Like many on this site, I am interested in beachcat hull design and with some information posted on another site I was able to build a crude 3D CAD model of the MM A2 hull. By crude I mean that you probably would not want to be making moulds straight from this data but its more than adequate for getting design information from. See picture below.

I am sure there are a few designers out there like me with CAD skills that could help produce and distribute 3D hulls.

These CAD models should enable us to make more objective comparisons of different catamarn designs, so when we talk about how much volume a Hobie Wave has or discuss the merits of wave piercing hulls we have access to real numbers, like hull volume, wetted surafce area, Cp and all the other ratios used.

So I have 4 requests

1. If anyone can send me or post links to any public domain drawings of catamaran hulls I would be happy to carry on creating these models. I need plan, side and section views. Such as was posted on http://www.morrellimelvin.com/a2/specifications.htm and http://www.geocities.com/patrik_elfving111/

2. How should I distribute these models?

3. Does anyone have suggestions how else we could present this data for people without access to CAD, i.e. Cp values, Volumes, Wetted surface area.

4. Is anyone else willing to lend a hand to create and post these models.

All the best

Gareth
www.fourhulls.com

Hi Grob

Wow ,really nice looking hull . thanks for posting the pic , I,m a fan of MM cat designs , their most recent A Class cat looks great .

I have something called Data cad 10 but hardly use it ,-I,m one of those dinosaurs that still use a pencil , so I,m not much help but like the idea.

Some classes like the Tornado for example are common knowledge and made available for home construction .Lines sections and volume calc -PC # wetted surface ,--CLR CE sail plan etc would be fun for those types ,--but we will have to be carefull about newer designs by individual firms and I suspect get their permission .
The intelectual property aspect I think would protect the authors rights on other designs so we should be carefull,--not a lawyer but this most likely would fall into this category even though all basic boats are published with {class} specifications ,though these just basics as per rated Length -board area- board depth- rated weight and rated sail area per ISAF requirements. The hull sections lines and actual design may be something the designer wants to sell themselves or not made available to a competitive design firm .

I,m sure the interpretation and laws may be different in various countries on intelectual property ,-It protects your fold up 4 huller as well, though you may have an actual patent and international patent on it ,

The intent is an excellent one -scientific research ,and this would really help many sailors to understand the basics of design and serve potential Formula Classes and help refine catamaran rating ,
-but again some newer designs may be off limits and may conflict with a design firms wishes .

Maybe someone more knowledgable on this would comment on designs off limits and those that are O K to examine in detail with hull sections and specifics of basic design .

We could start with the Tornado which is used as a benchmark of sorts for cat design , suggest just posting the pic , L and basic section dimentions then let others calc the various comparisons at various angles of heel and submersion from sail forces and crew weight .

There is some nice looking boat specific cad software available
http://home.att.net/~hcyoung/
http://www.vacantisw.com/
http://www.westlawn.org/student_center/yacht_design_software.shtml

A group just started a Formula 14 class concept with fairly open rules , just L 14,3 and a 24 ft mast with 300 sq ft max sail area in any configuration .

Unlimited beam ,-any sail plan configuration within 24 ft
any hull shape or number of -hulls .

Some have built ht type 14s that weight 107Lbs ,-
there are production types like Mystere 4,3
H-14 Trac 14 N 450 Wave -the production boats weight 240 and up ---Some design comparison and performance annalysis might be fun there ,--It would help future F-14 rules and the potential of sub categories as numbers grow in class --They may have 3 eventually that they find needed
due to potential speed differences .--antisipate a HT Category for lightweight designs that will include the more experimental type creations allowed within open rules ,like 14 ft beam craft with 2 masts ,-or a tri configuration or tunnen hull scow type .--a Production category for 230 and up weight boats that will be the majority at first in another category .

Beyond C Class cats with solid wing sails the only other development classes are the A Class with 7.5 beam limits and no angled boards ,150 sq ft sail ar ,-160 min weight ,
and the seldom built 18 sq class with unlimited beam on 18 ft L and 193 sq ft of sail . It would be good to see this type of class in this size renued maybe longer with spin added .

The C class designs at 25 ft and 14 beam with 300 max sail area due to its light weight to length ratio seem to concentrate on minimizing wetted surface area in hull design .
The new F-14s present a completely different set of problems due to human scale -weight ,-sail forces etc .

It might be interesting to take some existing hull design and just try variables out on it ,--like wider transom on an A class cat design ,then predict its speed potential comparison ,-though they wildthing downwind .

