What is with that 15% fixed bowsprit allowance?

Per Adam Farkas, Asst. Chief Handicapper:

Q. You introduced some kind of 15% allowance for centerline bowsprits. I do not understand how this works and is applied.

A. The idea is that for boats that have been converted to fixed bowsprit (and do not carry on board an articulating pole), a spinnaker pole / bow sprit no penalty band is provided to the extent of J+15% (JSPadj).

Any sprit that extends beyond this Adjusted JSP limit will be added to the girth of the spinnaker at the rate of 180%.

Eg: C&C 30 decides to install a fixed bowsprit.
J = 13.5

Scenario 1:

Girth = 24.3 (180%)

His Max Pole allowance (without penalty) is the greater of:
Girth /1.8  or 13.5 * 1.15 (JSPadj)
13.5 or 15.52

This means he can add a 2.02' centerline bowsprit and incur no penalty. So he does.

Bowsprit length = 15.52'

The math: If the pole is greater than the JSPadj. Then:
Adjusted Girth = Girth + (JSP - JSPadj)

In this case = Not applicable so Girth = 24.3'

Scenario 2:

Girth = 21.5 (160%)

His Max Pole allowance (without penalty) is the greater of:
Girth /1.8  or 13.5 * 1.15 (JSPadj)
11.94 or 15.52

JSPadj = 15.52

He decides to install a 4 (four) foot centerline sprit. So his Bowsprit length is now 17.5'

Adjusted Girth = Girth + (Bowsprit length - JSPadj)
21.5 + (17.5 - 15.52) = 23.48'

Note: that this does not apply to class standard sprits (J 105, J 109, etc.) as their SP is determined as per the standard class design configuration.

The reason:
To recognize the less efficient spinnaker performance when it cannot be squared back at broad sailing angles. This rule is to replace the previous arbitrary 6 s/m credit applied to all fixed sprit conversions.

This rule also facilitates that an undersized spinnaker area (based on the as designed class sail measurements) will enjoy the small sail area credit, and will only be penalized for an oversize non articulating (fixed) pole to the extent the pole length exceeds J+15%, or 1 ft, whichever is greater.  The excess pole length is added to the girth of the spinnaker at the rate of 1.8 times the excess beyond the ‘max pole’; the same way as the excess pole penalty is used in the articulating spinnaker pole arrangement.

Note however, that this does not mean that the no penalty girth or sail area is increased from the standard class value.