How to measure the speed of your jetski, what equipment you will need, how to prepare your jetski and what the ideal conditions are for speed testing. But first a little background on measuring speed in the marine environment. In the times of the old mariners, water-speed was measured in knots…literally speaking. A rope that had a knot tied every few feet was attached to a weight thrown overboard. The number of knots that were pulled overboard in a minute would indicate the water speed or “knots”. Obviously the rope measuring system has long since been abandoned, but it underlines the progress of measuring marine speeds not to mention the speed increases themselves. In the relatively short amount of time that personal water crafts have existed, changing technologies have also changed the performance measurement of pwc’s. These changes will eventually be important to every owner of a high performance watercraft.
UNDERSTANDING THE STAKES
Why does anybody care about speed testing your jetski?…lots of reasons! First and foremost, performance and speed “sells”. The boat manufacturers attract many new boat sales with big horsepower numbers. However many potential pwc buyers are aware that “claimed” horsepower and “real” horsepower are not always the same. Even if a claimed power figure is accurate, that says nothing of the boat’s true performance ability. To better demonstrate that performance ability, a manufacturer will provide a machine to the editors of a popular national pwc magazine. The manufacturers hope that the evaluations made by these editors will once and for all give potential buyers an accurate profile of performance. Unfortunately, there is often an inferred conflict of interest because the magazine accepts many thousands of advertising dollars from the manufacturer of the boat being evaluated. If this isn’t enough to put an editor on the hot seat, every manufacturer wants to be sure that their machine will scrutinized no more and no less than their competitor’s. For these reasons, the publications want to use a performance measurement format that leaves the least possible room for error.
If this hornet’s nest of politics wasn’t enough, at the same time these magazine editors are also asked to perform exactly the same types of impartial testing on dozens of aftermarket bolt on parts and modifications. Since a positive editorial test can result in a landslide of sales…the results of these tests are taken very seriously.
WHAT ARE THE PROFESSIONALS DOING?
For the first 8 years of pwc, there were only 440 stand up style boats. These machines ran about 28 mph in stock form and seldom ran over 45 mph in modified form. With speeds that low, side by side drag racing was as accurate a speed measurement tool as anyone needed. Riding these machines at 45 mph was a skill easily mastered by many riders. The performance shops of the time, however, needed a more accurate means of “on water” performance measurement. Radar guns soon became the standard tool for every serious race shop. Radar guns that read in one tenth mph increments could accurately show small “on water” increases. At the same time radar testing eliminated the logistical headaches of test sessions that required two boats and two riders.
Dynamometers have been used by many race shops for the development of products that they wanted to manufacture. The engine stand dyno is a terrific tool for “off water” prototype testing, but the costs and setup time involved make it impractical for all but the largest aftermarket manufacturers. Even for these shops, it’s often difficult to predict in a dyno room what part of the power band needs the most attention. Dyno technicians often find that while a dyno can accurately display horsepower output numbers, it can not simulate the loads of hull drag, or the jet-pump that the engine was working against. As a result of these variables, many shops with dynamometers often find engine setups that make big horsepower on the dyno… but for some reason run slow in front of the radar gun. To some technicians the conclusion is obvious…radar testing is the only test evaluation that’s “real”. For a long time, these technicians have been mostly right. However like all current technologies, the radar gun will soon see it’s time as an accurate evaluation tool come and go. For many performance technicians that time is already here, but for reasons that you wouldn’t suspect.
GETTING THE “PERFECT” RADAR PASS
In the ’80’s when most stand up boats ran in the middle 40 mph range, it was pretty hard to mess up a radar test. All you needed was a big piece of reasonably smooth water and a decent rider. Those days are long gone. Most high performance boats can run 50 plus mph. Getting the best possible radar pass is no longer a task, it’s an art form. To be sure, getting good radar passes are easier on sit down boats than stand ups, but a quality “radar rider” is always a clear advantage. That’s right, there are a small group of skilled pwc riders (and technicians) who know all the little tricks required to eke out an extra 2 mph that no one else can. These riders and technicians understand that the engine of a given test boat will turn exactly the same peak rpm in every radar pass. Despite this fact, they also understand that the way the boat is managed during those passes can make a 3-4 mph range of peak radar readings.
Here is a walk through of how a “perfect radar pass” or speed test is done:
- Wait for glass water… Absolutely perfect glass. Many race shops have isolated ride spots that they travel to only when good radar numbers are desired. These spots are usually well protected from wind and ripples. As added insurance, passes are usually made early in the morning or just before dark to assure the best conditions.
- No helmet. Most radar riders are 1/2 to 1mph faster without it.
