cary 50 offshore express

Konrad and Cummins already have a tie-up together. Maybe worth talking to your Cummins area rep?

How fast do you want to go and how much HP will the drives handle? I'd go with something like MAN v8 common rails due to weight and 900hp.....Cary did a few boats with twin 8v92's I believe back in the day.

I have heard good things about the FPT diesel engines, they have a kg/hp of 1.1-1.4.

The new FB Design (Fabio Buzzi) vessels are equipped with FPT engines.

http://www.fptindustrial.com/products/en-UK/marine/pleasure/Pages/N67_570.aspx

Hi. Perhaps the new Volvo Penta D8-600 (840 kg dry/Bobtail)? Have no idea if they will fit/match the Konrad though. I know Fountain offered the 38 or 42 with 480 hp Cummins and Arnesons but it seemed somewhat under motorized to me - just from the top of my head really since You wanted "ideas" - good luck!

The 48' Express cruiser was built with 3 450/480 HP Yanmars and Cummins and flew.....64 mph top end, 1 mpg at 40 knots.

Twin 250HP won't even get that boat on plane. What you need is a light/high rpm diesel with your gear ratio. I'd be looking at the highest HP Yanmars the drives will handle. They have the powerband and weight to HP ratio you need with those drives. Honestly I would've gone to heavier drives.

I went for a ride on a 50'/quad Cary many years ago when I was running yachts in FL. I still remember that with the drives tucked in & tabs down, the boat struggled very hard to get on plane & the bow was pointed skyward.
When I first started reading this thread, I wondered how the boat's running attitude was now that you removed all of the stern weight. I guess removing the extra ballast under the galley helped. Even with the ballast in place, the quad engine boats required 15-20% trim tabs down to maintain a somewhat normal ride, That much tab obviously slowed the boats several knots. I don't know how much money you want to start tossing into this but reworking the bottom is a big can of worms in my opinion. You're going to get into some serious money & I don't think you will see the drastic results you might be thinking of. Your money would be much better spent on the new diesels and drives. You are very limited with the drives you have & with the right setup, the bottom will be perfect just the way it is.
If it were me, I'd be looking for a minimum of 650hp per side but if you're doing all of this work, you would be much better off in the 800hp range. Pay attention to the torque, not just the HP numbers. Big torque can spin some big props! It's a heavy boat so either way, I'd go with Arneson surface drives http://new.arneson-industries.com/?product=1 Your drives right now are limiting everything & if you upgrade just the engines, I know you will be wishing you upgraded the drives & went to bigger engines.
With the right set up (big diesels, Arneson drives), you will have one hell of a nice, reliable, stable and fast boat! Good luck & keep us posted on your progress!

I agree with CTDave, reworking the bottom is a can of worms. I also agree the boat needs new drives and a lot more HP. Problem with Arnesons is you don't have a lot of lift from them, so need speed and big props to get and stay on plane.