AntonLargiader wrote:
I agree with everything you say, but you're not answering the question. I asked about the forces specifically between A and B caused by the jib halyard tension. Not the mast step reaction, not the mast support loads. Let me reword them so as to acknowledge the external forces:
1) Cleated at A: mast compression equals mast step reaction force
2) Aussie system: mast compression equals mast step reaction force plus 1/3 jib luff tension
3) Older system: mast compression equals mast step reaction force plus all of jib luff tension
Sound right? Or not?
Lets look at this another way. In a no wind situation (no reactive forces), without the main rigged, and assuming the main shrouds are not slack (rarely the case but not important for this discussion), you have the vertical component of four external force vectors acting on the mast. Three act downward- the forestay/halyard, both standard and Aussie, (includes the weight of the jib if rigged), and the two shrouds. You also have the weight of the mast but we don't need to include that for this discussion.
Those vectors are summed and applied to the mast
at the tang (A) regardless of whether the halyard is cleated at the tang, whether it is run through a block at the tang and cleated lower on the mast (B), or it is run through an Aussie halyard (multi-sheave block attached to the tang by a pigtail and a second multi-sheave block mounted lower on the mast) cleated at (B). The fourth vector supplied by the boat's structure acts upward through the mast step and
is and MUST be equal to the sum of the other three. The amount of downward force (mast compressive force) can be adjusted by tensioning the forestay/jib halyard. As you shorten the forestay/halyard the line (steel or fiber line) you will be attempting the stretch the halyard and bend the hull bows upward (through the bridle). The elastic modulus of the line and hulls will cause the tension to increase. It just doesn't matter where you terminate the forestay/jib halyard, at the tang (A) with a block, cleat, or Aussie pigtail, or at a cleat elsewhere else on the mast. The resultant total compressive force is transferred to the mast at the tang and acts on it between the tang and the foot/step. It doesn't matter how many sheaves you have, where on the forestay/jib halyard they are located, or where the end of line is cleated. Compression on the mast is the same in 1), 2), and 3).