WELCOME TO RIVER DAVES PLACE

Jet Boats...How Do They Work? (For guys who are new to jets)

Stalkaholic

Run but ya can't hide...
Joined
Dec 18, 2007
Messages
261
Reaction score
3
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.

pwr-curve-lg.jpg

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 and develop the proper fuel and timing curve for your application in order to achieve the best overall performance.
 

Oldsquirt

Well-Known Member
Joined
Dec 19, 2007
Messages
137
Reaction score
0
...........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............


Are you ABSOLUTELY sure of the statement I highlighted? Can you provide ONE concrete example of this? :hmm The reality is that the impeller's absorption curve intersects the UP SLOPE of the engine's power curve, except in the rare(probably non-existant) case that someone has decided to do something quite abnormal and allow their engine to run beyond the RPM at which peak power is produced.
The UP-SLOPE is the portion of the curve from Zero RPM to the RPM at which Peak HP is achieved. Downslope(or downside as you put it) is from Peak HP RPM on up the rpm range, where HP is falling off.
 

RiverDave

In it to win it
Joined
Sep 13, 2007
Messages
123,232
Reaction score
150,642
GREAT ARTICLE!! :)

(incidentally, not to take anything away from it, but I think oldsquirt is right on that little portion)

THANKS FOR TAKING THE TIME TO POST IT THOUGH! Definatley an interesting read!

RD
 

Stalkaholic

Run but ya can't hide...
Joined
Dec 18, 2007
Messages
261
Reaction score
3
Are you ABSOLUTELY sure of the statement I highlighted? Can you provide ONE concrete example of this? :hmm The reality is that the impeller's absorption curve intersects the UP SLOPE of the engine's power curve, except in the rare(probably non-existant) case that someone has decided to do something quite abnormal and allow their engine to run beyond the RPM at which peak power is produced.
The UP-SLOPE is the portion of the curve from Zero RPM to the RPM at which Peak HP is achieved. Downslope(or downside as you put it) is from Peak HP RPM on up the rpm range, where HP is falling off.

Hey Oldsquirt. That statement was derived from the answer to a question I asked a couple of people. Maybe you can better answer it for me.

Let's say you have a motor that peaks out at 500hp @ 5500RPM. However, you have a C cut impeller which only requires roughly around 375hp at 5500RPM. Even though the engine is capable of making 500hp at 5500RPM, with the C impeller it only needs to make 375hp at 5500RPM.

Even though you're not using up all the available horsepower at the peak RPM, will the engine not spin past the peak RPM?

The answer I got on this was that the RPM will still climb even though the horsepower is dropping past 5500RPM. However the engine is still producing more hp than the C impeller needs. Once the engine reaches the RPM on the downslope where it starts to produce less than the impeller needs, that's the RPM it will max out at.

If you have a better answer for this please post it. The more knowledge the better, and I know you're definitely one of the most knowledgeable when it comes to pumps and engines.
 

Oldsquirt

Well-Known Member
Joined
Dec 19, 2007
Messages
137
Reaction score
0
Let's say you have a motor that peaks out at 500hp @ 5500RPM. However, you have a C cut impeller which only requires roughly around 375hp at 5500RPM. Even though the engine is capable of making 500hp at 5500RPM, with the C impeller it only needs to make 375hp at 5500RPM.

Even though you're not using up all the available horsepower at the peak RPM, will the engine not spin past the peak RPM?.

Yes, it will exceed the peak, but I cant see anybody doing this intentionally. In your example it is clear that the "C" impeller is way too small. The idea is to set up the pump to absorb as much of the engine's power as possible without exceeding the Peak HP RPM. What purpose would there be to going for more rpm if it means using less power than you have available at a lower rpm? You'd be doing less work(which = less thrust) at a higher rpm. The goal is to maximize work at the lowest RPM possible. In most cases the power curve is flat enough in the area around Peak HP, that one could go up in impeller size, relative to the impeller that EXACTLY hits the Peak HP RPM, without loosing too much RPM, or giving up too much HP.

In stock form, jet boats had impellers that were large enough that they didn't allow the engine to spin to the engine's peak HP. A big block would typically get an "A" and a small block would normally get something along the lines of a "C".


