One of the great leaps forward available to us when using epoxy is being able to eliminate fastenings (screws,nails, bolts etc) from the structure.
Many builders now only use fasteners to temporarily hold things together while the glue sets up.The temporary fasteners are removed and can be re-used many times.
Eliminating fastenings also speeds up major repairs. See the companion page to this article for method and photos.
The Downside of Fasteners
1/ Fasteners are expensive
2/ Fasteners often make the boat a little bit harder to fair
3/ Over time fasteners may move a little, cracking the paint that covers their heads and allow moisture to get in.
4/ A glued structure has a waterproof glue line between parts. Fasteners penetrate the waterproof joins.
5/ Fasteners can make repairs difficult – where fasteners are eliminated from the structure and part of the boat can be cut, planed or sawn without risk of damaging tools
Where can Fasteners be omitted?
Where there is adequate bonding area at the joins between ply panels through filleting or gluing to solid timber structure (eg chinelogs, centreline structure, deck clamp, carlins, bulkhead cleats etc) there is no real necessity for fastenings at all.
If the designer understands modern epoxy construction they will have set up all the bonding areas of the structure so that they will be strong enough without fasteners.
All my Designs are set up in this way.
Where is it good to use fasteners?
The brief answer is, anywhere the timber may split.
Where a timber member is attached to the outside of the hullhull by only one face it could split away from the structure (gunwale or bottom skid). Some fastenings through the timber member are a useful split preventative.
Where a timber member has ply on two faces it is very unlikely that it will split.
Glassing will also prevent such members from splitting from the hull – so you can take your choice of fastening or glassing. Generally underwater members I would glass and visible ones that have an aesthetic contribution and will be left clear finished (gunwales) – I would screw or bolt.
On all canoes and dinghies and many boats under 20ft (6m) I would probably not bother about putting fastenings in the gunwale either – it is unlikely that the gunwale will ever be hit hard enough to split it away from the hull because of the lightness of the boat – it is very unlikely to ever hit something hard enough to split timber..
If a keelboat or trailer sailer is likely to spend a lot of time banging around marinas I would consider fastenings for a clear finished or painted gunwale. If the timber for the gunwale has straight grain (the grain lines meet the long edges of the timber at small angles then splitting is not very likely.) then a single screw through either end of the gunwale is probably enough – if a crack can’t start at the end it is even more unlikely to start anywhere else.
What abouut Masts, booms, yards and other spars?
To keep costs down it often makes sense to lash sails to spars – it means you don’t have to buy mast track or other fittings to attach the spars. You can simply run the ropes to tie the corners of the sail in place through holes in the end of the spar. However there is a small chance that the tension forces of the rope can split the end of the spar.
Wrapping a couple of layers of fibreglass tape prevents this from happening – the threads of the fibreglass take the splitting loads. The hole is later drilled through the mast tip in the middle of the fibreglass reinforcement. This approach works well when the rope will tie to the end of the spar. If the rope ties somewhere other than the end you will need to attach a fitting. See fig below
But All Boats need to be fastened don’t they – it can’t be safe?
I have built, or had direct acquantance with hundreds of boats built without fastenings over a period of 25 years. This includes Multis up to 55ft, Powerboats up to 50ft.
Engineering the joint.
Properly Engineered, the glued joint structure will be stronger than the adjoining plywood.
Tests indicate that a bonding area to both pieces of ply involved in a joint needs to be 2.5 to 3 times the thickness of the ply. If there are two different thicknesses of ply the bonding area needs to match the smaller thickness of the two. The bonding area can be made up by using a fillet or by having timber in the joint.
Some glass tape on the outside of the joint (eg chines or keel) or glassing the bottom of the boat and covering the joins in the process increases the strength of a bonded joint even further.
Using fibreglass tape only on the inside and outside of smaller boats is a good method too – for example one layer of 6oz (200gsm) 2inch (50mm) wide tape works along all the seams of my Eureka Canoe design. If the ply used is thicker than 6mm the join requires multiple layers of glass. This can lead to problems fairing the join which may lead to unsightly bulges on the outside of the boat
Some timber is difficult to glue well. Some timbers need to be treated differently.
An important exception is with timber that does not glue well – like teak or Australian White Beech – if they are being used for structural members they should be fastened with one of the standard fastening schedules that are traditional. These oily timbers can be bonded well enough for a deck overlay with a few precautions.
See my page on applying a teak overlay on a ply or fibreglass deck for information on the gluing procedure for teak and other oily timbers.
What temporary fasteners? … And Some Methods to make things EASY.
Generally we use phillips head countersunk drywall (gyprock) screws to hold things together while the glue sets up. For structures made of ply thicker than 6mm we move up to hex headed chipboard screws.
Where the area will be clear finished the screws can have a piece of ply under their heads to greatly reduce the risk of the fastener denting or marking the hull timber. There needs to be some plastic packaging tape on the pad so it won’t be permanently glued to the hull. I mass manufacture pads by cutting a long strip of ply, covering it with packaging tape and then cutting it into shorter pieces.
Removing the temporary fasteners – don’t wait toooooo long.
Remove the temporary fasteners (which can be re-used) within a couple of days so the glue doesn’t bond them in place as well.
It is no problem if a fastening is accidently glued in – heat it with a soldering iron for about 30 seconds and it will screw out OK – it just takes some extra time – so best to pull them out when the ‘pox is hard.
This fastening was well and truly stuck. After about a minute heating it with a soldering iron – out it comes!
This is usually overnight – in cold conditions sometimes a couple of days.
The epoxy should feel hard and not yield when you attempt to indent it with your thumbnail.
Repairing Wooden Boats is Easy when there are no Fastenings
I have just been doing a repair job to a small boat I built some time ago – someone put their foot through the bottom – the plywood was defective and had a number of serious core voids.
I decided to remove the whole bottom and replace the ply – having no fastenings to worry about made this major job quick and easy. I was able to use fast cutting power tools to remove the bottom without running the risk of hitting a screw or nail and damaging the router or router bit or taking a chunk out of the framing.
Method – it looks long, but it is REALLY FAST! – only a day to replace the complete bottom of a small sailing boat and have it in the water the following day.