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| Syndication for December 2006 Understanding Prop Pitch Fine tuning the performance of your boat with a change in propeller pitch can allow you higher top end speed, or get your boat up on plane faster, or provide more control over slow trolling speeds. The rub here is you can’t have it all. When it comes to prop pitch it all depends on what ranks highest on your totem of fishing priorities. A propeller has two major features that determine its performance, diameter and pitch. Diameter is the measurement across the face of the prop, or the circle it would make when it turns. A 14 1/4 X 21 prop has a diameter of 14 1/4 inches and a pitch of 21. The diameter of the prop your boat will handle is limited by the clearance space between the prop shaft and your boat’s hull or cavitation plate. The definition of pitch is the theoretical forward movement of a propeller during one revolution. By theoretical we mean if there is no slippage between the propeller and the water. Imagine a block of Jell-O about 1 foot by 1 foot and 3 feet long. If you could take a prop that has a pitch of 10, and shove it into the Jell-O at one end and turn it one revolution, that prop will rotate forward through the Jell-O exactly 10 inches—no slippage here! If you do the same with a prop pitched at 14, it goes 14 inches in one revolution. Now let’s imagine there are two identical boats attached to our props. Which prop would be easier to turn? The answer is the prop with a pitch of 10, because it spins easier and moves the boat a smaller distance forward. Which one will go faster at a given RPM? The 14 pitch, because you get four additional inches of travel with each revolution. If these boats are sitting side by side, the boat with the lower pitch prop (10) will plane faster because with less resistance the engine can build RPMs more quickly. However, when both engines hit 4,500 RPM, the higher pitch prop (14) will give more top speed. So which prop pitch will allow us slower trolling speeds with more control, the 10 or the 14? Well, let’s work through this in a perfect world again with no slippage. It’s spring and we are slow trolling for Coho with the prop turning at 500 RPM. With a pitch of 10 our prop will travel 5,000 inches per minute (4.7 mph). With a 14 pitch we are advancing 7,000 inches per minute (6.6mph). So the lower, 10 pitched prop is better for slower trolling speeds. In addition, the ability to vary the speed in smaller increments is also a characteristic of the lower pitched propeller. This ability to vary speed in smaller increments is called “resolution” by engineers. Lower pitched props have higher resolution ranges at slower speeds than do higher pitched props. So what is the perfect prop for your boat? To start, your boat should be propped so that the engine reaches the manufacturers’ recommended maximum RPM at wide-open throttle. This is the baseline from which to start for those wishing to specialize their performance by changing props. From here you can shift up or down to suit your individual taste and needs. One of the most helpful and informative websites for propeller technology is the Michigan Wheel Corporation website www.miwheel.com. This is a great website covering everything about props from one of the oldest names in the business. Strike Vision is Sending Great Lakes Fishermen Back to School Strike Visions underwater camera, which is available in both black-and-white and color models, is an entire downrigger system with a special spool containing 150 feet of 380-pound tensile-strength cable with a video connection at the end. The downrigger ball or weight hooks up below the video connection, and a tiny, high-resolution camera attaches above. The cable diameter is .055 inch, which offers little trolling resistance and can be easily power-set and retrieved with electric downriggers on Walker’s special spool. The camera housing is only 2 inches in diameter, which contributes to low drag and helps the unit to track straight. Individual 12-volt batteries supply power to the camera and the camera is easily detached for storage and handling. Other system features include an 80-pound-test breakaway lead and a heavy-duty slip-jack connection that automatically disconnects in the event of snagging. If a snag occurs only the cannonball is lost. Trollers watching a TV/monitor on board can see their lure’s action and how fish react to them. Resolution is so sharp they can actually discern brown trout from lake trout and salmon from steelhead. “It’s the only underwater camera system you can seriously troll with,” concludes Captain Stopczynski. “If there’s a downside to Walker’s new technology, it’s the frustration that comes when fish you can see don’t always strike. But hasn’t that always been the challenge of fishing?” The website, which contains 30 minutes of underwater action video, is www.walkerdownriggers.com Duo Prop spare Wwhala Q: Does anyone make aluminum props that can be used as spares on a Volvo Penta out drive? I have a Doral250 powered by a VP 5.7Gxi DP. I’d like to save some bucks and get aluminum rather than SS spares. Thanks, Wwhala A: Wwhala, Sounds like you need to talk to Bob at Midwest Propellers. You can go to their web site (bob@midwestpropeller.com) and get a screaming deal on props for your Volvo. Don’t overlook the idea of composite props. They are almost the same price as aluminum but with replaceable blades. If you do a lot of skinny water boating they can pay for themselves in replacement cost and downtime. Replacement blades store easier than a whole prop as well. |
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