Catamaran beams New! Improved!

[bob the builder]

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.

[bob the builder]

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

Join Date: Aug 2007
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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]

bob the builder
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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

[bob the builder]

Rick Willoughby
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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]

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

[bob the builder]

Rick Willoughby
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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]

bob the builder
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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

[bob the builder]

Rick Willoughby
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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]

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

[bob the builder]

Rick Willoughby
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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]

bob the builder
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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.

[bob the builder]

Rick Willoughby
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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

[bob the builder]

Yesterday, 10:16 PM
Steve W
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Join Date: Jul 2004
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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]

bob the builder
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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

[bob the builder]

Rick Willoughby
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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

[bob the builder]

the last thread got taken over.
all rubbish will be deleted.
all drivel to the old thread, ok?
is that fair?


and,
once again, thanks guys, to all helpers.
mal.

lets continue.
box beam on the front, triangular, 3ft wide on the top, 60cm high, and whatever underneath

truss on the back, T6 aluminium currently 4.4m long 70 x 100 x 4mm with 45 degree webbing of the same.

[bob the builder]

ok all, here's how my two beams are progressing.

if 7ft in from the truss transom i put in another aluminium beam under the solid deck, say 70 x 70 x 3.5 T6 all the way across and bolted on bulkheads in either hull, the solid bridge deck i believe has now been transformed into a beam member 210cm long.

i suspect the only way for me to know for sure is to build a model;
of the truss transom and bulk heads, 7 feet of two stiff hulls, the deck, and then the bulkhead and aluminium cross beam.

rick suggested enclosed volume for resisting twisting in quartering seas.
this should then enclose space. and have large, non buckling, very stiff planes in all 3 plane axies. x, y and z.

if the depth of the beam is doubled, the deflection will be reduced to just one-eighth, a normal round mast section is say 3mm walls, 20cm high. the truss i'm looking at is 80 cm high all up. is the truss going to have 4 cubed more stifness than a normal mast section?

also, if you are a wandering engineer, wandering past, do feel free to coment on the truss specs. i suspect they are way too heavy.



transom specs
80 cm from outside top to bottom
100 x 70 x 4mm x 440 on top
100 x 50 x 4mm x 440 bottom and for 45 degree webbing

and bolted onto 20mm ply bulkheads in the hulls


210cm in from the transom,
70 x 70 x 3.5mm T6 all the way across and bolted onto 20mm bulkheads in either hull

deck 15mm kiri plank, 600db/epoxy either side

all this to resist twisting


Front triangular box beam,
90cm wide kiri plank on flat top,
60cm high 20mm ply,
15mm kiri underneath
ply stiffners every 75cm
an i might stick some aluminium 70x70 beams across as well,
as there will be large holes in the ply beam for storage

please comment,
mal.

[bob the builder]

Yesterday, 09:25 AM
bill broome
Junior Member

Join Date: Jul 2008
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Location: sydney

a bridge deck with no holes is the best structural connecting member. it is strongest for the weight, but useless for any other purpose.

they are often used in catamaran design, but we call them 'beam boxes.'

a bridge beam with accomodation space must have doors, windows, and utility ports, and each must be re-enforced for local load to prevent tearing.

if you are welding aluminum tubes, why not build a real box structure, human-high, of triangulated tubes? this is very strong and stiff for the weight, and you can drape a thin ply and/or grp skin on it.

[bob the builder]

that's exactly what i'm currently considering doing. the default option is still a mast section in front, but i'll post pics of the box beam i'm pondering welding up, basically as above (ply) but with aluminium tube welded to form the frame of the box.

[Richard Woods]

This is a really confusing thread, unfortunately not entirely solved by splitting it. So I may be repeating what you already know. I have written some thoughts on beam design on my website, so won't repeat it (much) here

Anyway a few comments

As others have said, there is no point CALCULATING the beam scantlings until you have DESIGNED the beams.

