Optimal RPM range 150 GT

[92 spectrum 150]

What's the recommended RPM range for my 1990 150 GT? Currently on 15x17 aluminum stock prop spinning at about 5200 rpm. With my OMC 14.5x19 SS I'm at about 4900-5000. I'm looking for top speed performance. Hoping to reach close to 50 mph. With the SS OMC, I'm able to top out at about 45 mph @ 4900 rpm.

I need to know if I should prop up or down for maximum top speed.

Thanks!!!

 

[emdsapmgr]

That engine is factory rated to operate between 4500 and 5500. Since it's a GT, it will actually still make reliable hp up to 5800. Current thinking is that the engine should be propped to run at the upper end of the normal rpm range, or 5500. You are below the 5500 target, so propped the way you have it, the boat is considered "overpropped". You are lugging the engine the way it is. By at least one prop size. You need to go down at least one prop size. When you do that, the engine will make more rpm's have a 10% better holeshot and will get better fuel economy throughout the entire rpm range. You should pick up a few mph. I'd borrow a stainless 17. That would get you up to the 5200-5300 range-still low. The next thing is to see if the engine can be raised on the transom one bolt hole. That will also yield an rpm increase. Less lower unit in the water=more speed.

 

[Bob_VT]

Need more information. Is this a new to you motor? New to you boat? What hull is it on? There is an "X" factor for props which is the height of the motor combined with the trim. When was the last real tune up?

 

[92 spectrum 150]

Need more information. Is this a new to you motor? New to you boat? What hull is it on? There is an "X" factor for props which is the height of the motor combined with the trim. When was the last real tune up?

It's on a 1706 Spectrum aluminum boat with full windshield deep v. This motor is relatively new to me. Only had it for about a couple months. This motor is mounted as high as it'll go.

 

[Bob_VT]

Aluminum boats (fishing hulls) are hard to break 50. I have ran a 16' aluminum with a strong 140 and the best I was able to get was 47-48 and I had an aluminum bass boat with a 115 that would struggle to hit 44. Make sure you check the motor and also confirm your tach. You are right ....what you have does not sound right.