The quest for speed
 

Hi!

This is maybe a impossible question. But its holiday, so I try anyway :cool-smiley-027:

I think I have run my engines too low. I can not use powerlifts, due to the overall balance of the boat (an extra 4-5 inches setback will be too much on my narrow boat).

I have two Merc 2.5 EFI SS engines with CLE lower units on my Phantom 25. Merc Hi Performance reccommends an engine mounting height of 27,75-28 inches with twins running speeds 85-90 mph. This translates on my engines running the propshafts approx. 0,6-0,8 inches below the bottom (with no setback).

I have the propshafts mounted 1,2 inches below. The engines also have 2 inches built in setback, so the effective propshaft height should be lower, approx. 1,45 inches.

If I rise the engines one hole, which is approx. 0,67 inches, the effective propshaft height will be approx. 0,78 inches (measured 0,54 inches below).

I know that only testing will tell, but how much faster do you think the boat will run with the engines 0,67 inches higher (if I can hold the bow up)?

The boat runs 87 mph spinning 25 pitch Hydromotive Quad X-O/B props at 7.400 rpms (1:1,87 gears).

Cheeers! Toffen G