If you have your beams too near the ends of the boat it will be a stiff boat, but there may not be enough "boat" to take the loads.

Conversely if too close together then the boat will twist, basically the bows will wobble up and down, at best bad for speed. (That was one of the root causes of the Team Phillips failure)

Don't make the beams "strong" enough, make them "stiff" enough. Beams made of rubber will never break, but it won't be a very good boat!

If you have a solid bridgedeck floor/cuddy/cabin you need stiffer beams than if you have an open deck boat. Otherwise the deck joints will crack, trampolines are flexible! And the rig proportions play a part as well. A genoa generates far more loads than a mainsail does, for a given area.

Very roughly, the bending loads are about 3 x the fore/aft loads. In other words the vertical inertia needs to be about 3 times the horizontal.

I am surprised that you think a truss beam is necessary on a catamaran. Maybe yes on a trimaran as the truss will have less "material" and more "open gaps" thus is better for cutting through waves. But remember that Manueva (sp?) had this type of beam and it was beam failure that most people consider caused the loss of the boat.

I agree with others, why not use a standard tube? Having said that I would avoid a round tube if you can, as it is hard to stop them twisting.

When I design a catamaran the position and design of the cross beams are one of the first things I decide on. I am a bit concerned that you seem to have started building before designing your beams.

As they say, "if in doubt use your eyes and plagarise"

Hope this helps, as I say, I write more on my website

www.sailingcatamarans.com

Richard Woods of Woods Designs

[bob the builder]

good onya richard.


before we get into it,
been reading all your stuff for years,


"Don't make the beams "strong" enough, make them "stiff" enough."
exactly where i am at present.
definitely got them strong enough, now trying to get the balance between STIFF, and weight. i'll put up my weight calculations here soonish (bit busy for just one more day).


"Very roughly" !
exactly what i was after. thanks!
you have to start somewhere.
got the design, now just have to do the engineering. see if it's possible.

rick suggested torsion in a quartering sea was something that needed to be seen to. after making a little model, i had to agree. have a look at the pics below. i'm putting in another aluminium beam where the cabin starts to bolt onto solid bulkheads 210cm in from the truss transom, and so turning the solid floor into a usefull beam. works brilliantly in the balsa model i JUST right then made and tested. if i can keep the planks straight without buckling, and the hull sides from buckling, (probably with stiffening frames in the hull bottoms and sides) it resists twisting really well.

the floor has so been turned from non-contributing to usefull. bargain.


"the vertical inertia needs to be about 3 times the horizontal"
onya.
i wanted the truss, i spose you read, just because of utility, ease of welding 15 devices on it (rather than bolting. i want this boat to last 30 years, so my interest in as few bolts as possible)

but the truss turns out (from all my beamboy and truss calculations) to be astoundingly stiffer than a normal mast section, the life time of the boat thus increased dramatically (i really hate foam boats, as you might have read previously. disgusting feeling when they get old)


so,
if i can cure the twisting, my boat is finished.


"I am a bit concerned" etc
don't be. i can hold my own. lived on a yacht before, the design i'm doing now is the culmination of many years design, calculating etc etc.
plain vanilla round mast section was of course used for initial design. but for utility, which nobody understands, or takes seriously, i'm going to do everything in my power to have a truss on the back. the astoundingly larger stiffness is a pure bonus.



this thread please.
just ignore the other thread. it has been so diluted now, i just mirror back what others are saying. i don't mind, some people are interesting, all by themselves, and it's great to see the diversity of people, but this is the serious thread.

and great to see you here!
mal.

