Have you looked into Small Step XPBD (introduced in the paper "Small Steps in Physics Simulation ")? It's the most efficient nonlinear dynamics integrator/solver I've come across, and luckily one of the simplest, too! (Keeping in mind that simplest nonlinear solver is a relative metric.) I've been able to simulate very stiff materials like bone by updating the sim at 6000 steps/second. The exact number of steps/second you'll need will depend on both the desired spatial resolution and physical accuracy of your sails, but I wouldn't be surprised if 6000 steps/second is more than sufficient. And computers are fast enough where you could simulate quite a few large, detailed sails at that rate in realtime.