solver [This iterative problem doesn't always work with Solver; can it be solved with VBA]

If my first impression is correct, h is a quadratic function of WL2 (h=a*WL2^2+b*WL2+c). With a little algebra, I would expect you to be able to arrange the equation in that form, then you can use the quadratic formula (much to the joy of our algebra teachers, to whom we swore we would never use the quadratic formula in real life) to find the two roots for WL2 and decide which one is the correct "real" value for the problem at hand. With the correct use of relative and absolute references, you should only need to enter these formulas into Excel once, then copy them down. An approach like this will eliminate the need to use Solver and the potential for Solver's numeric algorithms to diverge like that.

If you need a quick refresher on the quadratic formula: http://www.purplemath.com/modules/quadform.htm

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MrShorty, there is no square term so it is not a quadratic equation.

When you multiply w by the second derivative (w''), you will end up with a w^2 term (among others). If I followed the algebra correctly:

wi''=wi'-w(i-1)'
wi'=wi-w(i-1)
w3''=w3'-w2'=w3-w2-(w2-w1)=w3-2w2+w1
w3*w3''=w3^2-2*w2*w3+w1*w3

Plug that back into the original formula and work through the rest of the algebra, and you should end up with something like h=a*w3^2+b*w3+c, then solve the quadratic.