Catamaran beams

hi all


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.




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


thank you for any advice,
mal.

 

Alloy tubes or mast sections can make great beams for a small cat, and your boat certainly is light for its length, but fabricating the way you suggest, I dunno, you may be better to consider a couple of wood and ply box-beams, glassed over; and a wood or alloy forebeam. A suggestion; Find a tube cat the size and config you want to build, buy the study plans and get the info from the materials list.

 

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

 

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

 

a sketch would help

 

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

 

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

 

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

 

"...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..."

No, it is about establishing the loads in the first place. resolving of forces is the easy part, what the magnitude of each load to apply is another matter.

"...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..."

No. Worse case scenarios are not individual load cases, but a combination or a summation of them, but must be demonstrated to ones satisfaction and not assumed.

"...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..."

No. It is a matter of establishing what is the actual load to apply.


"...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..."

To understand a concept or "action" that is not clearly understood, yes. But, it does not provide the magnitude of the loads since to establish the magnitude of load and its location there must be exact scaling and the exact same conditions. Which is next to impossible since the scaling of 3D structure is not linear

 

Ad Hoc
thanks. great stuff. does your philosophy also involve my truss specs?

(which would have taken a single line instead of 20)
ie truss 70x100x5mm 60cm apart, 45 degree webbing, gives a beam with 1200Kg @ 0.3% deflection

and do you think your undoubtedly brilliant philosophy will ever bear a single piece of fruit? linear or otherwise?



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

 

Mal

Your questions are rather like, how long is a piece of string. Hence my reply in #5. No point discussing further without seeing what the actual proposed structure is like (application is everything) and where/how it is proposed to be attached, and to ensure "words" have not been misused/misunderstood etc, as so often happens, which leads to mistakes/errors. However, you selected to decline, that's your prerogative.

 

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

 

"..The bridge deck is the ideal structure for rigidly linking the two hulls providing it does not have any holes..."

This is such a gross generalised comment and is in essence nonsense. Only those that do not understand structure nor design structure would make such a statement. Structure is all about establishing the loads, what/where the load paths are and what localised discontinuities exists that may affect the transfer of said load to the surrounding structure and then compare against the allowable design values and what is the environment.

Catamarans all over the world, including the many I ahve designed, all have holes. The size of the hole, or whether one wants a hole is dependent upon the magnitude of the load, the geometry and if production time (ie manufacturing costs) coupled with saving weight (ie a proper weight estimate has not been done at the beginning).

To place a hole through any structural member can be done and safely too, if you know what you're doing....how else would you be able to look out of a window when you're flying??? If is wasn't safe, there would be no windows, QED!

 

So, with this substantiating position, could you be so kind as to post a collection of examples of your work, as it would apply to the original boat and question asked?

Since I am a guy with no formal engineering training, it would be real solid info for me (and the rest of the folks here) if you would consent to sharing your, I'm sure, exemplary work for our reference.

I'm most interested in drawings of your design work, coupled with completed examples of each craft, to illustrate just how it was completed.

Thank you,

 

Chris

Unlike many on this site, I'm not here to say "look what I can do...aint i brilliant". I just offer advice based upon experience. This is to short circuit the many years of training and education as well as studying that that would other wise be required.

However if you PM me your details, I would be happy to provide you with details, citing reference books and magazine where the designs are featured. I design in the commercial field, not luxury/leisure.

But the information given, is just about the boat, not details of hard engineering/structural design as you appear to want. I have written and co-authored many technical papers (all published), however, if you wish to read some of the papers identifying significantly more detail than the very brief summaries I have given above and elsewhere, i am happy to do so. Since you want some 'hard' details, i can send you some of these, rather than "glossy data" in magazines.