The answer I got on this was that the RPM will still climb even though the horsepower is dropping past 5500RPM. However the engine is still producing more hp than the C impeller needs. Once the engine reaches the RPM on the downslope where it starts to produce less than the impeller needs, that's the RPM it will max out at.

If you have a better answer for this please post it. The more knowledge the better, and I know you're definitely one of the most knowledgeable when it comes to pumps and engines.

I think you may have partially misunderstood what you were told. An engine will turn any impeller to the point where the Engine HP curve intersects the impellers theoretical HP Absorption curve. Although you CAN go past the peak HP rpm by using a much smaller impeller, that is not the norm. Impellers are chosen by there intersect with the UPSLOPE of the power curve. Find yourself some dyno curves(without exceeding about 500HP) and plot them on the Berkeley chart you posted. That should make it fairly clear.

As far as yesterday's question regarding 1 motor and how it would perform with one of either an "A" or "C" impeller, the first constraint in the comparison would be that neither of the impellers would allow the engine to exceed the Peak HP RPM. Think about the case of the "C" exceeding and the "A" not. Your question could have three possible answers. If both exceeded, then the answer would be the opposite of the case where neither exceeded.
 

bp298

abracadabra
Joined
Dec 20, 2007
Messages
23
Reaction score
1
Yes, it will exceed the peak, but I cant see anybody doing this intentionally. In your example it is clear that the "C" impeller is way too small. The idea is to set up the pump to absorb as much of the engine's power as possible without exceeding the Peak HP RPM. What purpose would there be to going for more rpm if it means using less power than you have available at a lower rpm? You'd be doing less work(which = less thrust) at a higher rpm. The goal is to maximize work at the lowest RPM possible. In most cases the power curve is flat enough in the area around Peak HP, that one could go up in impeller size, relative to the impeller that EXACTLY hits the Peak HP RPM, without loosing too much RPM, or giving up too much HP.

In stock form, jet boats had impellers that were large enough that they didn't allow the engine to spin to the engine's peak HP. A big block would typically get an "A" and a small block would normally get something along the lines of a "C".




I think you may have partially misunderstood what you were told. An engine will turn any impeller to the point where the Engine HP curve intersects the impellers theoretical HP Absorption curve. Although you CAN go past the peak HP rpm by using a much smaller impeller, that is not the norm. Impellers are chosen by there intersect with the UPSLOPE of the power curve. Find yourself some dyno curves(without exceeding about 500HP) and plot them on the Berkeley chart you posted. That should make it fairly clear.

As far as yesterday's question regarding 1 motor and how it would perform with one of either an "A" or "C" impeller, the first constraint in the comparison would be that neither of the impellers would allow the engine to exceed the Peak HP RPM. Think about the case of the "C" exceeding and the "A" not. Your question could have three possible answers. If both exceeded, then the answer would be the opposite of the case where neither exceeded.

yup, i thought that was what i said???

you really never want to cut an impeller to the point it will allow the engine to exceed peak hp. you can, and you can exceed n/a peak with n2. but if you cut the impeller so small that you exceed peakhp, and it revs to an rpm that is on the downslope, the pump will not have the ability to absorb peak hp; it just allows the engine to rev beyond it, which is a considerable loss in efficiency and performance. i don't know if that makes sense or not, but like craig said, overlay a dyno curve with a pump curve and hopefully you'll understand what i'm trying to convey. whether it's a c, an a, or something else, it's best to have it set up so hp is on the upslope.
 

460

Well-Known Member
Joined
Dec 19, 2007
Messages
18,545
Reaction score
3,930
We need more of this around here. Great read
 

jetboatperformance

Well-Known Member
Joined
Jan 1, 2008
Messages
7,393
Reaction score
14,800
Simply put .... Water is drawn in through the intake in the bottom of of the drive intake by the rotating impeller (turned by the engine) ,The Jet Unit containing an impeller or impellers (internal propellers) then compresses and drive or forces the water out through the stator housing behind the impeller and then thru jet nozzle at the rear of the boat with great force like a fire hose . The force of this water from the nozzle propels the boat forward . Steering is achieved by turning or pivoting the jet nozzle which changes the direction of the water forced out the back of the boat and the craft changes direction. Jet Boats can also "brake" (to some degree) and reverse/back up using a deflector or reverse bucket similar to redirect the water from the forward thrust, backwards underneath the boat
 

