That's wicked looking Huie I can imagine making some mayhem let loose with some of those, what are the limitations on size etc ? 

 Is it true that you can dial up the shape for customers from a computer drawn shape ?

 How about bagging paulownia on ?    

It looks like the rocker increases as it moves from the apex back to the tail , not a good idea imo as it slows the board down, so I agree with Strangebrew.

It's impossible to tell what sort of curve one actually has with that sort of design tool though, which is one reason why I don't use them.. ..  they can't make mathematically defineable curves.

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  I just got some pictures back today of an 11 year old balsa board, with butt joins. It was surfed a lot when I knew it and it looks like it has been  since. 

 The board is only 44 mm  thick, with 4 layers each of   11mm, it's a solid balsa multi directional laminate, no chambering, glassed with WEST system and a single layer of 6oz glass.
 
 Anyway, it looks like the butt joins are still sweet

let's remember as a moderator to keep things in moderation.

Quite so, which is why I had to object to Kits comment that my boards are just 'lumps of wood' and not properly engineered.. . . that sort of flaming is unnecessary.

the lattices you use are elegant roy. I admire the time it takes to put them together. They don't absorb energy in the same way a low density core does under the skin though, nor do they have very little resistance to shear, which is what makes the "composite" construction people often refer to here effective; flexy, light and resilient. It doesn't work as soon as the core resists shear. Then again, I don't know...maybe your lattices aren't actually locked up on the inside; are they?

The lattice core is extremely flexible without the skins, and the finished structure is excellent for flexible boards. . . it is optimal for flexible longboards, your eps skinned boards are better for flexible shortboards. both systems work.

Weight is a red herring in this discussion, let's not go there. 

 

I'll back you that if the wood had to bear a significant structural element, that yes; butt joints = bad. For your purposes, this is obviously the case. For the most part, in the construction being described by people here, skin thickness RARELY ranges over 3mm. I have built skins with butt joints out of wood and corecell. With proper attention to the adhesive used to connect the pieces, I have experienced zero failures at the butt joints...and that includes a few kiteboards that routinely get landed on from 15ft up.

Yes I can see that.   At 3mm the core is taking a fair slice of the load but it seems that it's ok provided that the glass fails before the core, that would be the critical issue. 

oh yeah....I think the ski industry was on to pressing curves in to flat material QUITE a while before you even started growing a beard....

Lol I was also pressing curves into flat boards before I grew a beard. . .. ..  main point though is that the structures I make are sandwiches.

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I tried to input the values zfennel was using in the formulas for flextural rigidity in the sandwich handbook and end up with the glass taking 6 times the load of the core. So give that the numbers zfennel are using are correct, the core takes up a significant part of the load, but nowhere near half of the load. Interestingly the balsa has similar modulus to resin research epoxy.

What kind of glass schedule was there on the boards you had the cracks on? How where the butt joins glued? Why do you think they failed?

Interestingly with a 12mm skin and the same lam schedule and properties as in zfennels calculations I get a 50% load on the core. Which is something I was pretty sure could not be true. So I've checked it a few times and it seems to be correct. Which means that the ratio of skin thickness to core thickness is more important than the sandwich thickness. Atleast I learned something...

That's interesting thanks for taking the time to run a few calculations.

What I have been thinking is this:

 I assumed that if the glass and the wood are sharing the load that the laminate will be weaker where the butt join is because the wood is weaker there, thus making the combined strength of the wood and glass weaker at the joint.

  Thinking about it some more after talking to Kit I realised that if the glass fails first prior to the butt join failing, and if the butt join is as strong as the wood until it is very close to the point of failure then it's likely that the butt join isn't weakening the structure in that case. Does that make sense ?

 If on the other hand the glass and wood failed together or the wood failed first then the butt join would have weakened the structure. Which way it goes will depend upon the relative thicknesses of the glass and the wood.

 So the question I have is this: We know that a butt join weakens the wooden panel, but how suddenly does this  occur ? Does the weakness occur very close to the point of failure or is the join weaker at lower loads as well ?  I imagine that the butts fail suddenly it's not as if they stetch much prior to failure.

