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Old 07-29-2021, 01:02 PM
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Rocketboy Rocketboy is offline
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Lt-1 Max Rpm

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Was working on the boat last week and someone said that my top speed is slow…….. I get 44.

What is the MAX RPM? Maybe I am not getting all the RPM I should.

I run about 4950 wide open.

Thoughts?


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1993 S & S 190
LT-1 EFI
Stock prop- 13x13
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  #2  
Old 07-29-2021, 01:45 PM
97 Sammy 190 97 Sammy 190 is offline
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I think I get about 5,100 and 47 or 48 on the Stargazer. Have you cleaned your air filter recently?
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  #3  
Old 07-29-2021, 01:52 PM
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bret bret is offline
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I've only done it a couple of times, around 5000, 45mph with 4 blade, 47mph with 3 blade.
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Old 07-29-2021, 02:10 PM
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Rocketboy Rocketboy is offline
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Quote:
Originally Posted by 97 Sammy 190 View Post
I think I get about 5,100 and 47 or 48 on the Stargazer. Have you cleaned your air filter recently?
I cleaned it this winter and oiled it. Boat has always ran the same since I got it 3 years ago.
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  #5  
Old 07-29-2021, 02:24 PM
88 PS190 88 PS190 is offline
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#1 verify actual RPM - tach's get lazy when old or if connections get bad.
#2 prop changes things greatly - your boat is like a fixed gear bicycle it can only pedal so hard and if it has extra ooomph to give you can go steeper/bigger/more cupped and the boat will go faster. If it lacks for extra oomph a steeper prop will drop the RPM go slower and overload the engine.
#3 With boats there is an allowable top end range we use this instead of a rev limiter, an LT-1 should be propped to get to atleast 4200 and no more than 5200 rpm - ie the prop should not allow the boat to rev past 5200.

So everything sounds about right with your engine, you might want to get into a modern prop from OJ or ACME, basically the newer props are slightly smaller, slightly less pitched, with slightly more cup, and slightly more surface area. The result is the prop moves water more efficiently which lets the boat rev up a bit easier to have better hole shot but still have "enough" prop load to not let the engine rev past 5K. Sort of a best of both worlds type game.
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Old 07-29-2021, 03:04 PM
chrislandy chrislandy is offline
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The indmar manual says 5200, MC owners manual says 5000 - I reckon you're right on the money
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Old 07-29-2021, 11:32 PM
mikeg205 mikeg205 is offline
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You have to also pic' low end torque our wot speed. I picked a prop for string low end torque but loat 1 to 1.5 mph at high speed. My girl is 26 years old so WOT happens very infrequently.

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Old 07-30-2021, 08:23 AM
88 PS190 88 PS190 is offline
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Quote:
Originally Posted by mikeg205 View Post
You have to also pic' low end torque our wot speed. I picked a prop for string low end torque but loat 1 to 1.5 mph at high speed. My girl is 26 years old so WOT happens very infrequently.

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I tell people there is no reason for a prostar to go more than 45mph - even at that speed a bit of surface chop makes the hull unstable.

That said if you compare say an original 13x13 OJ or michigan wheel cast prop to a modern OJ ski pro or ACME 541/543 prop - I've gotten more top end and more low end out of these products. So its not a simple pick torque or pick speed. The new props have less pitch, more cup, and bigger blade surfaces so they can give you a bit of both.
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  #9  
Old 07-30-2021, 09:30 AM
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All good info.

Not looking to win speed races, but if I was short 4 plus MPH- I wanted to make sure everything was working like it should. Appears it is and I just have an outdated prop.
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  #10  
Old 07-30-2021, 09:44 AM
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waterlogged882 waterlogged882 is offline
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Quote:
Originally Posted by Rocketboy View Post
All good info.

Not looking to win speed races, but if I was short 4 plus MPH- I wanted to make sure everything was working like it should. Appears it is and I just have an outdated prop.
For the 1" diameter keyed shaft, Acme 541. For the spline bore - Acme 843.

Nettle Props dot com for the best price and availability. Both CNC finished and will smooth out the drive line noticeably. If you have a larger shaft in diameter, shop Nettle Props as well. They have a lot of props in stock. You'd be looking for the 13.5 x 17.5 3-blade....Acme 847 I think.

But if you need more than 45 MPH...this is the ticket for Rocketboy.

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Last edited by waterlogged882; 07-30-2021 at 09:56 AM.
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