The variables are really interesting and think evan experienced designers often fool themselves with test tank type and formula data between theory and practical function.
Inspiration and insight being wonderfull unknown human quantities.

Maybe we could pick a Length class category and start there and examine one a week . It is very boring stuff for some ,--maybe Rick would add a design -build specific forum category for those interested.

I,m strictly an amature ,but always interested in learning more

a few other boat CAD packages..
First is an Aussie package .. I like it because its Aussie and more importantly its free.. in the basic version for students..
http://www.hullform.com/

Another I have seen used is Maxiform from http://www.formsys.com/

You are right about this thats why I only want to use stuff in the public domain, so don't send me plans unless they are freely available, and its probably best to tell me the source of the drawing so I can make my own judgement.

I don't think I am doing anything wrong here, this is something we do all the time in the auto industry. Indeed many companies make a living out of collating and selling this type of information.

All the best

Gareth
www.fourhulls.com

I am totally ignorant regarding design. Can either of you propose a good primer as a starting point?

Hi Pete

its dated but think it was Skene,s elements of yacht design that got me started ,-
http://www.sportsreading.com/Skenes_Elements_of_Yacht_Design_0396079687.html

It goes thru the basics ,though applied more to sailing yacht than beach cats ,--but the basic calc and formulas apply ,--just differently with two hulls and no lead mine keel below.
model building or the actual thing , then application of these and final design becomes all to real as any error becomes obvious ,-this is the hard way but best way to really learn . Hope we see a resurgence of cat designs and interest encouraged by limited development type racing classes to test then out in.
You read about a number of home builders or catsailors planning a cat project.

Thanks Grob --boat designs in public domain

it is kind of a grey area ,-wasn,t sure .

VoloView by Autodesk (AutoCAD) used to be availble for free download. It has ability to open dwg files, and should have the ability to "orbit" your 3-d model. Hence, you could distribute dwg files if you wished. I am not very familar with VoloView so I maybe wrong.

Hi Carl, thanks.

What I'm really after is a boat with the stability and seaworthiness of my Tiki 21 with the performance of a Hobie 16.

It's probably a pipe dream, but I'd like to be able to form a more educated opinion.

Any comments?

Hi,

I have all the data for my own design for an A-Class hull. Hopefully the attached image files work.
I used a spreadsheet attempting to minimize wetted surface (elliptical-circular-ellptical sections), maintain a target prismatic coefficient, while minimizing the rate of change of displacement/frontal area from fron to back. It ened up with a little bit of rocker, which I thought was nice, but can't explain why. There are much more sophisticated ways of doing this, such as using Taylor Series expansions to minimize/maximize certain variables, but I don't have time yet for this. Anyway, E-mail me and I'll send you my spreadsheets...

Steve Bellavia
Hobie FX-1 Sail #211

Hi Grob,

IGES seems to be the most universal format for surface data. Multisurf, Rhino ICEM and many CAD systems support IGES.(you are using Pro-E?)

Modeling is a nice way to get hydrostatics on a hull.

Without CAD maybe... for catamarans maybe bonjean curves or longitudinal metacenter vs displacement would be useful.

Cool project as long as nobody gets mad.

Talk to you later
-colin

Pete, I would reccomend

Principles of Yacht Design by Lars Larsson, Rolf E. Eliasson
http://www.amazon.com/exec/obidos/t...103-7971006-7374220?v=glance&s=books

All the best

Gareth

Colin, yep I use Pro-E, that background is a giveaway

Hi Pete

My guess is Grob,s recomendation on the basics of design is better,--Skene,s is very dated ,-got it more than a few decades ago

On over 21 ft cruising cats ,- H-makes a 21 sport ,buts its heavy ,--The Stiletto 23 was a nice platform with not quite enough hull volume for a berth though .
The real problem getting performance in a larger cruising cat is having enough sail area ,-but then a capsize and righting become a concern .

A modified Supercat20 or Arc22 with a smaller cruiser type or reefable sail plan may be worth while ,-Bill designed the neat stay extender lever hardware to solve the righting problem ,--have a SC 15 for the kids ,--it works great .

You might find a used one very reasonably that needs sails or minor repair , Maybe a slighter shorter mast and sail plan on it would make an excellent fast coastal day cruiser that numerous people could sail without being overloaded .

There are some newer mid sized cats on the market ,but extremely expencive .

hope that helps .

Thanks to all. I'll order the book, see if it makes any sense to me (had a tough time with college algegra!).

In the mean time, I'll start pricing those two boats.

