Mast Compression with Aussie Jib Halyard

Think of the jib halyard (at least the part running from the block to the cleat) as a bowstring.

With the conventional halyard, the bowstring has three times as much tension as it does with the Aussie halyard.

More tension in the halyard, more compression in the mast.

The mast is more likely to have an induced bend from the conventional halyard.

If you run a line down the center of the boat, then the port and starboard sides of a Hobie 16 are mirrored images except for where the jib halyard is tied off (the port side of the mast). The jib halyard now pulls the upper jib block (at the end of the pigtail) slightly more toward the port side. It also tends to impart a slight twist and bend in the mast (only in that section of the mast between the mast tang and the cheek block near the base of the mast). If the jib halyard was centered on the boat (by tying it at the forestay's lower chainplate or directly on the front of the mast) instead of on the port side of the mast, then the boat should sail identically whether the wind is from the left or right.

The real question should be, does pulling the upper jib block slightly to the port side and the additional twisting and bending forces in that section of the mast in favor of the port side really make a difference when sailing to the port or starboard? If they do, then as Matt and I pointed out, using the bow string analogy, the Aussie jib halyard reduces these forces by about 66%. If not, then I wouldn't see the benefit.

_________________
Tim
84 H16
82 H16
87 H14T
Tortola Sails: 115222
Blue Prism Sails: 88863
Clearwater, FL

I thought the compression issue was manifested in mast rotation. If you find your mast is rotated to the position desired for the tack you are not sailing then there is a good chance the mast has a bend in it which is causing it to twist and not rotate freely. I can say without reservation that the Aussie jib halyard reduces this tendency of the mast to take the wrong set. Therefore, somewhere there must be physics involved but I am only looking at the evidence.

Physics 101 plus Statics and Dynamics. I believe we are generally in agreement, except the mast is still being driven into the mast step (actually visa-versa)! There is no other place for the force to go! It just doesn't matter what is going on at the tang or Aussie blocks- it all boils down to mainly four vectors that MUST all balance each other. The vectors are (1) jib halyard/forestay (including Aussie rig and bridle), (2) L shroud, (3) R shroud, all being opposed by (4) the mast (from the step to the tang. ALL other forces (jib halyard from cleat to tang/Aussie block), etc. become one of those four.

If you don't believe it, put a tension gauge on the forestay/halyard just above the bridle and a compression gauge/load cell on the step under the foot of the mast. Tension the forestay to exactly the same reading, first with an Aussie rig or any combination of blocks you want, then do it with only one block. The compression reading at the step will be the same in both cases- "equal and opposite" (vector sum).

This is a little simplification, because there are a few other vectors involved, like gravity, wind, and main sheet (when tensioned), but ALL those get combined and act on the mast compressing it against the step. And, yes, there actually is some mast compression that is totally within the mast, but that is between the main halyard cleat + the boom downhaul cleat (both are a part of the same vector) and the mast head sheave since the beginning and end of the vectors are applied to the mast only.



Nope, the tension on the vertical part of the jib halyard is transferred via the jib block(s) (Aussie and non-Aussie) to the forestay/halyard. If you could raise the jib and cleat it at the mast, and also cleat it at the Aussie block or tang with tension still on the vertical section, it would apply compression to the mast between the cleats only, but since the upper end runs through a block it is still a dynamic force which is ultimately transferred to the forestay/halyard and therefore must be countered by upward force from the step to the tang.

Again, when you talk physics in a static system (nothing is moving through space) all vector forces MUST be balanced against each other or the same fixed reference which in our case is the boat. If you want I'll draw this up in SketchUp and post it here.

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Alan
'86 H16, Sail #89057

I normally don't pay much attention at the boat when rigged up as I rig it, go for a sail, get back to shore and unrig. This past weekend, we stopped at a beach for a break in the middle of the trip and the wind was coming straight from the shore, so left the boat rigged facing the wind. I noticed that back of the mast (where the sail track is) was rotated towards starboard and there was a bow in the mast with the concave part on port side.

