Considering prop change. Seeking input.

[chasxjs]

Hoping for some input from those more educated on boat performance than I am.
I have a 2000 Chaparral Sunesta 252 with a Volvo Penta 5.7 GSI PEFS connected to a SX-M outdrive. It currently has a 14.75 x 17 Viper SS prop. ( # 3858026).
Overall, the boat runs great other than an occasional slight surge between 3-3,800 rpm.
Reading reviews on-line I am seeing this should be a 50+ mph boat. The best I am able to achieve is 32 mph (using gps) at 4200 rpm. This is on a glass smooth lake at wot.
I would appreciate any input you think may be helpful.
Thank you.

 

[racerone]

I do not believe a prop change will boost speed by 18 MPH.-----How did the boat run for you last season?----You might want to do a dynamometer test on the engine to determine it's condition.

 

[jimmbo]

50mph from a 5.7 in a 25ft boat is asking a lot, maybe attainable with a Big Block.
That engine has a WOT Range in the 4800-5200, so if 4200 is what you attain, then it appears to be over propped, or the engine is not up to snuff.
Do what racer suggested, and/or some basic checks, like compression, Timing Advance, Fuel delivery
How long have you owned this boat?

 

[jlh3rd]

you are 1000 rpm's below your max operating rpm range. Worrying about your supposed speed from "internet" opinions are the least of your concern.
Either you are way over-propped and lugging your motor, or your motor has a problem.
If the motor is healthy, then your looking at a 3-4 inch pitch reduction, keeping diameter, blade count, and SS construction.
As said, better check that motor.

 

[Scott Danforth]

agree, no way that 4000# dry weight boat is going to do 50 mph with a 5.7 unless the 5.7 is in a truck pulling it down the highway

with the boat being 25-26 years old, time to look into a few things.

have the boat weighed. if you are carrying extra weight such as saturated flotation foam, that will slow you down

verify your tach

do a compression test

verify the fuel system.

 

[bajaman123]

I always ask first when these threads come up...are you trimming it correctly? Many times people respond back with, 'Uh, trim?" Starting with the drive tucked in, you hammer the throttle then start trimming the drive up as you get on plane?

My 252 Baja weighs around 4,600 pounds with all gear and fluids, and I run a 21" pitch Revolution 4 prop, 4600 rpm nets me around 55 mph on GPS...with a basic Mercruiser 454 MPI engine, Bravo One drive, 1.50:1 ratio.

as others have noted, SOMETHING is seriously wrong with your WOT rpm and associated performance.

 

[cyclops222]

The engine is way too small for that boat. Need to remove the prop to get the information stamped into it.
Then call the www.propgods.com Ask for Ken. He knows boats & prop combinations very well.

 

[bajaman123]

Your prop slip works out to 29%! You are either not trimming it correctly or have a spun hub or something major. That prop @ 4200 rpm, assuming you have a 1.50:1 gear ratio, will give you 42 mph with a very realistic 10% prop slip.

 

[Bondo]

Hoping for some input from those more educated on boat performance than I am.
I have a 2000 Chaparral Sunesta 252 with a Volvo Penta 5.7 GSI PEFS connected to a SX-M outdrive. It currently has a 14.75 x 17 Viper SS prop. ( # 3858026).
Overall, the boat runs great other than an occasional slight surge between 3-3,800 rpm.
Reading reviews on-line I am seeing this should be a 50+ mph boat. The best I am able to achieve is 32 mph (using gps) at 4200 rpm. This is on a glass smooth lake at wot.
I would appreciate any input you think may be helpful.
Thank you.

Ayuh,...... Welcome Aboard,...... How long has it been since the fuel filter was changed,..??
Were the contents of the old one checked for water contamination,..??
As noted above, yer rpms @ Wot should be 'bout 5000 rpms when properly trimmed out,....

 

[airshot]

A " surge" was mentioned.....sounds like you have an engine problem, that needs fixed before anything else !

 

[cyclops222]

Do you check several motor parts for wearing out ? Like correct sparkplugs and readjusting the gap if out of specification ? Had the injectors checked out for correct flow rates ?