Thanks again.

Is there any way to post any dwg. or dxf. or iges. file on the forum. I use Rhino and Multi-surf. I have some hull models ive built for the F-16 class, The open rules for f-14 looks like fun!
Ricky

render it as a jpg from within autocad.

attach that.

Hi Gareth,
Let me explain briefly about hull/boat design. Boats are designed with numbers, many many many numbers and ratios of many numbers and numbers raised to powers, etc. Long before the first hull line is put on paper, that hull shape is described in terms of numbers which are necessary to meet various requirements for the hull design. The same is true for the mast and sail plan and the centerboard and rudder. It is all numbers. The last step is to draw the picture, the geometry, that satisfies the numbers, the ratios, etc.
To draw use CAD and draw pretty pictures of hull shapes without knowing and understanding the numbers, the rules and the goals, of the hull design leaves many many unknowns which could be called guesses. The beauty of running the numbers first is that if some performance or behavioral characteristic of the design is undesireable, change the numbers, iterate, until a satisfactory solution is found. It is much quicker and less expensive to do it this way.
Bill

Hi Bill

Great racing recently , running aground ,snapping a rudder, capsizing ,-righting and sailing well to win O A always makes the finish a little more rewarding.
race story -
http://www.catsailor.com/Stories_Temp/Stplchs03.htm

Agree --a general design discussion of hulls only of various sizes and different intended functions will not be benificial unless targeted to a specific type and with a broader understanding of design .

The knowledge and experience gained from designing and producing a series of successfull cat designs ranging from 15 to 30 ft --SC 15 --17 19-- 20 --Arc 21 22 RC 25 -27 and a 30 --and I,m sure a wide range of experimental type craft in between , now allows the ready applicable calc. or formula for a particular cat. size and desired function within a targeted design criteria. From going through this catamaran design exercize successfully numerous times you have the wonderfull ability to apply the correct series of engineering calc required , and can readily recall them having real models of each to relate to .
SC link http://www.aquarius-sail.com/

I like Gareth's idea , I,m here to learn more , think your correctly stating that if the design comparisons are to be benificial they should target a specific type ,but also that Naval architecture is complex and it will take some time to comprehend the intrical aspects .
--It would still be interesting to discuss design, several do have auto cad to help visualize it all .
There are numerous sites with basic lines and jpg pics for comparison also , this from the Aussie C cat challenge group .http://www.lacaustralia.com/design/challenge.htm

It may start with the A Class per example used per pic .
Some great video from the Worlds here
http://a-class.org.nz/photos_2004_worlds_video.htm
Defining all basic elements of catamaran design then applying them to targeted design criteria and type like A Class cats ,-then other design development class categories .

Like the A Class intnl reference to {big pond }
as opposed to a big fish in a small pond
http://www.users.bigpond.com/aiadca/index.html

a Class rules and basic boat specs are here -
http://www.usaca.info/

One interesting aspect of this class is many designers also race in it . So sailing techniques and practical insights instincts and intuition also come into play .
The downwind sailing technique of hull up with crew back may lead some larger crew to desire more volume towards the stern for example.
The A-s are highly refined and specialized type with matching mast flex and sail to crew weight as well.

It would be very interesting to run through variables of design in A-s - typical design --then add volume in the sterns and run through the numbers and a visual pic to relate it to ,--then canted hulls - etc .

It might be best if Rick set up a Design build specific category on Catsailor .

All the best Bill -& Gareth

Carl R

Hi Carl,
My point is that if you don't know what you are doing, you don't know what you've got.
Adrenalin is an example of a boat that was built to an exciting idea, pivoting amas, with incomplete analysis and understanding. Pivoting amas should move through the water with less drag than fixed amas. Less drag means more speed. What a great idea. Evidently no forward stability calculations were done. This proved to be the boat's downfall. The boat originally had a 76ft tall mast on it. A few downwind runs/reaches and the mast was shortened to 56ft. Why??? When the amas pivot as forward pitching moment increases, they provide no increase in pitchpole resistance. The boat wanted to dive all the time. This design was very deficient in pitchpole resistance due to the "pivoting" amas. A few minutes of forward stability calculations would have said, "hey, wait a minute; we have a problem".
Sailing World magazine said this boat was a "world beater, a breakthrough design for trimarans". The last thing I heard about the boat was that it sits abandoned in an old boatyard with weeds growing up around it several feet tall.
A few minutes of calculations and analysis could have made a big difference here in time and effort and dollars spent.
Bill