RULES of THUMB

[mdatrpz]

Hello fellow sailing catamaran designers.

Experienced designers generally use engineering rules of thumb to guide their design.

Please share the any rules of thumb that you have come across:

Here are three to get this thread started. Please post in regards to mast locations, twin wing sail surface areas, hull strength, etc...

Any engineering RULE OF THUMB is fair game as long as it has to do with sailing catamarans.

To determermine if your catamaran is strong enough:

1) Can you support it in cantilever with the edge of one hull fixed and the other floating? (maybe Derek Kelsall told me this)

2) Can you support it at two points, front left and right rear or left rear and right front. Basically, will the catamaran break in half if you support it at two extreme points. (maybe Dave Gerr told me this)

3) Can your cat take an frontal impact of one hull at 15 knots and the other hull won't shear off. (maybe Dave Gerr told me this)

4) Don't build a boat (maybe my mother told me this)

Ok, try to keep the rules of thumb serious! A simple RULE of THUMB can reflect years of experience.

[masalai]

Something like - - "If it ain't broke, don't fix it"

or - - "A fool and his money is soon parted"

and - - R.T.F.M. - (Read The F.....g Manual)

and - - "If you gotta ask, You can't afford it" - - followed closely by - - "A boat is a hole in the water, into which you pour money"

But in your case - read lots of big books - (similar to RTFM)... You are asking questions that require extensive knowledge to understand the answer (assuming you can ask the right question).... Good reading...

[Guillermo]

"You'll do it level at an increased price"

Cheers.

[Pericles]

It'll cost 3 times the estimate for money and 4 times for hours of construction.

Sailing is more fun than sanding.

Everyone gets seasick and they'll all prefer death at some stage before recovery.

Don't picket, it'll never get better.

http://www.thejaywalker.com/pages/****_happens.html

Pericles

[mdatrpz]

Well, that was unexpected. None of the RULES of THUMB offered were actually usable engineering guidelines. Moderator, please snub these!

The three that I offered are actually test conditions that you can run on a design to see if your boat strength is up to the challenge.

Come on guys, especially those doing design, you can't all be solving the differential equations for the perfect solution to multi-variate systems like sailboat strength, propulsion, stability, hull efficiency, etc....

[Pericles]

"You can't all be solving the differential equations for the perfect solution to multi-variate systems like sailboat strength, propulsion, stability, hull efficiency, etc....

Yes they can , but absolutely, not me.

This should work for you. Courtesy Kach22i.

http://www.catamarans.com/news/2006/...Comparison.asp

Pericles

[mdatrpz]

Ok, its really easy to say "Yes, they can". So can I, but it does not reflect experience, just abstract math solutions to who knows what exact problem.

But upon talking to many, whom tend to be very guarded in their answers, nine times out of 10, they will claim experience first and never mention mathematical prowess.

Thanks for the link, but it deals with power cats instead of sailing cats. I will read it again for any nuggets, but there is little signal in that noise from my standpoint.

So not very many engineering rules of thumb yet. Only 3, and they were mine.

Cheers, ya'all.

[Guillermo]

Sorry 'mdatrpz' (my God, I hate refering to people by such nicknames!), I was just joking.
These may be of interest to you:
http://www.john-shuttleworth.com/Articles/NESTalk.html
http://www.john-shuttleworth.com/Dogstar50-article.html

Cheers.

[masalai]

Mdatrpz, You asked for it.... What you got is usually referred to as "rules of thumb". Your questions asked for serious content requiring greater clarification - operating environment - designed performance range - specific detail on hull to be analysed - the mass of the boat, material & engineering criteria - There are so many variables that must be considered in boat design your questions CANNOT be answered with simple "rules of thumb"

Get some books on yacht design, learn how to use Rhino or FreeShip/DelftShip and learn how to interpret the data needed to input into Michelet/Godzilla, consider going to Westlawn and do some formal study. Then you may realise there are NO short-cuts & yacht design requires a detailed understanding of the many complex compromises involved in the design and analysis processes.

There is NO quick fix.

A gentleman of patience and understanding, being a very polite person has pointed you to some starting points and I respect his views greatly.

