Here is our current thinking.
Jet pumps are not able to draw water in on their own; they need to be fed water. A jet boat feeds water to the pump by three means.
The shape of the keel has the most influence on loading the pump, but only while at speed. An object moving through the water displaces the water, causing a point of low pressure immediately following the displacement. Everything in nature attempts to return to equilibrium and the water rushes to fill this low pressure area, attempting to return to equilibrium. In a jet boat, this point of low pressure is immediately following the keel, leading up into the pump intake, just before the loader/rock grate pad. The Coanda Effect describes how the water “entrained” to the surface of the intake follows it up into the pump. The Coanda Effect is described as the phenomena in which a jet flow attaches itself to a nearby surface and remains attached even when the surface curves away from the initial jet direction. If the surface is too severely curved, the water will not follow the surface, but will bend slightly and detach from the surface.
The shoe acts as a splitter to scoop water into the jet pump. A high pressure area is created above and slightly in front of the biting edge of the shoe and a low pressure area, as evidenced by the cavitation burn on the ride plate, is created below and slightly behind the shoe. The high pressure area just above the shoe feeds the bottom of the impeller.
Note: the cavitation burn pattern on the ride plate is also influenced by the parting line between shoe and ride plate, the countersunk fasteners holding the ride plate on, and the biting edge of the shoe itself. Condition of the biting edge can also contribute to the ride plate cavitation burn pattern.
A loader is a compilation of these two effects. The leading edges and top surfaces of the ramps act like a shoe, whereas the bottom surfaces resemble the curvature of the intake and assist in loading via the Coanda Effect.
In most cases, a dropped keel will load the impeller harder at the top than the shoe can load the impeller at the bottom. The design of the loader should complement the type of hull, intake design, and installation of the intake/pump in the boat. There are many variables to loader design: length of the ramps (both leading and trailing edges), angle of attack, location, etc. One design of loader does not fit all applications. Not all loaders fit all intakes either.
With that being said, our philosophy on loading jet pumps is; every effort should be made to load the impeller appropriately for the horsepower available without overloading the pump. A properly loaded pump helps to develop more lift on the hull and build more thrust.
Some comments were made about losing RPM with the addition of a loader. If the pump is not being properly loaded, the inlet eye of the impeller is not receiving enough water to bite into. Air ingested into the pump is compressible as is aerated water. A mixed flow pump, such as a typical Berkeley jet pump, works by accelerating the velocity of water within the pump and converting that energy to pressure. Any time air is ingested into the pump, the impeller stops accelerating the water, bowl pressure and thrust drops, as does resistance against the impeller. The result is an increase in RPM.
Aerated water can be ingested by the impeller from many sources. The first and most obvious is when the intake opening physically loses contact with the water. For this reason, white water and circle boats that operate in rough water will work better with a different loader design than a drag boat. Much less obvious is driving through aerated water as in the case of a drag boat driving through the wash of the boat that made the prior pass. More importantly to this discussion is aerated water that is the result of a disturbance in the laminar flow of the water ahead of the impeller. Rock grates and turn fins are just such examples. One racer campaigning a Rogers in NJBA picked up substantial intake pressure, and therefore bowl pressure (see GT’s earlier post regarding NPSH), by cutting off the turn fin with a Sawzall in the pits. This made such a drastic difference on performance that the rough cut was further dressed and filled for additional gain.
This brings me to my strong dislike of rock grates. Rock grates are constructed with six 1/4" by 1” flat stock welded vertically to a pad that bolts to the intake. If you add up the restriction created by the rock grate, it is actually more than the effect of a 1-1/2” bar covering the opening of the jet intake. Each of the six bars of the grate creates its own turbulence and aerated water, immediately in front of the inducer (if so equipped) and impeller, depriving the pump of a clean(er), more laminar flow of water. This explains jyeager’s loss of 200 RPM. The rock grate is disturbing the water flow to the impeller, allowing it to slip. A car analogy is “riding the clutch.” The clutch pedal is partially depressed while driving which partially disengages the engine from the transmission. There is less resistance on the engine and the RPM increases, however the same amount of power is being transmitted to the wheels to drive the car.
In capogniracing’s case, the MPD loader is a blocker style and the B1 Racing loader is not. A blocker style loader does exactly what the name implies, it blocks some of the flow to the impeller. The concept is that a ramp is incorporated into the loader do deflect a portion back out of the jet intake. This has the effect of creating more lift of the hull. Another effect, and depending on your perspective, can be either positive or negative, is the impeller is deprived of water. The blocker ramp creates a shadow as the water is deflected out of the intake. This shadow is an air cavity in the intake and suction housing which is not helping to load the impeller. When a naturally aspirated pass was made, the partially loaded impeller slipped, allowing the engine to RPM higher, where it made more horsepower. More work was being done. While making an NOS pass, the engine was producing substantially more power, causing the impeller to slip more. The non-blocker loader loaded the impeller harder, more efficiently converting the NOS horsepower to thrust and the boat increased speed. While running the non-blocker loader naturally aspirated, the pump absorbed more horsepower than what the engine was capable (compared to on the NOS). The result was a loss of 200 RPM and 2 MPH compared to the blocker style loader. While making a NOS pass, the boat liked the non-blocker style.
In both cases, the pump was working more efficiently and therefore requiring more horsepower to turn the higher RPM. Our solution would be to cut the impeller so that the pump would operate at the RPM where the engine makes peak horsepower. If the pump stalls the engine at peak horsepower, more work is being done. Ultimately, the boat will be faster all around. In some cases, depriving the impeller of water (pseudo cavitation) can actually cause damage to the pump. Our opinion is that blockers are dated technology, however, if you absolutely have to have one... the customer is always right.
Really, this just scratches the surface of loader theory. There is a lot more to setting up a boat than just bolting in a loader. I would like to emphasize a word of caution here. In the case of a heavy mini-day or day-cruiser, cutting the impeller will drastically hurt launch. It is true that a smaller impeller reduces efficiency at every RPM compared to a larger impeller. But it is also true that a pump operating at a lower capacity it is designed for is also inefficient. In this case, it is better to error with a larger cut impeller. Consider the application, just how much time is spent at peak RPM? A larger cut impeller will launch harder and have better cruise characteristics. If this is where you spend most of your time, but with an occasional full-throttle blast, select an impeller cut to enhance your boating needs.
ANOTHER WORD OF CAUTION!!! LOADERS, AND THE ATTACHING HARDWARE, NEED TO BE CHECKED FREQUENTLY! Loaders are the one part where upon failure, the WILL crash. There is a tremendous load being placed on the loader, the bolts holding it, and the intake itself. You should inspect your loader frequently for cracks that might form, in the verticals, just after the loader pad where it attaches to the intake. Corrosion is another issue and if not kept in check, can render a loader unsafe. The intake should be drilled and tapped all the way through to 5/16-18UNC 2B. Inspect the torque on the fasteners every trip. Only grade 8 fasteners should be used to attach the loader. Stainless steel fasteners are between a grade 2 and a grade 5 for strength and should not be used to attach a loader. At least the rear two fasteners should have nylon locking nuts installed on the inside of the intake. Install nylon locking nuts on all four fasteners if there is no interference with the oil pan. Replace the fasteners every season to avoid corrosion fatigue. Use a lot of anti-seize to facilitate easy removal of the fasteners.
Cheers,
Joe