[bob the builder]

here's some beam boy stuff, just for those that have never seen it.
http://img232.imageshack.us/img232/4831/image13d.jpg

http://img29.imageshack.us/img29/6346/beamboy2.jpg

http://img10.imageshack.us/img10/973/beamboy3.jpg

http://img10.imageshack.us/img10/7615/beamboy4.jpg

there's no way i need 20cm (2 square tubes, each 10cm high, 60cm apart, total truss height 80cm, 4.4m wide) high al
as;
beam boy suggests the two beams (50mm wide x 10cm high, x 4mm walls) will have roughly 35% more stifness than the round mast section (20cm diameter, 4mm walls). (the 50mm you see is for the transom that the outboard sits on)

when the truss webbing is added (roughly an extra 6m of same tube cut into 60cm lengths), for small extra aluminium, the stiffness sky rockets. many multiples stiffer.

30kg truss should give 3000 kg point weight in the middle of the 4.4m span with <0.3% deflection. astoundingly stiffer.


Using a second T6 aluminium tube 70 x 70 x 4mm x 440cm wide, 210cm in from the truss transom, bolted to 20mm ply bulkheads, to create a non-buckling torsion resisting structure;

here's a balsa model.

point of the exercise was to find out if by making the floor non-buckling, would the torsion resistance increase? (ie, adding beam, as above)

so,
truss transom and hull sides without floors,
http://img91.imageshack.us/img91/7409/transom1.jpg

bottle has 150g water, deflection on opposite to held corner = 12mm
http://img223.imageshack.us/img223/82/transom2.jpg

solid deck and floors added to the hulls,
http://img36.imageshack.us/img36/732/transom3.jpg


1Kg!!!! of water gives about 7 and a half mm deflection.
http://img268.imageshack.us/img268/483/transom4.jpg


i think the concept of turning the last 7 feet of boat into one huge box beam has merit.
just have to make sure the deck and hulls stay stiff and non-buckling.

worst case beam load of fully laden hull weight of 600Kg is (W= 0.5 on 73cm long section) gives 7mm total deflection. flying a hull gives 0.159% deflection. super stiff.
http://img17.imageshack.us/img17/374/truss.jpg

safety factor astronomical

[Rick Willoughby]

Mal
Your simple model is a handy way to demonstrate what is going on.

I have not followed everything you are doing but I would be interested to see the deflection test done with the proposed box beam incorporated. And a picture of the model with it.

You should be able to do the calculations for your model to confirm your method. Then you use the same method for the real thing. Data for balsa is around or you can do tests to calculate it. I found elastic modulus 2.2GPa (on grain) and shear modulus 106MPa.

You could also make your model a bit more realistic and set it up with diagonal support per linked diagram with some weights on the middle of the hulls to reflect the distributed weight of the hulls:
http://www.boatdesign.net/forums/attachments/multihulls/29829d1236301819-load-paths-catamaran-presentation1.jpg

Rick W

[Rick Willoughby]

Mal
I have had a look at beam boy and it looks OK. It does have the ability to add moments and also to set moment carrying supports. You will need to use these capabilities to get a meaningful result with you fore/aft beam configuration rather than just being simply supported as you have shown.

BeamBoy does not have the ability to handle torque tubes but I am sure you can find the formulas or another simple calculator.

Rick W

[Saylaman]

Chris White, in his book 'The Cruising Multihull' on page 73 has a photo of a weld failure on a truss-design beam. He gives a warning regarding welded beams. If I remember right the welding reduces the strength to half the original material strength. Chris comments that he decided there were places to try to save weight, but the beam was not one of those places. It seems as if he decided not to use welded truss beams again, but went to manufactured sections.
Just a suggestion, not from me, but from an experienced man...

If you do go for a welded truss, make sure you've got enough of a safety factor for welding.

[whoosh]

Chris White, in his book 'The Cruising Multihull' on page 73 has a photo of a weld failure on a truss-design beam. He gives a warning regarding welded beams. If I remember right the welding reduces the strength to half the original material strength. Chris comments that he decided there were places to try to save weight, but the beam was not one of those places. It seems as if he decided not to use welded truss beams again, but went to manufactured sections.
Just a suggestion, not from me, but from an experienced man...

