Impeller ?'s - Why do they use 2 different pitches? - AR230

[Jeff Nelson]

Simple question, does anybody know why there are 2 different pitches between the port and starboard impellers? Has anybody tried to use the same pitch on both pumps? My impellers are fine, my boat runs perfect and I am simple curious because I am bored.

 

[Julian]

It has to do with the water flow into the intakes and how that flow impacts the impellers and the direction that they spin. Ideally you'd have each engine spin each impeller in opposite directions....but that makes engine manufacture more expensive. As the water comes off the center line of the boat into the intakes it goes into the intakes unevenly. On one side that water flow matches the rotation of the impeller so they can put a higher pitch on that side, where the other side, the flow into the intake is opposite of the impeller spin, so they put less pitch on that impeller.

 

[Rwittmann]

I want to put aftermarket impellers on my 2008 AR230. But I can’t find the impellers with the different pitch. Everyone says get the same one for both sides. That doesn’t seem right since they are different from Yamaha. Andy advice?

 

[Speedling]

I want to put aftermarket impellers on my 2008 AR230. But I can’t find the impellers with the different pitch. Everyone says get the same one for both sides. That doesn’t seem right since they are different from Yamaha. Andy advice?

I bought a pair of solas dynafly 14/20 impellers and did a very very slight impeller bending to get it close enough. You can solve the impeller loading and rpm difference with the lucky 13 cone as well.

 

[Rwittmann]

I bought a pair of solas dynafly 14/20 impellers and did a very very slight impeller bending to get it close enough. You can solve the impeller loading and rpm difference with the lucky 13 cone as well.

What is the lucky 13 cone?

 

[Speedling]

What is the lucky 13 cone?

It's an aftermarket pump cone that you can extend to create more or less pressure in the pump. Read about people's results by searching lucky in the search bar. Some good write ups for sure.

 

[Ronnie]

I remember asking this question long ago on a web site (Yamahajetboaters.com) seemingly far away now. The most basic response I got and believe is that by varying the pitch slightly between impellers, the engines’ rpms will better match each other throughout their throttle ranges, the amount of water pressure produced by each pump will also match which results in the boat tracking (goIng straight) more reliably / predictably throughout the throttles’ ranges when both throttles are in the same position.

Summarily, the impellers are pitched differently to keep the boat going straight. If the impellers had the same pitch one engine would over or under rev compared to the other and cause it’s pump to produce more or less thrust compared to the other and the boat would turn/drift in the direction of the under reving impeller. The best analogy I can think of is having mis-aligned wheels on a car.

To address this, I like many others, bought matching Aftermarket impellers to improve performance (either acceleration or top speed) but have one of the impellers repitched to accomplish the same thing that Yamaha did tracking wise. I’ve been told that twin engine Yamaha jet boats started coming with matching impellers in 2015 (when the second generation of the 240 series came out).

Have you noticed that your boat turns sharper in one direction than the other? I believe that this is caused by the torque of the engines / impellers spinning In the same direction.

 

[Inthrustwetrust]

I believe the correct term would be “P-Factor” if you want to google it, it will explain it better than I can. Spark notes-as a propeller spins it actually creates more thrust on one side of each rotation than another, this is turn causes the vehicle to yaw. Most noticeable is at low speed with big power inputs.

Yaw effects how the water flows into the pump, to much yaw can mess up the flow and cause cavitation, or just a overall lack of power with “dirty” water.

Most my knowledge of this comes from airplanes. It’s pretty noticeable on prop planes, bigger props have rudder boost to help counter the effect. Jet/Turbine aircraft also have this aerodynamic effect with turbine spin but it is pretty much a non factor, I don’t remember exactly why, but something to do with science

 

[tdonoughue]

(on a jet the thrust is very disproportionately high relative to the spin, so the effect is negligible. on a prop you have a much closer ratio and therefore noted effect)

 

[Julian]

I read in that other site, that the reason for different pitch impellers was that the water coming into the pump was flowing different directions based on which side of the keel they were. Imagine the flow on the port side would create a water spin exactly the opposite of the spin on the starboard side. Ideally each pump (engine) would spin in the same direction as that water flow, but it costs too much money to have engines/pumps that spin opposite directions. So the side where the water is spinning opposite the impeller direction gets a lower pitch impeller. I recall reading that, but is that accurate? I have no clue.....but seems to make sense?

