Adding Side Stringer on Shell Plating

[petlily]

In what occasion should a side stringer be added to the shell plating says, for a small steel tug abt 26m LOA x 8.0m B x 3.6m D? Does the inclusion of side stringer governed by the depth or due to the operational requirement alone? Suppose that we add the side stringer on the shell plating, is that mean that we can reduce the scantling of the side transverse and web frame as well (for a transverse framing system)?

I have checked numerous class rules for small vessels regarding this side stringer issues but none of them clearly stated when should a vessel requires a side stringer. Can anyone shed light on this please?. Thank you.