If you do go for a welded truss, make sure you've got enough of a safety factor for welding.

well you are completly correct on the welds on beams
it is ok to weld ALONG a beam , chassis railetc, but never across it
I am always frightened when I see, say a chainplate welded over a beam. on a Cat Yet when you look at a boom, with the vang lug welded along it, its ok
There are other ways like using plates riveted on with high tensile aircraft rivets
Built a lot of large high ten al al road trailers and I always bolt to chassis rails
OH and B the B, where are you? I am in Buderim, I posted in the other thread that I thought you may be, another member(given your ockerisms) but now I can see that you are not

[Saylaman]

Whoosh
Thank you very much for sharing your valuable information and experience. I have wondered about some of the things you've mentioned, you've shed some light on the subject for me.:)

[bob the builder]

hi Saylaman and whoosh

i've seen that same photo.
i am gunna use the truss, but i'll confine all welding to the flanges, so the beams will be intact

mal

[bob the builder]

thanks rick,
didn't know it could do moments.
will play further.

"make your model a bit more realistic"
ta da!

http://img259.imageshack.us/img259/374/truss.jpg



truss photo
single balsa 6.5mm x 6.5 mm x 30cm long supported both ends
1Kg water hung in middle gave 15mm deflection, severe risk of breaking it.

two pieces stacked one on the other gave 4mm deflection
1.5 Kg gave 8mm deflection

6:1 truss from same 5mm2
3Kg of water gave 1mm deflection.

i think this is astounding.

so just to let you all know, the truss is on no matter what.

[whoosh]

Whoosh
Thank you very much for sharing your valuable information and experience. I have wondered about some of the things you've mentioned, you've shed some light on the subject for me.:)

thanks, it is nice to be appreciated

[bob the builder]

and before you say anything,
yes i sanded the b!
yes i used bog, spray putty, high build (1 full can of each)
in the end, i just gave in

lessons learned?
get it right before you start sanding.
and, more importantly,
get other people to do the sanding, backpackers, friends, relatives, etc.

vital lessons i feel.


http://img406.imageshack.us/img406/51/model1n.jpg


http://img41.imageshack.us/img41/46/model2k.jpg


http://img139.imageshack.us/img139/1627/model3.jpg

http://img190.imageshack.us/img190/2599/model4u.jpg

http://www.freeimagehosting.net/image.php?9f3c6180da.jpg


http://img404.imageshack.us/img404/2362/model7.jpg

http://img406.imageshack.us/img406/3435/model6.jpg

http://img440.imageshack.us/img440/301/model9.jpg

[bob the builder]

http://img406.imageshack.us/img406/2455/model8.jpg

[bob the builder]

Design at the moment.

(trying to save weight, comment if you can!

(all sections from this G J James (http://www.gjames.com.au/cgi-bin/priv?url=/marine/gjmarine-metric.pdf) marine extrusion catalog (they give section modulus in the catalog)
920.145 is an example part number

450cm wide
31 feet long
hulls 90cm wide at the waterline, flat, no rocker, 15cm deep
60cm bridge deck clearance

boom 3 feet from deck, 390cm feet long
mast 10m high
sail area 15m2 main, 15 jib (center of effort 4m above the water, quite low.

Front Box Beam
is still a problem. can't decide what to do. mast section, ply box beam, or complicated welded aluminium structure.
there's going to be wood all around that triangular shape anyway. . .

at present, sorta keen on a builders tressel plank extrusion. astoundingly cheap! ten times more inertia than a mast section
(typically 200 or 300 x 50 x 3mm T6, Ix above 20 000 000) then build a box beam around this. thats what all these pictures show,
the rectangular beam at the front, for the jib, box beam built around it.

just a thought.

please comment.




