Scott 912 - 9.1" Two Stage with Trim

[FlyLow]

My 852 with tall HP in front of it shows no sign of this, however it is in a very light 16ft boat. Hole shot is violent with the rpm jumping to max and stays put. Will find out next spring as we are going to set up ballad thrust testing, basically full rpm sitting stationary pushing on a scale pad. If it is going to flare / cavitate etc, this should make it happen.

This sounds like a fun test, I’d love to be around for it that’s for sure. I’d also be courious what my setup would pull.

[viking]

Regarding cavitation pitting, my Berkeley exhibited significant pitting even though no flare was present. Adding a second impeller (inducer) eliminated this pitting, as well as eliminating air-ingestion-related slip. If the cavities collapse while not contacting the impeller, then good. But if they are contacting the impeller, then pitting will happen; the question is how much.

It's possible that cavitation is more problematic in the 212 because the front and rear sets of blades are very close together, so cavities don't collapse before reaching the rear blades. Then they get worse.

[mgrant]

One thing to add; the Scott has an intermediate stator between the impellers to straighten out the flow and build pressure, the turbo impeller 212 does not.

Another point to be made, when my 212 cavitated, slipped, blew out, etc, that meant that going 0 to 100% throttle at a dead stop, the tach would hit the Holley HP rev limiter at 6500 rpm and the boat would not move while the 212 frothed, and would only recover when the operator brought the throttle back to 50-70%, and the 212 would then start pumping water. Once the boat broke over the bow-wave, then you could apply 100% throttle, however going from 4000 to 4400 rpm, no appreciable speed or thrust was gained. The Scott 912 solved all these issues with my boat, plus gave the added feature of a trim nozzle to optimize boat performance and planning speed.

[lovetheedge]

My 212 with 4.0 impeller attached to an LSA does not cavitate thanks to BIll at White water marine.

[mgrant]

I took my 212 to Intermountain Machine, twice to pitch it up. Roy told me that I'm simply putting too much power to it, a recommended stepping up to a 241. However, when the Scott 912 came out as a direct swap, I decided to give it a try and it proved to be the best upgrade I've ever done to the boat.

[Riverjohn]

My 212 with 4.0 impeller attached to an LSA does not cavitate thanks to BIll at White water marine.

Aren't they just squeezing down the nozzle? I thought I read somewhere on here that is what they did. I did this to the 3 stage boat I used to have. It was blowing out and this cured it.

[mgrant]

My 212 with 4.0 impeller attached to an LSA does not cavitate thanks to BIll at White water marine.

Aren't they just squeezing down the nozzle? I thought I read somewhere on here that is what they did. I did this to the 3 stage boat I used to have. It was blowing out and this cured it.

I brought that question up when I had the 212, he said it would still blow out with the power this mopar is making, as he found with the BWC LSAs...apparently they were playing with supercharger pulleys at the time.

[Upacreek]

Thanks viking I was hoping someone would say something. With some knowledge of pumps and water this isn't very complicated to understand. I dont think people really look at the intake pressure enough. Intake pressure drops would be an obvious speed related cause of rpm flare. Considering the 912 has a 241 sized nozzle and a 212 sized intake everything adds up to be expected.
The glory to the scott pumps is the unlimited ability to stop this flare by using the correct impeller and nozzle insert combination. If you dont like the flare, get the right impeller or nozzle. I have installed an 852 with an LSA in a 24' boat that had a cuddy cabin and Alaskan bulkhead which had zero flare or cavitation. But I had many impellers and nozzle rings to play with when we water tested the boat. I installed another 852 into a 22' boat with an 8.1 and when I delivered the boat to pocatello I brought several impellers and nozzle rings with me and spent 4 hours dialing the boat in at the elevation the owner lived at. This is the glory to the scott products and the reason they wont provide kw ratings, there are just too many impellers to choose from and nozzle rings to choose from. Scott has a great product and a simple impeller selection improvement is most likely the proper solution.

[Scottwaterjet]

Some great user feedback.

Firefish 20.5' Hammerhead with a 7’ 8 degree bottom. Kodiak LSA engine. Scott 912.

