR style monos of the 70s and 80s were very much sailed this way. The big Farr type sterns had to have crew way forward - the boat did not have to lean more than if the crew stayed aft - they just had to get the big fat stern out. According to one theory it should have slowed the boat down because it was shorter and fatter but it didn't. Waterline was only important when the boat sped up.

In fact isn't that one of the fundamental principles of boat design, shown by the metre boats and any boat where LOA is not rated. Shorten the LWL for rating but put huge overhangs on it. So when the wind is light the fat hull is fine but when the wind comes up the overhang is immersed and the hull has less resistance.

For the boats Ray design I am sure that a fat - 6:1 or 8:1 hull shape is correct. Small boats have their length determined first and then they need cross sections determined by weight and therefore volume. For large cats that always move with engines the designs are different - LOA may be able to be changed and well developed hull ratios stayed with.

I maintain that a proper cruising cat should not have hulls thinner than 13:1 in less than 40ft for a cruiser. I base this on the fact that every thin hull cat I have seen cruising has it bum deeply immersed. An immersed bum is very slow and bad for tacking etc. In fact many cruising cats with even 11:1 hulls have immersed sterns. Then there is also how we sail. In 10000 miles of cruising my cat I got passed twice on passage - that's not much and both times by racing monos on delivery - not by fast cruising cats. That is because cruisers slow down in rough stuff. So to go back to the design here I would suggest that with all the stuff to go on board each hull has to have about 2500 litres volume. This is going to be pretty hard with very thin hulls of a semi-circular section. Do the spreadsheet and then add a heap more volume - say 10%. Every second cruising cat I have ever seen has had a stern extension because the designer, builder or owner got something wrong or more probably just forgot something.

cheers

Phil

3. Joined: Oct 2007
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### masalaimasalai

The ones I have seen "overloaded" mainly because they "needed more stuff" than the design they bought/built should have been carrying so to carry that stuff a mould of a large oxy bottle was used to extend the bows and the stern was extended to allow the transom to be clear of the waterline.... Very stupid people... The design was the design not a supertanker... or a bulk cargo carrier... use what is there for what it is designed to carry/do - or buy a different boat more suited to your hoarding nature and desire for lots of heavy additional stuff...

A boat that can be readily disassembled cannot be called "seaworthy", for by its very structural form has weaknesses that are not necessary nor advisable in a boat - why compromise your safety with a design that is "engineered" to do something else? - - There are enough compromises in designing a well found boat, let alone requiring it to do and be something else...

4. ### apex1Guest

Stop that mate! You have crossed the line where delivering some compliments can be spicy! You are just offensive!

Your own private benchmarks and star ratings btw. show you as a unknown quantity! Even in your direct neighbourhood!

5. ### Chris OstlindPrevious Member

Richard,

Perhaps the potential for fairness in your criticism is something to be pursued? Frankly speaking, I do not see any difference between Ray's comments and those of Ad Hoc in this matter. Yet, you have singled-out Ray.

To be honest, I'd rather see both of them chill with the invective and listen to each of their honest responses to the questions that have been asked rather than ignoring that which is obviously still without acknowledgement. Ray could simply say that yes, KAPER has its limitations and that some of the data as he supposes it, could be flawed in that regard. He could then go on to using another piece of software designed for the purpose and produce evidence that is more supportable. Ad's disdain for software in this regard is a Reddish sort of Herring as I'm sure that computer buttons are pushed all day long in that office of his.

Ad H, on the other hand, could very easily address the obvious here, rather than launch into a circuitous route that only avoids that which so many of us already know. What can one say? Many of us here have done a lot of actual sailing under a wide variety of conditions and purposes and the issues being "discussed" have long ago been sorted by the obvious results on the water. One can push numbers around all day, but the real deal happens out there where the boat is being used.

Failing that, perhaps it would be better to refrain from dosing Ray's commentary while letting Ad H present his own version exclusive of your distaste?

I know I speak for several of the guys who are reading this thread when I say that we long ago, as one of my esteemed friends has so ably put it, "Jumped the Shark", on this discussion thread. Apologies to those who do not know the expression. It can dosed here: http://en.wikipedia.org/wiki/Jumping_the_shark

Ad... Ray, it's time to let it go and get this back on the thread. You each have something valid to say, but I'm certain that it could be said in a whole lot more respectful fashion.

