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[RoyStewart]

 We see a  lot of butt joined balsa with compsands, whichever way you look at it they are a very weak point, particularly 4 to 8 inch wide ones.

 Yes and I have done them in the past on flexible boards, 4 inch wide balsa planks just like builders use here with glass cloth and yes they eventually caused problems, but even if they don't they are a weak link in the chain.

Really they should be slash joins or scarf joins.

[Kit Sidwell]

I've never used them.... full length paulownia rules
wow, i'm suprised the butt joins are a problem when well glued? never realised

To use up the short pieces, the 'ol fishbone pattern works well, but time consuming

Otherwise, i've used this trick and variations of it - makes laying up the skin fast, looks better than butt joins

[Kit Sidwell]

Whats a slash join?

[StrangeBrew]

respectfully, roy, you're not entirely correct.

it depends on the physical properties of the materials you're using.

if the adhesive used in the butt joint has similar material properties to the material being joined, after it's cured, I SINCERELY doubt (in fact, I have evidence to support this), that you'd be able to distinguish between the joint and the bulk material.

you are entirely free to continue using whatever methods you'd like to use. Scarfs and slashes are time consuming, and not all that necessary IMO...and that's what the opening post in this thread is; opinion.

[huie]

We see a  lot of butt joined balsa with compsands, whichever way you look at it they are a very weak point, particularly 4 to 8 inch wide ones.

 Yes and I have done them in the past on flexible boards, 4 inch wide balsa planks just like builders use here with glass cloth and yes they eventually caused problems, but even if they don't they are a weak link in the chain.

Really they should be slash joins or scarf joins.

              while i totally agree whith you roy full lenght timber is cosmeticly
far more eye appeal  stagerd joints are fine
 i worked in a yard where 70-90 ft     boats were the normal build.
no planks  90ft long  ( butt straped)       
back to the boards  firstly balsa is shit    but even whith it the iner glass is the butt strap
but i agree i gota stop doin pergolas
me thinks you just poking a stick in the cage
       be     happy

[jeff matsuno]

Stagger your joins, use a 45 degree cut and I think you're OK.  A scarf joint with 1.5mm or 3mm balsa might be hard to pull off.

[RoyStewart]

respectfully, roy, you're not entirely correct.

it depends on the physical properties of the materials you're using.

if the adhesive used in the butt joint has similar material properties to the material being joined, after it's cured, I SINCERELY doubt (in fact, I have evidence to support this), that you'd be able to distinguish between the joint and the bulk material.

It's a well established fact that glued butt joins are much weaker than scarf or slash joins, it's not simply my opinion.It's not possible to provide evidence to support the idea that a butt joined plank is as strong as a scarf or slash joined plank because it quite simply isn't. . . . sincerity has nothing to do with it.

Try an experiment if you don't believe me, the butt joined plank wil fail at the butt every time regardless of the nature of the glue used, and it makes the plank dramatically weaker.  Scarfs and slash joins can actually increase the strength of the plank.

http://www.storerboatplans.com/Faq/Birdsmouthmast.html

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[RoyStewart]

Stagger your joins, use a 45 degree cut and I think you're OK.  A scarf joint with 1.5mm or 3mm balsa might be hard to pull off.

Yes, but that's a low angle slash joint you have there, not a butt joint.

If the angle is increased it will gain strength.

It's not simply a matter of whether or not the join will be 'ok',  as ideally we don't want to weaken the plank at all . . . . it's like not wanting a weak link in the chain . . . in order to get the most out of the materials the joint shouldn't weaken the plank.

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[RoyStewart]

We see a  lot of butt joined balsa with compsands, whichever way you look at it they are a very weak point, particularly 4 to 8 inch wide ones.

 Yes and I have done them in the past on flexible boards, 4 inch wide balsa planks just like builders use here with glass cloth and yes they eventually caused problems, but even if they don't they are a weak link in the chain.

