Why does my 78 Aristocrat tunnel hull only go 40 mph

[Michelinman]

Im trying to locate the lack of speed problem with my boat. Its a 78 Sleekcraft Aristocrat tunnel hull, has a pretty much stock 350 engine in it with a few added goodies, alpha 1 drive(250.. yep 250, made in '77 only), and a 24" prop on it. Boat is tuned and runs great. Problem is that max speed is reached at a little over half throttle and 3000 rpms,trimmed up and not porpoising. Max speed is 45 mph. Thats gps and factory speedo. More throttle does nothing to speed, rpms, or backfiring.... nothing. It does rev much high when not in gear, and im sure the max rpm range isnt 3000 for a small block chevy engine. Thats gps and factory speedo. Read somewhere that the boat came with a 305 and was rated at 65 mph. The guy I bought it from had a built 400 sbc in it, said it would do 80 mph, for what its worth. 80 seems waaaay off. Id be pleased at 65 mph, maybe even 55. Any help is greatly appreciated.

Please tell me this boat isnt meant to go 45 pmh top speed???
Michael

 

[Bondo]

Re: Why does my 78 Aristocrat tunnel hull only go 40 mph

Ayuh,... Sounds like you're lacking the Horsepower to spin the 24" prop,...
try a 21"...

 

[Michelinman]

Re: Why does my 78 Aristocrat tunnel hull only go 40 mph

Wheres the local swap n shop or prop swap shop? Lowering prop size was my next step but the boat mechanic said it wouldnt add any speed, so I postponed it until I could get some real advice from you guys here. This site is more informative than any manual or mechanic. The guy that said it wouldnt go faster with smaller prop also said he had worked on mercruisers for 30 years and never seen a 250 model drive. Maybe we both learn something here.

Thanks,
Michael