[Public-list] Boom bails

[John Birch]

Absolutely there Big Don, I now use a wide leather and dacron saddle with 
webbing reinforcements and two D rings - it is about a foot wide to straddle 
the boom - works great and spreads the load so there are no peak loads in 
one spot.strap. Harry Grigat had warned me about the bale hoop I had before 
even though there was a compression sleeve. The upside is that when it broke 
it was such a clean slice that I only lost about a 1/2" of boom length - it 
is now sleeved, twice as strong and will not likely break again.

Steel bales are great for simplicity sake, but when they fail, and it is the 
thing attached to that fails - like the boom, well the results can kill ya - 
I was lucky, concussion, separated ribs, a month of recouping or so, June 
had to sail the boat 89 miles to safe harbour under bare poles and a fair 
bit of hubris knocked out of me (maybe not such a bad thing). Could have 
been worse.

Yea, I know, you don't intend to be caught with the main up in a sudden 
storm that becomes a sustained gale in less than 5 minutes, while you're 
working a mayday for another boat on the radio - and you only buy stocks 
that go up.

Use a webbing ...

Cheers,

John



----- Original Message ----- 
From: "Don Campbell" <dk.campbell at sympatico.ca>
To: "public list, Alberg" <public-list at alberg30.org>
Sent: Friday, April 21, 2006 10:57 AM
Subject: [Public-list] Boom bails


> 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
>
> _______________________________________________
> These businesses support your Association:
> http://www.alberg30.org/store/A30supporters.html
> Please support them.
> _______________________________________________
> Public-list mailing list
> Public-list at alberg30.org
> http://alberg30.org/mailman/listinfo/public-list 


 1145635036.0