Prop help for a beginner

[metal249]

I just bought a 1986 seaswirl with the 3.0L Omc. The manual says the full throttle operating range for the motor should be 4200-4600rpm. At the lake, I could only get 3400rpms out of it, empty, at full throttle. The motor was just tuned , oil changed, and its only 2yrs old. The prop is a little chipped and says 13 1/2" X 17. I think I need a different prop, but I do not know what to get. The sizes I keep seeing for my boat are 13 1/2" X17-13 1/2 X19. It also lists 13 1/2 X25-13 1/2-27" for bass boats. Any help is appreciated.

 

[chiefalen]

Re: Prop help for a beginner

that outdrive have a big rubber boot around it?or rams to lower and raise the outdrive? what size boat is it?

 

[metal249]

Re: Prop help for a beginner

It has hydraulic rams and its a 17"ft seaswirl, open bow.

 

[Don S]

Re: Prop help for a beginner

You have other issues other than props. There is no way you can make that much difference up with a prop change.
Here is a list of items that can cause low WOT rpms.

Engine Won't Reach Operating RPM. Check

• Fuel condition. Type and Octane possibly old fuel
• Propeller pitch or diameter, damaged blades
• Restricted fuel pickup tube or anti siphon valve Fuel System Test
• Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
• Marine growth on hull and outdrive
• Wrong gear ratio in outdrive
• Restricted carburetor air intake (clogged flame arrestor)
• Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
• Poor cylinder compression Compression Test
• Carburetor defective, or wrong type.
• Fuel pump pressure and vacuum
• Boat overloaded, improperly loaded, or improperly trimmed.
• Engine Overheating
• Engine timing and ignition system operation
• Remote control cables and linkage for proper travel to open throttle plates fully.

 

[metal249]

Re: Prop help for a beginner

The motor was rebuilt 2 yrs ago and appears to be running very well. My fuel is 3 octane points higher than recommended( 90 octane), but when running 87 octane, I only pull 300 less rpm's(it just knocks a little). I tried to check the timing, but I could not get a reading, but it doesn't knock or ping, so I must be pretty close. It takes me a long time to get it to 3000rpm's. It seems like I need a prop that has less pull on the motor so I can achieve higher rpms. If I went from a 13 1/2X17 to 13 1/2 X 17, could I get higher rpm's?

 

[Don S]

Re: Prop help for a beginner

If I went from a 13 1/2X17 to 13 1/2 X 17, could I get higher rpm's?

I doubt it

I tried to check the timing, but I could not get a reading, but it doesn't knock or ping, so I must be pretty close. It takes me a long time to get it to 3000rpm's

Why can't you get a reading, sounds to me like you have more problems than you think and are trying to blame it on props. You are about 1000 rpm low in rpm, and 1" of pitch change or Diameter change is only going to get you 100 to 150 rpm of change.

Deal with it. you have an engine problem.

Did it ever work right since you have had the boat?

 

[chiefalen]

Re: Prop help for a beginner

go down the list.one by one; best list i have seen.each one very important.post what you have as a result ie:low compression than well be able to help you.good luck