Regarding my 12 footer with the failed butt join my opinion is that because the wood was taking most of the load  the butt join failed first in tension and then the glass couldn't handle the constant strain of flexing with the entire load on it and failed afterwards. It took a year or so of hard surfing ( surfing full time, every day of every swell ) before it happened. The join was glued with WEST sytsem epoxy and WEST 403 Microfibre blend, it was only one of many butt joins in the planking, all the others were tight. The one which caused a problem was right in the middle of the board where it was subject to more load.  Oh and it was glassed with a single layer of 6 0unce cloth

.At the time we made a lot of boards that way ( with butts) and most of the boards had no problem at all. 

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 Kit, the structures in question are not just 'just lumps of wood' calling them that is  uncool and simply not true. They are well designed flexible wooden boards.

Take the 12 foot balsa board in question . . . a multi directional wooden lamination only 2 inches thick with great flex properties. . . . it's just a different kind of structure in that it uses the wood to take most of the load.  In my opinion this is better ( but each to their own) because wood doesn't fatigue like glass and resin does, it can just keep on flexing. 

The point is that when wood is taking most of the load it needs to be joined properly, and butt joins are not valid structural joints.

So I don't think that it is reasonable to react by saying in effect that any structure which relies on wood for most of its strength is invalid and not 'properly engineered, and is not a sandwich structure etc, in my opinion it would just be better to realise when and where a proper sructural joint should be used  . .. . .. .. . .  that's useful information, and doesn't require you to claim that your way is the only correct one, or that all wooden boards are incorrectly engineered 'lumps of wood'

I can assure you that my boards are properly engineered,  the butt join flaw was eliminated many years ago.

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PS the boards I am building now have wooden cores of a density equivalent to 1 pound eps, and wooden skins which take the  majority of the load, they are sandwich  structures.

PPS I have to add that I was the first as far as I know to use the FLAT BOARD system which some people here have adopted and which is so supposed to be so 'hush hush'  that it isn't to be spoken about. . . . . . . we make flat panels which we called 'SANDWICH BOARDS' from day one in 1995

And they are composites too. . .. . wouldn't get far without epoxy resin glue. Wood and resin composite, sometimes wood resin and cloth.

OK ?

Circumstance already showing that that butt joins are not an issue in this case, by evidence of testimony.

Experience tells us that butt joins are a problem when the balsa skin core gets thicker, and not when it is thinner.

I'm like you, learning, and not here to convince anyone of anything necessarily . . . . I have my own system sorted, just passing on what I have discovered, which might or might not  be applicable to the  regular compsand structure.

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Soulvoid I'm talking from experience:   

14mm balsa  skins glassed both sides, WEST system resin and glass, with butt joined 4 inch wide 36 inch long planks. In such a scenario the butt joins are prone to failure. 

5mm balsa skins with butt joins, also prone to failure

With thin balsa in the skins experience from people here suggests that it isn't a problem.

That's the third time you have told us to just 'look at the math(s)' so maybe its time, it should be interesting. Of course I realise that it takes time to crunch the numbers especially when one is busy.

Here's the page with the post from zfennel, you are also on the thread:  http://www.swaylocks.com/forum/gforum.cgi?post=222973;search_string=bert%20burger%20wood%20balsa%20flex;guest=20552178#222973

By the way the failures I mentioned were not snapped boards, just fatigue due to flexing, so that the butt joins and glass across the joins were cracked and 'working'  as the board flexed.  The most memorable one was on my 12 foot balsa and  I surfed it for 2 years with the butt join weeping through the crack. I glassed over it several times and it would just crack again within a session or two.  There was no buckling of the panel at all and the join was on the bottom, it failed in tension.

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 Hi Kit, better get in your fox hole because here comes another absolute statement:

One plus one always equals two. . . . scary ?   

 Anyway there's plenty of interesting stuff to answer here but I'm nose down tail up with work so I'll have to answer in detail a  little later.

 I'm quite willing to agree that the loss of strength due to a butt join is likely to be insignificant when the wooden skin is really thin, but insignificant still has a number above zero. . .  and at least to the purist ( or the pedant ?)  it is  real. 

