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I was told that biscuits were merely for alignment and the joints are no stronger than a butt joint.  I was also under the impression that a butt joint will be stronger than the wood, if done properly, and scarf joints' biggest advantage is that when bending at the joint, you get a fair curve.

As to whether biscuits add strength to a join, it is a question of what kind of joint.  For mitered trim, like door and window casings, a biscuit ads significant strength to the joint.  When I did residential finish work I always biscuit joined the pieces before installing.  We called the process MSG for mitered, splined and glued. Biscuit joints have their place, just not for boat building.

Scarf joints have 2 biggest advantages.  Not only does the new joined piece bend more fairly, the joint is many times stronger.  The 8/1 ratio scarf yields more than 8 times the glued surface of a butt joint.  In the case of plywood for boats, the joint is probably as strong as the plywood. 

Many people find the prospect of scarfing plywood intimidating.  All I can say is try it.  Many builders in this forum never tried it until their boat projects and surprised themselves with the results. 


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Biscuits, the store bought variety, are designed to swell inside it's slot, with the traditional aliphatic resin type glues (yellow water based, wood worker stuff), making for a reasonably tight joint when cured. As a structural element they are far weaker then the surrounding wood, but the increased gluing surface the slots and biscuits provide help a good bit. Used with epoxy, they offer little more then joint alignment. In fact, I suspect they're machined a little too tight for thickened epoxy to be really effective, within an epoxy joint.

Epoxy joints are best made by a "wood butcher", that is a sloppy joint, with poorly fitting surfaces. This offers the goo a place to live, rather then be squeezed out with clamping pressure, like other glues. This is one of the bigger mistakes I see novice users make with epoxy. You want space for the epoxy, inside the joint.

On joints I want a tight fit, for appearance sake, I'll "back" out the joint, but leave the edges perfect. The hollowed out inside area of the joint, will accept the epoxy and the tight fitting edges will clamp with minimum glue line. I can often get a nearly invisible glue line using this trick, if the grains are well matched.

Scarfs have been covered here dozens of times. They're not so tough if you keep in mind the sloppy fit requirement for epoxy. The hard part is a clean feather edge on brightly finished work with a scarf. I usually cut the scarf long, then use a razor and straight edge to "clean" up the edge. Some times I'll cut a thin relief to receive the razored edge on the other part of the scarf, so I don't sand through the clean edge. Others will use a notched or nipped scarf to accomplish the same task, but I find them more trouble to make then worth the bother.

Posted

Seeing you already have a joiner I would buy a block plane and a good spare blade - I really like my Hock. You can thickness stock on a table saw and clean up on the joiner. Or hit it a couple of licks with your block plane. So many different ways of cutting scarphs -belt sander, circular saw, power plane to mention few but your block plane will come to the rescue when it is time for for the final clean up. That is one tool I really get a good use out of. Don't buy bunch of stuff up front- do with what you've got  or borrow first. Buy after you have a real need for something. Save yourself from having a lot of stuff you get a little use for. If you go the kit route you should be able to put the boat together with a few hand tools. Personally, and I do have six hand planes- I would spend money on sanders first. You will spend a lot of time sanding. LOTS. Lot more sanding than planing goes into boat building.  I have six different size sanders ( plus a long board and a hand block) and and use them all well when boat building. PeterP

Posted

I bought Stanley's low angle block plane for my first.  I'd say a low angle block is almost necessary for building with plywood.  The Stanley's not really a good tool, but it works.  If you can get an old one, they're much better.  I think it was like $35.  Given how much I used it, I'd say the one by Lie Nielson, $95 from Wooden Boat, is worth it, if you plan to build more than one boat.  I've never used a Lie Nielson, but they have an excellent reputation.  I just like using hand planes, however.  I used it when making my oars and on one household project so far.  

Posted

I'll keep my eyes open for planes and sanders.

Does anyone with a Core Sound live within an hour or two of San Antonio?  I will be there the first week of May for a conference my wife is attending and I would love to take a look at a boat.

Thanks

Joel

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