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PostPosted: Thu Jul 07, 2011 8:17 am 
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I do not understand how the use of the Aussie Jib Halyard results in a 66% reduction in mast compression. Can anyone explain?

Thanks!


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PostPosted: Thu Jul 07, 2011 1:57 pm 
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It's a 3:1 line. If there is 90lbs pulling on the jib, then there is only 30lbs on the halyard running down the mast.


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PostPosted: Thu Jul 07, 2011 2:18 pm 
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Less tension from the mast tang(or upper block) to the halyard cleat. Same tension from the mast tang to the briddle.

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PostPosted: Thu Jul 07, 2011 7:28 pm 
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That is not quite how it happens. If you do the free body diagram for the set up, you are getting 1/3 of the total load with the halyard down the mast and a portion of the load going down the jib luff (cosine of the angle to the mast). So someone needs to do the geometry again. I haven't done it so it just may be the quoted 66%, but I doubt it.


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PostPosted: Thu Jul 07, 2011 9:59 pm 
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I ain't buying it, its hogwash. It doesn't matter whether you have a 1:1, 2:1, 4: 1, 6:1, block assembly on your Aussie rig at the tang, or no Aussie rig at all. The force and angle (a vector) on the jib halyard between the bridle and tang doesn't change no matter how you rig it. It is only changed by how much you tighten it when you raise the jib or by wind forces. All the Aussie rig does is reduce the force required to raise the jib (the force on the downhaul part of the halyard.) It DOES NOT reduce the amount of force transmitted to the mast at the tang or compression on the mast. It can't, it ain't possible from an engineering standpoint Since the mast doesn't shrink under pressure the force it applies upward MUST equal and balance the force applied by the jib halyard (and the shrouds)!!!! The downhaul line or the location of the cleat doesn't play in this at all either! It would be no different if once the jib is raised and tensioned the downhaul line was cleated at the tang!

How did this fiction start anyway? Was it a marketing ploy by someone who doesn't know statics and dynamics?

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PostPosted: Thu Jul 07, 2011 11:00 pm 
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I have to agree with Alan. Now that I have done the freebody, it looks pretty obvious. It's a salesman thing. They are always telling the customer, "yeah, we can do that." And then they talk to the engineer and learn that they have promised something that physics doesn't allow. Happens all the time. Reality has nothing to do with selling something.


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PostPosted: Fri Jul 08, 2011 4:54 am 
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Location: Saint John, NB Canada sailing on Washademoak Lake
I do agree that the tension between the tang and the bridle does not change, but if you have to use less force to pull on the halyard, that has to put less pressure between the mast tang and the halyard cleat (IE down the mast).

If there is less force on the halyard from the tang to the cleat though, doesn't that put less compression on the mast? Forget the halyard rig for a minute. Just put a ratchet strap between the tang and the cleat and tighten the hell out of it. Does that create any negative effects? I can't see how it would create any negative effect. If however there was a negative effect, then that opens up potential benefits with the aussie rig as there will be less tension on the mast than the standard rig.

PS: It's more about physics than geometry.

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PostPosted: Fri Jul 08, 2011 6:18 am 
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Kind of an interesting problem, and it's been a long time since I've done much statics, so I'm not 100% sure on this one. However, I think this is what's going on.
In the stock setup, there is one pulley for the jib halyard up at the hounds. Therefore the tension on the halyard is constant everywhere (negating friction). So the tension along the luff equals the tension running vertically down the mast to the cleat. Assume this is 600lb. The mast compression resulting from halyard tension is then the sum of these two component forces acting along the axis of the mast. So it would be 600lb (vertical component of halyard running down to the cleat) + 600lb x cos a (tension along luff wire) where a = the angle between the jib luff and the mast. If we assume a=30 degrees then the mast compression due to the stock jib halyard equals 600lb + 519lb = 1119lb.

With the Aussie setup, the tension along the luff remains the same, however the tension running down the mast is reduced to 1/3 the luff tension due to the 3:1 purchase up at the hounds. Therefore given the same 600lb luff tension the resulting mast compression due to the jib halyard is 200lb + 519lb = 719lb or a 35% reduction in mast compression.

Keep in mind however that we are only looking at mast compression induced by jib halyard tension. In reality, there are several other factors that influece mast compression, namely downhaul tension, shroud loading, and trapeze loading. These are all going to add hundreds of additional pounds of compression on the mast. So while there is a reduction in mast compression from using the aussie system, it's pretty minimal - certainly much less than the 66% claimed by the advertising.

sm

56kz2slow wrote:
PS: It's more about physics than geometry.


PPS: Physics, geometry, and trigonometry are all interrelated. You can't solve a physics problem without having a solid understanding of geometry and trig.


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PostPosted: Fri Jul 08, 2011 6:52 am 
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srm wrote:
PPS: Physics, geometry, and trigonometry are all interrelated. You can't solve a physics problem without having a solid understanding of geometry and trig.

Fair enough.

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PostPosted: Fri Jul 08, 2011 7:04 am 
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I ran for years with the stock assembly on my 1983 H16 and more recently with the aussie setup on my 2008 H16.

