I custom built a bracket similar to this. Not at the
boat now and I don't have a
photo, but I'll try to describe it. Instead of the
curved arm coming off the pivot, I put an arm about 2" wide going straight down ending about level with the bottom of the
alternator. The curved arm goes from there to the bottom bolt on the
alternator. That holds the tension just fine and the belt does not slip. I first made it just like the Universal bracket but as the OP says it wouldn't hold tension. The rest of the bracket is just like the one shown here. I couldn't use the Universal bracket because my manifold is a different style and the dimensions are not the same. You should be able to modify the Universal bracket like this.
As Roland mentioned above I ended up using a turnbuckle instead of the arm. Holds tension better, doesn't require you to overtighten the bolts and is real quick to adjust. Didn't use a
marine turnbuckle, just an
aluminum and stainless one from Lowe's. Cost about $4.00