Catamaran beams New! Improved!

hi all, moved all good stuff here, left all the DRIVEL behind. please continue.





building a catamaran, 31ft long, 14ft wide, strip plank, epoxy and glass. 1200Kg fully loaded for cruising, 30M2 sail all up.

need advice on aluminium cross beams for the;

1. front

2. and back. the transom (50Kg outboard hung in the middle)


instead of single round tube, wanna use two tubes, 60cm apart, one on top of the other, with welded webbing connecting them.
ie a truss for the transom and a box beam for the front



so then,
the question is,
who suggests what? (square, rectangular,or round)
and how thick should they be.


thank you for any advice,
mal.

 

07-09-2009, 12:11 AM
jaydh
Junior Member

Join Date: Aug 2007
Rep: 10 Posts: 19
Location: down unda

mal,

An option is to bend up your own if you can find a wide-enough press close to you. We used 4mm sheet for the front cross beam and a full box transom (I realize you need a round beam/section). The transom takes minutes to fold or bend if the operator knows what he's doing. The front cross beam can be custom shaped to to suit your needs also...flat on top for chainplates seagull striker, etc...then curved nicely to a flat bottom or whatever. You can easily weld in gussets/beckets to the ID shape and stiffen the heck out it. If you look into that, make sure the bender people have a 30mm or so diameter round bar on the press. The angled ones used for steel make little chop lines and they can also split alloy depending on the material thickness and severity of the fold. just make up a cross section template on plastic or cardboard and they should be able to match it near perfect. It almost pathetic how easy and quick it is....and how affordable IF you don't have a pre-extruded mast section that fits your needs.

As a side note for a stab in the dark reference....our old 35 foot Seawind cat had slightly oval shaped back, mid and front beams about 200mm x 150mm in 4mm wall. they were encased in glass roughly 300 mm in the mid part and all the way though the hull in the front and back beams. No dramas with it and it had alot of use/abuse/miles. ...vague info I know!

have fun - Jay

 

bob the builder
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Join Date: Jul 2009
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onya j

measured a large tinny here an thats what they used for their transom, 4mm box beam, to hold a huge outboard.

a seawind 24 full of toys would weigh about the same.
and they have 5m x 190 x 128 x 4mm wall beams
weird how the 24 and 35 have such similar beam strengths




instead of folding,
i was thinking rectangular tube T6 80 x 40 2mm walls for the top and bottom

maybe round tube 40 with 2mm walls for the truss webs

easier. just buy it and weld it together.





"
(I realize you need a round beam/section).
"

NO. thats exactly what i don't want.

the reason for this post is to try and spec a space frame/box beam/house truss thing to do the same job as a mast section,






if the hulls weigh 600Kgs each, 2.5m apart, is this a good space frame for them?

should i up the strength


tell me more about your front cross beam and what is it for (please)
whats the all up kilos of aluminium per meter, what does a hull weigh?
(and why did you fold you own?


cheers,
mal

 

Rick Willoughby
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Join Date: Jan 2007
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Quote:
Originally Posted by bob the builder View Post
.......
instead of single round tube, wanna use two tubes, 60cm apart, one on top of the other, with welded webbing connecting them.

......
mal.



Mal
A catamaran in quartering seas generates relatively large moments between the two hulls as one bow digs in and the stern on the other hull is pressed below its waterline. The opposite bow and stern are unloaded in this condition.

The cross beams have to resolve the moment without undue flexing. With two beams, the moment will be resolved through the combination of vertical bending and torsional shear in the beams.

Your proposed beams do not make the best use of material to provide a rigid connection for this twisting moment between the hulls. The lightest section, to provide adequate rigidity, will have large enclosed area with the thinnest wall that will withstand the localised impact loads.

The type of beam that Jay describes will result in a light beam with high torsional rigidity. A standard section is preferred though because it avoids welding and the associated reduction in allowable cyclic stress range.

It is quite easy to appreciate the difference in rigidity using cardboard taped up to form the different sections and make comparisons.

