[Public-list] Boom bails

[Don Campbell]

There have been several comments about adding bails to the boom of an
Alberg 30 for both vang and main sheet attachments as improvements over
the original configuration. Let me preface this discussion by saying
that it is difficult to ascertain just how much force is exerted on
either the vang or the end boom by the sail but the accepted size of
blocks recommended suggest that we are well within a safe range with
"mid range" equipment,  that for a 4:1 purchase system are spec'ed about
2800 pounds for a safe working load with up to 500 sq. feet in the main.
Since we are less than half that area on our mainsails, we should be
looking at somewhere between 1500 and 2000 lbs on the end of the boom
without considering shock loading on a jibe! While we usually check the
safe working load of the blocks, do you check the shear for the bolts?
    If one were to add bails, consider first that there ought to be a
compression spacer within the boom for these systems, particularly for a
bail for  the vang. The original boom was not designed to accomodate a
bail for the loading that occurs on a vang system, which, as Gord has
pointed out, can be quite heavy ( because of leverage developed due to
the length of the boom). All the load is on the wall thickness of the
boom and there is always a trade-off between the hole size for weakening
the rigidity of the boom (particularly laterally since that is the
smaller profile compared to the profile  vertically)  and the increased
surface area that supports the load with the use of a bigger hole. John
Birch can attest to the dangers of breaking a boom at a bail when the
load exceeded the design. The further that one locates the boom bail
aft, the greater the lateral force possible and the more total
resistance is required from the boom structure to offset the added total
leverage from the vang system.
    The same holds true for the attachment of the mainsheet block and
topping lift. Drilling holes in the side of the boom for bails develops
a weakening system compared to the original set up, particularly when
the accepted upgrade of replacing the rivets holding both boom endcaps
has been done with machine screws. The aluminum casting in the end of
the boom is a far stronger section than the boom for accepting the
forces applied by the mainsheet, and thus if you want to increase the
strength of the  system, consider increasing the bolt size there before
drilling the side of the boom. The torque that is applied to the boom
from the sail at any angle ( on any point of sail except close hauled
with the boom as tight in as possible )  is borne by the machine screws
on the whole interior circumference of the boom and the resistance of
the surface area of the lip of the oval section of the end cap against
the inside of the end of the boom. This twist transmitted to the hull
via the mainsheet system is minimised in the original system because of
the single pivot point of only one bolt holding the force, - so the pull
is longitudinal not twisted,  where with a bail,  there is twist and it
is solely on the bearing surfaces of the holes for the bolt, (top on one
side and bottom of the other,  so only the surface area of the
circumference of one hole)  with added leverage because the width of the
boom acts as a lever. There is  a much much smaller surface area for
resistance with the bail system and it is my guess, that that hole will
be off round within one good sail with winds over 20 knots.
    Unfortunately, the physics of these systems is still the main
governing factor and it is just a question of doing the engineering to
see the problems. Most engineers use a safety factor that is somewhere
between 4 and 10 and so I ask whether your safety factor has been
considered or is as great as the original? The fact that the mid range
blocks are for mainsails of 500 sq. feet already adds a factor of >2 for
us, but as I originally said, how big is the  shear force on the bolt?
The system requires a balance, or at least an understanding of the trade
offs.
Don #528


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