De-ja-vu :D . . . . my boat was the exact same way. I mounted mine under the swim platform just about in line with the floor of the actual deck of the platform. Sounded awsome but yea it is a bit much sometimes, almost viberated the boat it was so deep. I installed silent choice and could switch it to the props when i needed it hushed down a bit, worked like a charm. I did turn some heads though coming into a dock with the loud switch on, most were like "whats in that thing?" :D

I'd say you are under rpm anywhere from 500 to 700 or 800 rpm. These last rpm will really give you some speed! second thing, if I were you, I'd try some other props. And yes, maybe even other 21 ssteel props. If the one that are on it were worked, or damaged and not fixed properly, it may be a part of the solution. But engine Rpm are the biggest concern. I picked up 2 mph for 30-40 more total hp (260 vs 275) My guess is you should run...210 more hp / 30 hp = 7
so 7 x 2 mph= 14 mph faster. so max max would be 70 mph but I think realistic number would be 65-67 mph... just my guess... but you sure have to spin these engines!

Clarify one other thing that would drop your speed,
Is the bottom of the boat Clean? No growth or anti-foul paint? Is it smooth and slick, no residue that needs an acid wash?
Often over looked and a real speed killer.
If you changed your tach's there are settings on the back for 4-6 or 8 Cylinder, make sure you have them on the correct setting.
These two items could be checked before looking into the cam with your mechanic. RPM Seems way off if the HP is correct. Most 260 w/ Merc 260's were 58 mph at close to 5000rpm when new with light fuel a little choppy water an empty beer cooler and Jockey at the helm. The best money spent on a Chris Craft is on the Stereo and Polish, not the engine room. No speed demon whatever u spend. It Is, what It Is.... enjoy her! Good Luck!
VeeCat

glassdave how hard is to install silence choice system in my boat?
how much money are we looking at?
any information helps,

Do you still have the Y pipes that go down to the transom assembly? if yes than it is a pretty simple installation. They mount in the rubber tubes that go from the riser to the transom tips. you also have to install an air pump to operate the cylinders. going rate for a complete setup for twins is in the 4-5 hundred dollar range. well worth it.

i need to know what to buy to make a sylence choice

i am thinking in letting go my stinger to get me a faster boat,
trades up or down with money are welcome.
anything over 25 feet is ok but needs to go in the mid 60s mph.
single engine is ok too.
any body intersted please let me know.
pictures are available of complete restoration of the boat.

how long have you had the boat?

its been in the family since new