I manually rotated the mast to the port side and the bow in the mast snapped the other way.

Now what I don't know if the mast was bowed because of tension on the jib itself or the halyard cleated onto the mast. I guess one way to find out would be to try to tie the jib halyard at the shackle on the bridle wires.

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Marc...
1978 Hobie 16 Keoke, sail# 36 84

Nope, the forestay/halyard + shrouds forces must be counteracted by the mast. It doesn't matter how you tension the forestay/halyard (single sheave or multi-sheave blocks) you are still increasing the tension on it and likewise the shrouds. If you do that you also increase the compressive force on the mast by the same amount between the tang and the foot/step.

The bowstring analogy would only apply if the vertical halyard was not a halyard and was fixed to the tang- not connected in any way to the forestay/halyard.

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Alan
'86 H16, Sail #89057

Agree.


Agree- all external forces to the mast system.


Jib halyard load between the mast tang and the jib cleat is also one of these forces - it has to be. If it wasn't, then the jib halyard cleat would be pulled up to the mast tang.


Disagree. The section of jib halyard running between the mast tang and the jib cleat imparts a compressive load on the mast that is independent of the compressive load from the jib luff. Again, this component of halyard tension DOES NOT drive the mast down into the mast step, but it creates a compressive load that is internal to the mast system.


Her's another way to look at it. Imagine your mast standing vertically. Your forestay is connected to a distant structure such that it extends horizontally forward. The shrouds extend horizontally aft (fixed to some other structure). The mast is held upright by these wires but they impart zero compressive load on the mast because they are all horizontal. Now, you tie off your jib halyard to the same structure that the forestay was connected to so that it extends horizontally (forestay disconnected). The jib halyard enters the block at the mast tang horizontally and then runs vertically down the mast to the cleat at the base of the mast. Once again, the horizontal wires (jib luff section and shrouds) impart zero compressive load on the mast to drive it into the step. However the vertical section of jib halyard imparts a compressvie load on the mast between the cleat and mast tang in exactly the same way that the bowstring compresses a bow.

It all comes down to internal vs. external forces. The section of jib halyard between the mast tang and the cleat imparts internal forces and the section between the mast tang and the bow tang imparts external forces to the mast system.



Go for it.


sm

?? Didn't you mean wouldn't be pulled up

Unscrew the cleat and it would be pulled up.





Whatever compressive load is created by the halyard IS NOT independent of the load from the jib luff- they are part of the same system. The load from the vertical halyard is determined by the number of sheaves. It can never be more the 1/2 the total applied by the forestay. There will be a slight amount of compression between the cleat and tang, sure but it is very small - something on the order of 1/2 X 1/3 = 1/6 of the tension on the forestay (or less if using an Aussie rig), and much, much less than the compression imparted by the forestay and shrouds, roughly 1/18, on the mast. With an Aussie rig you are reducing the already low cleat to tang compression but not doing anything to the main compressive force applied by the forestay and shrouds between the tang and the mast step.



In your analogy you have replaced one EXTERNAL diagonal force vector (forestay/halyard) with two EXTERNAL component vectors at 90 degrees that are one piece through a block- the resultant vector is exactly the same as a single, diagonal one- the diagonal forestay/halyard has horizontal and vertical components. The vertical component of that vector is applying a force downward on the mast. There are no "internal" forces unless the part is acted upon, if there were, the mast would fly apart when derigged. The mast resists deforming from external forces, because in a vector sense they are applied equal and opposite. You can't apply an EXTERNAL force to a object that isn't fixed without it moving unless you also apply an equal and opposite EXTERNAL force! The object can't just absorb the force- it transmits it to the other side! Push on a box and it moves unless friction or something pushes back.

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Alan
'86 H16, Sail #89057

Holy crap!


Mast compression aside its a nice tool for fine tuning rig tension. If you're working a 1:1 system before and in a range of two inches, that just became a range of six inches. That alone makes it worthwhile.

