This guy looks like he's got a pretty nice setup:

http://texasboatforums.com/viewtopic...=200&hilit=ls3

Sweet!!!!!

I was searching for some weight numbers,,
The 350 mag SB from mercury with iron block, heads, and Exhaust
on Mercury's website is i am guessing fully dressed at 885 lbs

From what i can find.. the LS3 SB
Aluminum block, Heads, composite intake, cast iron exhaust manifold..
set up for a car. weights in at 403 lbs..

to Dress this up for a marine engine.. and I would use a sport tube header... guessing to add 150 lbs
that's a 330 lbs saving on weight times 2 engines.. that is good.

However, on the Bam speed calculator ( given HP of the motor is not changed) it shows up as a 3 mph increase..

I scrapped a 305. Block-heads-intake. iirc it was 504 lbs. I think that 885 is transom assy, drive, prop etc.

Ya, Merc weights are generally for the full power package.

http://www.lsxtv.com/tech-stories/en...e-engine-dyno/. Just take a look at some LS3 testing.
Baseline and also a few cam
Options. These motors make plenty of torque down low. Just a factory LS3 461hp@5900 and 455@4800 tq. Down at 3000 rpm 378 tq already. This is stock.... First mild cam swap 500hp at 5900 488tq at 4800. And a very broad curve at that, no "dyno queen" spikes. Very useable broad power. Mix in a head swap with cam and a useable 600hp isn't a far stretch at all. All NA. I love these LS's..... Data don't lie

In my merc literature, a fully rigged bobtail 350 MAG MPI is 946lbs. New big block closed cooling 8.2L bobtail, is 990lbs, and 6.2L is 862lbs. Thats fully rigged with all accessories, coolers, etc, ready to bolt in and go. Not including the transom assembly, or outdrive.

Impressive for sure for its size. Almost as much power as a 454 old school engine with a set of AFR heads makes with a shelf crane cam...

http://www.airflowresearch.com/engin...-265cc-bbc.php

Right.... Strong little "under dog". Throw on a set of AFR's and a shelf cam on the LS.... Now we're well in built 496/502 territory no question for not much money... And lighter to boot.

If it was me i would pull some weight off the back of the boat like a previous poster suggested and see how the boat responds. Free and conclusive. On a light layup 2500-3000 lb single stern drive its not nessesary.but for this guy it does make sense it could put more wetted surface into play. Have to see a na ls dyno 100 over say a scorpion vortec based sbc on same dyno same exhaust in person to believe it myself .great motor but put wet cast on a 6.0 vett witb a auto and it may run high 14's