Index

Golden Rules for Fastening Things.

INDEX - Fastening, Galling, Deck Rot, Wood Rot, Bolt/Screws, Roberts, Glue, Forming.

1. Prevent the fasteners from moving with respect to the material it is fastened to.
2. Act as a bedding compound to fill the low spots and to prevent the high spots from taking the majority load. Another way of expressing this is to prevent fitting distortion by distributing the stresses equally over all the available surface area rather than just the fasteners.  This is huge advantage that can't be stated enough.
3. Protect the connection from distortion during freeze by keeping the water out.
4. Prevent marine growth and electrolysis between the surfaces by keeping the water out. (this applies if you have two dissimilar metals touching each other, like stainless steel and aluminum).  If you live in a desert you will likely never experience this problem.

1. This phenomenon is especially important for stainless steel to stainless steel as it applies to a turnbuckle on a mast that is never lowered.  But it applies to all similar metals.  If the barrel of the turnbuckle is not turned every once in a while, the thread will gall, preventing future movement.  Lubricated thread turns much easier under load.  For a mast that is stepped seasonally it is imperative to easily adjust the turnbuckles on the launching ramp. 
    

1. Seal the deck hole from water ingress to prevent wood rot.  This might require a tapered wood plug.
2. Create compression strength around the deck hole so you can't crush the deck with the fastener.  Having a solid deck around the fastener also ensures the fitting stays tight.
3. Seal the wood to prevent galvanic action with the metal bolt.  This is generally an issue with very long term exposure.

While the above are crucially important, you must also seal the inside of the hole with sealant.
 

1. If the fastener is in a usually wet area then seal the screw hole from water ingress.
2. Install a 1/4" thick spacer of none permeable material between the fibreglass and the wood.  Surface tension can creep water up 1/4" to wick into wood. 
3. Cap end grain and never expose it to water.  It can soak up water at an alarming rate.
4. Seal the wood with epoxy to prevent galvanic action with the metal bolt.  This is only an issue with very long term exposure.
5. For soft wood, use epoxy to create compression strength around the hole or fastener so you can't crush the wood with the fastener.
    

• Through bolting is much stronger than using a wood or sheet metal screw. 
• When bolting something to fibreglass, use a backing plate with bedding compound between to distribute the load.  If the backing plate is made of wood, use solid wood, round the corners and taper the edges.  Also seal the wood on all 6 sides to protect it from humidity.
• Use the shear strength of the bolt rather than the tensile strength.  Much stronger.
• If you have to screw something to an aluminum mast, it is much stronger to thread the hole for a machine screw than to use a sheet metal screw.  Coarse thread holds better than fine thread through the thin aluminum of an SJ23 mast or boom.  However, the mast foot or head is about 1/4" thick to tapping a fine thread is OK. 
• I have installed lots of hardware to the boom and mast using pop rivets with a marine sealant.  None of my hardware has ever come off or even suggested coming loose.  A fitting installed with machine screws is more serviceable though but a rivet is real easy to drill out and replace using the same hole.  Its worthwhile to buy a quality pop rivet gun.  Buy aluminum rivets with aluminum shanks.  Steel shanks rust, leaving an ugly stain.
• Always apply a thin coat of marine sealant under a fitting to prevent movement, marine growth or corrosion. 
• Always consider how you can access the fitting after everything is assembled.  This is especially important for a fitting attached to the mast or boom.  For instance, a long backing plate inside the mast will necessitate lowering the mast to remove the track.  Individual screws threaded into the mast can be removed without lowering the mast.  Pop rivets can be  drilled out without lowering the mast.  But the next time you lower the mast, remove the mast foot to remove the ends of the pop rivets you drilled out.
• Always keep the inside of the mast clutter and snag free if you have internal wiring or halyards. 
   
• ROBERTS SCREWS - Many times I refer to Roberts screws, thinking that everyone understands what I mean.  That is not necessarily the case outside of Canada.  While the expression, "Throw me a #8 Roberts," is very common jargon in Canada and maybe in the northern US, not so in the rest of the US.  To understand the situation with Roberts screws in the US today it helps to know some history of this drive system.
  
  A Canadian by the name of Peter Lymburner Roberts (1879-1951) recognized a problem with slot screws.  In 1908 he invented a new screw that was self centering and couldn't fall off the tip of a screw driver.  This new drive design was intended to serve the new assembly line industries to speed up production and save a lot of money.  It is called a Roberts screw after the inventor but in the US I understand many people call it a square drive screw.   You can hold a Roberts screw horizontally on a screw driver and it doesn't fall off.  Try that with a Phillips or a slot drive.  Nobody has been able to improve on the design since.  Henry Ford had one of the first assembly lines where Mr. Roberts thought it would benefit the most.  I have no idea what transpired at the meeting but suffice it to say that it was not a meeting of minds for a common benefit.  Rather than buy the screws from Mr. Roberts, Henry wanted to buy the company.  This didn't go well with Roberts and the meeting went sour from that point on.  To make a long story short this screw was not used in the US because Roberts wouldn't sell his company.  Roberts had a patent on the design and Henry had a lot of influence in the US.  That's about the jest of this story.  The Roberts screw was introduced in many other countries in the world with good success.  I think the Phillips drive appeared after this and was used extensively in the automotive and appliance industry driven by pneumatic tools.  Mechanics ever since have cursed this decision with each screw they skin their knuckles on.  Much later, hex drive showed up (driven by Allen keys), spline, Torx, and the list goes on.
  
  As far as the Roberts screw being used on a San Juan toe rail, you only have to experience doing this job once to understand the true benefits of the Roberts drive system.  My guess is that the Clarks must have heard about it, being close to the Canadian border, and hopped across to Vancouver to buy a zillion boxes of #8 Roberts screws and screwdrivers.  When San Juan boats were built in South Carolina, boxes of screws and screw drivers were shipped down there to be used in the assembly line.  This is quite a common event repeated in other places in the world when they discover the benefits of the Roberts drive system.  Fact is, this screw is so good that when I get frustrated with a Phillips (which doesn't take very long) I toss it in my metal recycle bin and replace it with a Roberts.  Hence the expression, "Throw me a Roberts."
  
  Roberts screws come in 4 sizes with a colour coded screw driver to make it easy to identify.
  #4 - yellow handle
  #6 - green handle
  #8 - red handle (used on the SJ23 toe rail)
  #10 - black handle
  
  Hope this explains it.
• Everything you ever need to know about fasteners - http://en.wikipedia.org/wiki/Screw 
   

• For a laminated assembly you must use a brittle glue that does not creep with respect to the laminates.  Two types that are ideally suited for the task are waterproof marine glue (powder form) or two-part epoxy.
• For a butt joint always create a fillet with the epoxy that oozes out from the end.  The epoxy seals the end grain and the fillet seals the edge of the crack from water ingress.  The fillet doubles the strength of the bond.
   

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