Scarf, Butt Joint or... - Woodworking Talk - Woodworkers Forum
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post #1 of 20 Old 02-09-2020, 08:11 PM Thread Starter
SawDustMan
 
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Scarf, Butt Joint or...

Howdy Yall. In a month or so Im gonna be neck deep in a stitch & glue duck hunting boat build. I will need to join two pieces of 4x8 marine grade plywood on the 4 ends to make the finished piece 16 long.

I could do a butt joint, or, I can scarf em (8-1), or...whats to keep me from doing a dovetail on both pieces and snap em together like a jigsaw puzzle? That would make a secure joint and it would increase the glue area.

Thoughts????

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post #2 of 20 Old 02-09-2020, 08:22 PM
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On boat building, a scarf joint is usually a 10 or 12 to one slope. You will not get the same strength from a dovetail.
If you go with a butt joint, it is necessary to glue or epoxy a backing board to hold the 2 pieces in line as well as together. I never tried a stitch and glue boat, it looks like it would be fun.

If you tell us what town/state you are in we might be able to help you find lumber and fasteners.

Are you building from plans or are you winging it? If by plans who's? I have built several boats from Glen-L plans and a small pram from my own plans. Funny story goes with that. Many, many years ago.
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post #3 of 20 Old 02-09-2020, 09:59 PM Thread Starter
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Thanks Tony. Ive two boat plans from Clark Craft and one from Glen -L. The stitch & glue duck boat first in the queue is called the Black Brant. You are correct, I was mistaken. The scarf is a 12-1. I live in Fabius, Alabama, which is in the NE corner of Alabama. Im an hour from Chattanooga, TN and 1.5 hours from Huntsville, Alabama.

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post #4 of 20 Old 02-12-2020, 09:02 AM
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I never used Clark Craft because I was so happy with Glen-L.
Going back around 30 to 35 years ago, their customer service was excellent. I'm sure it still is today because cust. svc is everything when building a boat.
I also used nothing but West System Epoxy By the Gougeon Brothers somewhere up north but readily available in areas where there are marinas. It is an excellent product. They too have excellent customer service. I used to have their videos which were excellent with a capital E. Their products come in very handy in woodworking and furniture repair. Read all of the labels and info on their website very carefully. Tons of info packed into it.
Have fun with your build If I can help, just holler. Not sure how much help I will be, it's been a long time. If I dont remember something, I will say so. I wont lead you down the wrong path.
Take lots of photos and post them on here as you go along. I'm sure there are several other boat building projects going on right now also. One currently in progress is by @faith michel. He is building a sailboat
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post #5 of 20 Old 02-12-2020, 04:32 PM
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In my search for boat building links for Absolutely Free Plans, one of the sites had this signature:

"Boats float, no need to re-invent any wheels"
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post #6 of 20 Old 02-12-2020, 04:43 PM
where's my table saw?
 
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It's hard to beat a half lap .....

Make equal size rabbets on opposing sides of each 48" end and glue them with waterproof glue:
https://www.amazon.com/dp/B00006LAAQ...dDbGljaz10cnVl

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #7 of 20 Old 02-12-2020, 06:01 PM
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Here’s a tool specially made to cut scarf joints in plywood for stitch and glue.


https://www.jamestowndistributors.co...ct.do?pid=4378
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post #8 of 20 Old 02-12-2020, 07:58 PM
where's my table saw?
 
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That link is incomplete ...

Quote:
Originally Posted by Quickstep View Post
Here’s a tool specially made to cut scarf joints in plywood for stitch and glue.


https://www.jamestowndistributors.co...ct.do?pid=4378


It will not show the enlarged version of the tool. Can't get it to do anything else.....?


Ok, I found this, a better link:
https://www.jamestowndistributors.co...ct.do?pid=4378


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The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

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post #9 of 20 Old 02-12-2020, 11:41 PM
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I'm not sure if you would get a smooth curve when bending a lap joint. I think the 2 faces glued would be a little stiff because the glue would not have much give to it as compared to a single long piece of wood.
Two pieces glued along side each other, long grain to long grain, with a 12 to 1 slope angle pretty much acts as a normal long grain to long grain edge glue up with all the properties of a single board. Also, if a lap joint would work as good or better, I'm sure the scarf joint would never had been invented.
Especially if you had to make an emergency repair in rough seas. I have seen sailboat masts glued with scarf joints and they bend and flex under quite a bit of strain.

