will be 1.34 x the square root of waterline length in feet.
My IP 38 for example has a 33 ft waterline length, so my hull speed
works out to 7.7 kts. that is pretty much the max speed I could go, above that drag just goes crazy, takes huge amounts of power to exceed hull
speed by much.
I have found on my boat that about a kt off of hull
speed gives me a good economical cruise
, that last kt increases fuel consumption
by about 50% for me.
So find your hull speed, back off of that a bit and that should be cruise
speed, then you have to cruise for a good long time and then re-fill your tanks
to see what your fuel
I'm going to throw out a guess of about 3/4 to 1 gl an hour per engine
, and especially if you can tilt an engine
out of the water
it's likely that running one at a time will be your best MPG.
But I am not Cat owner so I can't speak from experience, but if nothing else cutting the running hours on my engines in half would be attractive to me.
Let me edit this a bit before others chime in.
Being a Cat due to your narrow hulls, I do not believe you are as constrained to hull speed as my fat ole mono hull is, but I think it is still a viable number for you to determine an economical cruise speed when motoring