Rick W

 

bob the builder
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rick,
i grabbed me little balsa truss and did you know you're right? the little bugger did flex a bit. fantastic in up and down though. bad in twisting

i got a different picture from what jay said. i thought he meant his transom was folded to be 40mm wide and say 60cm deep, and if so, then why not use two tubes?, but i think you and he are talking one thing and i'm picturing another boat



the front i'll make into a box beam then.


the back i might have to add something in addition to the truss to handle torsional shear

i sorta wanted a normal aluminium transom so i could just drop an outboard on with no fuss



hmmm

 

Rick Willoughby
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Join Date: Jan 2007
Rep: 680 Posts: 2,997
Location: Melbourne, AU


Quote:
Originally Posted by bob the builder View Post
rick,
i grabbed me little balsa truss and did you know you're right? the little bugger did flex a bit. fantastic in up and down though. bad in twisting

........

hmmm





Mal
Understanding how to resolve moments can be a challenge for experienced structural designers. I have seen some world renowned engineering firms produce structures that fall apart prematurely because there was inadequate means of resolving moments. Five minutes with some cardboard and sticky tape can be illuminating.

Anyhow I saw you other post on the beam analysis. The analysis and diagram shown are not relevant to the cat cross beams as the beam was simply supported. The ends of your beam are rigidly supported so whole other case. There is a thread somewhere here that goes through some of this analysis.

Analysing the combined condition of torque and vertical bending to determine deflection is complex. If you work them as separate cases rather than combined and take whichever is the stiffest you will have a conservative design for deflection.

Another load case you have to contend with is a high speed collision of one hull against something solid. One hull shudders to a stop while the other tries to keep going. This is extreme case and an ultimate condition but the fatigue condition with the leeward hull slamming into waves on a regular basis will be as demanding on the beams. This produces bending of the beams in the horizontal plane.

Another significant load case for one of the beams will be the vertical bending from the mast compression.

None of these calculations are trivial - unless you are experienced with structural design. Many good designs have been arrived at through trial and error because establishing the load cases and doing the maths (with adequate precision) is time consuming. One-off professional designs that are pushing the limits will often be fitted with strain guages during initial trials to verify that the load cases used in the design were appropriate. The other way is to fix the things as they break but this can be soul destroying or much worse.

Having a simple cardboard or balsa model that you can twist, force and press by hand to simulate different loading conditions is useful for a first-off design.

Rick W

 

bob the builder
novice

Join Date: Jul 2009
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Location: mooloolaba

this the current truss specs;
70x100x5mm 60cm apart, 45 degree webbing, gives a beam with 1200Kg @ 0.3% deflection
please comment, that means you, yes you out there in cyber space


j - i just saw your cat
(nice! - (saw an aluminium jaguar once, no paint, just mirror polish, please consider))

,and understand now about your beams. good idea.



rick,
truss transom on it's own ain't so good with torsion, but if it's on a strip plank glass epoxy floor, bolted onto large bulkheads in the hull, isn't the floor a beam that's 3m deep? isn't this then 20 times more inertia than needed?

(full bridge deck cat , deck 15mm kiri 600gsm db top an bottom)

(really want a truss, but also want the thing to stay together at sea!


thanks all,
mal

 

Rick Willoughby
Just my name!

Join Date: Jan 2007
Rep: 680 Posts: 2,997
Location: Melbourne, AU



Quote:
Originally Posted by bob the builder View Post
....

rick,
truss transom on it's own ain't so good with torsion, but if it's on a strip plank glass epoxy floor, bolted onto large bulkheads in the hull, isn't the floor a beam that's 3m deep? isn't this then 20 times more inertia than needed?

(full bridge deck cat , deck 15mm kiri 600gsm db top an bottom)

(really want a truss, but also want the thing to stay together at sea!


thanks all,
mal





mal
You need to have an enclosed area to transmit torque. A floor and sides will have limited ability to do it. A floor, sides and roof will do it but it brings in a lot of issues in stress concentrations around any openings. As the hulls work relative to each other, the openings distort and experience stress concentration at corners the result in cracking unless suitably strengthened.

I am reminded of a structural design engineer's comments when I asked for some openings for cables to pass through. He commented that I was no better than a plane passenger wanting windows. The fuselage is the perfect structure and then some clown puts holes along it simply so that passengers can see out!

The bridge deck is the ideal structure for rigidly linking the two hulls providing it does not have any holes. Most people want holes for various purposes and there is a lot engineering detail around the openings to avoid problems. Simple test - make a cardboard mock up of the bridge deck without holes - twist cross wise. Now place all the openings and compare.

