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PostPosted: Sun Aug 12, 2012 8:44 pm 
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Joined: Wed Jun 01, 2011 8:37 pm
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Location: Vancouver Island, Canada
Hey all
Rivets are good, right? Well that depends on the material used to make that rivet.
Aluminum Rivets are compatible with the crossbars and should not cause a lot of electrolysis unless the materials have different make ups. In that case, the softer material will become the sacrificial anode and corrode before the harder material. Rivets also come in the Monel or Stainless Steel variety but come with a set of problem the same as the next product below.
Marine machine bolts are made from stainless steel but have to be tapped into the crossbar to make compatible female threads. That will require the use of a male Tap; the absolute correct drill; and some oil to lubricate the Tapping tool. Kerosene works well for Aluminum as does sewing machine oil. However there is the issue of the stainless steel being far harder than the material in the crossbar which will cause corrosion.
There are a number of ways to prevent this problem. The key is to isolate the harder material from the softer one (ie: the Crossbar):
Silicone not only works in this regard but also works as a waterproof flexible glue to prevent electrolysis.
Loktite comes in various strengths. The Red is a permanent lock and will isolate both materials if used per instructions. This also means it is a forever bond. (That may require a messy process to drill out the fitting in case of replacement. The blue Loktite is medium strength)
Teflon Tape can also be used but it tears easily unless multiple wraps are used.

Why bother with this stuff if all you need is a few Pop Rivets to fix something that came loose well before it should have?

Quite simply, you can 'fix' a connection that will last a few years of hard use or spend the extra time and money and make it permanent.

A good Tap and Die set can cost you over $300. But if you are only using a certain number of Machine Bolts, buy those taps with their respective drill bits and a Tap Wrench. Remember also to clean your threads and DO NOT over torque your new 'Nut'

Cheers
Tri

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PostPosted: Mon Aug 13, 2012 4:43 am 
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Location: Moncton, New Brunswick, Canada
Stainless steel rivets are permanent.


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PostPosted: Mon Aug 13, 2012 7:38 am 
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Joined: Fri Jul 15, 2005 10:43 am
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Location: St. Louis, MO
The use of SS, Monel, or aluminum rivets is dictated by the loads the rivets will be seeing. For low load situations, use aluminum rivets in the aluminum crossbars. For high load situation you need to use either Monel or SS. Monel will not corrode aluminum but is more costly than SS. Using stainless rivets or screws will require isolating the SS from the aluminum. I have found that Tefgel works really well for this application. It does not glue the connection together like Lok-tite nor does it leave a mess behine like silicone.

Harder metals do not corrode softer metals. It depends on thier relationship on the galvanic chart. For example, titanium is very hard but does not corrode aluminum.

You should choose your fastener by what is appropriate for the situation. Often blind fasteners are used where threading thin walled materials is ineffective, you do not have access to use a nut, or if you do not plan to remove the fastener. Machine screws are used if you need to be able to remove the fastener.

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Current Boat
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Previous boats owned
'74 Pearson 30
'84 H16
'82 H18 Magnum
St. Louis, MO


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PostPosted: Mon Aug 13, 2012 6:20 pm 
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Location: Vancouver Island, Canada
Hobie Nick wrote:
The use of SS, Monel, or aluminum rivets is dictated by the loads the rivets will be seeing. For low load situations, use aluminum rivets in the aluminum crossbars. For high load situation you need to use either Monel or SS. Monel will not corrode aluminum but is more costly than SS. Using stainless rivets or screws will require isolating the SS from the aluminum. I have found that Tefgel works really well for this application. It does not glue the connection together like Lok-tite nor does it leave a mess behine like silicone.

Harder metals do not corrode softer metals. It depends on thier relationship on the galvanic chart. For example, titanium is very hard but does not corrode aluminum.

You should choose your fastener by what is appropriate for the situation. Often blind fasteners are used where threading thin walled materials is ineffective, you do not have access to use a nut, or if you do not plan to remove the fastener. Machine screws are used if you need to be able to remove the fastener.


