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Old 30-12-2010, 21:42   #16
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Originally Posted by boatman61 View Post
So I take it a centre sheeted boom is of inferior quality to the other... hmmm.. still not sure I buy it
The opposite actually - a mid sheeted boom must be stronger, because it takes more load.

The ideal way to attach the main sheet (from a structural perspective) is at several points between the clew and where the last reef meets the boom.

spreads the load and minimises the distance from the loading
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Old 30-12-2010, 23:05   #17
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Thanks Daddle

Just thinking purposes... The fact that the traveler is a fixed components and it is being located to higher grounds, you do not think it would act as a lever on the boat as it is being pulled by the main sail, making it more sensitive to cross wind (tilting the boat)?
I know what you are thinking. But that's not the way the forces work. The forces acting on the hull as a whole will be identical for identical sail trim no matter where the mainsheet is attached.

Arches do cause considerable wind loading from all that structure up in the breeze.

The boom breaking loads while reefed are an interesting problem. A spar-builder would know better. They see all the broken stuff.
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Old 31-12-2010, 00:27   #18
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My Buchan 37 originally had boom end sheeting, the second owner (a rigger) moved the traveler to the cabin top to give mid boom sheeting and less clutter in the cockpit. The sheeting loads at the traveler are higher for obvious reasons but the point loads on the boom are lower because:

1) They are distributed to blocks at three locations on the boom instead of one (plus the goose neck at the mast), spaced some distance apart, and ...

2) There is less bending moment imposed on the boom because it is no longer restrained only at the ends.

For the boom, you just have to ensure proper attachment of the blocks (inner backing plates), and if the cabin structure is up to it, installing the traveler to the cabin top, also with husky backing plates.

You also get a bonus in that the traveler length is more effective due to the booms swinging arc is now shorter due to it's closer proximity to the pivot point.

But I would still recommend an opinion from a knowledgeable source on the suitability of your cabin structure for this option.

Peace,
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Old 31-12-2010, 01:05   #19
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There is less bending moment imposed on the boom because it is no longer restrained only at the ends
It looks like a good solution - and similar to my set up - but this particular statement needs a bit of clarification.

Moving the sheet to a mid boom position doesn't reduce the moment, it increases it considerably

However, since you have a number of attachment points, that moment is now resolved at 2 (or 3?) different locations on the boom.

So, you may have* reduced the max moment acting on any particular point of the boom by having several attachment points, but certainly not by moving the sheet away from the clew.

* It's a calc rather than a given and the distance really does make a big difference.
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Old 31-12-2010, 02:28   #20
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Recently did a very similar exercise in moving the traveler from the center of the cockpit (where it was a real PITA) to a targa over which the end of the
boom would traverse. The targa was built to conform with the arc described by the boom.
The Santana 39's boom is sheeted to a traveler about 1/2 way along the boom. Sheeting it at the end will not be a problem.

I even considered doing away with a traveler altogether by using several pad eyes.
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Old 31-12-2010, 05:45   #21
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On the plus side, you'd make quite a saving on main sheet with the mid-boom rig - unless it's used up in the extra purchase needed.

I notice that multiple point boom rigging is popular on a lot of sports boats these days, and on cruisers (seems they use the winch rather a lot). It's an idea with merit but not new of course; various dinghy classes have used this rigging to allow lighter boom sections.
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Old 31-12-2010, 11:46   #22
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Hey there Bewitched,

I believe, and someone correct me if I'm wrong, but the overall load/stress on the boom will decrease because:

The bending moment, which puts the greatest stress on the material (assumed aluminum) is greatly reduced not only because the loading point (sheet interface) is now spread over multiple locations, but all of these locations are now moved closer in line with the center of effort imposed by the sail (somewhere forward of boom center?). Think of it as stretching a line across a ten foot span and hanging a 100 lb. anvil in the middle with the ends of the span representing the goose neck and the main sheet at the other end of the boom, and the anvil representing the load from the sail ( a little extreme and not truly representative, but for the sake of argument you can humor me). Now try to imagine how much force is required to keep the line straight, this represents the material tensile loads reacted from the bending moment. Now, if you merely suspend the anvil with the line hanging vertically the greatest load the line will see is the 100 lbs. (except at the knot but we won't get into that).

So mid boom sheeting is much kinder to the boom all around, the down side however is it is much more harsh on the traveler and it's related attach points, and of coarse the guy hauling in the sheet unless more purchase is used or a winch is employed.

Let the fur fly!
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Old 31-12-2010, 12:36   #23
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A correction!

My apologies Bewitched,

I reread your post only after posting my response and I think we agree on the increased point loading at the sheet interface thing, the only thing I wanted to clarify was the benefits of reducing the bending moment. You clearly don't need an explanation on loads, moments and stress, I just miss interpreted your post and then went off half cocked. I tried to edit mine but was already past the 30 minute barrier and don't know how to contact the administrator to correct my error. So again, with a deep bow, please accept my humble apologies.

