The mercruiser manual states that I need 13 inches from the water line to the top of the riser. I measured 3 times and got 12.5 inches. I also have the 6 degrees of slope as well as the 4 inch drop from riser to exhaust tip. Will I have water ingestion problems not being at 13" or do I need a 3 inch spacer?

Is your tips level with the water or under water when sitting still?

i've done one conversion on a mid eighties chris 23 scorpion. only diff was the noise level. not a half a mph top speed increase. but, if you're gonna do it, first look under engine hatch and see if you have the 3 inches for the risers. i have a 27 Nova now. i am seriously considering scaring up a couple of y-pipes and putting the exh thru the props. especially these days with marine cops running around with sound level meters and ticket books...

Where ever you decide to drill - make a good job better by using thru-hull exhausts with a replaceable internal flappers and maybe externals mounted on vertical tips ..... If you have headers might have to make different arrangements . you can still use external flappers on headers ---------------------------
You do not want to ingest water into the cyclinders ...Even if the motors are not running there is a ever so small chance, at the very least the starters will give up the ghost or worse!!!!!!


Good luck


Jim

With what you stated here it won't be a problem at all . I have done some with less drop and never a problem in 13 years of running with STOCK power of coarse . If your tips are at water level or close flappers are a must . I have sacrificed some drop on some boats to gain distance off the water still never a problem . 1/2 inch won't make you or break you as long as the tips are above water .