Main Bulkhead Cross Beams
aluminium square tube bolted to the tops of the main 20mm ply bulkheads, and epoxied to the cabin ceiling, and another the same under the deck.
931.148 76.2 x 76.2 x 3mm 2.313 Kg/m Ix 750513




Mast Cross Beam
sandwich of 931.127 1.211Kg/m Ix 208870

60 x 60 x 2mm T6 x 250cm
deck in between (12mm kiri plank, 600gsm glass epoxy both sides
60 x 60 x 2mm T6 x 300cm

unusual i know
didn't want the mast cross beam to spoil the deck so much, so put some underneath the deck and some on top. put some bolts through them.
mast rotating for the same reason, smaller to have the little cup than the typically large deck step plate.

rather than bolted to a molding on the inner hull (like the seawind) i'm going to weld the under deck beam to flat aluminium strips,
920.145 80 x 8 1.7 kg/m Ix 341333
or
980.055 99 x 5.35 1.7 kg/m Ix 607000

and then bolt 1.5m long strips to 100 x 25 kiri plank on the top of the hull sides where they meet the deck



Wing Seat cross beam
931.065 50 x 50 x 2 .994Kg/m Ix 137 094
(is this reasonable? or should i up it a bit?)


Truss Transom
(perhaps a bit heavy.)
50mm wide is so the outboard can just fit straight on, no messing around building special fittings.

page 19 of the catalog,
941.360 50 x 100 x 3 2.223 Kg/m Ix 1011942

webbing (45 degree truss members)
931.065 50 x 50 x 2 0.994 Kg/m Ix 137094

the only real question left is should i stiffen up the truss in the z plane?
(ie welding same section on horizontally all the way across the bottom)

should a plate go on as well? 30 cm deep 40 wide 1 cm thick? (for the outboard)

beamboy (for 3m long 3mm 200mm deep T6) gives 6561590 mm4 inertia.
(which is more than a normal mast section for same job, as square section provides a 70% increase in stiffnes for 27% increase in weight,
or, 34% increase for the same weight.)
and
115mm deflection cantilever, (3m long, 600Kg load on the end, equivalent to lifting the entire hull on the one beam)

truss is more than 16 times stiffer, so
1/621 deflection / length
thats 0.16% deflection lifting a hull, (on this single beam!) and i can weld crap all over it (which makes me extra happy)



why all the cross beams?
if i want that huge open cabin space, and the truss transom,
the deck and hull sides have to be kept rigid and non-buckling.

what about the torsion resistance reduction?
the idea is to make torsion affect the entire structure, NOT a single isolated beam

all these cross beams bolted to bulkheads should do it.



please comment on anything!,
cheers big ears,
mal.



http://img158.imageshack.us/img158/8962/profilew.jpg

http://img514.imageshack.us/img514/7046/planopw.jpg

http://img338.imageshack.us/img338/3924/hullsection2.jpg

http://img441.imageshack.us/img441/1415/hullshape3.jpg

http://img113.imageshack.us/img113/6523/hullshapewl120.jpg

http://img204.imageshack.us/img204/9041/image2ak.jpg

from here (http://uk.geocities.com/fnsnclr@btinternet.com/yachts/spreads/fred.htm)
http://uk.geocities.com/fnsnclr@btinternet.com/yachts/spreads/fred.htm

http://img441.imageshack.us/img441/6193/boxbeam1.jpg


http://img156.imageshack.us/img156/3318/boxbeam1bh.jpg

http://img156.imageshack.us/img156/5843/boxbeam2.jpg

http://img194.imageshack.us/img194/224/mainbh.jpg

http://img521.imageshack.us/img521/2531/mastcb.jpg

http://img158.imageshack.us/img158/4448/seatcb.jpg

http://img158.imageshack.us/img158/8247/transom.jpg

[bob the builder]

station 1
30cm back from bow edge
nose

station 2
90cm from front
box beam front

station 3
180cm
box beam rear

station 4
390
main bulkhead

station 5
420
cupboard

station 6
630
wing seat

station 7
840
transom

station 8
930
stern

[apex1]

Design at the moment.

