I see a lot of misconceptions regarding how jet boats work. Lots of guys have a misconception that they work just like prop boats, which work very similar to a car. Lots of guys apply the same tuning principles as they'd apply to a car to tune the engines just because it's an automotive engine, however the same rules do not apply. In this article I will explain the differences between cars, props, and jets and what their differences are in operation.
Before you go any further, this article explains the basic physics and concepts of jet boats assuming an ideal/perfect world. In actual operation, there are other variables which are not explained here that the pros can better fill you in on, but for simplicity this article will give you the basic understanding of the concept of jet boats/pumps and arm you with the knowledge to properly tune the engine to work with the kind of load a jet pump places on it.
In my first example, let's talk about what goes on with the engine and powertrain/drivetrain in a car. A car at a dead stop only allows the torque converter in the transmission to spin up to the converter's rated stall speed. Let's say you launch the car at wide open throttle (W.O.T.) and your torque converter stalls at 2000RPMs. The engine will only swing up to 2000RPMs until the car starts moving. Once it starts moving, the wheels start turning and "unload" the engine. The faster the wheels spin, the more the engine unloads, allowing the engine to pick up RPM and spin faster. The speed of the vehicle and the speed of the engine continue to climb...once the engine reaches the RPM that is near the end of the engine's power band (the range of RPM where the engine makes power), the transmission shifts to the next higher gear, which reloads the engine, dropping the RPM back down into the low side of the engine's power band. However, the car continues to pick up speed due to the higher gear ratio.
As the car picks up more speed in the next higher gear, again the engine unloads, picking up more RPM the more it unloads, until the transmission shifts to the next higher gear...so on and so forth. Transmission gear ratios, rear axle gear ratio, and circumference will determine the RPM/MPH relationship. This never changes...however, the weight of the vehicle will determine how much torque has to be made at that RPM to move the vehicle and keep it moving at a constant speed. The more weight you add, the more torque will need to be made to move the vehicle.
Typically when you select a cam for a car, you select a cam that matches the torque converter's stall speed. The stall speed allows the engine to swing up to the RPM where the engine starts to make big torque, then loads the engine at that RPM in such a way that causes the engine to make big enough torque to get the vehicle moving. Most car engines are set up to make big torque in the lower range of RPM...this keeps engine wear and fuel consumption minimal, since the engine doesn't have to spin as fast, requiring less fuel and exerting less strain on the engine.
Prop boats are very similar. A prop acts very similar to a torque converter and a tranny all in one. The prop size/blade pitch will control what the boat's max RPM will be at a dead stop...i.e. the "stall" speed, just like a torque converter. As the boat picks up forward speed, the front of the prop unloads, allowing the engine to pick up RPM as the boat speed increases, very similar to how turning wheels assist the engine in removing the load for a car motor to pick up RPM.
Jet boats...totally different animal. Jet boats work off of thrust, which is a perfect example of Newton's 3rd Law: "For every action there is an equal and opposite reaction". Jet pumps pump water into a bowl, where it is pressurized into a bowl and leaves the pump outlet at a high rate of speed. This creates a backward moving thrust, which pushes the boat forward. Think of a balloon...you fill it up with air and let it go. The balloon flys across the room. The air is exiting the balloon in a direction opposite of the balloon's flight direction, thus creating your equal and opposite reaction.
The weight of the boat and the amount of thrust the pump is capable of generating determine's the thrust/weight ratio. The weight of the boat along with the hydrodynamic design of the hull will determine how much thrust will be needed to move the boat at a certain speed. The impeller size ultimately controls how much thrust is generated at a given RPM (there are other factors as well that control this, but for simplicity we'll get into that later). The impeller size also governs how much horsepower is needed to spin it to a given RPM. The faster you spin the impeller, the more horsepower will be absorbed. Different size impellers have different horsepower requirements. For example, we'll use Berkeley brand impellers since they seem to be the most common.
Berkeley impellers are identified with letters: AA, A, B, C, so on and so forth. The higher the letter, the smaller the impeller size and the less it loads the engine, which means the less horsepower required to spin it to max RPM. In other words, it takes less horsepower to spin a C impeller to 5500RPM than it does an A impeller.
However, an A impeller spun to 5500RPM will generate more thrust than a C impeller spun to 5500RPM, but will require more horsepower than the C impeller will to spin it 5500RPM.
Take a look at figure 1 below. This is the Berkeley Impeller Power Curve chart, which will show you how much horsepower is required to spin the different impeller sizes at a given RPM.