Did you see the estimate done by zfennel that with a 3mm balsa core the wood is taking about half of the load ? That is significant.   

Whatever the percentage of load taken by the core of the skin it is always above zero, it always takes some of the load, that's the first premise.  . . . . it's  impossible for the core to take none of the load, that's the mistake that soulvoid seems to be  making.

Is that first premise making sense to you ?

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 Not so, I was initially talking about the butt joins which we see in compsands with wooden skins. . . . these quite clearly do not use end grain panels. 

 Thus I haven't changed my position at all, if anyone has moved the goalposts it's yourself , as I wasn't talking about end grain panels.

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 I just want to mention the following to those  who think that this is not a worthy subject:

 Ever since flexible boards with wooden skins have been built there has been a lot of discussion abut how grain orientation in the skins affects flex response. . . .  for example there are many posts on Swaylocks on this subject.

 Now if the grain orienatation, thickness and density of wooden skins affects the flex response ( which it does ) then it follows that the wood is being used not just as a gap filler but as a flexible structural member. .. and in any such case  butt joins are relevant as they reduce the strength of such members.



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In case this photo was missed, here it is again....

Many many many boats made with contour cut balsa, millions of butt joins....
I guess all those boats must have sunk then?

Kit those are end grain panels and thus the joins are not really butt joins, I use end grain panels joined that way on my rails on all my boards.

The glue lines are running with the grain, a different story altogether.

The kind of joint I'm talking about here is where the ends of planks are butted together, making a break in the grain  which weakens the plank. End grain is quite different.

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  Unless you place the joints in the test panel at the position of maximum load then the panels will not necessarily fail at the butt join, so the results can be misleading.

  So if you want to do a reasonable test place the joins right in the middle of the panel.

 It is well proven that butt joins weaken a panel, so there's really no point in arguing about it, as it's not possible to win.  The main point made by those who are in favour of butt joins boils down to saying that as the wooden core of a panel becomes thinner the loss of strength is less, to the point where it is not relevant. It is is true that the loss of strength is less as the thickness of the wood in the skin decreases , however whether or not a weak point in the structure due to butt joins is important or not is certainly  a moot point .

 My opinion is simply that if one is going to bother to use wood in a structure one might as well take full advantage of its properties  by avoiding weak butt joins.  This does not mean that I am expecting everyone to share my view, or to stop using butt joins, but it is still 100% true that any surfboard structure will be stronger if such joins are avoided.


PS   Your method of using gps to measure the length  of time it takes to make a glue joint is a novel idea 

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As mentioned previously; this argument is as pointless as your overarching statement regarding butt joints. I was pointing out to people who might have read your initial comment and taken it as gospel, that it actually wasn't so.

In fact it is true that butt joins in a panel are always a weak point in the panel, this cannot be avoided, because the wood panel   always takes some of the load, and the wood panel is always weaker where it is butt joined.

By reducing the thickness of the wooden core and increasing the thickness of the glass the amount of load taken by the core is decreased, however it still takes some load and as long as it does the overall strength of the panel will be reduced by the butt join.

It is also incorrect to say that butt joins don't have 'squat' (??) to do with deck failure due to pressure dings, as butt joins also weaken the panel when it is under under stress from rider weight.

 As long as the skin flexes one side must be in compression and the other in tension, it's a red herring though as butt joins are weaker no matterwhich way the load is applied to the panel.

This is not true, if the board flexes the deck skin must stretch, and the bottom skin must compress, it's not possible for the board to flex otherwise.

Here's a quote from zfennel on Swaylocks:

 "The relative load carried by the balsa and glass in the skin is mostly a function of their modulus and cross sectional area.

-glass modulus:approx 2.5x10^6 psi
-balsa modulus: approx .5 x 10^6 psi
-2 layers of 4oz on the deck is probabably .018" thick
-basla skin: approx .100" thick
....
the glass has 5x the stiffness of the balsa,
but the balsa has 6x the cross section area.
so, if the above numbers are close, it would appear that the balsa cores are carrying more than half of the load traditionally assigned to the glass 'skins' alone. "

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