My original message could have been written more clearly, I think.
Bails for vangs can be attached securely with several different fasteners:
rivets, tapped machine screws, or thru-bolts. All work well with a bail
because the force on the fastener thru the bail is a shearing force rather
than a tensile force. That's in contrast to using an eyestrap that is
attached so there is a tensile load on the fastener. The tensile loading
will cause failure of the fastener in an unusually stressful situation.
On boom hardware for small boats, you see thrubolts used frequently, but I
have rarely seen a thru-bolt used to attach the vang bail to a mast. SS
rivets work just as well as thru bolts on a boom, IMHO, unless you're
attaching a part that might need to be replaced frequently. (My results with
thru-bolting hasn't always been great. I haven't always been successfull in
getting the holes for thru bolts lined up exactly opposite each other using
my hand drill -- somebody want to donate a drill press to me?)
I don't truly know why you don't see thru-bolts often on the mast. Usually,
SS rivets are used on thin walled masts and tapped screws are sometimes used
on thicker walled masts.
I would hazard a guess that the larger diameter of the mast would require a
longer bolt and therefore larger diameter bolt and you don't want a hole any
larger than necessary in the side wall of the mast. Also, you don't want to
tighten the thru bolt on a mast enough to create a compressive load on the
side walls of the mast; it is sometimes recommended that a compression sleeve
be fitted over the bolt inside the mast.
Another reason for not using thru-bolts is that you do not want the threaded
part in contact with the wall of the mast; the threaded part is weaker and
the crevices promote corrosion. That's probably why you don't see thru bolts
on masts; they would have to be custom fabricated for the mast.
In all cases, if you're attaching two different alloys (SS bail, SS fastener,
aluminum spar), you need to insulate them from each other using tef-gel (best
but expensive, available from West Marine or EB) or 3M 101 (good and cheap)
or marine grade silicone (cheapest, not quite as long lasting) to prevent
galvanic corrosion .
(BTW, it's funny you should bring this topic up at this very moment -- when
your email arrived, I was leafing thru my books, trying to find the
guidelines re: wall thickness for tapping/machine-screws vs. rivet to attach
my new fixed gooseneck. I'll let you know when I find it; it's somewhere in
one of my books)
Best,
Judy B.
In a message dated 6/15/99 4:38:09 PM Pacific Daylight Time, rich@gorts.com
writes:
> Hi Larry and Webgang,
> >
> > I don't trust vangs that are attached with rivets that can be pulled out
> > easily (with tensile force along the axis of the rivet). That's why I
> have
> > always advocated using bails to attach a vang. The stress on the
fastener
>
> > that holds a bail to the boom or mast is a shearing force, not a tensile
> > (pulling force)
> >
> > Eyestraps attached with rivets are too easy to pull out ( a tensile
stress)
> .
> > Bails attached with thru bolts all the way thru the boom and rivets or
> > tapped machine screw into the mast are the most reliable way to do it.
>
> Judy,
>
> Why do you say that rivets or tapped machine screws are OK to attach the
> bail to the mast, but a thru bolt is necessary on the boom? I would think
> that the stresses would be about the same on each. Do you recommend not
> using a thru bolt on the mast? If so, why? Do you think it might weaken
> the mast? I've got the "heavy" mast and was planning to use a thru bolt
> on it to mount the bail for my new vang.
>
> Rich Gort
> WWP19 #202 SAYOKO
> Lake Stevens, WA
> Sailing Northern Puget Sound and North Cascades Mountain Lakes
>