(trying to save weight, comment if you can!


Just one,
find a good NA, what you are doing is not designing, you play designing!

[Ad Hoc]

"...Just one,
find a good NA, what you are doing is not designing, you play designing!.."

First sensible comment :)

[waynemarlow]

You could also use the bimini rear top section as the upper part of the truss, mount the main sheet and traveller on it as well, just probably only 2 diagonal connecting braces needed to make the single rear lower beam and top beam a true load bearing truss.:)

[apex1]

"...Just one,
find a good NA, what you are doing is not designing, you play designing!.."

First sensible comment :)

:D well, I have bright moments! Though with decreasing tendency.........;)

[bob the builder]

the model was to see if i liked it.
i like it.

so i'm going to build a strip plank model and i'm looking to finalise the design now.

(on the scale model i'll be moving the mast back or forth as the mast placement i think is the only radical idea in the whole boat. theoretically it should be 2 1/2 feet forward, but i've seen so may different mast placements that i'll really be surprised if it makes a whole lot of difference to the balance. (weather/lee helm going upwind) and there are books showing how to trim for auto sailing without autohelm or wind vane or rudders)


any ideas, let me know.


Why is it designed this way?
Why is it built this way?
the reasoning.


Simple full sloop rig. The mast goes behind the hard top. The main sheet track along the transom. Jib sheet to the rear cabin tops.

A forward of the mast hardtop, and high foot jib for great visibility.
Unusual yes. Built for the pacific. Lots of reefs. Instant visibility all round is important. Any person in bed is only 2 feet away from all instruments and all round visibility. And dry in all weather.


Large open areas instead of cabins? Lots of flat open areas? Huge open spaces, and huge! storage for toys?
i've lived on a yacht before. stinky. fumes, smells, airless. you sweat all day and all night. this is built for the tropics. the entire front opens up as well. great for when you park on a beach.
Standing headroom in the kitchen/toilet/shower area, and hard top cockpit. 4 1/2 in the main cabin, bedrooms.
(all hulls save the kitchen/toilet/shower area are closed over)
good to live long term on.
lots of flat open area to DO things on.
even the large professional cats i've recently seen are far more cramped than this design.
anything nice to live on is far too large for me to feel comfortable handling all by myself.
top of my list was good to actually LIVE on the thing. it's no fun if there isn't enough room.
this small boat has more flat usable room than all the large pro cats i've been on or looked at.

Huge windows on the front that open all the way (fold over backwards)
nice area. for chairs and stuff when parked. may even have a tent and poles over it.
the deck/walkways to the sides of the windows are to stiffen the top of the window frames.


Stronger than average? (lots of aluminium beams and wood.)
strong enough is easy, long term stiffness harder.
felt old FRP (aka expensive modern foam cats). feel like walking on jelly fish. they loose 10% of their stiffness in the first year etc.
been on modern foam cats. remind me of yachts. i don't like the tunel feeling.
weigh far too much to beach without stretching something (no foam panel can take 5 tons on a single point)
built to last 30 good years of cruising, and still be stiff.
beaching often



Why the wing seats?
Vital, thats why.



3/4 bridge deck gives more hull slam
Not if you don't sail against the weather.
how often do you intend sailing in storms? this is not built to win races in any weather, but as a camper for south pacific island hopping.

Single centerboard
will give indifferent to weather pointing.
true. having sailed against the weather before, and now having all the time in the world, i prefer to wait for wind and weather, or turn the engine on.
centerboard 7 feet from the transom?
gives surfing stability in the open ocean, rather than turning in a shorter distance. hopefully i'll never see a marina.


