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Whereís my speed at?

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Whereís my speed at?

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Old 02-16-2019, 11:08 PM
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Iíve got a 25í 2í 1990 Larson Senza Spectre with the 454 mag, bravo 1 with a 23pitch prop (365hp and 350hp at crankshaft). With 3 people (about 550lbs) and 3/4 tank (80 gallon tank) I run at 53mph (on GPS) @5500RPM... the RPMs seem a little high and speed seems low (even for an older boat). The 300hp test boat ran at 58mph on radar and the 230hp test boat ran at 41mph. This step hull runs 10mph faster than conventional hulls. Havenít had a ton of time to mess with the boat and really play around so this is pretty much all of the info I have. What can I do to regain some speed?
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Old 02-16-2019, 11:30 PM
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Originally Posted by Prestigeworldwideee View Post
Iíve got a 25í 2í 1990 Larson Senza Spectre with the 454 mag, bravo 1 with a 23pitch prop (365hp and 350hp at crankshaft). With 3 people (about 550lbs) and 3/4 tank (80 gallon tank) I run at 53mph (on GPS) @5500RPM... the RPMs seem a little high and speed seems low (even for an older boat). The 300hp test boat ran at 58mph on radar and the 230hp test boat ran at 41mph. This step hull runs 10mph faster than conventional hulls. Havenít had a ton of time to mess with the boat and really play around so this is pretty much all of the info I have. What can I do to regain some speed?
This is my favorite calculator, plug in your numbers.

Propeller Calculator / Prop Calculator
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Old 02-16-2019, 11:33 PM
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Have you checked gear ratio? It may be a 1.65
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Old 02-17-2019, 06:51 AM
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53 at 5500rpm sounds like a ton of slip if you have a 1.5 ratio drive.....something doesnít add up.
I dont think the ECM will allow a 454mag to turn that fast unless itís been reprogramed. Tach off?
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Old 02-17-2019, 06:53 AM
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Scratch that last statement, just realized you have a carbed engine.
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Old 02-17-2019, 11:16 AM
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Originally Posted by liberator221 View Post
Scratch that last statement, just realized you have a carbed engine.
YEP 1990 stock OEM was all carb engines. And T Bolt 4 ignition so no rev limiter for any those. CARB engines T bolt 5 ignition did have a rev limiter and were 100 rpm over max WOT from the OEM.
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Old 02-17-2019, 11:23 AM
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compression test ?

condition of the props - any redone - Some prop shops do chitty repairs inreturn cost lower performance running condition. My area that is the norm - repaired props junk work

Timing and advance ?

Full Ignition tune up needed --- possible ?

check tach rpms to compare how accurate they are. The tach possible set on the WRONG pole setting would be a good guess here. Or inaccurate

Bottom of the hull spotless clean and do you have a painted bottom ? or a repair bottom. Check for hooks in the bottom of the hull.

IMPORTANT -- holding extra weight ? water logged ? or bringing water into the boat - ? Rotted wood ?

Is your Larson the one with the single Step hull bottom ( DDC) that Harry Schoell designed ?? If I recall correctly that hull bottom for Larson was in 1992 or 93 or 94 ? Forgot when that happened

Last edited by BUP; 02-17-2019 at 12:41 PM.
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Old 02-17-2019, 12:20 PM
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Assuming your numbers are correct and it’s a 1.5 drive that’s a crazy high slip. I’d want to borrow a box stock prop and go from there.
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Old 02-17-2019, 02:05 PM
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I havenít checked my gear ratio, I assume itís 1.5 and itís one step. The prop is the original 3-blade stainless steel quicksilver prop which seems to be flawless. I think everything is original on this boat, if itís the 1.5 gear ratio, thatís means Iím getting like a 33% slip. I donít know if the RPMs are right but thatís what Iím seeing. I canít do anything right now as itís winter here.
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Old 02-17-2019, 02:22 PM
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Get a good timing light with a tach and you can check the accuracy when you get better weather. I would still try a box stock prop. Easiest thing to do. To see if the numbers change. The prop may look fine to your eyes but people knock and ding them a lot and get them repaired. Very easy to ruin a prop. Next I would go to a waterlogged type situation.
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