PROP SIZE?

QUESTION: I have a 1993 Conquest V-hull that had a 454-cubic-inch, 330-horsepower motor with a Bravo One outdrive and a 21"-pitch Mirage three-blade propeller. The boat now has an ATECO 540-cubic-inch, 650-hp engine equipped with CMI exhaust and MSD ignition. With the old engine, the boat ran 56 mph at 4,700 rpm. With the new one, the boat now runs 72 mph at 5,900 rpm using the same propeller.

I only care about midrange and top-speed performance. I’ve tried a four-blade 21"-pitch Mirage and ran 75 mph at 6,000 rpm. I went to a four-blade 23"-pitch and lost 10 mph and 1,000 rpm. What prop size do I need to get the best top speed? The bigger I went, the slower the speed and rpm.

ANSWER: You have made a significant increase in the performance of your boat. At this point, it would be wise to add full hydraulic steering if you don’t already have it. I also would take a closer look at the condition of your gimbal ring and transom assembly to make sure it is in good condition.

Your boat was probably originally equipped with a Mercury Mirage propeller because it was one of the better props available back then, especially for boats with lower X dimensions. Typically, Mirage propellers are only available in odd number pitch sizes. The Bravo One propellers are available stock in even number pitch sizes. If you tried a four-blade 23"-pitch propeller, it might have been a Revolution 4. The “Rev 4” works well in some scenarios but acts like a larger propeller when it is running deep in the water.

I’d stick to the Mirage Plus three-blade prop for your boat unless you go to a shorter drive. Depending on how much wide-open-throttle rpm you want, I would try a Mirage Plus with a 23" or 25" pitch. If the 25"-pitch prop is a little too much, purchase a Mercury lab-finished version. The lab-finished prop will allow your engine to run a little more rpm and result in a few more mph at the big end.

With your lower X dimension, the three-blade propeller will “manhandle” your boat less than a four- or five-blade propeller would.

HOW LONG DO I FLUSH?

QUESTION: I look forward to your tech articles every month in Powerboat magazine. In the past I have found them very informative. I have a few questions that I hope you can answer for me.

Last year I purchased a 1998 Sonic 386. It is a relatively heavy boat because it has a generator and air conditioning. Over the last year I have rebuilt the 415-hp MerCruiser 502 Magnum motors. In the process, the cylinders were bored .030" over. I also installed Edelbrock marine aluminum cylinder heads and GIL Marine exhaust. The boat, which currently has Mercury Mirage Plus 25"-pitch propellers, does between 65 and 67 mph on the GPS at approximately 5,000 rpm.

So here are my questions: I feel the need for a prop change with more pitch. I’m looking for better midrange and top-end. I would like your advice on whether to go with a Bravo One 26"-pitch four-blade propeller or stick with the three-blade Mirage Plus and go up to a 27" pitch? I would like to know what other ideas you might have.

My second question relates to flushing the motors. Since these motors are saltwater cooled and flushed after every use, how long do you think the motors have to be flushed before the saltwater is out of the entire motor?

I would greatly appreciate your help on any of these subjects.

ANSWER: Usually, boats that work well with three-blade propellers have their drives installed with a conservative X dimension (with the drives running deep). Nine out of 10 times you will gain midrange efficiency by going to a four-blade prop, but you might not gain any top-end performance. My experience is that a 26"-pitch Bravo One four-blade propeller is about the same “pitch-wise” as a 25"-pitch Mirage Plus propeller.

So, if you went to the Bravo One in a 26" pitch, you might gain a little performance in both of your desired areas. The other prop that might work well, especially if your boat is really heavy, is a Maximus five-blade.

The standard diameter for this propeller is 15 5/8", which is usually too big for applications where the propellers are running deep in the water. The Mercury Maximus propellers perform as if they are a higher pitch than stated. If I was going to choose a Maximus propeller for your boat, it would be a lab-finished 15 1/4" diameter with a 24" pitch.

How long you should flush your engines depends quite a bit on how good your water supply is from the garden hose. One thing is for sure: It does not hurt to over-flush the engines so long as you have an adequate supply of water. Many times, inadequate flow from a garden hose results in the engine getting overheated when it is being flushed.

When we are flushing the race boat, we stick our hand in the exhaust water after the motor has run a while on the hose and taste it to see if there is any salt. I am not suggesting that you do that, but it is what we do.

IN OR OUT?

QUESTION: I have a 2000 Formula 353 FAS3Tech with Mercury Racing HP500EFIs. I'm spinning 30"-pitch Bravo One four-blade propellers. I plan on swapping props to spin them in. My goal is to improve bowrise and increase performance in terms of throttle response and top-end speed. What do you think I can expect?

ANSWER: It is likely that you will gain some top-end speed with your FAS3Tech by turning your propellers inward. However, the gain in speed may not be worth the possible deterioration of handling characteristics that you may experience.

Generally speaking, the boat will come on plane faster with the propellers turning outward. With the propellers turning outward, there is usually more transom lift, which helps to reduce bowrise. This is especially true with side-by-side big-block installations with all the weight aft on boats that are 35 feet or shorter.

If your boat has a slight tendency to chine walk, the additional bowlift realized by turning the propellers inward will probably have an effect on the intensity for better or worse. In some cases, control is improved. But more often the result is a boat that is a little more difficult to drive at top speed.

Another drawback of rotating your propellers inward is the effect that it has on how the boat handles around the docks. If your propellers are rotating in, you may find that you have to steer the boat in addition to shifting, especially if wind or a current are present.

—Need some guidance from Bob Teague, Powerboat's lead test driver and owner of Teague Custom Marine? To send him an email click here.

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