You've made some great points in your reply. Thank you. Most people do not have the knowledge (or inclination) to alter their own boat by leaving any mods to their dealer. Hopefully anyone that has read both of these posts will benefit if they live in rural areas far from their dealer.
My choice of machine bolts is based on the fact that fittings will wear out as time goes by. Why change something that has proven itself to me for many years on much larger vessels than a 13 foot roto moulded Catamaran?
Have a 'Whale' of a day...

http://www.youtube.com/watch?v=Ndppa4Vj37E

Best Regards, Hobie Nick
Tri

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PostPosted: Wed Aug 15, 2012 9:11 am 
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Joined: Mon May 09, 2005 10:25 am
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Location: Jersey Shore
Hobie Nick wrote:
Harder metals do not corrode softer metals. It depends on thier relationship on the galvanic chart. For example, titanium is very hard but does not corrode aluminum.

You should choose your fastener by what is appropriate for the situation. Often blind fasteners are used where threading thin walled materials is ineffective, you do not have access to use a nut, or if you do not plan to remove the fastener. Machine screws are used if you need to be able to remove the fastener.


This is exactly correct with both regard to galvanic corrosion and choice of fastener. The extent of corrosion is based on the electrochemical properties of the metals in contact, not their hardness.

I would also add that removal of rivets (even stainless rivets) is really not that difficult as long as you understand the process and have a sharp drill. In the case of installing fittings onto a thin-walled extrusion like a crossbar or mast, the use of a rivet is often preferred as it will generally provide a stronger connetion than a threaded fastener.

sm


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PostPosted: Wed Aug 15, 2012 6:19 pm 
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Location: Vancouver Island, Canada
Thanks for your reply smc.
You are correct in my incorrect of the use of the word 'hardness' of a metal as when their conductivity should have been mentioned.
However, in the Marine World, we basically deal with 4 common metals:
Zinc
Aluminum (in all it's forms from cooking foil to Aircraft quality grade)
Steel and Nickel alloys (stainless steel)
Ferrous Iron (due to older boat engines on older vessels, and galvanized Iron alloys used for anchor chains)
(Any other metals are usually used in modern marine electronics that require hard grounding to prevent a mess)
The isolation of conductivity of these materials is what prevents slow or very fast corrosion by electrolysis. (In case you wonder, look up the sinking of the USS Thresher...)
As to your point about the use of a machine bolt versus a rivet, obvious considerations have to be place on the material being threaded and it's thickness. The size of the bolt and the thread used also make a huge difference.
Rivets are not the automatic answer as both methods of fitting attachments need 'human feedback'.
A bad tap will give you better feedback as to the grip than when the mandrel snaps on any type of rivet. Their long viability is based on isolating two dissimilar alloys if a proper grounding system is not available.
That has been my experience, and so far over 40 years, I have never had to replace any of my fastenings due to my lack of attention to detail to my own or my clients' vessels.

You may find this hint of use: If there is a question as to using a Machine Bolt as opposed to a Rivet, if a smaller (or equal) diameter bolt is placed (that can handle the long term stress more so than the rivet), you can also simply remove the bolt and replace it with the Rivet without having to re drill the hole.

Regards
Tri

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PostPosted: Sun Aug 26, 2012 6:27 pm 
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Joined: Sun Jun 04, 2006 7:09 pm
Posts: 122
Location: Buffalo, NY
Hobie Nick, great points from a former H18 owner. Been a long time since I've heard from you. After my purchase of my H18 about 7 years ago you're assistance got me up and running in no time. Glad to hear you're still on the water.

t-bone


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PostPosted: Thu Aug 15, 2013 8:56 pm 
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Joined: Thu Aug 15, 2013 8:51 pm
Posts: 2
Old post I know, but always important - Monel rivets hold up to corrosion and are stronger than 304 Stainless Rivets (which most are) - they key to avoiding Galvanic Corrosion is separating the Monel and any Aluminium - try Tef-Gel - it's the best and all the kiwi riggers use it now;
http://www.anzor.com.au/chemical-produc ... el/product


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