phughes69

I just finished redoing my parents 1984 242 LS. I installed a 2006 300 hp 350 MPI Magnum/ Alpha Gen II into it. The existing prop is a 3 blade 21" pitch Mercury Vengeance prop. I am seeing 46 MPH @ 4800 rpm on GPS with 50 gallons of fuel, me and one other person in calm conditions. Does this speed seem reasonable to others that have a similar boat.

sprink58

With that prop you are about as good as it gets. You might see 50 with a 19" Mirage Plus and trimmed out to the max.

Mr Maine

46mph at 4800 rpms is about 30% slip which is quite high. I think the diameter of that prop is 13 3/4 which I think is too small for that heavy boat. try more diameter like a 19 or 21 mirage (15 1/4 and 14 3/4" diameters I think) like Sprink said.
I think that boat would run 46 with an old style 350 260horse. With a 300 horse 350 I would think you would be 50-52 but maybe 1 person and half fuel. Thats my guess.

tpenfield

I had a 242SS with 330 hp 7.4 and saw 47-48 mph. LS should be a bit quicker, but 50 mph is probably your max.

The 454 Mag 'supposedly' was a 60 mph boat, but I would think maybe mid-high 50's