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Scott 912 - 9.1" Two Stage with Trim

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Re: Scott 912 - 9.1" Two Stage with Trim

Postby nwticerider » Sat Mar 30, 2019 9:20 pm

Looks pretty impressive.
On one of the videos it showed what appeared to be WOT at 42-4300 rpm.

I was wondering about a bit less impellor to let that engine get up on the curve ?
Just about 100 horsepower from 43 to 5300 left on the water.
Then the cruise speed wopuld be at the torque peak for max efficiency. 3800

Now that would be sporty. If the tip speed didn't induce massive cavitation.
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Re: Scott 912 - 9.1" Two Stage with Trim

Postby Scottwaterjet » Sun Mar 31, 2019 2:55 pm

nwticerider wrote:Looks pretty impressive.
On one of the videos it showed what appeared to be WOT at 42-4300 rpm.

I was wondering about a bit less impellor to let that engine get up on the curve ?
Just about 100 horsepower from 43 to 5300 left on the water.
Then the cruise speed would be at the torque peak for max efficiency. 3800

Now that would be sporty. If the tip speed didn't induce massive cavitation.


The cavitation testing video was for a recent retrofit of an LSA\212 4.0T to LSA\912 that yielded the following outcome:

So peak torque for LSA ~ 551 lb-ft @ 3800 rpm and Peak Hp ~ 556 @ 6100.
What is to be remembered is that you are fundamentally dealing with an impeller hp demand curve that increases to the power of 3 as rpm increases. Therefore, getting a 1000 rpm increase requires significantly less impeller. For example an LSA\212 with 4.0 WOT ~ 4350, and a 212 with 2.8 WOT ~ 5100

A. LSA\212 4.0T - WOT 4350 ~ 57 mph

B. LSA\912
1. 16s/19s impeller stack - WOT ~ 5100 ~ 64 mph
2. 19s/19s impeller stack - WOT ~ 4850 ~ 63 mph
3. 19s/19p impeller stack - WOT ~ 4600 ~ 62 mph

The 19s/19p impeller stack (highest pitch) gave a slightly lower WOT speed than the two smaller combinations. However, it was cavitation resistant everywhere no matter what you did, had the best mid-range punch, best hole shot, highest speeds thru the whole rpm range (other than WOT). The other lever we have to tune the 912 is the nozzle insert size. This was left constant in the above testing.

[More RPM did yield a better top speed but a lower rpm actually produced the most sporty and driveable (the difference was significant) boat at the expense of 1 - 2 mph top end speed loss. And let's face it you probably only use that top speed less than 1% of the time you are boating :-). So you are far better off with a lower rpm in relative terms, and bigger impeller stack in this case for the LSA\912]

We have run this same impeller stack with a 6.2DI with good results also. We will provide some data on that in the near future...
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Re: Scott 912 - 9.1" Two Stage with Trim

Postby nwticerider » Fri Apr 05, 2019 8:17 pm

Sooo, I need another 3-400 hp. LOL
That is at a 100+:1 ratio so $30-$70,000. :Drink: That will go slightly faster.

The testing results helps out a bunch of us trying to figure out impeller combo's on all kinds of engine/pump combos.
It appears the hull speed is the hull speed and the push of the pump has more influence on load carrying and acceleration.

Thank you for posting the numbers.
I am going to try a bit more impeller in my 3 stage.

You are right about the wot time.
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