I generally tell it like I see it, from the "school of hard knocks". I would not call yours as "rules of thumb" - more likely a retort to some fool with an idiot question who deserved no further serious consideration - brought forward by this "awed person" to become as folk law

1) Can you support it in cantilever with the edge of one hull fixed and the other floating? (maybe Derek Kelsall told me this)
Any beach cat can do that but try it on a supertanker and you will have a mass of scrap iron.... depends on design criteria and purpose etc

2) Can you support it at two points, front left and right rear or left rear and right front. Basically, will the catamaran break in half if you support it at two extreme points. (maybe Dave Gerr told me this)
Don't jiggle it - many would... depends on design criteria and purpose etc

3) Can your cat take an frontal impact of one hull at 15 knots and the other hull won't shear off. (maybe Dave Gerr told me this)
Hobie cat will pass - not many others it is a matter of mass/inertia and what and where does it hit... depends on design criteria and purpose etc

4) Don't build a boat (maybe my mother told me this)
Your mother was right - she must know you well - listen to this wise lady as your life may depend in it...

Can an idjit design and build a boat? - - Of course. - - Don't ask me to go sailing, I am a sensitive person and would not be happy to hurt that persons feelings by refusing...

[mdatrpz]

masalai,

No short cuts, supertankers, or power cats in my world.

As for my mother, she was too late. I ALREADY built a 48 ft catamaran. The problem was that I was not the OWNER. Or the DESIGNER.

What I must change is the OWNER and DESIGNER part.

Please don't mention HOBIE again. None of us has time for that.

Westlawn?

Dave Gerr was a lead architect at Westlawn!

And he gave me 2 RULES of THUMB off the cuff at IBEX Miami 06 or 05(?)

But there are loads that must be accounted for on a sailing catamaran.

I will open up a new thread and see if that gets any bites....

You started to make some intelligent points. Let's finish those.

"depends on design criteria and purpose etc".

What design criteria would you have for a cruising sailing catamaran of 48ft Length/24ft beam.

That frontal impact test was restricted to the leading tip of one hull. Then the momentum of the other hull will generate forces on the briddgedeck. Will the bridgedeck shear or buckle?

Cheers,

Mark Diener

[the1much]

Originally Posted by Pericles

It'll cost 3 times the estimate for money and 4 times for hours of construction.

Sailing is more fun than sanding.

Everyone gets seasick and they'll all prefer death at some stage before recovery.

Don't picket, it'll never get better.

http://www.thejaywalker.com/pages/****_happens.html

Pericles

O MY GASH!!!!!!!!!,,,,"something" just "HAPPENED" to me,,,,,i havent laughed so hard since i broke my neck,,,,OOOOOWWWWWW,,,,,now i know why,,hehe
TOO funny Pericles!

[masalai]

Sorry Mark, As usual I type before researching/thinking - A typical "red herring tosser".

But as "rules of thumb" they struck me as such...

My "final-boat" design criteria is still evolving, (an old bastard has some needs, in waiting the long sleep), for me to be as self-sufficient/independent as spare capital $$$ allow.

Another conservative voice that read your post and made an interpretation. Possibly Mark, Your original post was too easily misinterpreted?
Pericles - On the topic!

[mdatrpz]

Hey, no problem. The only thing that will risk my life is bad engineering and the inability to take good input.

The only other risk to my life is that I am a professional taxpayer with years of experience. Now I need to start directing those dollars away from Uncle Sam and there is NOTHING like a BOAT to consume DOLLARS.

DOLLARS are falling in value anyway, so I might as well do something.

I started a thread called catamaran loads, maybe that is more understandable than RULES of THUMB.

[PsiPhi]

You toss around the phrase RULE OF THUMB - but have you ever stopped to think what Rule of Thumb actually means?
I have.
It is taken from an age gone by when men were allowed to beat their wives for, well, for whatever they thought necesarry.
Any way, there were rules, it was a civilized world, you could only beat your wife with a switch no thicker than your thumb - hence Rule of Thumb.

I now declare this thread well and truely dead!

[mdatrpz]

Yes, this thread is dead, a bad use of the phrase, but any opportunity to beat my wife, hot damn, I will take it!

Can I use my THUMBS?