Rick W

 

bob the builder
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see the specs above? done by me! so imagine betting your little bum on that baby a thousand miles from shore.

so if you can,
please improve my specs. is my math correct? i wouldn't put a single dollar on it.)


thanks rick,
you've helped me heaps.

maybe talked me into a normal mast section for the front beam, so this is half a victory for reality Vs me at any rate.

as for the transom truss,
after i have a enjineer say "Great Work Mal!, but, you're truss calculations are OUT! (Shocked! tone), you need 4mm T6 to get 0.3% deflection with 1200Kg right in the middle."

then i'll try improve the twisting.

i think all the guys here should be able to come up with a good solution, but in the end it may be like the front beam, and just be more sensible to fit a big round tube, say 150mm round 4mm walls? then attach the 50Kg outboard with bolts in the middle of it.

right now i'm thinking maybe another truss the same say 7ft in. great! idea hey?

thanks all,
mal

 

Rick Willoughby
Just my name!

Join Date: Jan 2007
Rep: 680 Posts: 2,997
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Quote:
Originally Posted by bob the builder View Post
...
after i have a enjineer say "Great Work Mal!, but, you're truss calculations are OUT! (Shocked! tone), you need 4mm T6 to get 0.3% deflection with 1200Kg right in the middle."

...
mal



Mal
To comment as you suggest requires a designer to do a design check, which involves work on their part. Most people seek payment for work.

If you pay them there is an obligation and the result has value. If you do not pay them there is no obligation and the result is just an opinion that you may or may not value. It was free after all.

There are examples on the forum where the opinion changes in subsequent posts from the same individual once the opinion is challenged.

I take any opinion offered in this forum with skepticism. Unless you have the means to validate it it is worthless. After all you did not pay for it. If payment is involved you seek references and look at past results.

Most opinions are given in good faith but often from limited experience or a myopic view of things because that is the way it has always been done. Hard for people trained a certain way to untrain themselves and be open minded enough to accept improved methods and new ideas from what they were taught.

I have learnt that no one knows what they don't know AND the more you know the more you know you don't know. So learning is an ever expanding horizon.

Given all of the above, I accept that anyone can build a boat. If the design is poor the launching will mark the start of a lengthy, frustrating and possibly expensive development process. If the design is good then expectations are met. You have to work out how much you need to know to achieve your ends.

This thread might give a better idea of my point:
Build yourself a boat and do a lap, crazy or not?

Rick W

 

bob the builder
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Join Date: Jul 2009
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Location: mooloolaba



rick,

i don't wish to appear light hearted, but i'm used to flippantly juggling abstracts all the way in one direction, and then all the way in the opposite.

so, although i'm a nerd and my verbiage is torrential and light, i have lived on a yacht before, and realise hard facts do exist.

it's not that big a deal to me, to ponder one way, then change to a more traditional method. i haven't looked at your other posts here on this forum, but from your tone, i would guess you do serious boat stuff for work. ie you have to take other peoples engineering seriously.

"To comment as you suggest requires a designer to do a design check"
no it doesn't. a design check implies hours of work.
a simple beam or truss inertia is a matter of a minute at the outside.
some people do formulas everyday, and might take some person here 10 seconds in a truss program to check my calculations. that is what i was after, all the rest is gravy.


you have to start somewhere. i started all by me little lonesome, after YEARS of reading and making other designs, seen lots of plans, and in my mind, think i can build much better than a foam cat. (professionals and food) http://www.boatdesign.net/forums/multihulls/good-reasons-not-design-13162-6.html
been on expensive foam cats recently, and refuse to even listen to these people anymore. won't let them put their biased info into my mind. walking on jellyfish. felt professional boat builders work after 20 years.

payment! absolutely not. all you get is one persons opinion. an i much prefer a gift like what you gave me. you had nothing to gain, and so spoke freely.

i prefer a menu, like what's happening here. a smorgas board of different opinions from all over the world. brilliant. and then choose the one that appeals.


my brother used to build drag racings cars. he's not an engineer. he would typically buy parts from engineers and then put them together. he didn't want a plain vanilla honda.

i talked to you and now i'm possibly doing a plain vanilla round beam on the front, MUCH better and easier.

i'm very happy. now to continue the refining process.

sure, i won't end up with a shiny! General Motors car, and that makes me very happy.

i just repaired a mazda e2000 van 9! years old. the paint fell off the roof, and there were rust holes big as your hand all through the roof.

professionals. all yours mate.

 

Rick Willoughby
Just my name!

Join Date: Jan 2007
Rep: 680 Posts: 2,997
Location: Melbourne, AU

Quote:
Originally Posted by bob the builder View Post
...........
it's not that big a deal to me, to ponder one way, then change to a more traditional method. i haven't looked at your other posts here on this forum, but from your tone, i would guess you do serious boat stuff for work. ie you have to take other peoples engineering seriously.

