Reply With Quotejbone,
JPC's speed estimates are based on a generic drag curve fitted to your measured speed/rpm or drag point. Speed extrapolations for large, heavy or deep v hulls can fall below that curve, smaller light-weight tunnels may rise above. The farther you extrapolate (or up your power) the more magnified these differences will be.
Your 21' shockwave is a heavy/large jet so projected speed may fall short if your hull drag curve is significantly different from the generic curve in the 58 to 70 mph range you're considering.
That said I have an example of a 21' Daytona w/an AT AA that matches the JPC's drag curve very well from 23 mph all the way to 88.8 mph (top end). In fact, for that jet JPC under predicts top end by 2.8 mph if the on plane drag point of 23 mph/2500 rpm is used, and over predicts top end by only 0.2 mph if a 78 mph/5000 rpm drag point is used. Not bad considering a 21' jet or even a smaller jet. I can also give an example of an 18' jet falling well below the curve because it had a bad hook in the bottom...throwing the drag curve well off-norm for that size jet. The more power he added the more it plowed water...not something a speed predictor can account for.
But even with the generic drag curve, the JPC's a good speed predictor for small changes (100 hp) and not bad in some cases for larger changes (300+) By comparison, the speed predictor used by the prop guys is based on a bogus "prop slip" factor and no drag curve, so in many cases less accurate IMO.
Finally, as with any calculator, the result only as good as the numbers in and how well you understand what comes out.
People often throw numbers around with few details then ask what's up? A recent impeller thread mentions 800 hp and A vs. AA without including impeller mfg or rpm. Then hope for an informed response? Others quote corrected power and expect to see that spin their pump to xxxx rpms. And they boat at 1500 feet in temps equating to 4000 DA w/oversized impellers and wonder why? Without details, any predictor can churn out junk. Pump curves btw are based on actual power to the pump not corrected.
In your case, an AT A @ 4800 means 350 hp spinning it according to AT pump charts*. Is your quoted 415 hp based on a dyno test, mfg rated at xxxx rpm, or a software program. Is that number corrected and does your engine produce it at 4800 rpm? If you're gonna accurately predict a new speed you need to know details as best you can then compare apples to apples.
Let's assume 415 is a corrected number and your engine reaches that power at 4800 rpm. Now with upgrades you estimate a new corrected (apples-to-apples) power of 540 hp at 5400 rpm. That's 125 more.
Enter your drag point (58 mph @ 4800) in JPC. At 4800 your AT A absorbs 351 hp. Based on your predicted power increase: 125 + 351 = 476 hp. (Or you can calibrate the impeller to corrected numbers either way same answer) Enter 476 hp on the right and you get a new speed of 69 mph at 5316 rpm. Will your new engine develop peak power at 5300, 5400 or 5450 rpm? If you're impeller's not matched to your new engines peak power then you won't see the full power gain. A small performance jet running off-peak can loose as much as 5 mph top end. For a bigger jet maybe less but things add up.
If your drag test point was on a cold day and your new engine top speed measured on a hot day...things add up.
JPC can't account for everything and by nature it's ballpark. Just a tool to help give direction not to predict your next time slip. But I think it's a notch above the prop slip wag published in HBM. At least JPC takes a stab at how hull drag drives performance. If I had time I'd program an option to enter a custom hull drag profile. That and corrected to actual power calculations. Those two issues alone account for most of the diferences in opinions and misunderstandings in power vs. speed talked about on this forum. Then it'd be done. Maybe.
jer
*Note: AT pump charts are published. As for how accurate they are, or how well someones raggedy a** dinged impeller...or for that matter fully-detailed race impeller with tight clearances matches AT pump charts, you decide. There's a calibrator if you choose. More importantly, pump charts are a good starting point and show trends (the cubic relationship of power vs. rpm) based on impeller design and cut. Even with real world pump wear and assembly, trends adjusted for variances can still help predict performance.
