Catamaran beams New! Improved!

NO more DRIVEL here NONE! ok?

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.

 

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.

 

Yesterday, 09:25 AM
bill broome
Junior Member

Join Date: Jul 2008
Rep: 10 Posts: 55
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.

 

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.

 

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

 

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.

 

beamboy and truss stuff

here's some beam boy stuff, just for those that have never seen it.








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,


bottle has 150g water, deflection on opposite to held corner = 12mm


solid deck and floors added to the hulls,



1Kg!!!! of water gives about 7 and a half mm deflection.



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.


safety factor astronomical

 

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/at...301819-load-paths-catamaran-presentation1.jpg

Rick W

 

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

 

Remember to factor for loss of strength in welding

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

 

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.

 

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

 

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

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





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.

 

thanks, it is nice to be appreciated