No rocker
it's not that daring, so don't get excited.
when your cat weighs 6 tons, then you need rocker to turn the thing.
least wetted surface area from rocker curve? true. but this also has the largest underwater cross sectional area pushing water ahead.
slower in calm winds, faster in average winds. there are online hull resistance calculators, and the small amount of less wetted surface area of a round/rocker hull is unimportant at any reasonable sailing speed, especially compared to pushing all that water out of the way.
should surf better than any cat. (i just like surfing boats. good fun)

Flat hulls give a rougher ride.
at what speed? 20+ knots? certainly not at ten.
mainly visiting atolls, reefs.
hull depth 15cm
beaching often, so weight spread over a larger area


Why is it so small? Why is she so light compared to others?
it is really only 21 feet long.
plus
1ft for cupboards in the middle bulkhead,
6ft on front for more forward floatation
3ft on the back sugar scoops to sit on while traveling
No interior furniture. Hull framing in kiri which is half the weight of ply.

A minimum of 35 feet to go cruising?
referring to either living space, making way, or load carrying ability
This boat is nothing special and displaces 200 Kg for each centimeter over the waterlines.
Because it's a small boat, flat hulls (in the fore-aft direction) equal much larger round hulls (rocker) in terms of good passage making, surfing etc.


Boom is very low.
so stability is higher.

the center of sail effort is only 4m above the water, and 2.25m from the side. the boat weight is 1500Kg, it takes 750 kg on the sails to lift a hull. on 30m2 of sail, this is 25Kg/m2 (wind around 35 knots)

nice feel. gives that "yes we are sailing" that you might not experience on larger cats when the boom is 7 feet off the deck. they are also much wider and heavier ( and more expensive) to compensate having such a high center of effort.

lowering the boom and lowering the mast hight is a cheap way of increasing stability.

high rigs give better performance?
bugger. i won't enter a race then.


Center of gravity
unusually for a catamaran, is below the bridge deck floor, as there are no walkways in the hulls. instead these areas are for stores, water, fuel, batteries, toys etc. there's only empty volume above the deck.

hinged storm board gate in the transom bulkheads to repel solid water but allow it free escape


To Reduce the build time as much as practical.

Straight sides and single level deck

Bulkheads and frames are included in the strongback - so when the boat is turned, it's almost finished.

Strip planked whole - can take a day to strip plank both hulls and deck. stronger, faster.

Lots of aluminium beams - good time saved, just bolt them on the bulkheads, all done in a day.

Have a think about the build time for this design after you turn the boat over. There is not much left to do.



Weight Calculations
i've used 12mm strip plank kiri, 400gsm glass/epoxy either side, 80m2 for the 416Kg

what else do you have on board?

Wood/glass/epoxy 416
Aluminium beams 70
Bulkheads 60
Hull framing 40
Paint 25

Fit out and Rigging: 60
Bollards on front
anchor runner
anchor
chain
rope
jib self furler
front stay
shrouds
cleats x 4 on transom

Mast 2.3 Kg/m 23
Pumps plumbing 22
Engine 15hp o.b. 55
Water 100
Fuel 50
People 240
Batteries 20
Solar Panels 11
Food 24

Toys 50
Odds n ends 100

Total 1370 Kg
Normal cruising average


note the minimum i'm considering is 10mm. as will be parked often. remember i'm a novice, so please give your opinion.
i'm not really convinced 400gsm is strong enough. may do 600 outside n 400 inside. maybe 600+400 on the hull bottom exterior.
yes? no?
after 30 years of life, it would be nice to have everything still stiff.

here's a pallet of scantlings for a south pacific island hopper
kiri plank..........................................glass/epoxy
.........................600gsm= +2.4kg....400gsm= +1.6kg...300gsm= +1.2kg
15mm= 4.5kg/m2.............= 6.9 ................ = 6.1................ = 5.7
12mm= 3.6kg/m2............ = 6 ...................= 5.2................ = 4.8
10mm= 3 kg/m2.............. = 5.4 .................= 4.6............... = 4.2

80m2 in total =
..........................552....................488 ................ 456

...........................480...................416..................384

............................432..................368..................336



Changes for the next version.

the hardtop is ill conceived and poorly integrated at present.

deck camber
to build camber on just the back deck would be a real pain in the arse. any ideas anyone?

