To any chippies out there - are biscuits strong enough?

Roach1948

Roach1948

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www.dallimoredesigns.nl
I am planning the joinery of the backrests on Roach. The problem is that in order not to intrude on interior space too much, I will need to push the backrests out to the hull as much as I can requiring a lot of trail and error . Normally I would make mortise and tenon joints, but I think the biscuit technique would suit me better as I can work within a confined space of the cabin without a workbench. The thing is that the largest biscuits that I could find are no. 20 and that seems pathetic compared with a traditional M&T join. Is this OK strength wise? Having never used biscuits, I presume they go in horizontal (rather than a vertical elipse) and that I can use PVA for bonding?
 
Biscuits go in long edge first. I don't think they'll like the wet, (only pressed cardboard or MDF) and are only meant to align two boards butted together, as in a long window sill, which are otherwise supported. could you perhaps groove both pieces top to bottom, and join with a tongue? In that case it could be all hardwood. Perhaps use a bit more sophisticated glue than PVA.

(Usual terms and conditions apply, ie I may be writing bulox)
 
'Fast grab'. Does what it says on the tin mind, and loves water. I glued three pieces of Sapele, each 10x4x60 inches, together with it, then gave it to a man who turned it into a 5ft x 9in diameter round billet. Brilliant stuff, but get the joint right first time.........Do a test piece to see what you think.

Edit: don't confuse it with tubes of the 'No Nails' variety. This stuff is a mix like expanding foam and superglue, in a plastic bottle.
 
Basically for any alternative the way to work out the strength is to see how much timber will be damaged if the join breaks.

Biscuits have little strength of their own and rely on gluing along the join. Any strategy that increases the glue join area will increase the strength of the final item.

Also you have to be aware of the tendancy of timber to split and crack along the grain and make sure that the type of bending and tension loads that cause this are taken up by other structure or are at a low enough level for the timber to handle.

If you were suggesting using normal PVA glue I would strongly suggest you do some more background reading before doing the job - you don't yet understand the requirements. Have a look at how other boats are put together in books and magazines.

Boatbuilding is not even slightly related to furniture building. Furniture has to be stiff but not terribly strong. Give me a chair and I can pull it apart with my bare hands. It is stiff to sit on but not strong if loads come at it from odd directions.

Just about any bit of boat furniture will need serious efforts with serious tools to break it. It is both strong AND stiff.

Furniture methods and materials will usually not be adequate in a boat structure.

See standard building texts like Rabl and Bud Macintosh if wanting a traditional appearance. Or the Gougeon Brothers on boat Construction if you are using epoxy and modern methods.

Best Wishes
Michael Storer
 
Good advice so far, so not much to add really, except IMHO, "biscuits" are really only locators, to allow accurate alignment of two pieces of timber which then rely on the strength of the glued joint to hold them together, with respect to that, why not try this...........

http://www.axminster.co.uk/product-Humbrol-Extraphen-Fully-Waterproof-Glue-21681.htm

It's not cheap, but I have had great results with it myself, the timber will break before the glue joint does, so you need to be sure that it is suitable for your application, plus it is totally waterproof.
 
Biscuits are for cheep crap and DIY and a quick way to do things in "the new Yankee work shop" and even "Norm" will do a proper job if it is required. If it is for a real wooden boat Sail or motor there is no substitute for a proper made joint and if done correctly it will need little or no glue.
 
What you say is absolutely correct of course, but it sounds like he is a bit strapped for space or summat? Interesting profile, 1905 Gaff Cutter eh? sounds nice? That's about where my heart is too, but me wallet is with a 1968 Westerly Nomad /forums/images/graemlins/grin.gif
 
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Biscuits go in long edge first. I don't think they'll like the wet, (only pressed cardboard or MDF)

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Biscuits are compressed beech and with a good glue join will not have any contact with water. A biscuited joint is strong in shear but weak in tension. The biscuits, which are kiln dried, expand in the slots with the moisture from the timber and glue to provide a mechanical as well as a glued hold.
The slot for the biscuit is cut with either a dedicated biscuit jointer or a slot cutter in a router, the slot is normally cut in alignment with the grain. (A good Biscuit Jointer is a lot easier to use and more versatile than a router.)
Allow roughly a minimum of 3mm or an eighth of an inch between slots vertically and horizontally if you want to cram biscuits in on a joint but normally 1 or 2 bicuits per joint will suffice.
I gave up using PVA after too many joint failures and use either a Polyurethane like Balcotan or Elch which have a D4 rating (resistant to seawater) or an Alphatic resin like Titebond.
The Polyurethane is stronger but the Alphatic cleans up easier.
A good biscuited joint with a polyurethane glue will tear the wood fibres apart rather than split the glue line when stressed to breaking point.
For speed and ease of joint forming you cannot get much better.
 
Well thanks for your input. Biscuits will now only be served with the tea -preferably cotaed in chocolate!

The problem with the joints is that it is where these joints take place that they will be up against the curvature of the hull (and possibly needing to be shaved back in two planes) as I want the backrests as outboard as possible to allow for more interior volume and more bunk width for the sleeper. I will also be working in a very confined space and don't want to be walking back to the workshop a million times (involving climbing out of a boat on dry land) - so prefer if much of the work to be done is in the cabin. Hence my attraction to biscuits as they can be put in place relatively easily with a router and special biscuit bit.

But now that I see that this may not be strong enough, I will look at an exaggerated tongue and groove join and will heat the interior somehow so that I continue working with epoxy glue as that has been very successful results on the rest of the interior joinery.
 
Now this may sound daft, but have you looked at the way kitchen worktops are butted together? Bolts and brackets in sockets within the thickness of the material on the underside, if your timber is thick enough. (Change to stainless, obviously) Need dowels or some such to locate. Depends if you are a purist I suppose.
 
You can also get plastic biscuits with teeth that grab into the wood , these would be better imo than normal biscuits . You could use the normal ones to locate your work then use the plastic ones when you are ready to fit .
 
If you have sufficient length in the joint you can use your biscuit jointer or router & slotter to cut a continuous groove and insert a high strength loose tongue e.g. 4mm birch ply. If there is enought depth in the joint you could use two or more tongues in parallel to increase the glues area within the joint. In my opinion an aliphatic glue like Titebond III would be appropriate, as it provides stronger than wood joints and is easy to use and clean up.
Whilst the construction of the interior fitout needs to be robust I do not think you need to achieve the same standards of structural integrity as you would in the hull. If in doubt make some test joints and break them - if you can.
 
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(Usual terms and conditions apply, ie I may be writing bulox)

I stand corrected. What's your opinion on the 'fast grab' glue?

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Had a look on the Rutlands website and the 'fast grab' glue looks the same as the Elch polyurethane glue that I use, I have yet to have a joint fail using this glue and its economical to use.
The polyurethane seems more flexible than epoxy, the only downside is trying to get it off your skin or clothes.
 
I made a six sided hardwood frame joining all the edges (60 degrees) and used waterproof wood glue and hardwood biscuits for the edge to edge connections. Under load the joints snapped at the biscuit, I had to reinforce them with a shaped packing piece bridging the joint.
 
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