Pump worked amazing!! Packed 350 gals of fuel and wall tent, kitchen, two guns each, and all gear for four guys to hunt for 8 days! We figured about 6000lbs!

Loaded cruised ~ 35-37 mph in shallow water @ ~ 4000 rpm.

[viking]

Regarding squeezing down the nozzle, there is a design "minimum flow" for pumps, but I'm unable to find a clear description involving cavitation. But visually, if you completely block the outlet, then no flow can happen and cavitation is guaranteed. Now start opening the nozzle, flow will rise, and eventually the cavitation will stop. Except, with enough power, cavitation will immediately resume due to low inlet pressure. So... above some engine power, there is no nozzle that will do the job. (Different for each pump.)

[Scottwaterjet]

On the left - SWJ912 LSA/Raptor575 <> On the right - HJ212 4.0 Turbo LSA/Raptor575
The 912 blades have not been machined.
Thoughts???

[Smoke]

Off the top of my lil head and only wrench knowledge (as opposed to engineering knowledge as I have none) I would guess the 912 is designed for to move more volume and the 212 is designed for more nozzzle pressure. It won't hurt my feelings if you tell me I'm dead wrong though.

[viking]

The HJ is very progressive pitch. At the exit, the blade angle is very steep. This implies more cavitation on the back side, and more rotational velocity that the sator must straighten.

Do you have photos of the Scott intermediate stator?

[Fishbucket]

It almost looks like you could cut 2” off the back of the Scott impeller and install it into a grey pump, while increasing nozzle size a little. This could be cool. Thoughts? Do they have the same size drive shaft? How much does a Scott impeller cost? I want to try.


Pete

[chariotdriver]

On the left - SWJ912 LSA/Raptor575 <> On the right - HJ212 4.0 Turbo LSA/Raptor575
The 912 blades have not been machined.
Thoughts???

My thought is "that is one stout impeller!"

Looks like it could pump concrete.

[Scottwaterjet]

912 Stator

[Scottwaterjet]

Scott WaterJet 912 Two-Stage Stack

[Scottwaterjet]

A couple of shots of a 912 alongside a 212

[viking]

Regarding Fishbucket's suggestion, I'm thinking (but don't know for sure) this may not work well. As a stage gets longer ("stage" being a distance without a stator, regardless of impeller count), the water picks up more rotational velocity, so you're stuck changing the impeller pitch; otherwise, near the exit, the impeller is (sort of) just chasing the water around in a circle (at the same speed the water is already moving). Additionally, as the water picks up rotational velocity, it is thrown outward (as per a centrifuge), which causes low pressure at the center and potential cavitation. Hamilton appears to deal with this by building the body outward, which effectively removes the center and requires the impeller to shrink. So their design is actually very clever, but apparently not optimal.

Atmospheric pressure being about 15 psi, and the low pressure boiling occurring at roughly 2 psi, you only need to reduce the center pressure by 13 psi to induce cavitation. That's not much.

An alternative to prevent cavitation is to drive a submarine. Pressure increases approximately 1 psi per 2 feet depth. So 30 feet doubles the pressure to 30 psi. At 1000 feet your cavitation problems are gone.

[viking]

Also, here is a Q & A from page 3 of this thread.

Based on the charts, it appears that the Scott turns more easily than the 212 at high RPM. Do you know what design feature causes this improvement?

The whole design from the intake to the nozzle are different resulting in everything working together to create a better product.

It appears that a better answer is that the HJ has a progressive pitch as a consequence of its absent intermediate stator. Speaking loosely, as RPM increases, the water is stirred more in a circle, rather than being pushed out backwards. It also implies a larger axial velocity gradient between the blades (uniform velocity will be most efficient), and it explains why the HJ seems to have an upper RPM limit above which cavitation is pretty much guaranteed. (Any pump/impeller setup will have this type of limit; however, the progressive pitch lowers it.)

My experience is primarily with Berkeley impellers, where cavitation develops from the front of the pump. Cavitation pitting is primarily at the eye of the impeller, and near the eye on the trailing edge of blades. My guess is the HJ cavitation may actually propagate from the back, rather than the front, which may explain mgrant's observation that the HJ tends to stay blown out once blown out. What we need is a high quality CFD analysis...