Looking at the chart, you can see that 400hp is required at 5050RPM to spin an A impeller to 5050RPM, but only 280hp is required at 5050RPM to spin a C impeller the same speed. However, the A impeller at 5050RPM will be generating more thrust than the C impeller will at 5050RPM because more water is being moved by the A impeller at that speed. The more water the impeller moves at a given RPM, the more of a load the engine will see, and the more horsepower you will need to spin it up to that speed. However, since it moves more water at that speed than a C impeller, it creates more water pressure in the pump, which means more thrust generated at that RPM.
Now we'll get into the how the engine operation differs from how it operates in a car.
In order to generate large amounts of thrust, the pump needs to be spun a lot faster than the drive train in a car. Below about 3000RPMs, the pump doesn't provide much of a load for the engine to make any kind of torque...therefore not much thrust is generated. Thrust is generated not only by how fast the pump is spinning, but also how hard it is being spun. Once the pump reaches the RPM at which it 'hooks up', it's starting to move a lot more water, which loads the engine accordingly, necessitating the need for the engine to make big torque at the RPM where the pump really loads up. This usually happens at about 3000-3500RPM. For this reason, most jet boat engines are tuned/cam'med to make big torque throughout the 3,000 - 5500RPM range. As the pump RPM increases, the amount of water the pump is moving also increases, which loads the engine more and more, causing the torque/horsepower demand to also increase.
At some point, the impeller's horsepower curve will cross the engine's hp curve on the downside of it. The RPM at which the two curves meet on the downside will be the max RPM the engine will be able to spin that particular impeller. At this RPM, the impeller's HP demand is still going up, while the engine's hp curve is going down. Because the engine can no longer make anymore hp, the engine is incapable of spinning the impeller any faster than this RPM.
The engine is capable of spinning a jet pump as fast as the pump will allow it to regardless of whether or not the boat is moving or how fast or slow the boat is moving through the water. Boat movement has no effect whatsoever on the load that the engine sees from the pump. The pump and the engine have no idea that they're even moving anything. As far as the pump is concerned, it could care less if it were mounted to a boat or a stationary platform...all it does and ever will do is pump water. For this reason, pump RPM is completely unaffected by the weight of the boat or by the boat's forward speed. The impeller cut, tightness of the pump tolerances, nozzle size, etc etc...will determine how much thrust is generated at what RPM and how much of a load the engine will see at given RPMs. The max available RPM is determined by what impeller you're running and how much horsepower the engine makes at what RPM.
The advantage of being able to spin the pump at 100% RPM from a dead stop is that you achieve 100% thrust immediately, which makes the boat a lot more responsive than a prop. Quicker holeshot and more responsive turning are a couple of things that result from this.
Think of the pump as a high stall torque converter. According to our impeller chart, if we had a 500hp engine built to exactly match the curve of an A impeller, your max RPM would be 5400RPM. This means that our "torque converter" would stall at 5400RPM and would not allow the engine to spin any higher than that. However, since max RPM is unaffected by the boat's forward speed, you could have the boat at a dead stop or running balls out at WOT, and the pump will only allow the engine to spin up to 5400RPM. The weight of the boat, passengers and cargo and design of the hull would determine what the boat's max MPH will be.
You could also think of the pump as you would a jet engine. Jet engines have N1 and N2 RPM...N1 is the first stage fan and N2 is the second stage fan. The RPMs of N1 and N2 are expressed as a percentage (ex. 50% N1RPM or N2 RPM). Regardless of forward speed of the jet aircraft, 100% RPM=100% RPM...you can go no higher (with the exception of having afterburner on a jet aircraft). And just like a jet pump, a jet aircraft engine's RPM has nothing to do with the forward speed of the aircraft, but everything to do with the amount of thrust generated (more RPM=more generated thrust). And just like a jet boat, a jet aircraft's weight and design will control max available speed, depending on how many bombs and munitions are loaded (not only the total munitions weight, but the added drag imposed by the loaded munitions also plays a part in max available forward speed). The only difference being that a jet pump only has 1 stage and it's an impeller, not a fan, you could say 5400RPM = 100% RPM. If your max RPM=5400 and you cruise at 3700-3800RPM, you're cruising at 70% RPM.
Another thing you'll notice as you drive a jet boat is that it takes time for the boat speed to 'catch up' to the RPM. Unlike a car who's RPM increases with vehicle speed (due to the unloading of the drivetrain), when you accelerate a jet boat, it immediately swings up to 'X'% RPM (depending on how much throttle you give it) and stays there while the speed of the boat increases. Once the boat reaches the speed at which the amount of thrust applied = momentum, the boat stops accelerating and maintains that speed until you either add or subtract the amount of thrust applied by increasing or decreasing RPM. This action supports the fact that engine/pump RPM and boat speed are not related to each other in any way.
Knowing how this type of a load works an engine should help you to determine how to properly tune the engine to work with this kind of a load on it in regards to timing and fuel curve.