I would only use epoxy on the exterior of a wooden boat. I would however, use a water resistant glue on the interior. There is a definite accepted criteria for using the term "water proof" and I dont think Titebond meets that criteria. It pertains to submerging something to a certain depth for a certain period of time with no measurable affect. It's easy enough to look up. My internet is way to slow tonite for me to even think about looking it up. Im happy to just be able to get on here
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post #10 of 20 Old 02-13-2020, 12:57 AM
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Regardless of which you choose, fiber-glass the boat.
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In furniture 1/32" is a Grand Canyon
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post #11 of 20 Old 02-13-2020, 06:05 AM
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We built a small boat with half deck out of masonite. My father made the oars from wood available at the time (just after the war).
He used two short lengths joined by an Araldite glued scarf reinforced by copper nails. never gave us any trouble.
We made other boats the same way. I will look out some pics for you.
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post #12 of 20 Old 02-13-2020, 06:39 AM
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Quote:
Originally Posted by woodnthings View Post
Make equal size rabbets on opposing sides of each 48" end and glue them with waterproof glue:
https://www.amazon.com/dp/B00006LAAQ...dDbGljaz10cnVl

On no account should you do this.Chances are the plywood will split when bent to shape.



The original question was scarf or butt and both of those are well tried and sound solutions.In fact both of them are permitted by Lloyds register of shipping while no other joint form is.Of the two,scarfing is a little more time consuming but the easier solution as far as keeping the inside of the boat uncluttered.Butt blocks can make it hard to keep the boat drained and they impede glassing the inside joints and painting.


Some CNC cut kits come with interlocking puzzle joints but this won't be an option for a plans built boat.Another option for an amateur built boat is the "glass weld" developed by Jack Chippendale and referenced in these two links https://smallboatsmonthly.com/article/classic-17/ https://www.simplicityboats.com/epoxyknowhowcarnel.htm . It features the transition of stresses that a scarf would give and the ease of alignment of a butt. The only real downside is that is is very visible under varnish-not a problem if you paint the interior.
I have a sort of attachment to the joint as the plane Jack used to cut the prototype is in my workshop as I type.
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post #13 of 20 Old 02-13-2020, 09:21 AM Thread Starter
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Thanks for all of the assists! Right now I’m gathering supplies. $$$ ��

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post #14 of 20 Old 02-13-2020, 09:23 AM Thread Starter
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Bought some good reading material...
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post #15 of 20 Old 02-13-2020, 09:39 AM Thread Starter
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Thanks Dude! I like the invisible butt joint for its simplicity. I think Im gonna give the scarf joint a shot and if I bugger it up then Im gonna give the invisible butt joint a go. 👍

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post #16 of 20 Old 02-13-2020, 09:51 AM
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Won't the finished piece be less than 16' long, by the amount of the scarf joint's overlap?
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post #17 of 20 Old 02-13-2020, 10:31 AM Thread Starter
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Quote:
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Won't the finished piece be less than 16' long, by the amount of the scarf joint's overlap?
Yes, by the length of the scarf as I understand it. I think the side panels Im making are like 14ish so Im good on length. Width is no issue.

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post #18 of 20 Old 02-13-2020, 11:12 AM
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I was thinking about this, because at first I was curious as to why a glued joint would be acceptable for boats. (I know nothing about boats other than what I learned from rowing them in college). If you take a scarf joint to the extreme, plywood is, itself, practically a scarf joint that's glued together. Or a laminate is an extreme scarf joint. The higher the ratio (8:1, 12:1) the stronger the joint.



Problem: solved. Carry on.
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post #19 of 20 Old 02-13-2020, 12:55 PM
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The term 'glue' is used roughly. Most importantly, they are referring to epoxy. Back up until maybe the 1970's, there was a waterproof glue. I believe it was 2 component and some powder might have been involved. I remember reading about it when I was using epoxy and it faded away. Cant remember the name of the stuff..
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post #20 of 20 Old 02-13-2020, 01:51 PM
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Quote:
Originally Posted by Tony B View Post
The term 'glue' is used roughly. Most importantly, they are referring to epoxy. Back up until maybe the 1970's, there was a waterproof glue. I believe it was 2 component and some powder might have been involved. I remember reading about it when I was using epoxy and it faded away. Cant remember the name of the stuff..
Would that have been Resorcinol, it is the glue used in marine grade plywood, not sure if it is still available in small quantities for the home builder.
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