Aramid? instead of a second layer of glass on the bottom of the hulls. (it would hold together when on a reef. . .

small curve on the bow, just enough to survive any floating debris such as logs, or large steel containers



i'll be away for a week,
but looking forward to reading all your thoughts when i return.

cheers all.
mal

[whoosh]

bob you are kidding yourself
get your self round to my house I,ll give you some software and hopefully some pointers

[bob the builder]

onya whoosh.

kidding myself?
design it, engineer it, repeat till ready.

?



but i'll take you up on the offer!

i'll drop in for a chat one day. soonish.

(i'll send you an email, and thanks for the offer!)

cheers big ears,
mal.

[bob the builder]

wayne suggested
"bimini rear top section as the upper part of the truss"

i pondered this (and i'm still pondering making it even more so the upper member of a truss)

good idea. thank you wayne!

have just rebuilt the model with aluminium framing across the top back.



(also;:(
about that um, :( sorta lip i gave you on that other thread, :( um,
you know,
about you bein a sorta father figure to me n all that. . .
um,
this here is sorta um,
an apology.:D

)

[bob the builder]

true planing sa/d higher than 37 (according to US sail

330 sqft an 1100 kg gives me 31.5

400 sq feet gives me 37.8
so i'm thinking of upping the sail by 33%

the reasons being;
i seem right on the edge of good planing




at present, (1400Kg) i've got
115 Kg/m2 weight/ water planing area

this is roughly the same ratio as a large surfboard (7 feet long 45cm wide)

(and thats the heavy version of the boat)

if i stick to pure displacement hulls, i'll never fly, never have blue sky performance, never have some open space above, never break free of the sticky 2 dimensional drag,




fiona sinclairs and michlet are in close agreement,
for a single (700Kg) hull
around 35 kg drag at 10 knots
70 kg drag at 15 knots

30m2 sail area gives driving force
1.7 kg/m @10 knots wind = 51 kg
4 kg/m @15 knots wind = 120 kg
7.5 kg/m @20 knots wind = 225 kg

see?



i'm so close to planing,
i've decided to busta nut trying to lighten the boat, (1100 is achievable)
and up the power/weight by increasing the sail area.




who's got two cents?


mal.






ps
(michlet for a single hull 700kg, 9.3m long, i keep getting 1.12 wide so i know i'm doing something wrong), total drag 5 Kg higher than the hull i optimised with fiona sinclairs applet

the weight/area means it will start to surf at 10 knots, so i am trying to optimise for 13 (no idea really, just anything over 11 knots for good surfing)

i'm more than happy to have Worse slow speed performance (if you've got light airs, you're going slow anyway), and get the occaisional brick in the face FUN in stronger winds

you can see from my hull shape that i've been trying to maximise dynamic lift rather than low speed displacement hull drag optimisation.


if i stick to displacement, a la michlet, i'll never get blue sky, some open end performance.





?less form drag, instead lift the nose more

?water tanks at the back (and empty them when running home as well)

[bob the builder]

finished the beams

going to stick it into delftship now, then polish it, then take it to a engineer or architect.



Rick
torsion, lots of good advice, etc

thanks rick.


Wayne
turn the hard top into a giant truss
brilliant idea. have redone the mast beam to integrate in as well. whole thing is now many multiples stronger.
great idea.


Richard Woods
bending loads, lots of good beam advice
thanks richard!


Whoosh
you can't ask for more help than to be invited over for coffee.
thanks whoosh.

and all the other people who chipped in. jaydh, Saylaman
thank you all,

i stuck the probable design in a zip if you're interest (beam specs etc)

mal.

[Saylaman]

Good luck. Let us know when the design is finished and you start building.