Prop up or down for Volvo Penta

[SeaEsta-12]

Hello VP boaters.
So, I have a question.
I recently purchased the "correct VP props" for my boat that were shipped with the boat from factory. They are the F4 series.
The previous owner had installed Quicksilver props for what ever reason and it was way over propped.
When I changed them out to the F4 series, I was able get to 4000 RPM at WOT.
My engine is a 5.7L GXi with the VP DP outdrive and SS props.
If I want to gain more RPM's, would I go to an F3 or F5?
I am not sure the diameters and pitch of VP props.
I also boat at sea level.
Boat is a 6500lb sport cruiser Chris Craft.

Thank you,

 

[ThomW]

As a general rule, for each inch you drop the pitch of the prop, you will gain about 200 rpm's. I know Volvo doesn't like to share the exact details of what the F series are pitched at, but I believe there is about a 1.5--2 inch difference between the pitch of an F4 and and F3, That being said, if you go down to an F3, you will gain about 400 rpm's, going down to an F2 would gain you about 800 rpm's. What is the boat's max rpm's rated at? I do know those duo-props are not cheap, so maybe try and find one you can try before buying?

 

[SeaEsta-12]

The boat is rated at 5000 RPM at WOT. Which should get me around 41-43 mph or 37 knots.
Currently, I can get about 27-29 knots at around 4000 RPM.
that is with a full tank (99 gallons) and about 4 people on board.
Boat weighs 6150 dry.
Am I being picky, or should I really try and change props to reach 5000 RPM at WOT

 

[bruceb58]

You definitely need to go to F3. How in the heck did someone even get quicksilver props to fit?

 

[SeaEsta-12]

I am not sure, but they fit on there and I have them for sale! haha
SS Quicksilver duoprop blades.
So, you think if I go to F3, I will get the WOT and gain speed for sure?

 

[bruceb58]

You will gain RPM and likely lose some speed.

 

[SeaEsta-12]

Well, that goes against my goal.
What I am trying to achieve is more speed.
What is the advantage of more
RPMs and less top end speed?
I don't do any towing either. Just cruising.
I'm trying to reach my 41-43 mph range.

 

[bruceb58]

You are currently lugging your engine. The only way you are going to get more speed is put a different engine in your boat.

You will never get 41MPH out of your boat/engine combo.

Is your boat slipped? Bottom paint? Growth on the hull?

 

[SeaEsta-12]

What is strange is that the factory specs from my boat and engine and outdrive combo show that I should be able to reach 5000 RPM with a speed of 41-43.
I cant get my boat over 4000 with the new F4 props.

Boat is in fresh water, but its bottom painted. Hull is clean.

 

[bruceb58]

Bottom paint will definitely slow you down. Might only be 8%-5%

How many hours on the engine?

Compression?

And don't believe factory specs. Probably not a full tank and likely one person.

 

[SeaEsta-12]

180 hours. Compression is all good. Just changed spark plugs, cap and rotor last week. DID not change wires yet. They were not in when I bought the parts.
Will do that next weekend.
You think that might make a slight diff?

 

[bruceb58]

Probably not. I still have original wires on my boat.

What plugs did you use...please don't say Champions!

You have timing set properly?

 

[SeaEsta-12]

SH*T! yes......the Champions

 

[bruceb58]

SH*T! yes......the Champions

To be honest, I don't have first hand knowledge of how bad they are. It's all DonS used to say is don't use champions. I would use the ACMR43LTS

rockauto carries them. $1.60/each

 

[muc]

you need to find what's wrong with the engine and/or drive before spending more $$$ on props.
you don't tell us what boat or engine you have (year and model) so can't give much more advice other than just props won't get this boat where you want it.

 

[SeaEsta-12]

I have a 2001 Chris craft constellation.
Motor is a 5.7 gxi. Gear is 1.95

 

[ThomW]

If you moved down to an F3 prop, wouldn't you gain rpm's and some speed? It seems like if the engine is turning over faster, you would gain some additional speed/power from the motor. I know that at a certain point, you start to gain rpm's and lose top speed, but if the boat is only at 4000 rpm's, it seems like it would gain rpm's and a little speed too? No?

 

[SeaEsta-12]

That was my exact thought........more RPMs = more speed at top end.
I hope someone on here can explain that better to me.
Im just wondering if I should even bother going to an F3, or just leave it the way it is and be happy with it.
I just feel like there is a little more I could be getting out of my engine.

 

[muc]

You won't be happy with it because running this engine like you are "lugging it" will cause the valves to tulip or burn out the head gasket.
I recommend more testing, a proper compression test both cranking and running, maybe a cylinder balance test.
You are low on power or there is something wrong with the hull.
This wasn't a good year for Chris Craft and this model in particular was kind of problematic.
Maybe hull is waterlogged?

 

[SeaEsta-12]

Hull is in perfect shape. I had a full marine inspection before purchase. I think it's either compression or my tach is not reading correctly.
I'll be doing a manual check in the next few days.
Will also be doing more testing as my tank empties and I carry I lighter load.