Really they should be slash joins or scarf joins.

              while i totally agree whith you roy full lenght timber is cosmeticly
far more eye appeal  stagerd joints are fine
 i worked in a yard where 70-90 ft     boats were the normal build.
no planks  90ft long  ( butt straped)       
back to the boards  firstly balsa is shit    but even whith it the iner glass is the butt strap
but i agree i gota stop doin pergolas
me thinks you just poking a stick in the cage
       be     happy

 Huie I'm a purist that's all, am very happy thankyou  and am merely trying to help improve the methods used and eliminate weaknesses,  that's a  good motive , and not just 'poking a stick in the cage'

What you are talking about is butt strapped joints, these are not simple butt joins.

The  glass either side of the wood isn't a butt strap as it is continuous, it's a continuous strap. . . . so once again you have a dramatic weak point in the plank.

I'm not necessarily advocating full length timber, just scarf or slash joins. . . . if you want to make the best possible boards butt joins are out, no question about it.

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[huie]

    roy you do understand what a butt  block is?
 by flipping the skin over &glassing the inner it is one big butt block
your argument is not releavent in a timber skined composite  shortboard
 but as the length of the board grew& the timber became thicker so to would your argument grow.

[huie]

   roy you do understand what a butt  block is?
 by flipping the skin over &glassing the inner it is one big butt block
your argument is not releavent in a timber skined composite  shortboard
 but as the length of the board grew& the timber became thicker so to would your argument grow.

    hey roy i am whith you the next board i do i will do neat scarfs
 & get rid of that f##ken horrible pergola look     but i  doubt it will make the board any stronger

   jezz man i gotta foil those fins yet       yer f##ken whith me brain.   heeee

  be happy

[RoyStewart]

Hi Huie yes I know exactly what a butt block is.

Butt joins do weaken  the laminate significantly even if it is glassed both sides, and if the laminate fails it will definitely fail at the butt join. If you scarf or slash join them this won't happen and the board will be stronger. Making the board stronger with the same materials by simply improving the plank join in a very simple way is a good thing.  

Yes you are correct that the difference is less if you use less wood, and more if you use more wood, nevertheless surfboards do snap and any increase in strength without a gain in weight is therefore an improvement to the product.

Anyway take it or leave it hopefully it's up to the individual to decide whether or not it's worth the small amount of extra effort, if there's an official line which has to be towed on the matter without good reason then that's not a good thing

.

.

[RoyStewart]

   roy you do understand what a butt  block is?
 by flipping the skin over &glassing the inner it is one big butt block
your argument is not releavent in a timber skined composite  shortboard
 but as the length of the board grew& the timber became thicker so to would your argument grow.

    hey roy i am whith you the next board i do i will do neat scarfs
 & get rid of that f##ken horrible pergola look     but i  doubt it will make the board any stronger

   jezz man i gotta foil those fins yet       yer f##ken whith me brain.   heeee

  be happy

 Maybe you can show me how to do a good scarf then I'm just a slash join butcher.

 

[soulvoid]

Hi Huie yes I know exactly what a butt block is.

Butt joins do weaken  the laminate significantly even if it is glassed both sides, and if the laminate fails it will definitely fail at the butt join. If you scarf or slash join them this won't happen and the board will be stronger. Making the board stronger with the same materials by simply improving the plank join in a very simple way is a good thing.  

Hi,

Have you done the math to support this theory with sandwich theory? I sincerely doubt that it is correct as the glass is the load bearing part, not the core. The core is in general only responsible for keeping the glass apart and as long as it does not compress or flex it doesn't really matter what the density is or how it varies.

Throw flex into the mix and you might get a different answer though. The math certainly becomes interesting. Also, as the core get's stronger/denser and the glass is reduced there will be a transition point where the sandwich theory stop having effect and you end up with a pure classic beam. However looking at the (lack of) strength in balsa I doubt this happens until you've practically removed all glass. If you do get a break at the joint it might be due to how the load is distributed because of the variation in flex rather than the joint being a weak point.

Think of a layer of glass with a 'core' of 3mm flexible (not brittle) epoxy resin then another layer of glass. Easy enough to do if you  laminate a layer of glass with epoxy on a flat surface then tape off the area and flood it with  epoxy, sand and laminate on top of it. Compare this to a layup of glass and 3mm balsa. I somehow think the epoxy core would be plenty stronger than the balsa core, but if you compare the strength to weight ratios, the balsa core win by a mile.

[Kit Sidwell]

My opinion is that if you're relying on the balsa, then the glass has already failed, and ya screwed