cave

Well-Known Member
Joined
Dec 22, 2007
Messages
3,049
Reaction score
21
Simply put .... Water is drawn in through the intake in the bottom of of the drive intake by the rotating impeller (turned by the engine) ,The Jet Unit containing an impeller or impellers (internal propellers) then compresses and drive or forces the water out through the stator housing behind the impeller and then thru jet nozzle at the rear of the boat with great force like a fire hose . The force of this water from the nozzle propels the boat forward . Steering is achieved by turning or pivoting the jet nozzle which changes the direction of the water forced out the back of the boat and the craft changes direction. Jet Boats can also "brake" (to some degree) and reverse/back up using a deflector or reverse bucket similar to redirect the water from the forward thrust, backwards underneath the boat

Thank you Tom for Summarizing! :D
I tell people a Jetbote pulls water in from the bottom and forces it out the back! Therefore creating thrust. To which one could steer by simply turning the steering wheel in either direction. :D One could also add a device known as a diverter. To which one could create one hellofa Roost. See Below :skull
 

jetboatperformance

Well-Known Member
Joined
Jan 1, 2008
Messages
7,393
Reaction score
14,800
Thank you Tom for Summarizing! :D
I tell people a Jetbote pulls water in from the bottom and forces it out the back! Therefore creating thrust. To which one could steer by simply turning the steering wheel in either direction. :D One could also add a device known as a diverter. To which one could create one hellofa Roost. See Below :skull

Just one of those moves to "Keep it Simple" , having worked on a lot of complex stuff over the years one of the things I like about jets is the simplicity of operation and design :cool and your right I left out the TRIM !
axial_pump_pre_cav_ani1-1.gif
 

allblowdup

Well-Known Member
Joined
Jul 20, 2011
Messages
916
Reaction score
851
Well!!!!!. Just to throw a few wrenches into the mix. A jet drive in a boat such as a AT or Berk with a properly designed intake system will NOT pull water in but will be supplied water under pressure. The key to jet boats matching propeller boats performance is this. The jet pump is only a stepup compressor and the higher the inlet pressure the higher the outlet pressure hp asside. Another thing that happens is when you achieve high intake pressures without loader grates and the sq footage of the intake it will actually starts lifting the back of the boat out of the water. When this happens and the boat is setup correctly a great increase in speed is seen. When not correct a bow hook is seen. When perfect balance is met speeds with marginal hp are amazing. Over 100mph with around 500 hp. There is so much to jet boat performance that anyone who tells you they know it all is lying as many things are still being discovered. The true jet pump boat is a well balanced and one with the boat and is ten time more difficult to setup than a prop but much more rewarding when you hit the sweet spot. Just my.02
 

RUNNINHOTRACING163.1

Well-Known Member
Joined
Jan 24, 2012
Messages
2,819
Reaction score
19
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.

View attachment 1899

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 and develop the proper fuel and timing curve for your application in order to achieve the best overall performance.

Great Post ,very imformative :thumbup::thumbup:



ROCK ON !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

http://www.ebay.com/itm/LED-BOAT-DR...t=LH_DefaultDomain_0&var=&hash=item2c6a87c5ea
http://www.ebay.com/itm/NEW-BOAT-TR..._Accessories_Gear&hash=item3cb4089d56&vxp=mtr
http://www.ebay.com/itm/LED-TRIM-TA..._Accessories_Gear&hash=item2c6501c609&vxp=mtr
http://www.ebay.com/itm/JETSKI-STOR..._Watercraft_Parts&hash=item3ccea52093&vxp=mtr

ITS A SKATER NATION !!!!!!!!!!!!!!!!!!!!!!!!!! :champagne :champagne
 
  • Like
Reactions: JBS

Jet Boat Maloney

Well-Known Member
Joined
Oct 9, 2013
Messages
77
Reaction score
18
I'm confused. I recently purchased a sw18 with a 496 stroker 772"cam 13:1 comp dart allum heads. Max rpm is 6400 has a berk with a C impeller and top loader and inducer. Is this the correct impeller? So far I've ran it to 6200 with two people and a full load if fuel 22gal it hit 87 mph on a GPS. Should I be changing to a B impeller ? Thanks Tad


