Pitch Perfect

As seen in Speedboat Magazine’s
April 2021 Issue.

Recently, a customer asked me for some advice about choosing a prop for his 2003 Cigarette Gladiator. The boat was equipped with staggered stock Mercury 575 SCIs and Bravo I XR outdrives, with 2006 style SportMaster lower units on ITS gimbals. He’d purchased the boat with 30”-pitch Bravo Is; after testing it, he bought a set of new 2016
Mercury Maximus ST 15.3″x32″-pitch Pro Finish (shiny). The 32″-pitch Maximus choice was based upon the prop choice of other Gladiator owners on the online forums, who claimed they were able to reach 92 mph using this propeller.

The customer told me he’d achieved a top speed of 85 mph with the Maximus props in choppy conditions, and the boat was very stable. The boat was great out of the hole and came on plane quickly using the Maximus props, he said, and his top speed was achieved at 5,400 rpm. He never got to run the boat flat out, as it was always over-revving in the choppy 3-4 foot seas.

Still looking for that 95 mph, the customer was looking at other props, including a Hydromotive P5-X in 15″x35″ or 15″x36″ sizes. He wondered if changing the prop’s diameter in a twin V-bottom setup would yield any kind of appreciable results. He also
reported that trimming the boat had almost no effect on the performance, attitude or speed of the boat.

First of all, valid testing must be done in ideal conditions. The best water for his Gladiator would be in 6-inch to 1-foot wind chop without rollers. Testing should be done directly into the wind and directly with the wind. Testing with a wind and chop to the side will reduce the speed and controllability.

Unless the ECMs had been modified on his 575 SCis, my hunch was that he was probably just beyond the rev-limited rpm. The 575 SCi engines have a flat-tappet hydraulic lifter camshaft set up with relatively low valve spring pressure. The other Gladiators he’d been comparing his boat to likely had different engines—I think the most popular setup was the 525 EFI. It would be better for engine longevity to prop the boat so that the WOT rpm is about 5,200 in ideal conditions. Excessive rpm is likely to result in valve float and potential engine failure.

All P5-X propellers were available only in a 15″ diameter. And the nonlab finished Maximus propellers only come in the full diameter of 15 5/8″. I think that a 15″ diameter in almost any prop is going to be too small, which would result in slip coming on plane and reduced mid-range efficiency. The only way one could run the full diameter Maximus propellers is if the X-dimension is very high. Maximus propellers are used in a full diameter lab-finished version on some Fountains (and other stepped bottom boats) that have the drives mounted high. The Maximus propellers have large blade surface area, which is why they are very efficient in the midrange. So, if you like the Maximus propellers, the 15¼” diameter is about right. (As a side note, the higher your prop shafts are on a Bravo setup, the more likely you are to have propeller longevity problems. This is because a propeller blades that are surfacing are subject to flexing, cavitation which can cause cracks and the loss of a blade.)

Mercury Racing has continually strived to improve its high-performance propeller offerings. One of the latest very successful creations is the MAX-5. Originally, it was offered only in a 15″ diameter because it was intended for use on the 400R outboard motors. After good success on our sterndrive applications, we had Mercury Racing build us some 15¼” diameter prototype propellers. We found that they were significantly superior on sterndrive applications. Because of our experiment, Mercury tried them on the Racing outboards and found them to be better there as well. The result is that 90 percent of the MAX-5 propellers that Mercury Racing makes are now 15¼” diameter. We are now looking into trying a little more diameter. The MAX-5 propellers are also made from the Maximus casting and are “Pro-Finish” with the diffuser shortened 5/8″ from the original stock length.

Our customer’s Maximus propellers were “lab finished” because that’s the only way you can get that diameter in a short tube version. “Pro-Finish” is Mercury’s latest term for lab finished. The lab finished propellers are usually satin and the Pro-Finished propellers are shiny. Basically, his Maximus propellers are 15¼”x32″ pitch. Maximus propellers are great for getting on plane and midrange efficiency. However, they are probably not the best top-end speed propeller. The MAX-5 blade design is intended for better top-end speed performance while still providing decent midrange efficiency. If your boat was provided with 30″-pitch Bravo I propellers, going up to a 32″-pitch Maximus was a big step because of the blade surface area of the Maximus propeller. I was frankly surprised to hear that our customer had hit the rev limiters easily with the 32″ pitch Maximus propellers. The MAX-5 propeller has high blade rake, which will provide more bow lift. For our customer’s Cigarette, I recommended the 15 ¼”x32″ or 33″ pitch

MAX-5s. Increased diameter results in getting the boat on plane easier and better trim control on boats with the drives mounted higher. I could not recommend a 15″ (or less) diameter on any make prop for his boat.

I could not promise that he would achieve 95 mph out of his boat, but it’s not out of the question. I tend to not believe everything that people say on the forums about their performance.

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