1993 150 Evinrude RPM's To Low?

scottpellegrin · #1 July 25th, 2007, 08:18 PM

I have a 1993 Evinrude Intruder 150hp on an 18ft Stratos 275 PRO XL. My trouble is when it's at wide open throttle (WOT) the motor is only turning 4700 rpm's. It has a 14 1/2" X 24P Raker prop on it that gets 58 mph on the gps. Any ideas on why it only gets to 4700 rpm's? Will this rpm hurt the motor? Is it because it's a Raker prop? Should I change the prop? What prop do you guys run on your 150's? What do your rpm's run at? Any info will help, I don't want to buy and try a bunch of props because the cost will kill me.

tashasdaddy · #2 July 25th, 2007, 08:22 PM

you should be turning 5000 to 5500, you do not develope the full Hp till 5000. you are lugging the engine and yes it causes extra wear. you are over propped.

Dhadley · #3 July 25th, 2007, 09:57 PM

First, verify your tach. If you were indeed only turning 4700 your theoritical speed is only about 58 with 0% slip, which isn't possible. Rakers are a little conservative with their numbers but not that much. The GPS will be reasonably close too.

On an 18 Stratos we generally see good props in the 24 to 25 range with
60* 150's so you're probably close. Which brings us right back to the tach.

Dhadley · #4 July 25th, 2007, 10:03 PM

5500 and 14% slip puts you right at 58 actual.

5200 and 10% puts you right at 58 actual.

scottpellegrin · #5 July 25th, 2007, 11:53 PM

Have you seen tach's wrong in the past?

walleyehed · #6 July 26th, 2007, 02:56 AM

I'm gonna step in on that question...."YES"....quite a bit more than you may think.
Good to see ya DH!....
As I think Dhadley would suggest, I feel the OMC/BRP tachs are the best choice, and I also believe the tach should be changed out every 5 years-my own opinion.

Dhadley · #7 July 26th, 2007, 09:18 AM

Agreed! The OMC / BRP tachs have proven to be some of, if not the most accurate tachs available. We find the analog tachs to be within about 50 rpm or so. The digital is within 10 rpm or so.

Had a guy from an aftermarket tach manufacturer tell me once his tachs were within 5%. That means on a twin installation with both motors turning a true 6000 rpm, it was acceptable for one tach to read 6300 and one to read 5700.

I know what I told him, what do you think?

scottpellegrin · #8 July 26th, 2007, 06:32 PM

Thanks for the help I'm going to get a new tach asap. I do have another question for all of you out there, how much will a jack plate affect the boat as far as speed goes? I want to get a hydraulic jack plate so I can really raise the moter when idleing in the river through the shallows. Any info you have to offer will help in me getting one. What kind should I get? I've been looking at the CMC style but dont know how much set back is to much. Again any thoughts you have will help.

walleyehed · #9 July 27th, 2007, 02:50 AM

Depends how much set-back this rig will handle with consideration given to the weight of the engine....C.G.-wise
Hyd. or manual, I've tried about 4 plates now on my own boat and am very pleased with the Bob's Machine plate....impressed enough I became a dealer.
Dhadley may have some data on that rig...let's see if he may have a suggestion.