16' Cobia only getting 3200rpm

[foofan]

Hi All,
I just found this site today. I am really looking forward to checking it out.
My question:

I bought my first boat ever about 2 months ago. She is a 16' Cobia with a 120hp chevy motor. OMC is the drive system and has a 15" x 15dg prop.
So full throtle I am only getting 3200rpms and about 25mph. The motor runs very smooth.

I am very new to boating but am sure this motor should be getting more rpms and we should be moving much faster than 25mph.

Thanks for the imput.
foofan

 

[steelespike]

Re: 16' Cobia only getting 3200rpm

It appears the motor has a 1.71 gear ratio A 15"prop is about right.(pitch is measured in inches). The theoretical distance the prop moves forward in one revolution. Actual distance is somewhat less this is expressed as slip.
Your rpm range is 4200 to 4600. I think your motor should be able to turn the 15" up to 4600 and make about 34 mph. with 10% slip.
Its possible it could do a 17"prop with a speed of about 39.
You need to figure out why it can't do the 15" I suspect motor problems. Start simple Check that the throttle opens all the way.
Be sure all the plug wires are hooked up.Prop is in excellent condition.
If nothing obvious do a complete tune up with a compression check Be sure timing is set right and advancing as it should.
Bottom clean and smooth.
Does the boat respond to up trim?How is the hole shot'

 

[Don S]

Re: 16' Cobia only getting 3200rpm

With only a 15" pitch prop and 3200 rpm max, you don't have a prop problem, you have other problems.

Here is a list. http://forums.iboats.com/showthread.php?t=295070

Might want to post a year, model numbers, and type of outdrive you have. type of boat means nothing.