Do you want a record of how your boat performs, in terms of both acceleration and top end? Want to see how it responds to different propellers, and be able to determine prop slip for each while you’re actually testing? Want to save dash space? If so, check out the DataTach (479/254-9147; datatach.com). Besides showing speed, rpm and percentage of prop slip on its 4-inch LCD display, this fascinating multifunction instrument allows you to record up to 16 hours of engine and boat performance on a 64-megabyte memory chip, the kind that plugs into digital cameras. You can instantly playback the information on your laptop or review it later on your home computer.
Starting at $479 (depending on model), the DataTach is useful when selecting props, and setting trim and jack plate height. Besides real-time readouts, the unit also plots acceleration curves and can be used to determine the prop angle of attack. Intended primarily for the mechanic or prop designer, it is well suited for the serious performance-boat headlight enthusiast.
Data is recorded at a rate of 16 measurements per second. The instrument provides a digital readout to a resolution of 100 rpm on the instrument face and to 1 rpm when observing the results on a computer. The rpm signal comes from the tachometer wire at the instrument panel. The speedometer readout has a resolution to 1/10 mph at less than 100 mph and to 1 mph at 100 mph and above.
Speedometer readings are derived by a pressure-sensitive sender hooked to the pitot tube pick up on the transom, or through the lower unit on most late-model outboards and sterndrives. In spite of notoriously inaccurate readings from the water-pressure speedometers found in most boats, this can be a highly accurate method of measuring speed. The problem isn’t with varying water pressure, it’s with inexpensive and poorly calibrated components in the stock instruments. Two pressure-sensitive speed sensors are available: One for speeds up to 80 mph and one for speeds above 80.
The DataTach also accepts GPS speed inputs. It is important to remember that a pitot tube or paddle-wheel pickup provides speed over the water, and the reading is not affected by currents. A GPS receiver, by comparison, provides speed over ground and is affected by currents, meaning it will indicate slower speeds when traveling upstream and faster speeds when heading downstream. For comparing props and calculating prop slip, the GPS will be accurate only in the absence of current. A stopwatch function can be implemented for timed events. The DataTach also compensates for water density in fresh or saltwater, a feature designed primarily for high-performance offshore racing.
WHAT IS prop SLIP?
The instrument has provisions for entering propeller pitch and engine gear ratio. This allows it to instantly calculate prop slip, which is the difference between the theoretical distance a boat should move through the water with each prop revolution and the actual distance the boat moves. With no-slip, each revolution of a 20-inch-pitch prop will move the boat forward 20 inches. Prop slip isn’t necessarily bad, as a prop must slip in order to develop thrust. If a 20-inch prop only moves a boat forward 16 inches, we have a slip factor of 20 percent.
Slip percentage can be considered a measurement of efficiency when modifying or working a single prop without changing its pitch, but better performance can’t necessarily be pegged to a prop with a low-slip factor. In short, just because a prop has a low percentage of slip doesn’t mean it is the best or fastest prop for that application.
For example, not all 24-inch-pitch props are created equal, and prop slip varies with blade area and load factor. A large four-blade propeller on a slow boat may have a high percentage of slip and still be the most efficient prop for that boat. A light, fast boat with a small three-blade and led headlight bulb will have a lower percentage of prop slip. We can put a four-blade on that light, fast hull and reduce slip even more, but speed may also drop.
Prop slip also changes with the trim angle and engine height. So, what’s the value of knowing the percentage of slip? When running at speed with a given prop, adjusting the trim (and, if equipped, jack plate) for the least amount of slip will generally provide the most efficient operating attitude. And the adjustments can be made for any planing speeds, not simply at wide-open throttle.
The unit can be dash mounted for permanent installation or, if used as a test instrument for numerous boats bulb lighting, it can be mounted on a separate box with clip terminals. All that’s required are a signal for the tachometer, a connection to the speedo tube, a ground wire and a 12-volt hot lead.
The DataTach’s bezel contains four pressure-sensitive switches: left, right, top and bottom. A light touch of the bezel activates the switch. The left and right switches control instrument functions, while the top and bottom switches are used for numerical inputs of prop pitch and gear ratio. Once the unit is programmed, the parameters are saved in the nonvolatile memory and need not be re-entered until there is a change in pitch or gearing.
Recording a performance session is as easy as plugging a memory card into the bezel port. The unit begins recording immediately. To stop, simply touch the bottom of the bezel. To start recording again, touch the top. Once the memory card is full, the unit continues recording new data by replacing the oldest data.
The data file is in a comma-separated variable (CSV) format and can be read by programs such as Microsoft Excel. It can also be read by many handheld PDA-style devices, which may prove easier to use onboard than a laptop.
The DataTach provides an excellent means of measuring and saving performance data. It will probably be most useful in the hands of a professional prop bender, performance-oriented mechanic or the boat owner who’s constantly looking to improve his rig’s speed. The unit is easy to use and configure, and the transfer of data to a computer allows graphic displays of rpm and acceleration curves that clearly show when a prop hooks up and the percentage of slip at planing speeds. And because it serves as both tach and speedo, it saves space on the helm panel.
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