 

[Ronnie]

I know it would cost a ton to make the engine to spin the crank in the opposite direction but why not keep it simple and offset the drive shaft/impeller from the crank and reconnect them with 1:1 gears?

 

[tdonoughue]

Cost. Just cost. Even the simple 1:1 gears would have cost to that transmission, including a need to lubricate them, change that lubrication periodically, etc.

Now, what we need is someone who is rebuilding one of these to put in such a gearbox and try it out...

 

[Ronnie]

Cost. Just cost. Even the simple 1:1 gears would have cost to that transmission, including a need to lubricate them, change that lubrication periodically, etc.

Now, what we need is someone who is rebuilding one of these to put in such a gearbox and try it out...

They would also need to design and develop an impeller that spins/blades are pitched in the opposite direction.

 

[Dragon]

I remember asking this question long ago on a web site (Yamahajetboaters.com) seemingly far away now. The most basic response I got and believe is that by varying the pitch slightly between impellers, the engines’ rpms will better match each other throughout their throttle ranges, the amount of water pressure produced by each pump will also match which results in the boat tracking (goIng straight) more reliably / predictably throughout the throttles’ ranges when both throttles are in the same position.

Summarily, the impellers are pitched differently to keep the boat going straight. If the impellers had the same pitch one engine would over or under rev compared to the other and cause it’s pump to produce more or less thrust compared to the other and the boat would turn/drift in the direction of the under reving impeller. The best analogy I can think of is having mis-aligned wheels on a car.

To address this, I like many others, bought matching Aftermarket impellers to improve performance (either acceleration or top speed) but have one of the impellers repitched to accomplish the same thing that Yamaha did tracking wise. I’ve been told that twin engine Yamaha jet boats started coming with matching impellers in 2015 (when the second generation of the 240 series came out).

Have you noticed that your boat turns sharper in one direction than the other? I believe that this is caused by the torque of the engines / impellers spinning In the same direction.

If the 2015 + boats now have matching impellers, what else was done to correct the imbalance problem? Are those owners just living with it?

 

[Cambo]

If the 2015 + boats now have matching impellers, what else was done to correct the imbalance problem? Are those owners just living with it?

The engineers most likely gave input to the best operation on the early models but field testing probably determined that the impact was negligible so now they just use the same impellers with no noticeable issues. The newer models are really low on tolerance to how the entire jet pump assembly mates up with the hull there are lips steps and valleys that the older models don't have. Some members have used bondo to smooth out the imperfections .

 

[Dragon]

Do you happen to know what pitch the 2015+ impellers have?
I have a 2011 242 Limited S and it has a small lip on the port side intake just past the intake grate. Should I sand it down?

 

[Cobra Jet Steering LLC]

Well we do hear a lot about the right hand pull of the newer boats and I often wonder if non counter rotating pumps and the keel in the center of the boat are not contributing to that or even the cause of it.

 

[FSH 210 Sport]

@Julian is correct.

Yamaha was using two different pitches between sides and that kept peak rpms the same.. my boat came with identical impellers and the port engine always spun faster than the starboard. When I got my high altitude impellers from Impros the port and starboard impellers have a different pitch and now both engines spin the exact same rpms at full song.

As to the counter rotating impellers, have you noticed that your boat turns better in one direction? Thats the torque of both engines doing that, and it is amplified because there are two engines. Single engine boats will also turn better in one direction. In a prop driven air plane like a single engine cessna, during take off and climb you always have pressure on the right rudder pedal to deal with the torque from the prop.

The cost of putting of making an engine that spins the opposite direction would be cost prohibitive as well.. meaning that the cost of the boats would be higher to deal with R&D and production of limited number of engines that spring the opposite way… would have been easy to do with the two stroke motors though.

On a twin engine boat, say an outboard powered one, the starboard engine is usually just run in reverse with a reverse pitch impeller to counteract the torque. The boat turns the same in either direction. Having the counter rotating props also makes a big difference when in reverse, and especially when splitting the throttles. Our rented house boat last year had two outboards but they were not counter rotating, and when splitting the throttles it would not maneuver like it should have.. too bad really with that much windage the correct prop set up would have made handling that big beast a lot easier.