Sent from my iPhone using Tapatalk - now Free
 

jetboatperformance

Well-Known Member
Joined
Jan 1, 2008
Messages
7,393
Reaction score
14,800
I'm confused. I recently purchased a sw18 with a 496 stroker 772"cam 13:1 comp dart allum heads. Max rpm is 6400 has a berk with a C impeller and top loader and inducer. Is this the correct impeller? So far I've ran it to 6200 with two people and a full load if fuel 22gal it hit 87 mph on a GPS. Should I be changing to a B impeller ? Thanks Tad


Sent from my iPhone using Tapatalk - now Free

Tad to better understand why the boat was set up with a C impeller it would help to know more regarding the engine what kind of HP it makes and where it makes its power (RPM) also what SW hull ?
 

Jet Boat Maloney

Well-Known Member
Joined
Oct 9, 2013
Messages
77
Reaction score
18
Motor makes approx 750 HP was told the cam works 3500 to 7400.
Max rpm on the motor 6400 44deg timing
Max rpm on nitrous 7600 34 deg timing
Dual 750 quick fuel carbs
200 shot nitrous haven't hit it yet
Also haven't moved timing up yet either
77' SW 18' not sure what the model is
Here are some photos having trouble with photos


Sent from my iPhone using Tapatalk - now Free
 

jetboatperformance

Well-Known Member
Joined
Jan 1, 2008
Messages
7,393
Reaction score
14,800
The cam used on your build combined with some other features would suggest why the builder opted for the small impeller (with the motor making power up to 7400 ) we tend to try to make power between 2k and 6k and run a much bigger wheel My "example" boat is similar to yours in design , size and HP , runs an AA @ 5600 and high 80's maybe a difference in philosophy ..
 
Last edited:

23kachina

New Member
Joined
Jun 22, 2014
Messages
1
Reaction score
0
yup, i thought that was what i said???

you really never want to cut an impeller to the point it will allow the engine to exceed peak hp. you can, and you can exceed n/a peak with n2. but if you cut the impeller so small that you exceed peakhp, and it revs to an rpm that is on the downslope, the pump will not have the ability to absorb peak hp; it just allows the engine to rev beyond it, which is a considerable loss in efficiency and performance. i don't know if that makes sense or not, but like craig said, overlay a dyno curve with a pump curve and hopefully you'll understand what i'm trying to convey. whether it's a c, an a, or something else, it's best to have it set up so hp is on the upslope.


Can a impellor be too big for a engine? Or not enough hp to turn it? I have a 22' kachina jet boat 5.7 pushing 280 hp (approx) won't turn over 4100 RPM and tops out at 40 mph, any ideas?
 

Wheeler

I'm just here to bitch about others negativity.😁
Joined
Dec 25, 2007
Messages
24,921
Reaction score
39,106
Can a impellor be too big for a engine? Or not enough hp to turn it? I have a 22' kachina jet boat 5.7 pushing 280 hp (approx) won't turn over 4100 RPM and tops out at 40 mph, any ideas?

Don't go away, he'll be here to answer your question.
 

jetboatperformance

Well-Known Member
Joined
Jan 1, 2008
Messages
7,393
Reaction score
14,800
Can a impellor be too big for a engine? Or not enough hp to turn it? I have a 22' kachina jet boat 5.7 pushing 280 hp (approx) won't turn over 4100 RPM and tops out at 40 mph, any ideas?

Yes and Yes The Boat is a bit big for a small block IMO , Got a hunch however that this was the way that boat was produced and honestly that RPM number is pretty consistent with an A impeller and your estimated HP you may be "down on power" just a bit for tuning altitude etc , Simply put that's about it until more HP is added , The numbers below are from our Impeller "Dyno" calculator , its a algorithm driven and very accurate I based the calculation on the assumption your boat has an A impeller (the boat size and weight has little effect on HP vs RPM issues) call if I can help Tom

rpm hp tq

4100 246.59 315.75
4200 265.07 331.34
4300 284.46 347.31
 

Enen

Well-Known Member
Joined
Aug 9, 2008
Messages
6,039
Reaction score
4,146
I though the simple answer was they suck and blow, kind of like a chick.....before you put a ring on her finger.
 
Top