I agree, and guess that was the main reason for the Aussie rig. Much easier to pull and make small changes when on the water. It sure is a pain having all that extra halyard to stow once the jib is up though. I've tried a number of pockets and pouches and haven't found one I really like yet, although I like stowing it under the tramp.

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Alan
'86 H16, Sail #89057

Yes, but each component can be viewed as an individual load. I.e., if you had one wire that had it's ends fixed at the mast tang and at the bow tang and another wire that had its ends fixed at the mast tang and at the jib halyard cleat, the end result would be the same.



This is wrong. In the case of the stock system, the tension in the vertical section of jib halyard is equal to the tension in the forestay (jib luff) not 1/2 the tension. Since it is a continuous piece of wire, the tension must everywhere be equal. In the case of the Aussie system, the 3:1 reduction occurs on the jib luff so the vertical component of jib halyard is reduced to 1/3 the jib luff tension.

Again, when you look at the segments of line in a pulley system, each segment can be viewed as an individual, independent line.



This is totally wrong. There is no external vertical component in this experiment. Do the experiment (it can be easily done with a piece of tubing, a pulley, and a few lengths of line). The result is that there is ZERO downward force on the mast. The mast is compressed between the halyard tie-off point and the tang (pulley), but since there is no external downward force on the mast, the end result that there's no change in force on the mast step. The mast is compressed, but it is not being driven downward. In this experiment, the only way the mast would be driven downward would be if instead of tieing off the halyard on the mast, you tied it off to the front crossbar or to the ground.

sm

This is getting old, so will be my last post

No, it absolutely won't be!! In that situation, unlike lines through a block, tension on one can't be transferred to the other! As long as you have one or more blocks at the tang pigtail the load will be shared as "a system". The blocks determine how the load is shared between the halyard downhaul and the mast. I have an Aussie but the standard rig has at least one purchase. But that still doesn't remove or lessen any of the vertical forces on the mast and step. What you are not seeing is the tension between the cleat and tang has no effect on the force on the step






NO, NO, NO. You are trying to tell me if you have a single block hooked to a wall, and pull on one lead, the tension won't increase on the other and be absolutely the same!?!?!?!? Or if you tension the forestay/halyard you are not adding tension to the shrouds!?!?!?!? Come on, you know better than that!





From your earlier post I thought you partially understood. ALL the vertical forces acting on or within the mast are EXTERNAL except one (the aluminum's resistance to compression is internal)!!!! The EXTERNAL ones are those applied by gravity, the tension from the forestay/halyard, the tension from the shrouds, the boat pushing up on the foot of the mast, and when tensioned, the main sheet! All those including gravity are connected to the boat and all are balanced by the force of the mast step pushing back- it is a "system" at equilibrium. If it weren't, something would be moving.

I misunderstood the setup of your "experiment", but that doesn't matter since it does not apply to the Hobie and therefore moot. Lets get back to the Hobie- the forestay/halyard isn't horizontal and imparts both vertical and horizontal forces (right angle vector components of the resultant angled vector.) Both must be balanced with equal and opposite forces for the system to be in equilibrium (which it is!!)- at the mast head, the horizontal vector components of the shrouds and forestay/halyard balance each other and the vertical components of the forestay/halyard and shrouds are balanced by the mast. At the foot of the mast, the horizontal forces are countered by the boat resisting compression between the chain plates and the bridle. The vertical force components of the forestay/halyard and shrouds are balanced by the foot of the mast (transmitted through the frame and hulls). There is a bending moment applied to the boat, but for this discussion we don't need to address it. It becomes part of the tension applied to the forestay/halyard and shrouds, anyway. Every vector force must be opposed by an equal amount of force! It is that simple! An external force can't just be absorbed by an object which in essence is what you are saying happens. It is contrary to physics!

I think maybe what you are missing is that while the compression between the tang and the cleat can be adjusted by changing the amount of purchase (Aussie) the total compression on the mast between tang and the foot does not change. It is always the vector sum of the forestay/halyard and shrouds.

