Just had my 200 optimax replaced with a new 225 verado and it appears that I will be switching from the 16p prop to a 15p. I am surprised by this since it is an increase in HP. What are the factors in place that would make this change necessary.
The motor sounds like its electric. lol It will take a bit of getting used to but the controls are sure nice.
Gear ratio is a very important factor. If the verado has a different gear ratio it will need a different prop. Another factor is where the engine makes it's torque. On a two stroke (optimax) it starts making torque at a lower RPM than a 4 stroke. Therefore it's able to turn a large prop at the low end when pushing your boat up onto a plane. 4-strokes typically need to rev up higher before they start making max torque and are therefore geared lower to accomplish this.
So in the end the 15p prop will be turning faster to achieve cruising spead at a slightly(2-3000) higher RPM(than the opti)? Im just surprised to move down on the pitch scale with a bigger motor.
The verado has a gear ratio of 1.85:1 while the optimax has a gear ratio of 1.75:1. So the Optimax turns the prop faster at any given RPM. You will have to rev the verado higher to maintain the same cruising speed.
For example:
Optimax: 16p @ 3000RPM with 10% slip = 23-24 mph.
Verado: 15p @ 3000RPM with 10% slip = 20-21 mph.
I don't know why specifically you would need the smaller prop with the verado. The prop I would recommend for the verado would be one that allows 5800-6400 RPM at full throttle. I'm guessing you're still in the break in period so you won't be able to do any full throttle runs yet.
The requirement to move down to a 15p came from the full throttle test where as you state, the motor should reach a certain rpm. Im my case it just barely made 5800 thus requiring a 15p prop. This is with the boat unloaded so It seams proper to move down to a 15 as the test suggests.
The break in requires that the boat be run at full rpm every 10 minutes for 1 minute for the first 2 hours along with the standard variation in throttle.
Yeah so it's going to be just the difference in torque profile and recommended max RPM between the two. Your top speed should still be very close to the same (I'm guessing around 43mph). The difference will be a higher RPM at both cruise and WOT.
I believe your opti would operate at max rpm of 5500 or so? The verado will operate best if you can spin the prop right up to 6300RPM. That difference will warrent a pitch change to at least 1 inch lower. I know on my grady we had a 225 verado with the 15 pich enertia prop and it was a great combo. Now that we have a 300 verado we are running the enertia in a 17 pitch and still get to the max rpm for the engine.
Have a read of the above link. Repowered a few years ago to a pair of 175 Verado's & had the same issue as your experiencing. Played around with engine heights and found the motors were buried (ie. too low a position on the transom) Bringing them up to where they are designed to run allowed spinning a larger diameter prop and increased top end RPM. The other important factor is propping, my old engines ran SS props, which were ditched after playing around to a set of Mirage's. See if your dealers comfortable allowing you test run a few of the different props available. Mine ended up with a better cruise speed and less likely to cavtate when running in swells. Comparing the two props the older prop was a small bladed unit, the Mirage's are larger diameter, fatter bladed unit. Allowing running a lower pitch, larger diameter wheel, which I was told is more efficient. Match the prop to the hull, load etc.
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