Something is wrong here??

B

birdy3897

Petty Officer 2nd Class
Joined
Sep 10, 2005
Messages
126
I am running a 14by19 cast prop with a 1978 Imperial 16ft Tri-hull and my speedo only reads 27mph at 4600 RPMs. My fishfinder reads even slower at 24-25 mph. This boat is a 2.5L 120hp. Shouldn't I be getting at least something like 35mph at that RPM?? It doesn't seem right, most people I hear get about 35-40 with these drivetrains. It doesn't seem like the prop is breaking loose but the motor runs great. Is this realistically all I will see for a top end on this combo? Any suggestions??
 
Don S

Don S

Honorary Moderator Emeritus
Joined
Aug 31, 2004
Messages
62,321
Re: Something is wrong here??

Are you using the drive trim trim? (assuming you have it since you forgot to mention what engine and drive combo you have.
 
B

birdy3897

Petty Officer 2nd Class
Joined
Sep 10, 2005
Messages
126
Re: Something is wrong here??

Yeah, boat is trimmed out and running 4600rpms. I have a 1978 chevy 2.5L with a mercruiser pre-alpha. Maybe I need more prop??
 
Don S

Don S

Honorary Moderator Emeritus
Joined
Aug 31, 2004
Messages
62,321
Re: Something is wrong here??

Are you sure your tach is correct? How about the gear ratio in the drive? Is it correct for your engine?
 
B

birdy3897

Petty Officer 2nd Class
Joined
Sep 10, 2005
Messages
126
Re: Something is wrong here??

I'll have to check the tach. It does seem right though because the motor is screaming at 4600rpm and my idle is right at about 600-650. As far as gear ratio, I can't imagine there would be a different gear than the 1:98:1 which I believe was the stock ratio for most 4cyclinders. I never heard of a higher than 2:00:1 being available.
 
You must log in or register to reply here.
Top