A detailed vector drawing will show you that. Maybe you can get some blocks and weights so you can model it or talk it over with someone who can explain the vectors and Statics and Dynamics to you in a different way.

_________________
Alan
'86 H16, Sail #89057

No, actually that's what I've been saying all along.


and




Again, you're either not reading my posts or you're trying to put words in my mouth. I said exactly the opposite - I said that the tension on the jib halyard cable (stock system) must everywhere be equal....





Actually, the "expirment" is totally valid in that it shows that even if we eliminate all externally applied vertical forces on the mast (i.e. compression between the mast tang and the mast step due to shrouds, mainsheet, forestay) we still have an independent compression force acting on the mast that is the result of jib halyard tension. This compression force exists between the upper jib halyard pulley and the jib halyard cleat and has no effect on the load at the mast step.



Again, it's the compession between the mast tang and the jib halyard CLEAT that is being changed. The resultant force at the mast step does not change and I never said it did. However, the force between the cleat and the mast tang acts to compress the mast (in addition to all the other forces listed internal or external). In the case of the stock system, this force is equal in magnitude to the force along the jib luff, for the aussie system, it is 1/3 the the jib luff tension.

Yet another way to look at it. If instead of the jib halyard cleat being placed at the bottom of the mast, you placed it at the top of the mast. Now there would be a compressive force on the mast between the top of the mast and the mast tang. There would be no change in force at the mast step, but there would be an increased compressive force between the mast tang and the cleat. Now move the cleat back to it's normal position at the bottom of the mast - this load does not disappear, it is still causing mast compression.




Finally, something you've said that I agree with...

sm

I just couldn't resist- you continue to miss the point. I'll try one more time!



Right, but irrelevant, because it is subsumed by the total compressive force applied by the forestay/halyard and shrouds



Wrong, for both examples! They are NOT internal forces. Bad analogy also, the vertical jib halyard shares its load with the forestay/halyard since they are one continuous line through a block or blocks and, unlike the main halyard, one end is connected to a structure external to the mast (bridle and hulls)!!!



Again since it is one continuous line, the halyard tension on the tang is shared by the forestay/halyard and the vertical part of the halyard- each one provides generally 1/2 the total load force in a standard rig.

I won't waste my time commenting on your faulty "experiment."



Could have fooled me. The change in the force between the tang and cleat (between standard and Aussie rigs) does NOT change the overall compressive force in the mast, not anywhere in the mast.



See my previous comment above. What you are missing is that the location of the cleat doesn't matter, the tension on the vertical halyard doesn't matter, it is all subsumed by the overall compression of the mast applied at the tang by the (A) forestay/halyard (and vertical halyard) and both (B + C) shrouds and opposed by (D) force up through the mast step.

(in a vector sense) (A = total force applied by the forestay/halyard + vertical section of halyard through the block(s))

A + B + C = D

There are no other forces being applied to or affecting mast compression (other than the previously mentioned gravity, main sheet, wind).

Try this- put a load cell between the foot and step, tension the jib, and cleat it normally- take a reading. Now un-cleat the vertical halyard while being very careful that it doesn't slip through the block (standard) or blocks (Aussie), then tie it to the bridle, tie it to the tramp frame, tie it to a rudder gudgeon, tie it to the tang, tie it to the mast head, heck hold it in your hand!!!. As long as the halyard doesn't slip through the block(s) the reading on the load cell won't change!!!!! Therefore the compressive load on the mast, anywhere on the mast, is not changing despite what you think!!!! Any compressive force applied to the mast by the vertical halyard is subsumed by the main compressive forces which are constant! If you cut the mast in half just below the tang and insert another load cell the reading there would be the same as the reading from the one under the foot, again regardless of where the vertical halyard is cleated!!

Just the facts, nothing but the facts.

_________________
Alan
'86 H16, Sail #89057

It's about time to get the Ouija Board out and talk to Isaac to resolve this.

_________________
Marc...
1978 Hobie 16 Keoke, sail# 36 84