Look around on the Crusader site for your engine's data sheet. I believe the max recommended speed for that engine is 4600.
So as Brownie said you will need to find the gear ratio of your v-drives then run the numbers and you'll have your theoretical top speed. I'm not sure what is typical for slip ratio on those props.

Magnum 35 side fact, The 35 Apache uses a Magnum hull. Bob Saccenti didn't even know it at the time. Thanks for that one Brownie. Read all about it,

http://www.***************.com/brown...es-like-topsy/

I had no idea they were 12' wide. I guess 16k is possible then and speeds are probably more like 50 max at WOT.

I think Ghost has it right or at least very close. Back in the early/mid 70's when Magnum was an Apeco company I knew the then president of the marine division. He had a 35 in Burnham Harbor in Chicago. I kind of remember it running in the mid/high 30's. He then got a 38' flybridge with a pair of 482's. I think that one was a bit faster and did reach 38.

Griff, I think they weighed even more than that. And I don't remember any of them getting close to 50. Think "floating Sherman tank"

Yes, HP will dictate the limit of top speed. Getting the pitch/gears right will allow making use of that HP to reach the limit. (AKA: "you can't work magic, all you can do is exploit your HP well. Or you can get it wrong and waste more of it").

Inboards are so much less efficient than sterndrives that I think many of us would naturally overestimate speed of a Magnum 35. Also, it's heavy and beamy, as has been said. Perhaps a not-unreasonable comparison might be a 35' Tiara. I think one of those would reach 33-36 mph with twin 8.1 Crusaders. I guessed 38 based on some rough calculations in my head based on a few memories/boats.

But as a data point to consider, my Nova Marine 24 just reached 50mph, with twin v-drive inboards and fresh 300HP small blocks, at 8'3" wide and closer to 5000 lbs. That was with the gearing and pitch set up right. (With the gearing very wrong, it would reach about 46.) It was a great beast and it felt like it'd go through anything. The 35 must feel like that plus a whole lot more.

I'm hopeful for 38 for this Magnum but honestly, that was at the high end of my SWAG range. I wouldn't be totally shocked if it were as low as 33. Such a cool boat--I've always wanted one. I wonder what one would do with, say, a pair of Konrad sterndrives or XRs and maybe 500-600 HP/side. I'm sure it would like the efficiency gains. I have no idea if it would completely throw off the COG and screw up the boat. But if it did well (and one had a BIG lift to keep the running gear out of the water) it feels like it might be pretty cool.

(With apologies to AB, as I have heard some references to his view of sterndrives. Something like "spawn of the Devil" comes to mind. )

Another consideration, depending on your budget, would be conversion to Arneson drives. They are expensive but Magnum has done many conversions to these on their v-drive and straight inboard boats. They claimed as much as 15% increase in speed and fuel efficiency. Some do come up for sale occasionally on Offshore Only. It would make the most of what horsepower you have.
If you ever re-power go for cubic inches. Higher torque motors always help.

Good luck!

I was going to mention Arnesons too but you beat me to it. Years ago a friend had one of these that he converted to Arnesons and diesels I believe but I can't remember details on the motors. It did run much better with that combo but that was pre gps. Sorry not much help...
Pretty boat, I'm right across the Sound from you more or less.

My brother has a 28' Donzi Sportsman that is a 28' x 101/2' beam with twin 454 Crusader engines & vee drives, it's a heavy hull around 13k that runs 45mph on a good day. The 35' Magnum looks to be a enlarged version so it must way in around 16k, performance 40mph ??. From the 35' Magnums I have seen diesel power would be a better choice for weight distribution as the 35' Magnums ride high in the transom with gas power..

using the zoom in button i see the "ghoul" direct from prama ohio in the right corner ...hmmmmmm

Nice boat. I like those.