The CG formula to calculate the HP rating (remember the regs apply to boats under 20ft in length- the mfgs determine their own limits on 20+ boats but am sure their ins liability carriers undoubtedly have a big say) the formula is:
factor = width(in feet) x length then this “factor” determines the formula- to use- if the factor is more than 52.5, which is any boat more than 4′ beam and 13′ plus length; then formula is:
(you can raise hp to next multiple of 5)
If boat has remote steer and at least 20″ transom, (2 x Factor – 90 = HP) Exmple 18.3″
If no remote steer OR transom less than 20in,
(0.8 x Factor – 25 = HP)
Width is maximum transom width in feet excluding handles, attachments and extensions, If boat does not have a full transom, th transom width is the widest beam in the aftermost quarter length of the boat.
Example- Remote steer- 96″ width(8′) 18′ 3″ length
8 x 18.4 = 147.2 factor — (2 x 147.2 -90 = 204.4hp) or rounded up to 210 HP but no 210 HP motors so set it at 200.
Tiller- Same size boat- (0.8 x 147.2 -25 = 92.76hp) or rounded up to 95hp no 95hp motors so 90 rating.