I have never been convinced or the need for a backstay adjuster on a masthead rig. I have owned two boats with this setup and I rarely used it on either of them. I have friends who claim that they can alter the sail shapes with this setup but the results have never been obvious to me. The only difference that I can see is that you can alter the Genoa luff sag. In theory, I think the luff sag should be consistent through most sailing conditions. In practice, I have never noticed that it made much difference on my boats.
I have a fractional rigged dinghy (Wayfarer) and altering the jib luff tension (it has a wire luff), which introduces mast bend the same way an adjustable backstay would, produces dramatic changes in mainsail shape. It does this by bending the mast around the forestay attachment point which acts as a fulcrum. You can flatten that main sail dramatically doing this.
If you were so inclined, you could make one by shortening the backstay and putting a block and tackle similar to a boom vang in the resulting space. I would add a wire attachment from the shortened backstay to the transom just to stop the mast falling down if the line pops out of a cam cleat.
I am in the process of buying a 240. Just have the survey left to finish it up. Let me know how the adjuster works if you go ahead with it. Maybe you will change my mind about them.
Cheers
david
