If the pump is properly loading (and doesn’t have other issues), the length of the impeller vanes, at the trailing edges, will be the only determining factor for peak RPM. Think of it this way, longer impeller vanes translates into a larger impeller diameter. The longer the impeller vanes are, the greater the leverage the water has acting against, and resisting, the rotational torque of the engine. In the course of repairing rock damage, I have cut back the leading edges of the impeller vanes by as much as 1/2 to 5/8 inch without any change to RPM. Only the trailing edges affect RPM.
About two years ago, I produced 6 inducer designs for testing. We did fairly extensive testing in drag boats ranging from 7 to 10 seconds in the quarter. The range of speed and horsepower in these test boats was broad enough to give us a good idea of the affect on performance that inducers have. To answer your question of “Is it true an inducer can eat a couple hundred rpm?” bottom line, we have seen no change in RPM from the addition of an inducer. We did see a change in the pressure curve of bowl pressure on the hit. Bowl pressure and therefore thrust comes up faster. MPH is gained faster but tapers off as the boat reaches top speed. This means that the gains provided by an inducer happen in the very beginning of the run, with minimal gains if any at the top end. At some point, in the fastest of boats, the inducer becomes a restriction. In some cases, jet boats can MPH higher without an inducer, but it will take much longer to get to that top speed. River racing is usually shorter than a 1/4 mile and even in quarter mile drag racing, jet boats are almost always quicker with an inducer than without, therefore ET is usually better.
To answer your next few questions, pumps don’t suck, they blow. Loaders, dropped keels, shoe biting edge, hull weight/design, and even the inducer all play a roll in how hard a pump loads. Jet pumps are not self priming and do not suck water in. Jet pumps require water being force fed into the impeller. I love the analogy, “you can not push a chain and you can not pull a liquid.” The typical mixed flow jet pump works by accelerating the velocity of water through the impeller and then providing an area of expansion towards the outlet side of the bowl and droop where the energy in the high velocity water is transformed into pressure as the velocity slows before the restriction of the nozzle. It is the restriction of the nozzle that restricts flow, generating bowl pressure. Pressure is the result of resistance to flow. Any given pump is capable of generating X amount of pressure. This pressure is directly added to the pressure at the inlet eye (suction) of the impeller. This equates to, the higher the inlet eye pressure, the higher the bowl pressure and therefore the greater the thrust.
There are quite a few dynamics going on with loading a jet pump. Anything done to increase suction pressure also increases drag as the boat moves through the water. There is a fine balancing act between loading the pump harder and increasing drag. If the pump is loaded harder and drag is increased, but the boat is quicker/faster, the net result is positive.
I see a lot of concern on the forums regarding overcharging the pump relating to lost RPM. We consider overcharging the pump the point where the pump is loaded so hard that it is not capable of processing all of the water it is loaded with. This excessive pressure build up in front of the impeller is then puked back out the intake opening, sometimes resulting in blowing the tail. We feel that it is good to find this point and know your limits. Some hull designs are simply not capable of reaching this point. Weight could also play a roll in that you never feel it, but the boat slows down. Keel shape and depth contribute the most. Too much keel depth, relative to horsepower, will overload the pump and create excessive drag. When we do a keel, we shape it for the intended horsepower. If a change in horsepower is made, a change in keel depth needs to be made also.
Here is the can-of-worms or the ripple effect. One change necessitates a whole series of other changes. You add more horsepower, so you add more keel depth. To balance out the suction pressure, top to bottom, you add more shoe depth. Now the leading edge depth of the loader ramps needs to change as well. This will probably also correspond to an impeller cut change and possibly a nozzle insert change. And then the slippery slope continues.
So far, everything that I have written about has been for the sole purpose of feeding the impeller. Increasing the impeller inlet eye pressure has another effect, it reduces cavitation. Cavitation has some effects that can be viewed as positive or negative. We, however, feel that the effects of cavitation are all negative and every effort to eliminate cavitation should be made. First, cavitation can cause damage to the pump. Second, cavitation bubbles are compressible, yet displace volume in the impeller and bowl that could be utilized by water producing useful thrust. Some view this second issue as a bonus in that the engine can pick up a few hundred RPM. Yes, the engine picked up RPM, but at the loss of thrust. If the boat doesn’t go any faster or worse yet, slower, it is a negative. If a change in RPM is desired, it is best to do so with an impeller cut change. Equate this concept to slipping the clutch of a drag car all the way down the track because it doesn’t have a low enough gear to allow the engine to come up to peak horsepower. The additional horsepower produced is converted to heat energy and dissipated into the water instead of producing forward thrust. You also don’t want to be slipping the impeller and burning up excessive fuel while simply cruising around.
Bowls do not affect RPM. We feel that bowls do make a difference in performance, but don’t recommend a change unless for an all-effort race boat. A fully-detailed Dominator bowl is worth about 0.10 to 0.15 seconds (in back-to-back testing) over a fully-detailed Berkeley G bowl. The last two seasons that 418 competed, it ran with an un-detailed Dominator bowl which replaced a fully-detailed Berkeley G bowl. The performance was roughly the same. Looking at the pressure trace data and the seat-of-the-pants feel, the un-modified Dominator bowl equipped pump struggled to pull down the engine’s horsepower as quickly as the detailed Berkeley G bowl. Once the engine came on the pump, the RPM ultimately remained unchanged. Though we have not done specific back-to-back testing of an un-modified Dominator bowl to a Berkeley G bowl, the performance is similar. For a river hot rod, I don’t think it is worth spending money, converting from a Berkeley B to a G bowl and for a race program, money invested in a modern Dominator bowl is well worth it.
For the most part, detailing whatever bowl you have can help improve pump efficiency and performance. Any dings and imperfections on the leading edges of the bowl vanes create cavitation. Even a simple 15 minute tune up, with a file, can make a positive difference where damage from ingested foreign debris can make a noticeable negative difference. Foiling the trailing edges of the bowl vanes assists in water control. Water leaving the bowl passages is less turbulent. The constriction at the convergence point where the vanes end is reduced. There is less swirling of the water in the droop and therefore less lost energy that is translated into useful thrust.
I really don’t think a river hot rod would notice any difference switching between a Berkeley and a Dominator Suction housing. The Berkeley suction housing does show better bottom suction pressure compared to the Dominator. A little Bondo filler in the short-turn of the Dominator does show improvements in suction pressure, however, it will eventually pass through the pump. The Dominator suction housing is a much heavier casting and is less likely to crack. The newer Berkeley suction housing with the cursive logo is the slightly heavier and stronger casting of the Berkeley housings. If you are not pushing 1,000 horsepower, it is not necessary to upgrade to a Dominator suction.
For now, what I will say about intakes is that how the intake is installed makes a significant difference to performance. Attention to detail and properly blending the opening of any intake into the keel will make or break performance. Of the 4-degree intakes, the later Berkeley intakes have a very affective contour. For race applications, the low profile intakes load really hard but require relocating the engine and could mean oil pan issues.
Hope this gives you some good food for thought.
Cheers,
Joe
