Butt Joint question

[Tom the rower]

My brother and I are reviewing my plans for the Core Sound 20 mk2, with Cnc Cut kit.  I am trying to do the "read it twice, do it once correctly" type of thing.  Fairly basic butt joints, only, here's the thing.  I have done crude butt joints on ply before on my last 20 ft home designed.  I simply laid the plywood on the garage floor, and butted up "factory to factory" ends till flush, added 6 inch fg tape and wet it out, added wax paper over top, then weights.  Went back the next day, flipped all of them over, and then did the second side the same way.

 

But, with the cnc cut boat, it should be even stronger and these factory ends each have kind of a finger joint partially cut in them so they can receive thickened epoxy in the fingers as well as tape on top and bottom.

 

My problem is that the plans seem to indicate this order.  1/8 on the ground, then plastic (or I assume wax paper would be equivalent), then a strip of wetted out fg tape, then pre wet plywood ends to be joined, then another pre wet strip of cloth, more plastic, then a whole nother level of the same, prewet cloth, another set of butt joined ply another pre wet cloth, release film and finally another 1/8.  Screw top and bottom 1/8 together to apply pressure to both butt joints encompassed by the top and bottom 1/8's.

 

Now, my grandmother always cautioned me to not carry too, too much groceries into the house for her, characterizing a well overloaded young boy as having a "lazy man's load".  It seems to me that I could do this with one set of boards at a time, and then only one side at a time.  Wax paper, join fingers with thickened epoxy, fiberglass on top, more wax paper, then massive weights till cured, flip over and repeat for side B.  Slower than doing 2 sets of boards at once, but, seems like I would be able to see more exactly what is going on on my joint.

 

So, could we discuss butt joining with the cool finger joints that the designer has cut into the ends?  Probably best not to rush till I have thought it thru.

 

Thanks

[Tom Lathrop]

If I remember what the finger bits do, they keep the joint in alignment so there is no possibility of the parts moving during the pressure stage.  That is the magic of the added finger to the step scarf.  It makes it possible to stack several joints together at one time.  I don't like wax paper for this job as it has stuck to the joint due to heat of curing epoxy and made a mess.  Get a roll of 6mil plastic from the building supply that will last for your lifetime and is useful for many kinds of jobs.

[hokeyhydro]

??? When I visited the B&B shop the CNC machine was cutting a "gear tooth" edge on joints such as the chine, and a "stepped scarf" joint for panel joins. I reckon Graham may have shifted to gear tooth for butt joins since even the stepped scarf edge is fragile for shipping kit parts.

Anyhow, to address your stacked panel join question: stacking insures that you make two perfectly matched halves. Alignment is critical because just a tad askew at the joint, say 1/32" off, becomes a huge target miss at the ends. You could do one, and then the other, but to make sure the halves match you should do #2 on top of #1. Stacking and gluing in one operation is faster, only have to wait for one glue set to kick off. As for glue joint observation you can't see the bottom side anyway, and the stack joints I've done came out well.

[hokeyhydro]

If I remember what the finger bits do, they keep the joint in alignment so there is no possibility of the parts moving during the pressure stage.  That is the magic of the added finger to the step scarf.  It makes it possible to stack several joints together at one time.  I don't like wax paper for this job as it has stuck to the joint due to heat of curing epoxy and made a mess.  Get a roll of 6mil plastic from the building supply that will last for your lifetime and is useful for many kinds of jobs.

Tom types faster than I do :-) So B&B has added a finger joint to the stepped scarf? Very clever idea.

Right about the wax paper - sometimes it will become "one" with the epoxy, annoying. I stretch and tape down poly to avoid wrinkles, gave up on cling wrap because it is near impossible to get that down wrinkle free, and for small joints I like those clear Mylar file covers available at an office supply store. One of these days I'll hunt larger pieces of Mylar since that creates glass smooth joints.