"To comment as you suggest requires a designer to do a design check"
no it doesn't. a design check implies hours of work.
a simple beam or truss inertia is a matter of a minute at the outside.
some people do formulas everyday, and might take some person here 10 seconds in a truss program to check my calculations. that is what i was after, all the rest is gravy.


......




Mal
I have owned a few boats and boats are a lifetime hobby. I currently play with design concepts and have built some reasonable models that are large enough to pedal. I started this thread to catalog my recent efforts and hopefully encourage others:
Pedal Powered Boats
http://www.boatdesign.net/forums/boat-design/pedal-powered-boats-23345-20.html
I have learnt about doing more with less in boats from these experiences.

My aim is to learn and have a nice retirement project for a boat I can get built and cruise the Australian coast and rivers economically and safely.

I am an electrical engineer and have managed multi-disciplined engineering teams for many years so I have reasonably wide knowledge across a number of engineering and business disciplines. I am currently paid by an insurance company to be the devil's advocate and work out how things might fail. I am in a position where I can have others do the detail engineering for my paid work.

Similar for boats. If I want design detail on a boat I pay a trusted Naval Architect to do the work as he is forced to know detail on crap that bores me to death. I can play with concepts and have acquired or developed some very good tools for preliminary design. I was suggesting you also find a good NA to bounce ideas off. A good one, like I use, does not bother charging for a casual minute here and there.

I give my opinions on boats in good faith and am glad you appreciated my input here.

I am not sure if you read much of that thread on Tin Can but one of the reviewers offered this:
Yet as Theodore Roosevelt said, …"credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood, who strives valiantly, who errs and comes short again and again, who knows the great enthusiasms, the great devotions, and spends himself in a worthy cause, who at best knows achievement and who at the worst if he fails at least fails while daring greatly."

There is nothing like getting the hands dirty. It is rewarding in itself and I find therapeutic. At present I build about 2 boats a year for myself and am helping about 10 others with various concepts they are developing. All input at no cost because it is all interesting and helps me learn. It rapidly amplifies my experience base on novel concepts.

I hope you take the time to post the progress on you boat and how you overcome the various challenges. Most here try to help and offer encouragement.

Rick W

 

Yesterday, 10:16 PM
Steve W
Senior Member

Join Date: Jul 2004
Rep: 19 Posts: 112
Location: profesional boatbuilder and occasional designer


Bob, on a small cat like yours im curious why you dont want to use the simple round aluminum tube approach, its well proven and cheap. As a reference i used to own a Macgregor 36 and all three beams were stock off the shelf round tubes between 16 and 17 ft long and 6 5/8 inch diameter, the mid and aft beams were 1/4 inch wall thickness and the fwd beam 1/8 inch. The mast beam had a substantal dolphin striker and the forestay used a bridle and no seagull striker.
Steve.

 

bob the builder
novice

Join Date: Jul 2009
Rep: 34 Posts: 16
Location: mooloolaba

hi steve.
the front bridge deck lent itself naturally to an enclosed triangular space 3' x 2', so o thought, why not have a ply or aluminium box beam, since the outsides of the box beam are going to be present anyway. thus saving on weight.

there's open deck at the back with open hulls. a truss style transom would suit perfectly.

i've got oodles of other round aluminium beams specs suitable to do the job, and plain vanilla engineering is the default option.

i may well use both a truss and beam together in the end.
the truss to weld on wing seats outside the cat, cut and weld outboard fitting on, bolt on to the thick ply hull bulkheads, weld pole holders on, weld sampson post on etc etc etc.

the utility and ease of attaching fittings is astounding compared to the delicacy of the round beam, which some people don't even drill for the main bolt holes.

mal

 

Rick Willoughby
Just my name!

Join Date: Jan 2007
Rep: 680 Posts: 2,997
Location: Melbourne, AU

Quote:
Originally Posted by bob the builder View Post
the front bridge deck lent itself naturally to an enclosed triangular space 3' x 2', so o thought, why not have a ply or aluminium box beam, since the outsides of the box beam are going to be present anyway. thus saving on weight.

......



Mal
Incorporating a box beam in the bridge deck is a good solution that I have seen others use. It is the ideal moment resisting member being near the centre of the hulls and with large enclosed space. Ideally the ends are enclosed and able to transmit the stress smoothly into the hulls.

Remember that any hole in this beam will see stress concentrations at the corners. If you can avoid holes all the better but generally such space needs to serve more than one purpose.

Rick W