Prop, Livewell and General Update

[spareparts]

yes, I wouldn't go with the 17. The reason I asked about the Mirage is teh earlier Mirage props had more blade surface than the Mirage plus, the Mirage plus also had ventalation holes in the hub. I figured a plus would give you a few more rpms than the regular Mirage. I'm still surprised you are only spinning a 15 5300 rpm, how low is the motor on the transom? What hole is the motor mounted on?

[reelapeelin]

Originally Posted by joe7670

As far as I know it's a plain Mirage. Not Plus but will verify tomorrow.

But sounds like the 17P would make the situtation worse? Am I understaing it correctly?

Spare's got TONS more experience than me and I wouldn't argue w/him...all I know is mine's a 17 and it got all the bottom end, mid-range and WOT performance there is to get outta my engine...your engine has a lower redline than mine and that may have something to do w/the difference...all I'm sayin' is, if you have the opportunity to TRY one, do it...

[joe7670]

I'm on a 24" set back bracket which is raised 1.5" per 12" of set back.

The motor is on the highest holes, in escence making it as low as possible without opening more holes on the bracket.

[RidgeRunner]

Originally Posted by joe7670

Please explain further. the 15P would cause the motor to work more when under load?

Pardon my ignorance but new to this and not sure i understand the full dynamics of prop changes.

I had someone on my first post state that this prob would make the motor more efficient or work less offshore or something to that extent.

I was not suggesting a 17p. Maybe just a 15p with less bite. Quicksilver props have less blade surface and they are known to turn more rpm than the Mirage with the same pitch, also true with the Tempest. Pitch is the theoretical distance in inches the prop would move the boat after one revolution. Going up in pitch means the motor will turn less rpm for a given speed and less rpm at WOT. Down in pitch allows the motor to turn more rpm at a given speed and more rpm at WOT. The outboard motors are more efficient and live longer when propped right. Your motor is within the range that Mercury suggests so it is propped within specifications(5250 to 5750rpm). I suggest that you may pick up some performance and possibly some efficiency by selecting a prop that puts your max rpm at WOT closer to the redline. If you were to load the boat heavy you may not be able to reach the minimum specified range of 5250rpm at WOT. That could cause engine damage in the form of a burned piston. I say "could" because I do not know if the Opti has a failsafe for this or not. The Opti may have a knock sensor that would tell the computer to back the timing off. Otherwise, piston damage occurs when the motor advances the timing all the way to max but the rpm is out of range on the low side. The result is predetonation(spark Knock) that causes a lot of heat in the combustion chamber and leads to swelling or melting of pistons, which have the lowest melting point of all components in the combustion chamber.
All props are not the same, every prop has its own characteristics. You may need to go down in pitch to get the max rpm to say 5600 or 5700. You will be turning more rpm for every given speed but that does not always translate into less mpg. Because you are putting less of a load on the motor it will turn more rpm, the motor does not have to work as hard and it will probably gain efficiency and MPH.
You need to look at motor height also, Spare is right on track. You need to have someone at the tiller and while the boat is underway say 35mph go back to the transom and look at the cavitation plate as suggested by Ferm earlier.
Every hole is 3/4" center to center. From the video, it looks like your too low, it has that funny spray. When you look at the lower unit while underway you will see why the water is spraying. If the cavitation plate is visable above the rushing water it is high enough. If it is in the rushing water you would benefit by raising the motor one hole. If the cavitation plate is below the rushing water you may want to raise it two or three holes. It is trial and error.
One more thing to try. Next time you are out with the 15p mirage, go WOT and start trimming the motor up. Keep trimming the motor a little at a time until you feel your speed drop off, look at your rpm. Then trim the motor back down just one or two stabs at the trim switch. Look back at the attitude of your motor, that is typically the max trim and often the most rpm and MPH. That is all the motor will do with the 15p Mirage.

[joe7670]

Sweet! Very informative.

I will do the cavitation plate check this weekend if I can get her out there. I somehow forgot to check that last time.

Then based on that if it's too low will proceed to change height.

If it's just right then may try the 15P quicksilver.

Question: Is a side effect of the motor being too low that the steering is a bit hard in one direction when wot and trimmed?
I remember my 18' that when I trimmed it at wot stearing was at a smooth as silk point.

Now I understand the concern with loading her up and possibility of RPM's being a bit low.

[RidgeRunner]

Yes, motor too low could increase steering torque. Prop torque forces them to steer right. There is a tab located on the antivent/cavitation plate. If your has one you can loosten the bolt and move the trailing edge of the fin to starbd. My opti has a torque tab without the fin but I am running hydraulic steering and steering torque is not that noticable. Just make sure the prop will clear.