Ignoring all the Trig, Statics & Dynamics, and Physics courses I took in college - The aussie rig is just a pain.

It takes longer to rig and you have to be so careful it is not twisted, and all that line to wrap/stow. Not to mention the horror stories on this site about the blocks failing. If there is a performance difference, I don't see it.

Were is the advantage for a day sailor like me?

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PostPosted: Fri Jul 08, 2011 11:24 am 
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The forces applied to the mast tang and mast base are basically the same with either the Aussie and original setup. What is different is the bending force along the side of the mast from the mast tang to the lower side jib halyard cleat (near the bottom of the mast) due to the location of the jib block or Aussie block . This is because the forestay pigtail extends the upper jib/Aussie blocks several inches in front of the mast so that the jib halyard, forestay pigtail and mast form a triangle. The jib halyard is now trying to bend the mast away from it (like the string on a bow and arrow). The Aussie reduces this bending force by 66% by reducing the tension in the jib halyard (similiar to less tension in a bow's string). On the older system you can also reduce this bending force (not as much as 66%) by using a jib halyard grip since it drastically shortens two legs of the triangle.

Professional Engineer

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PostPosted: Fri Jul 08, 2011 12:20 pm 
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I agree Tim H16 for the most part. The reduced tension in the section of halyard running down the mast as well as having the upper blocks located closer to the mast tang are both features of the Aussie system that help to prevent the mast from wanting to counter-rotate. They also help keep the jib battens from getting hung up. So overall, there is a performance improvement over the stock system.

However, the original question was how does the Aussie system reduce mast compression by 66%?

The answer is, it doesn't. The Aussie system reduces the mast compression that is induced by the vertical section of jib halyard by 66% (the section of halyard running between the mast tang and the cheek block). But the mainsheet, downhaul, shrouds, and trapeze also induce very significant compressive loads on the mast that are totally unaffected by the Aussie system. So the TOTAL compressive load on the mast when using the Aussie system is only reduced by a small fraction of the advertised 66%.

Another thing to consider. When your mast bends (from mainsheet, downhaul load, etc) the distance between the mast tang and the jib halyard cheek block gets closer together. This effectively loosens your jib halyard and causes the mast to rake back. On the stock system, the mast will rake back three times as far as it would with the Aussie system, so using the Aussie system would give you a more consistent mast rake setting.

sm


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PostPosted: Fri Jul 08, 2011 2:03 pm 
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If you want to remove all "mast compression" caused by the jib halyard (old or Aussie setups), then after tensioning the jib, you would either have to tie the jib halyard to the mast tang or to the chainplate (at the base of the forestay).

You still would have all of the other compression forces on the mast that srm listed.

Personally, I don't have an Aussie system or think it is worth the upgrade due to the cost and it is more of a hassle.

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PostPosted: Sat Jul 09, 2011 2:29 pm 
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Remember, once you cleat the halyard, the section running up to the tang is no longer in the picture because the force is transferred to the block at the tang. As the expression goes, you can't push a rope! You have a mast pushing up against the forestay/halyard and the two shrouds. It is simple as that.

The bottom, line, as I said above, there is absolutely NO way to reduce mast compression unless you have a sky hook! :lol:

For those having a hard time following, try this analogy:

Lets look at the problem upside down- attach a 10lb weight to the middle of a rope whose ends are both tied to a beam, say 5' apart making a 'V'. If the weight is in the middle, that 10lbs is distributed so each half of the line is supporting 5 lbs. You can replace either half with multi-sheave blocks, but as long as each half is cleated and are the same length, each half will still be supporting 5 lbs and the total supported will still be 10 lbs.

It our case the mast is applying and upward force against the jib halyard and shrouds. No amount of blocks or Aussie rigs will change the compressive force on the mast between the tang and the foot of the mast.

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'86 H16, Sail #89057


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PostPosted: Sat Jul 09, 2011 6:27 pm 
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aschaffter wrote:
Remember, once you cleat the halyard, the section running up to the tang is no longer in the picture because the force is transferred to the block at the tang.


Sorry, but I've got to disagree with that one. This is physics 101 - equal and opposite forces. The section of jib halyard between the block up at the mast tang and the cleat is under tension. As a result, the mast has to resist this tension by exerting an equal and opposite force which results in a compressive load on the mast. Note that this force is not driving the mast into the mast step (on the crossbar), but the mast is still compressed from the jib halyard load.

aschaffter wrote:
It our case the mast is applying and upward force against the jib halyard and shrouds. No amount of blocks or Aussie rigs will change the compressive force on the mast between the tang and the foot of the mast.


You're only looking at the external forces applied to the mast. The tension on the vertical part of the jib halyard is an internal force on the mast in the same way that mainsail downhaul load is an internal force. You crank on the downhaul and the mast is compressed from the load between the mast head and the downhaul cleat. The load isn't driving the mast down onto the mast step, but it is still compressing the mast. The load from the jib halyard acts in the same way.

sm


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