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6. ### M-SashaGuest

Chris, I do not see Richards criticism unfair, the opposite. He was repeatedly attacked by Ray in a pretty stupid and premature manner for a statement Ad Hoc made!
For a man of his knowledge and expertise in boatbuilding it is unacceptable to be named a "sock puppet" by a tinker toy hobbyist, whos lack of knowledge is not substituted by playing inappropriate software.

Moreover despite of being attacked, Richard was the one here who tried several times to go back to topic, and to provide a helpful hand for the thread opener. Whereas Ray is out for fighting, and he has proven that on other threads.

As an aside, I have noticed your affectation to patronize people and repeatedly Richard. Knowing him personally for many years I can assure you, that man does not need to be spoon fed by anyone around here.

Sasha

7. ### Chris OstlindPrevious Member

Sasha,

You can relax. I'm not patronizing Richard. I happen to like him and have enjoyed his posts a great deal. Both Ray and Ad can speak to their own motivations on these matters. They are both over the line, as far as reasonable decorum would indicate.

My take on Richard is that he's a reasoned and balanced dude. I'm just not seeing that here when he drops the hammer on Ray and not on Ad Hoc, as well. You can have any opinion you like, Sasha, but fairness is something that needs to be observed, or this whole thing will crumble quickly.

It matters not what experience, or letters, one has in hand if one acts in a way that is unnecessary. Trite phrasing, condescending sentence structure, the whole thing. If you think that Ad has not used that as a tool then go back, my friend and read his posts again. It's fairly easy, he's made 1500 of them since October. They are not all nice and a good deal of them are unprovoked.

I'm not saying anymore on this disturbance as there's a topic yet to be enjoyed. Surely you'd rather see a reasonable attitude expressed by all... as in everyone showing that kind of respect to one another...?

8. ### apex1Guest

If I may answer directly? That is very easy to understand Chris. this:

was just daft only. But:

was barefaced and presumptious.

I call a person claiming that I have the obligation to either proof others statements or i´ll be named unable, a mentally disabled or simply a complete IDIOT!
But to retrieve the toy tinkerers'** honour, it has to be said that probably someone just left his kennel door open.

And as you can see it was Mr. Aldridge who attacked ME! Ad hoc did´nt. So why should I react on his posts?
well, and where is your grave warning to Mr. Aldridge? Or was that a unilateral declaration valid only for the opponents of him?

Richard

**thank you Sasha well observed!

9. ### Chris OstlindPrevious Member

Your points are taken and understood, Richard. I have not defended Ray. He's in this game as much as is Ad H and it has been stated as such. I've also had several bouts with Ray and they always go nowhere, so there's no point in pursuing that avenue and he knows it.

Ray was out of line for stepping into you that way. Agreed. You banged him around some for that and perhaps that could have been enough. When Ray references Ad Hoc's sock puppets, he's only trying to get you engaged again. If you refuse, he has nowhere to go with that line of debate.

Surely you see that Ad can be an unnecessarily contentious guy with little, to no provoking? When you side with him, you automatically get lumped in that pile and that's just human nature.

I'm getting emails from other members who say they find the topic interesting, but will not post here unless the BS comes to a halt. They find it a waste of time and see no reason to be subjected to the flame-outs by either of these guys.

Bring back your fair and reasoned approach and ask whatever party to stop when they are being excessive. Stay away from the feeding frenzy mode and seek a sense of order and respect.

With this said, I submit the following from my most recent post.

Danke sehr, Richard

10. ### Guest62110524Previous Member

I agree with your balenced views Chris
You and I know that there are several other people who make their living from building, designing etc and those people will not come into a thread that is continually hijacked by individuals who will never ever credit anybody else or see anybody elses point of view
Anybody with a mediocre of knowledge of human behaviour, can see that these folk, scream for attention, perhaps because they have not achieved recognition, in a real life, but want that recognition in a virtual world
I do not have to name people here I promised the moderator that I would not, they know who they are if not whom they are

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11. Joined: Jun 2006
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### rayaldridgeSenior Member

Phil, actually, I wouldn't go less than 10 to 1 on any cat hull that hoped to exceed hull speed without fuss. In the two hulls I ran through KAPER, the fat hull was 10 to 1 and the skinny hull was 17 to 1.