Peace
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Old 31-12-2010, 13:31   #24
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Hey there Bewitched,

I believe, and someone correct me if I'm wrong, but the overall load/stress on the boom will decrease because:

The bending moment, which puts the greatest stress on the material (assumed aluminum) is greatly reduced not only because the loading point (sheet interface) is now spread over multiple locations, but all of these locations are now moved closer in line with the center of effort imposed by the sail (somewhere forward of boom center?). Think of it as stretching a line across a ten foot span and hanging a 100 lb. anvil in the middle with the ends of the span representing the goose neck and the main sheet at the other end of the boom, and the anvil representing the load from the sail ( a little extreme and not truly representative, but for the sake of argument you can humor me). Now try to imagine how much force is required to keep the line straight, this represents the material tensile loads reacted from the bending moment. Now, if you merely suspend the anvil with the line hanging vertically the greatest load the line will see is the 100 lbs. (except at the knot but we won't get into that).

So mid boom sheeting is much kinder to the boom all around, the down side however is it is much more harsh on the traveler and it's related attach points, and of coarse the guy hauling in the sheet unless more purchase is used or a winch is employed.

Let the fur fly!
I don't think the load from the sail transfers to the boom that way.
As mentioned before there are loose footed sails, all the load is at the end of the boom. Loose footed sails are not constructed very different from footed sails so the loads can't be very different. Remember shelf footed sails with a lightweight panel to take the load to the boom? Take a knife and cut the foot loose from your current sail. Do you expect the shape to change much? Less radically, take your finger and push on the sail just above the boom at various points. I don't think you're going to find much load. Better yet look at where they put carbon fibers in laminate sails, they don't have any going to the boom.

Unless you have a very small boat the reef points are going to have a much shorter span to the mainsheet attachment for end boom attachment.

John
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Old 31-12-2010, 13:47   #25
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Never mind.

"I don't think the load from the sail transfers to the boom that way."

Ok, I'm not doing very well today. You're right, my whole bending moment theory is D.O.A. and reversed. I'll take my "F" and go home now.
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Old 01-01-2011, 04:08   #26
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Seems to me that the horizontal loads on a boom arise from the clew/outhaul fitting, not much from the slugs along the boom. Nowadays, many mains are loose footed, and in them, there are NO horizontal (or vertical for that matter) loads anywhere along the boom. It is very hard for me to see that moving the sheet attachment to the end of the boom can do anything but reduce bending loads.
Right on Jim
You are absolutely right. As long as the Main is loose footed, the lateral stress on the boom is strictly related to the difference bewteen the clew fitting of the sail and the main sheet position on the boom. Since the Main sail carries the most significant force when it is not reefed, the clew point being all the way back. Mounting the Main sheet in that same location underneath the boom should be the arrangement offering the less amount of stress on the boom.

On a reefed main sail situation, the clew point is being moved forward to about half boom length, which is where the most significant stress on the boom would occur... Then if my traveler is located on the arch, nothing would prevent me from moving the main sheet position using a second traveler on the boom... In theory.
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Old 01-01-2011, 04:25   #27
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Recently did a very similar exercise in moving the traveler from the center of the cockpit (where it was a real PITA) to a targa over which the end of the
boom would traverse. The targa was built to conform with the arc described by the boom....


Laidback
This is a FANTASTIC idea! This way I dont need a traveler and can position the blocks lower on each side of the arch. This will reduce the stress on the structure of the arch and even better, it will distribute the load on two points creating less distortion (providing the boom is out passed one of the block)... I will put this on the drawing board tomorrow to see the pros & cons of this great idea!

Thanks for all the great inputs guys!
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Old 01-01-2011, 09:35   #28
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The issue isn't how hard you have to pull in to sheet. The issue is that the middle of the boom is not supported when the wind picks up. This is due to the longer distance between attachment points. Just have a surveyor review your boom and in a worst case replace or strengthen it.
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Old 01-01-2011, 10:24   #29
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Quote:
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Right on Jim
You are absolutely right. As long as the Main is loose footed, the lateral stress on the boom is strictly related to the difference bewteen the clew fitting of the sail and the main sheet position on the boom. Since the Main sail carries the most significant force when it is not reefed, the clew point being all the way back. Mounting the Main sheet in that same location underneath the boom should be the arrangement offering the less amount of stress on the boom.

On a reefed main sail situation, the clew point is being moved forward to about half boom length, which is where the most significant stress on the boom would occur... Then if my traveler is located on the arch, nothing would prevent me from moving the main sheet position using a second traveler on the boom... In theory.
Surely that only applies with in mast furling... or was that the OP... if so I'll retire from the discussion....
Normal reefing would not transfer in this manner... its more linear and only moves less than 1/4 the length of the boom
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Old 01-01-2011, 17:13   #30
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Hi, MBrault,
Just to show that the general idea of doing away with the mainsheet traveler is very practical and in use by boats that have huge main sails (this photo from the Volvo Ocean Race - Sunergy)
http://www.cowesonline.com/zonexml/s...y_id=1099;cp=0

The next photo shows The Bale Arm plate I made up for the end of the boom :-

The next is the stainless boom brake (simple design made with 8mm 316 round bar) welded and polished in less than an hour (if you want dimension drawing - shout)
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