- Don’t buzz the boat around first. A heated up engine will run 1 to 2 mph slower.
- Leaning forward to avoid nose bouncing, draw the throttle smoothly to full rpm.
- With the engine at full rpm, “settle down” the nose so there is absolutely “no” up and down movement of the nose. Even the slightest up and down motion can scrub off 1 – 2 mph.
- Find the “center” of the hull. That is, keeping the nose of the boat perfectly centered in front of the pump so there is no speed loss caused by the back end of the hull coming slightly off to one side. This is a tough one to explain and to master. However many riders have accidentally experienced it. Sometimes as your riding along a piece of smooth water, you’ll suddenly feel your boat surge forward as if you had a momentary boost of power. In truth there was no boost in power, but rather a big release of water resistance when your boat is momentarily perfectly “centered” on the water. Pump Stuffers can also be of some benefit.
- By now your going well over 50 mph. Keeping the nose settled down and the hull centered has become full time work. If you can, get down out of the wind. Get as low as you can and look up only to make sure that your headed directly at the radar gun. If you move through the beam at a slight angle, the indicated reading will suffer.
The riders who can accomplish all this in a 10 second pass are worth a good 2 – 3 mph to the radar pass and a whole lot more than that to the parties involved. But the use of these radar riders during testing makes “accurate” high speed engine performance comparisons almost impossible. Accuracy is only assured when you use the same rider in the same weather conditions, on the same caliber of “glass water”. For technicians who do on-water testing all the time, the weather and the water don’t always cooperate. Worse yet, for industry magazine editors, the changes in weather and water conditions during a day of evaluation testing can result in big inaccuracies that would misrepresent the merits of a bolt on part or modification.
IF NOT RADAR GUNS, THEN WHAT?
Waterproof, on-board, digital tachometers. Of course, digital tachometers have been around for a long time. However in the past, they have always been very fragile, very water sensitive, and very expensive. One relatively new tachometer, however, has changed all that. The PET 2000, available from several aftermarket shops, is the small instrument that will have a huge impact on how pwc testing and evaluation gets done. This durable, affordable, and waterproof unit is capable of enduring more abuse than anything that’s been previously available. The result has been a whole new level of accuracy that’s available to the pwc enthusiast as well as race technicians. It bears noting that many new models of watercraft come with tachometers as standard equipment. Unfortunately, the accuracy of these stock tachometers can be plus or minus 200 rpm. (not very accurate) The PET 2000 is accurate within 20 rpm (that’s accurate).
Most pwc technicians have long since abandoned their radar guns as the primary test evaluation tool and adopted these on board digital tachometers. When a pwc race technician goes testing, he doesn’t want an un-centered ride or a slight wind ripple to be a variable in his high speed tests. On relatively smooth water, these tachometers accurately show the most subtle changes in peak rpm ability. As mentioned earlier, technicians have found that five radar passes at 7100 rpm can yield speeds covering a 3 – 4 mph range. However for these technicians, that consistent peak rpm is the only information that’s “real”. Many technicians now feel that high speed radar readings can be more a testament of the rider’s finesse than the engine’s performance. This is not to say that radar testing is useless. However there’s no doubt among engine builders that 50+ mph radar readings are much less accurate and much less informative than tachometer readings.
In time, pwc magazines (like all other motor sport magazines) will likely make tachometer readings a standard part of every performance test they do. Performance comparisons made with the tachometer and an unchanged prop would give the readers definitive and repeatable peak rpm numbers. The results of such speed tests could then be compared from one issue to the next. Digital tachometers could also eliminate the controversy of water condition changes, for example, during an all-day pipe shoot-out test. Many publications have been very coy about doing performance product comparison (aka “shoot-out”) type articles because of the controversy surrounding radar test consistency through the weather changes of a day. With tachometers, a test editor could confidently print the beginning and end of power bands knowing that his tests are repeatable and not affected by small changes in water conditions. Manufactures who disputed test results would have to argue with the tachometer, not the editor.
Printing the peak rpm numbers of new model machines could also be a big benefit for both readers and editors. Magazine editors, as well as many boat manufacturers, go to great lengths to assure that the machines supplied for an editorial shoot-out test are “absolutely” stock. Knowing that many new buyers will get an affordable digital tach, nervous manufactures would be much less inclined to provide a magazine test boat that turned several hundred rpm more than a genuine production unit.
Having more instrumentation on future models of high performance PWC’s is certain to be labeled as “high tech”. If the durability and accuracy of these new instruments is anything like the PET 2000, that label will be well earned. However the current trend of instruments appear to be of the ’56 Ford truck motif…not really high tech yet.
By Harry Klemm – www.GroupK.com
- For more on hull and handling performance click here.