Select to expand quotemathew said...Imax said..
Ian , as an engineer and a heavy 120kg boom bending bastard I do understand metallurgy or carbon mallubilaty. As stated earlier longer and shorter boom use the same strength bar stock , so longer booms will be weaker than short ones all things being equal. The question was will a smige longer effect the strength , I think not as long as there is enough overlap as to not kink the structure , although there will be a slight more flex due to the length ,
Carbon doesn't have the notion of malleability - that term is largely limited to ductile materials such as metals... ie: specifically, it is the ability to change shape without fracture.... Carbon fibre cannot change shape at all, as it is an extruded/drawn fibre.
Back to the original question - longer booms are usually heavier (so all things aren't equal)... I'd third the statement that no calculations are actually done.
IMax notion of 'carbon' malliability refers to the composition of carbon, resin and its componant design maliability. Carbon fibre does bend without breakage, hence why it can be supplied as a cloth and transported on a roll, and why it can be used to make highly flexible components like masts. Carbon fibre filament(the smallest isolated strand of carbon) do stretch. High Modulus carbon stretches less than Low Modulus carbon..and in between those there are Medium and Intermediate Modulus.
To make a carbon fibre componant lighter and stiffer, you need to use High Modulus carbon. You achieve the required stiffness with less weight. To achieve the same stiffness with lower modulus carbon you need to use more carbon. The structure with therefore be heavier, yet stronger.
Without knowing the composition(pieces of string) and the properties of each element of composition(The sum of How Long is a chinaman) of the boom in question, it is impossible to provide an estimation on the durability of pre-posed tail piece retrofit.
I'd do it. 
The added stress of lengthening the boom is undoubtedly going to increase stress forces, but to what effect?
For an aluminium boom, it will increase the metal fatigue, so will shorten the life of the boom, perhaps by 10%??
But does stress fatigue occur in a carbon boom? If not, then no big deal about extending the length. But I know nothing about the long term fatigue characteristics of carbon.
You can't beat empirical testing for proof of concept. We need a guinea pig. Thanks Mark. 
Select to expand quoteImax said..
Good point Harrow about the boom flexing.Mabee they do that on purpose so that there is some pretention on longer settings so as to reduce flex when using , I've noticed on longer settings that it's easier to flex the boom inward than outward , Magee because of this .?
Pre-tension won't change the flex characteristics, Hooke's Law is linear.
Malubility is not even in the dictionary?
I dunno Mark, I think the heading you put on this thread is getting us in all sorts of strife. 
Select to expand quoteIan K said..
Pre-tension won't change the flex characteristics, Hooke's Law is linear.
...but it does increase the stress on the tube end in the mast clamp and therefore increases the metal fatigue in the area that often fails.
Select to expand quoteMark _australia said..
^^^^ yes but we are talking a few cm added to fit a small freeride on a wave boom. Like 200 up to 208 or soemthing
...
It wont be wide enough.
Model it in here you can see exactly how much more force it will experience: skyciv.com/free-beam-calculator/
Harrow yes you get stress fatigue in carbon booms & masts, they develop micro cracks, go soft over time & eventually fail. This happens over a much longer period than alloy added to the non corrosion properties. The limitation of carbon is it doesnt withstand shear forces as good as glass or spectras.
I believe weight for weight carbon fibre is stiffer than glass not stronger.U can use more carbon and achieve more stiffness with less weight . Carbon fibre is just burnt fibreglass.A carbon board for example will be stiffer but not stronger.
Select to expand quoteMark _australia said..
Can I make a boom longer by using the same front (incl the tubes of course) and just using a longer set of rear arms?
i don't see why not, a longer set of rear tubes will apply more load to the arms............ but not enough to break them as overall load is same.
I am talking say 10 - 15cm longer extension.
I guess......
Or am I dreaming............
Don't make the boom longer,
cut the sail to be shorter in the clew width.
You're welcome.
Select to expand quoteImax said...
I believe weight for weight carbon fibre is stiffer than glass not stronger.U can use more carbon and achieve more stiffness with less weight . Carbon fibre is just burnt fibreglass.A carbon board for example will be stiffer but not stronger.
Substitute 'stronger' for 'tougher' and you will be closer to correct
Select to expand quoteImax said..
I believe weight for weight carbon fibre is stiffer than glass not stronger.U can use more carbon and achieve more stiffness with less weight . Carbon fibre is just burnt fibreglass.A carbon board for example will be stiffer but not stronger.
Quick run out and sell this process of turning Silica into Carbon.
Strength is subjective, you can bend a fibreglass rod further than a carbon rod before breakage but yes the carbon will be stiffer. Possibly carbon is also happier doing moderate bend cycles, a fibreglass mast will get a permanent bend long before a carbon mast. Booms don't really experience the damaging sort of shear forces unless you crash, knock the boom or load the arms in a weird way such as when locating pins pop on one side.
Just make a back end from hell using parts from broken booms. If in doubt, just use more carbon. If you can make a tail piece that can take more load then the front arms shouldn't be placed under any more pressure.
Since it's a wave boom, it'll be the wrong shape for a large freeride sail.
But if you must, I'll supply you with the broken tubes needed but only on the condition that you strictly follow the plans below and post results,
The all new 2017 'Malubility'
Select to expand quotebarn said...
Just make a back end from hell using parts from broken booms. If in doubt, just use more carbon. If you can make a tail piece that can take more load then the front arms shouldn't be placed under any more pressure.
Since it's a wave boom, it'll be the wrong shape for a large freeride sail.
But if you must, I'll supply you with the broken tubes needed but only on the condition that you strictly follow the plans below and post results,
The all new 2017 'Malubility'
The promo video will need a soundtrack...something catchy...sing along if you know it...
Boom, Boom, Boom, let's go back to my room, so we can do it all night, and I can make you feel right..
Ps Barn you could call it the IUD model..
'Improvised Extension Device' works too..
Boom Boom
You guys have really twisted this thread into a pretzel.
Just lengthen the damm Boom and make sure you also lengthen the overlap same amount.
As for Malleability. I think you guys are thinking about the word Elasticity.
or maybe the term Modulus of Elasticity.
Engineers?? after owning and running a Metal Fabrication and General engineering business for 18 years.
the biggest obstacle to efficiency and a user friendly end product that worked, was an "Engineer"
That's what I rekon , its only a couple cm for gods sake ! Gotta love an engineer in a white labcoat , like in the Ponds Institute
^^^^ aah now I get it
Thanks Barn :) I will give you a bell with my list of things I'd like for free.
(BTW if I have a boom that goes to 200, and a 6.5 camless freeride does not have too much draft for it, surely a 7.2 will be fine 
)
