Alumacraft Navigator 175cs Performance/150 Optimax

[PredLur]

Picked up a new boat with a 150 Optimax and during the break in period, could only push 4500 RPMS with it and get a top speed of 44 mph. That was with 2 other guys and less that a half a tank of gas. Am running a 21" SS prop. I was only going to go up to 4500 RPMs, per the directions on the break in period but once I got up there, I noticed the throttle was pegged.

Ive read some of the other threads and know I should be getting anywhere from 5500-6000 rpms on this motor. My question is, will going to a 19 inch prop increase the RPMs that much (could i expect that much change) or is could there also be an issue with the throttle linkage.

[staylor]

The RPM range for your engine is 5250 to 5750 rpm- for best speed it should be running around 5600. Do not ever over-rev an Opti. The computer will log this as an over-rev which could affect your warranty, and you may well damage some expensive parts. I would check your throttle linkage to make sure its opened fully. Remove the cowl and take off the black plastic air intake on top of the engine. This lifts off with a bit of force and when you're finished, make sure you get the lines that go into the cover reattached. With it off and motor not running, shift to forward and push throttle control to the limit. The intake butterfly points upward and is at the front of the engine. Look down at the butterfly and it should be fully opened to the point where there is only a small amount of play before moving the butterfly by hand causes it to hit its end stop. If the butterfly is opening fully, then you're going to need a prop with less pitch. I don't think dropping to a 19 from a 21 will get you up into the operating range from the 4500 you're at now. I suspect you'll need to drop to a 17.
Doug

[TomP.]

I would get your tach checked first. Even with the 21 you should be getting more RPM`s. That 21 pitch is way to much prop for that rig. 14X19 would be better but your more then likely going to need to get down to a 18 pitch. Power Tech makes 18`s

[PredLur]

When you say over-rev, what exactly are you referring to, are you refering to opening up the throttle wide open?

I called the dealer who told me to go back to the lake, put the stock aluminum prop on the motor(a 21" pitch) and try that. I ended up getting around 5100-5200 hundred. He then suggested a 17" pitch SS.

He told me to open the throttle up and see how hig it got with the motor pitched up as high as I could, is that "over reving" the motor?

[Ray Champine]

An over-rev is anything higher than the manufacturers highest recommended rpm. For your Opti that's anything higher than 5750 rpm. Say you change props and hit 6000 rpm, you've just logged an over-rev.

Ray

[phowler]

Over revving is when your RPM's go higher than what the manufacturer recommends. If 5750 is the maximum RPM your motor should turn you don't ever want to exceed that number.

Edit; looks as though Ray & I were typing at the same time.

[PredLur]

Dealer recommended a 17 inch SS prop. Havent gotten it out yet with it. Anyone else running a 17 inch prop on a 150 opti?

[Chasen eyes for fun]

I have a 08 Navigator 175 with a 150 opimax.It has a 19 pitch factory prop on,and with 1 person 1/2 tank gas t8 kicker and two batteries for bow trowing motor I can get 51mph. My rpm's are about 5400 to 5500 with the factory alum. prop. I have ordered a 19 pitch Tempest Plus Stainless Steel but have not recived it yet. I would think going to a 19 pitch will make a big change on your boat.

[PredLur]

So do I run the risk of over revving the engine if the prop is too small and I put on the 17 inch?

Thanks for the help.

So any particular reason why they sent the boat out with a stock aluminum 21" as it sounds like that is too much prop for the boat.

Let me know how that boat performs when you get it out.....i may be looking at that exact prop then.

[teamlund]

a 21p seems like an awful lot of prop for a 150 hp engine. I am surprised the boat was even rigged like that

[smoker62]

I ran a 23 pitch trophy plus on my 150 merc. on my triton. Totally different animal I guess.

[Chasen eyes for fun]

Update on stainless prop.
I just had my stainless prop put on and got it out on the water.
With my factory alum. 19 pitch I would get about 51 mph at 5400 to 5500 rpms.
With a Tempest Plus 19 pitch stainless my rpms were about 5200 and my speed was 48.6 on gps.
I could trim it up alittle higher and possibley get rpms up to about 5300 and my speed would be colse to
49 to49.5. Boat has great hole shot and theres not muct of the tail end in the water at 48 mph. Prop gave
my boat alot of lift. I'm happy with it being alittle slower. If you wanted more roms you could go down to a 18 pitch. I think a 17 pitch would be too low and your rpms would be to high!! Hope you didn't buy a 21 pitch
as that would be too slow and no rpms. Hope this helps you out some.