Pitch Your Own Prop

Everyone who tests boats for a living, or anyone who has sea-trialed a number of boats while shopping for one, has heard a boatbuilder say, "We're still tweaking the props." This usually comes across as a catchall excuse for performance that, for some reason, hasn't met expectations. In many cases it really is the props that cause the problem. Now, however, there are props, called Interceptor Propellers, that can make "tweaking" a simple matter - so simple you can do it yourself. "Tweaking" the props on most production boats means finding a standard prop that will allow the engine to turn at the number of revolutions per minute at full throttle that the engine manufacturer has specified as the engine's operating speed. Many owners consider that number to be an ideal rather than a requirement and treat it as a guideline rather than as a hard-and-fast rule. Few owners realize that before a manufacturer will warranty an engine in a particular boat, the boat's propeller speed must be in synch (as dictated by the transmission) with the engine's operating speed - not under it or over it - because a discrepancy of as little as 40 to 50 rpm can result in premature engine wear. But simply achieving this goal doesn't guarantee that a boatbuilder has chosen the best prop for a particular model. While production boats under 40 feet usually make do with "standard" props, predicting which standard prop will be adequate is something of a crapshoot, and nailing it consistently is an art. Because of the number of different diameter and pitch sizes available, proper propping is far from a one-size-fits-all proposition. Just because a boat is a certain length, width and weight doesn't mean that choosing the best propeller to bolt onto its shaft is a simple matter. Standard props are mass-produced and their blades' pitches are constant from hub to tip. Most of the time they'll do the job but they usually leave plenty of room for improvement in a boat's speed, smoothness and fuel efficiency.

HEIGHT MAKES RIGHT. Differences in pitch are created by the differences in strip height; higher means more pitch.	MATCH, DON'T MIX. Here there are three yellows and a blue, just to show the variation, but strips must all be the same color.

To make matters even more complicated, props that meet operating-speed requirements when a boat is lightly loaded with fuel, passengers and gear may not do so when the boat is heavily loaded. The same is true in cold weather compared to hot weather, in frigid water compared to warm water and in fresh water compared to salt water. A boat from New England that cruises to Florida and then takes on a heavy load of gear to sustain fishing in some Caribbean hotspot will probably switch props because the New England set will not allow the engine to reach operating speed under the altered weight and climate conditions. At $20,000 a set, that's a costly adjustment. With Interceptor Propellers, made by Veem Engineering in western Australia, one set of props can suffice for both scenarios. Of course, carrying a second set is still a good idea in the event of damage.

COLORS COUNT. The different colors of the strips indicate their size and make them easy for a diver to identify under water.

Computer programs are able to vary the pitch of props during the manufacturing process so that in-stead of being designated "standard," they're considered "semi-custom." These props are more accurate but still often need alteration by a prop shop. Herb Jarrett of Treasure Coast Propellers in Stuart, Florida - who supplies props for such prestigious builders as Garlington, Jim Smith, Whiticar, Willis, Rybovich, Bayliss and Spencer - says that as few as 10 years ago, "new props had to be fine-tuned or the boat would vibrate. Many people accepted this vibration as a matter of course. If they did bring their props in for tuning, they couldn't believe how much more smoothly their boats ran. Back then, out-of-the-box props would be rated Class 2. Now they can be Class S [the highest rating; Class 1 is second-highest]." This improvement was initiated by such international prop makers as Teignbridge, Rolla, France Helices and Veem - firms that took the lead in CNC (computer numerical control) prop technology to develop more high-tech props. During this period, more powerful engines have increased boat speeds dramatically. "Fast" has leaped from 30 knots to 40 knots, necessitating stronger, more fuel-efficient props. Despite this evolution, even semi-custom props can leave owners frustrated by vibration, noise and cavitation. Instead of having them reworked by prop shops, which can cause metal fatigue, many owners are turning to custom props, despite their 15 percent to 25 percent higher cost. Unlike standard and semi-custom props, a custom prop is specifically designed for a particular boat. A prop guru like Jarrett gets all the pertinent information on a boat, including the designer's proclivities, and then confers with a custom prop manufacturer's engineer. "Based on their experiences and philosophies, naval architects have personal preferences in reduction ratios, number of blades, distance from tip to hull, etc.," says Jarrett. "Jim Smith likes a 2-to-1 gear ratio, while Bruce Angel at Hatteras prefers 3.25-to-1 and big props with a lot of blades. Donald Blount usually sticks to a lower gear ratio and likes to crowd the tunnel [have minimal clearance between blade tip and hull] with four- or five-bladed props." At Viking, senior project engineer Bill Gibbons has switched from conventional custom props to five-bladed Interceptor props from Veem. These are truly unique because they can be re-pitched by simply swapping one strip of plastic for another. Forget costly experimentation to settle on the proper prop. Grooves in the trailing edges of the Interceptor blades accommodate wedge-shaped plastic strips of varying heights.

WON'T BUDGE. Once in place, the trailing-edge strips will not come out of their grooves unless they are pulled from the hub end.	SNUG FIT. The strip fits in the groove so tightly that a hooker tool, used for hose and cotter pin removal, is needed to pull it along.

Veem makes extremely accurate CNC custom props, but, depending on the particular situation, they still may have to be adjusted up or down by 100 rpm. Now, instead of having the prop bent to change its pitch, you remove the colored strips on the blades and replace them with different colored strips. The grooves and strips are wider at the bottoms than at the tops, so the strips will never come off of their own accord. The strips add resistance along the trailing edge that provides more thrust while lowering rpm. A yellow strip, for instance, will add resistance equivalent to about 1.5 inches more pitch than if a prop had no strip. So, with a yellow strip, a 30 by 38 prop effectively becomes a 30 by 39.5 prop. There are a total of seven strips available, but having three should take care of an owner's needs - one on target (yellow, for example), and one on either side representing 100 rpm less and an additional 100 rpm. The cost of the convenience of a set of Interceptors is about 10 percent higher than the cost of a set of custom Veems without the grooves. "There seems to be absolutely no downside to this system," says Gibbons. "The props have been through exhaustive testing. They enable us to fine-tune full-load rpm in about 15 minutes." If a Viking owner decides to add a tower to his boat, he can effectively re-prop the boat himself in the same amount of time. And fuel efficiency will be maximized because operating speed will be spot-on over the entire life of the boat. Not only is there no need to remove the props, there is also no need to haul the boat, because the strip switch can be made underwater. If visibility is too poor and the boat must be hauled, strip replacement can be done while the boat is in the slings. Then, as Veem puts it, it's just "splash and forget."