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November 03, 2009

Props to You

Tips for choosing the right propeller for your engine and boat

The builder was none too happy with us," said the captain of a brand-new custom 64-foot sportfisherman. "He wanted the boat to be smokin' fast for marketing purposes, while we wanted realistic midrange performance, fuel economy and superior handling. That's why we chose the props that we did rather than a set that would maximize top end but leave the rest of the performance curve hanging out in the wind

With the computer technology available today, you'd think that choosing the right propeller for any given boat would be easy. Au contraire! Even today, you'll commonly see builders have an idea about what prop specs they want, but then they'll experiment with numerous sizes and pitches to find which matches their expectations. The process of fine-tuning starts with understanding basic propeller properties.

Terminology
Let's define the primary propeller terms. Diameter is obvious: the distance across the prop between the blade tips. Pitch is the theoretical distance (in inches) the propeller will move through the water with a single revolution. Larger-diameter props push more water (greater overall speed).

Pitch has several effects. A higher pitch number translates to better top speed but a weaker hole shot. Lower pitch improves acceleration but also lowers your top end.

Rake is the inclination of the blade forward or aft of the conventional position of 90 degrees to the propeller shaft. Low rake provides more power, while high rake gives the most speed and lift. Most commonly, recreational props have 20- to 30-degree rake angles.

Cupping is the curved lip near the blade tips that allows the prop to get a better bite in the water and operate more efficiently near the water's surface. Though cupping generally lowers the rpm, it also usually results in reduced ventilation and slipping and allows for a better hole shot.

Slippage basically equates to wasted energy. Spott Randolph at Treasure Coast Propellers (772-219-0881; www.tcpropscan.com) says you can calculate the amount of slippage by multiplying any particular rpm by your propeller's pitch. Divide that sum by your engine's gear ratio, and then divide that by 1,056 to arrive at the speed your boat should travel with zero propeller slippage. Subtract your real-world speed at that rpm from your theoretical speed; then divide by the theoretical speed to determine how much slip your prop(s) suffer at any given rpm. When selecting your props, you should target 10 percent as the ideal slippage factor.

And, of course, props also come with anywhere from two to six or more blades.

Are More Blades Better?
One reason for choosing a prop with more blades is its resultant smoothness. A prop with fewer blades vibrates more, translating through the hull.

Randolph suggests that flats skiffs can benefit tremendously from increased rake and cup. Rake is the angle of the blades in relation to the hub, and cup denotes the curvature of the blade. He suggests custom OFX props with a high-rake, high-aspect-ratio, X-cut blade with aggressive  cupping for a fast time to plane.

Jeff Whidden at PowerTech! Propellers (800-736-7767; www.ptprop
.com) claims that four-blade wheels have more blade area than stock props, making them inherently more efficient. PowerTech! also manufactures five-blade props for the new 350 hp outboards for ultimate efficiency.

Inboard Props
Inboard boats have different requirements, and, of course, you can't blithely (or easily) swap out a prop as the mood strikes. Propping a larger inboard boat depends on the weight of the boat, among many other things. For example, if you plan to take your boat to the islands or somewhere far afield and load it up with your gear, food, tackle, ice, fuel, people, bait and so on, it will require a different prop than when the boat is running a light load. But because propellers for such boats cost many thousands of dollars, you can't just keep numerous sets on hand for various loads. However, an Australian company - VEEM (www.veem.com.au) - has designed adjustable-pitch propellers. To change the prop pitch, boaters can easily slide plastic strips into the grooves along the tip of each blade. If you don't engage in such dramatic binge-and-diet boating, however, you need only change props if your inboard fails to run efficiently or you damage your prop.

Determining the correct prop for your boat may sound much easier than it is; I'm convinced that it's equal parts science and voodoo. Work with independent prop houses to improve your boat's performance through proper selection and fine-tuning.

***

Spares
One thing I always do is carry a spare set of props on my boats. With outboards, my spares are of a different size and pitch than standard. If I lose an engine, I may not be able to get up on plane and run home at a reasonable speed with my factory-spec wheels. But if I have a prop with two inches less pitch and greater blade area, I can plane much easier on one engine. On such boats, each inch of pitch equates to 200 rpm. For this application, if you want greater thrust, you need less pitch.

Spott Randolph at Treasure Coast Propellers suggests that "going from a three-blade to a four-blade to increase blade area helps with efficiency while delivering more thrust." And he agrees that dropping the pitch of your emergency spare prop by two inches may help plane the boat and get you home.

Jeff Whidden at PowerTech! Propellers says that his company has had considerable success replacing stock three-blade props on larger multiengine center-consoles with PowerTech! OFS four-blade props of the same diameter with two inches less pitch. "With these props," reports Whidden, "many boats can plane on one engine without trimming up the dead motor. While you could switch props when a motor dies, why not just prop a boat right to begin with? Fuel burn and engine load at cruise will improve, offsetting the cost of props."


Photos courtesy VEEM (www.veem.com.au)