New Prop Question

[NFA]

Hey guys, I've been reading this forum quite a bit the last few months with plans of upgrading my prop for the upcoming summer.

My current boat has a Mercury Optimax 150HP sitting on an 18' Maxum bowrider. My original prop was an aluminum 14.25 x 17. I use the boat mainly for cruising and some skiing/towing.

Yesterday after testing out 6 different props (Ballistic, Laser II, etc) I decided to pick up a 13.75 x 25 SS prop. I chose this one for a few reasons...

It added 10mph to my top end.
Better hole shot speedwise, and the nose stays in the water moreso than it did with the previous prop.
Better handling and driving - I noticed a significant difference with the steering and handling with this prop over all the other ones.

The bite on the "slow down" was great to see too. With this prop the boat seemed to slow down faster with minimal wash over the back.

My only concern is that my RPMs are at 4800RPM. I could still punch it more if I wanted, but it's fast enough for what I need. So even though I'm not topping out in the 5000RPM range, is the motor still okay revving at 4800RPM? Or is the motor "chugging", even though it's flying?

Any questions, advice, things to look for, would be fantastic. I know I've already bought the prop, but the guy I got it from said I could swap it out if needbe.

Appreciate it;



Scott

 

[steelespike]

Re: New Prop Question

What is the max rpm at wot. The motor would be happier at arond 5,600.
Once you have it propped closer to 5600 you can run it at what ever speed you like.
Marine engines are working hard like they are going up hill all the time when you prop too low
an rpm its like going up hill in overdrive.

 

[NFA]

Re: New Prop Question

What is the max rpm at wot. The motor would be happier at arond 5,600.
Once you have it propped closer to 5600 you can run it at what ever speed you like.
Marine engines are working hard like they are going up hill all the time when you prop too low
an rpm its like going up hill in overdrive.

Hey Steele, thanks for the reply.

I'm not sure what my max RPM at WOT is because it's just too damn fast for me and what I need. I was wondering the same thing in regards to the engine working too hard.

Just to clarify, I'm not at WOT at 4800RPM, there's still some room to go on the throttle, it's just that I don't intend on going that fast ever. My RPM gauge caps out at 6000RPM.

I'm sure I could push the throttle to the 5600 range, but after testing out the prop last night I have no intention of doing that.

Also, seeing as the engine is sitting at 4800 and cruising, is it still "bogging" down the engine, or making it work harder even though it's flying?

With my aluminum prop I was capping out at about 5000RPM, but going 10MPH slower. And same thing, I could have punched it more if I wanted, but I had no desire to.

Thanks!

 

[Robbabob]

Re: New Prop Question

NFA,

Seems you made a huge change in pitch, 8"? And from aluminum to SS to boot.

As Steel says, your engine is happy around 5600 (http://www.mercurymarine.com/engines/outboards/optimax/2.5L/?model=1). It's not about wanting to run it there more than once to make sure you have the right prop. It is important for the engine efficiency to have the correct set up... even when taking off and cruising.

Though you like the performance and feel, you're likely lugging the engine and causing excess wear. Do yourself a favor and take her out to WOT with good trim and smooth water. Also, make sure your tach is working properly and set for your number of cylinders.

Good luck and enjoy!