Bass Club Digest

Winter is the slow season for bass fishing. Presentations are worked deep at a snail’s pace and bites are soft at best. It takes patience—and the willingness to put up with the cold, wet weather—to be successful.

It’s just the opposite when it comes to our bass boats. They come alive during cool weather.

Outboards perform their best because cold, moist air is perfect for making lots of horsepower. Such air contains the highest amount of oxygen per volume, providing a power-rich fuel-air mixture inside those cylinder heads.

That’s why cooler weather is the perfect time to see just how fast you can make your bass boat run. Odds are quite good that your bass boat isn’t tuned to perfection when it comes to prop selection and engine setup.

That said, there’s probably a couple more miles-per-hour at the top and better fuel economy to be had, at least according to the experts.

The reason is prop “slip,” that phenomenon shared by everything that runs a propeller in water.

Slip is just that—slip. For example, a 24-inch-pitch prop would move 24- inches in a solid medium. But water is not solid. That prop might only move 22-inches. The difference between theoretical movement and actual is called slip.

“Bass boats with aluminum props will typically see 12-15 percent slip when they are setup properly, while those running stainless props will be closer to 10-12 percent,” says Jack Litjens, a 34-year veteran setting up boats for Mercury Marine.

“It depends on the boat. Slower, heavier, less efficient hulls will have more slip than lighter, faster hulls. For example, the Bassmaster Classic boats we setup earlier this year all were running around 9- to 10 percent with a 24 Tempest. That’s ideal.”

Most propeller and boat setup experts agree that the best the typical pad-style bass boat will see is going to be in the 8 – to 10 percent slip range. Bill Leonard, with PowerTech Propellers (www.ptprop.com; 318/688- 1970), one of the leading manufacturers of aftermarket performance props, says 12- to 14 percent prop slip is pretty average these days for a performance fiberglass bassboat that comes from the dealer.

“Bass boats running less than 8 percent are a very rare breed. So, what a bass boat owner should strive for is getting their boat setup so prop slip is down around 10 percent.”

How do you know if the setup on your boat is even in the ballpark of being close to ideal? You have to take it to the water and get real numbers in real-world conditions.

Load your boat just as if you were fishing—full of fuel, gear and a buddy in the passenger’s seat. Turn on the GPS (forget speedometers, they are worthless) and run the boat up to wideopen-throttle (WOT) and trim the engine out until you find the fastest speed. Don’t worry about rpm—concentrate on speed.

Note the best speed and engine rpm at that point. Then repeat the test traveling in the opposite direction and jot down the numbers again. Average the two speeds and WOT rpm. Put the boat on the trailer and settle down for the math part.

Figuring the boat/engine/prop efficiency is a straight mathematical formula that utilizes theoretical speed (no slip), engine gear ratio, prop pitch, and actual boat speed to determine the amount of slip.

Use this formula to find theroretical speed (no slip):

Prop pitch + 1 (for cup) X (Engine rpm / gear ratio) / 1056 = Theoretical Speed.

Once theoretical speed is determined, you can now find how much prop slip your particular boat/engine combination has by using this formula:

Theoretical speed – Actual speed / Actual speed = Prop slip.

(If you are mathematically challenged, you can visit the websites www.gofast. com/Prop_Slip_Calculator.htm or www.admboatworks.com/pitch.htm, which are just two of numerous internet sites that have built-in calculators that instantly figure prop slip numbers once you put in the basic information.)

“A bass boat with more than 15- percent slip is in need of some adjustments. Either a more aggressive prop or changing the engine height—or both,” says Litjens. “If slip is more than 20- percent, the setup is grossly wrong.”

Let’s say that your bass boat is running a Yamaha HPDI 150, which has an optimum operating range of 4500-5500rpm and a gear ratio of 1.86:. It ran 64mph, turning 5600 rpm with a 24-inch-pitch prop. Using the Theoretical speed formula, your boat should be able to run 71mph. Inserting that number into the prop slip formula shows a slip of approximately 12-percent.

Average slip, but not ideal. And, the engine is just over the “optimum” rpm range.

The ideal engine setup for winter tuning is to have a two-stroke turning in the upper third of the highest rpm recommended by the manufacturer when the boat is loaded, and have slip dialed in around 10-percent. That way when you are running light, the engine will be close to or at the peak rpm.

(Four-strokes should be setup so they are turning within 200 or so rpm of the upper limit because they need all the rpm they can handle at the top for best holeshot performance.) If the engine in our example was mounted on a jackplate, we’d drop engine height a halfinch and check the WOT numbers again. Odds are good that the engine was set a bit too high. Lowering it should bring engine rpm up and at the same time may increase top speed.

If the engine rpm were below optimum, we’d try raising the engine, or try moving heavy tackle and gear aft, so the bow would lift with less engine trim.

Many boaters forget that engine height and weight balance has a dramatic effect on a boat’s overall performance—and to find the perfect prop, engine height and weight balance must also be optimal.

If engine height adjustment weren’t a real option in our example, we’d try a prop with an inch more pitch. In general, for every inch change in pitch, rpm changes 200. So increasing pitch by one inch should drop engine rpm by 200 and vice versa. With the rpm change, slip will also change.

Another option bass boaters forget about is the local prop repair shop. You can have them change the amount of “cup” on the prop, which is another way of changing effective pitch, and usually cheaper than buying a new prop.

Whatever single change is made, repeat the on-water testing and calculations.

Finding the perfect prop and engine height is all trial-and-error tuning. Change only one thing at a time and keep track of how that specific change affected the boat’s performance.

When it comes to holeshot, if the prop you choose has removable vent plugs, play around with them. Popping out one or two vent plugs allows more exhaust to flow around the prop at low speed, creating cavitation that helps an engine spin up faster, reducing holeshot times. (Vent plugs have no effect on top speed.)

However, too much venting can have the reverse effect of causing excessive slip or cavitation. Finding that fine line between too much and too little slip is the secret.

In fact, that’s what finding the perfect prop is all about—getting that perfect balance between engine rpm, top speed and holeshot performance.

Then there’s the difference between propeller brands and styles. This is yet another story because not all props perform identical even among the same part number by the same manufacturer.

As for the perfect prop, stainless props are always faster and better performing than aluminums because they have less blade flex. Less flex is less slip.

Three-blades designs are generally the fastest at WOT, but result in more steering torque. In contrast, four- and fiveblade designs usually give the best overall handling and faster holeshots. And to make life really confusing, different prop styles have different effects on how easily the bow responds to trim.

Deciding which prop is the perfect setup requires trying potential candidates out and looking at the numbers each time a switch is made. That’s the beauty of being a bass club member—usually there’s an opportunity to try different props without any money changing hands.

When you find one that allows your boat’s outboard to operate at its peak operating rpm while providing minimum slip, a quick holeshot and good handling, you’ve found the perfect setup. It’s as sweet a feeling as hooking into a toad on a sunny winter morning.

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