--- In firstname.lastname@example.org
, "sweetthug" <sedirisinghe@...> wrote:
>With sheet tension on the balanced lug being relatively low (due to
> the boom being somewhat forward of the mast),
Due to the lever arm, effective load, and fulcrum... and, is it boom forward or mast aft? A boom of similar length might be in the very same position with the sail rigged differently.
> Newton would say that the tension on the sheet (before running
> through any blocks or anything)
Newton (or is it Euler?, etc?) might remind that it still must run through angles! At the attachment point of the rigid boat the tension or instantaneous (static) resultant force vector aligned with the sheet (can't much push or slide a flexible line) can be broken down into various sized force components, or vectors, pulling in varying directions in three dimensions.
> would be equal to the proportion of the force which is pulling the
> boat from the sheet/boat connection.
Sheet load at the "sheet/boat connection" is transferred to various load pathways through the hull, but "the force which is pulling the boat from the sheet/boat connection" actually is fully "the tension on the sheet (before running through any blocks or anything)" and thus is not proportional.
> So the rest of the torque applied by the sail will be applied to
> the mast.
The wind fills the sail. The sail cloth is in tension. At some point on the sail all the instantaneous aerodynamic forces across both sides of the sail that cause tension in the fabric are in cumulative balance - the centre of effort. The cumulative force at the coe acts in a direction normal to the tangent of sail curvature there. The cumulative or net sail force vector can be broken down into component vectors of sideways and drive, etc. The tensile force transferred from the coe to the anchorage points of the sail (ie. discounting any imposed strains, eg. outhaul) is proportional to their distance from the sail coe. The tensile or otherwise (spar compression, etc) force transferred from the sail rig to its attachment points on the boat is likewise proportional. The relative loads and load pathways from those attachment points to the cumulative point of balance between the coe and clr determines "torque" or rig dependent yawing force contributing to weather helm. The alteration in load pathways when a boom (or anything serving more or less similarly) is fitted to loose-footed sails like a D'lug or Sprits'l accounts for decreased weather helm and improved off wind performance more so than any change in sail force transferred at the clew.
> That's why I was asking if the CE location (on the lateral plane)
> could be approximated just behind the mast.
It's comparatively closer to the mast on B'lug rigs than on some other low aspect quadrilateral rigs as the mast is further aft relative to the sail.
> Suzanne, I'm not sure why you said i'm making assumptions about the
> underwater profile?
You mentioned "torque". What resists it?
> I was basically trying to focus on what's above the water.
What's above and below the water plane is not only connected physically it all interacts dynamically. That's why the B'lug sail and board on a small una rigged skiff are where they are.
> I suppose windage on the hull would move the CE slightly as heeling
> angles change, but i'm still a little confused with your response.
Heeling might adversly affect drive and leeway, so vmg, and "torque" etc.