[Fishman38]

Would one of you guys explain what a 1/8 is so maybe I can figure out what you're talking about?  Should I be able to find a picture of such joinery in say the Devlin or maybe Gougeon book.  I understand what a butt joint is, just cannot picture the setup described above.  Aren't finger joints normally used to join two short boards to make one long board and wouldn't you apply (a little) pressure parallel to the long axis?  Here I assume we're talking about joining two pieces of plywood edge to edge to make a larger piece of plywood?   I think my problem is I have no idea what in meant by "1/8".  To me it means 0.125...........     perplexed.

[Designer]

Tom,

 

There is no need to use glass tape on the finger scarfs. You will most likely glass the outside of the hull which will further reinforce the scarfs.

 

I will go over the instructions and correct them. With the rush to get you the prototype kit, this will not be the only correction. If anything does not seem logical, check with me.

[Designer]

Tom,

 

It just occurred to me that you may be referring to the glass tape on the dovetail joint at the forward end of the chine, the glass tape is necessary on each side of the joint or it might let go during the folding process.

[Tom the rower]

Yes sir, that is true.  I would always tape both sides, it just seemed easier to lay it down on my garage floor, join the pieces of the sides or bottom, depending on which one I am working on, tape only the topside facing me, and apply pressure, wait till that side cures, then turn over and repeat for the other side, rather that both doing both sides at once and adding the complexity of two sets of joins, all between 2x8's for pressure.

[Docpal]

Here is a picture of the CNC finger joint which Graham came up with last year. He may have even modified it further since then. As you can see it incorporates the true fit on the edges, with an "anti slip" finger so the joint can't move sideways when pressure is applied in the gluing process...

[Tom the rower]

Yes, that is what my kit looks like.  Seems like a really nifty way to make the long sides and bottom of the boat and seems like alot stronger than plain old butt joints.  Dang work for getting in the way of the much more fun boat build.

[Fishman38]

Wow!  Now I get it.  I think.  So 1/8 is the 1/8 inch ply and this is the "stepped scarf".  Thanks for the pics.

[Tom the rower]

No,  I do not think there is anything less than 1/4 inch.  In any previous post, if I wrote 1/8, I meant to write 1x8.  I only know one guy who builds boats completely out of 1/8 inch, but then he has to cover the whole outside of the boat with 6 oz fiberglass cloth.

[hokeyhydro]

Wow!  Now I get it.  I think.  So 1/8 is the 1/8 inch ply and this is the "stepped scarf".  Thanks for the pics.

Since the topic of this thread joining panels, as in butt joint, I believe "1/8" refers to the bevel ratio of a standard scarf joint.

[Fishman38]

  Screw top and bottom 1/8 together to apply pressure to both butt joints encompassed by the top and bottom 1/8's.

 

Thank you HH and I'm sure you're right. Never thought of that and when I go back and read Tom's opening post with that in mind it makes perfect sense.

[Scott Dunsworth]

My last visit to Grahams the week before the last mess-about he showed me these new joints he had been working on. He is light years ahead of most designers in his way of thinking about many things of boat building, performance and these joints are just one of them.

[Andy Woerner]

On 3/10/2013 at 5:38 AM, Designer said:

With this thread being over 10 years old, can someone confirm the guidance below still accurate, that tape is not needed on the finger scarf joints of the hull pieces, with the exception of the dovetail section of the chine at the bow?

And in case it matters, we're building a CS17, not the CS20 this post is about.

Thanks,
Andy and Grayer

 

 

On 3/10/2013 at 5:38 AM, Designer said:

 

Quote

 

Tom,

 

There is no need to use glass tape on the finger scarfs. You will most likely glass the outside of the hull which will further reinforce the scarfs.

 

I will go over the instructions and correct them. With the rush to get you the prototype kit, this will not be the only correction. If anything does not seem logical, check with me.

 

 

 

[Alan Stewart]

Hey Andy, 

Yes that is correct and the finger joint is plenty strong but for the cockpit sole where you're stomping around on it it's little effort to put some glass over the fingers and would be hidden under nonskid so i'd probably glass them on the bottom of the boat just because why not. On the side panels i wouldn't bother. And if you plan to glass sheath the outside of the boat which i'm pretty sure you are then the outside of all the finger joints will be glassed anyway. On the other parts like the longitudinal bulkheads those don't need any glass.