Mike Gillum wrote:Greenlake,
Pretty sure (not 100% but close to it) that the windward chainplate isn't affecting overall sail trim or performance as I don't see any movement in things like the Jib Sheet or Barber Hauler Block rising or falling relative to the aft top edge of the Cuddy Cabin where I would easily pick-up any movement if the windward chainplate was moving effectively "whipping" the Mast.
Keep in mind that the only chainplate under load going upwind is the windward one as the leeward shroud is usually loose and rolling around even under the high rig loads we run as the DS Mast will be bowed both towards the bow and to windward under most circumstances decreasing the true distance between the hounds and leeward chainplate thereby creating the loose leeward shroud.
Some people may think that the chainplates are being pulled upward creating the loose leeward shroud though I would suspect that the forestay stem fitting has a far greater chance of being pulled upward vertically because of the greater distance (re: lever arm) between the bow and Mast versus the chainplate and Mast.
100% sure that I don't want to know just how much #2772 is bending!
OK, let's see whether I can put this into my own words in a coherent way. That's the best test I know to make sure I've reached the same understanding.
Under sail, upwind, the sails impart a force to leeward on both mast and forestay (and the stemhead fitting, but I think we can ignore at least the leward component of that contribution). There are also leeward forces on the sheets, but we don't worry about them, either. So, the mast is pulled downwind, the sideways pull on the forestay increases its tension, pulling the mast forward (while the mainsail, as well as the swept-back shrouds will pull a bit backwards). Resisting this downing pull is the upwind shroud, which elongates a little, allowing the mast to give, which de-tensions the leeward shroud (and fore-stay).
If the hull near the cuddy opening acts like your solo cup, the chainplate comes a bit in (and a bit up). For the same tension on the upwind shroud, the mast would give a bit more, because the effect of the moving chainplate is the same as more elongation on the shroud. Essentially, the shroud acts like it's softer. If the boat heels, the buoyancy would concentrate under the downwind chainplate and the boat would deform on that side as well, if it's sailed flat, there's a more asymmetric deformation on the hull. Do I have that right so far?
As you mention, the forestay should be pulled up a bit as well, but I think lever arm alone is not a good consideration in predicting the magnitude of the effect. In the forward area you have the full-strength hull-laminate form a step V, which puts that laminate at nearly parallel to the plane in which the bow would have to bend. The equivalent example to your Solo cup would be the business card, folded length wise into a V and stapled on one end. That structure resists bending very effectively - any bending would be concentrated in an area near the "open" end. The foredeck being flat also may be stronger on compression than the curved cuddy-opening. For all these reasons, I would expect the hull to be naturally much "stiffer" in that direction (at least up to the mast step).
Now your stiffening the center of the cuddy has the effect of making the upwind shrouds stiffer, by maintaining the location of the chainplate. Because the shrouds are swept back, the upwind shroud counteracts the forestay pull better when it's stiffer, so mast-bend at the shroud attachment point should become less (and forestay sag should be less).
All things being equal, the forces on the shroud and forestay should be nearly the same as before, because their overall magnitude and direction is governed by the aerodynamic effect of wind on sails. Any change would have to come from better aerodynamic efficiency (effectively better trim) available with less forestay sag and different mast bend.
Is that about the size of it?
Now for the jib, I can get my head around that it might work more efficiently with less forestay sag. But how does the main benefit?