Slider's hulls are a little better than 10 to 1, but I had to increase rocker in order to get enough displacement for two people and a modestly luxurious camping outfit, on less than a 15 foot waterline. I'm sure that slows her a little, but on the plus side, she tacks in a fairly snappy and uncatlike manner, so for a little boat with no pretension to speed, I think it all worked out okay. In any case, she slips easily through the hull speed barrier.

I'd agree that for cruising 13 or 14 to 1 is a good practical limit. I, alas, don't base this on any deep knowledge of hydrodynamics, since I'm only a slight step above a complete ignoramus, but having looked at an awful lot of designs over the last 30 years, the most successful designs by the best designers seem to stay under that 14 to 1 edge. I think if you graphed all the best live-aboard cruising designs, the average would come out around 12 to 1.

Everyone else: My apologies for giving any scope to Ad Hoc and his clownish games. It seems clear that he's a fraud and only here to massage his ego, and I regret helping him. I'll add him to my ignore list, presently occupied only (and recently) by German Richard-- I really don't think the appropriate response to polite disagreement is to call someone a coward and tell him to shut up. The internet is full of tough guys who'd be a whole lot meeker in person.

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12. ### apex1Guest

Thank you! And the suffix "German" was the one to discover your intentions!!! fight...........

13. ### Guest62110524Previous Member

Richard
you hafta calm down, and walk away from what you do not like

you should not resort to personalities, I learnt that the hard way , frinstance there is a guy here, who is on abt 10 persons ignore list, thats his game to disagree, disrupt He remains anon. I,ll betcha thats his reply under, I,m just smiling , cos I can envisage what he writes
when I get peeved with someone, I offer to tallk it out by phone or some other way, same as I offered him early on
One man you guys ripped into, offered me the keys to his beach house

if I responded to every Turkey that posted nonsense or very Primer One Stuff abt my specialities, I would be a nervous wreck
Just today somebody gave me neg feed abt Frosty,s diesel post, yet I am a qualified diesel fitter getting to the stage where I post little, let the pics do the talking
Now go Hug a Mosque, mosquito? Turkish lady?

14. Joined: Oct 2008
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"..Just today somebody gave me neg feed abt Frosty,s diesel post, yet I am a qualified diesel fitter getting to the stage where I post little.."

That is the problem with fully open forums. Doesn't matter who or what you are, whether you post your real name or not...you shall always come across others that either:
1) Think they know more
2) Do know more
3) or simply not a clue what they are talking about either way

Either way, just because you do know more or don't know more...doesn't make them or you an expert. Just knowing a bit more or a bit less, that's all. In a normal world, debate is easy. There will always be disagreements.

But as noted on another thread and rightly said....what I write can be written with a smile or with a grimace...but they are just words...so how they are interpreted is all down to the person reading them. Since can they see me smile or frown when writing said words?

I have endless debates/arguments with my peers in my job. But it is a debate and a difference of opinion or exploring opinions, nothing more. We each enjoy it as fun too....there is no disrespect shown at all, quite the opposite extreme mutual respect.

But this is not the case on an open forums. Too many trying to look or sound knowledgeable and as such don't wish to be questioned or shown to be less than they portray. That is simply a personality disorder or education issue, nothing to do with the topic or the level of subject matter being discussed.. Why....go figure?

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### masalaimasalai

I get a feeling that this thread is starting to look like a "pissing contest" - - Lets start again, and try to point this bloke in the right direction???? If you reckon all the challenges in "Mission Impossible" were a cake walk, then continue with your original idea, and keep us posted with photographs and expenditures and test runs - I am sure many will delight at your exploits and adventures - - - - It does get boring herein occasionally... Otherwise build nearer to the ocean and build a well proven design with care and obey the designers instructions exactly and enjoy some good cruising...

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