4 blade for Volvo Penta 3.0 liter GS SX Drive; go from 19 3 blade to 18 or 17 4 blade

[insttech1]

Boat/motor are 1999 Regal LSR 1700 (17ft) with 135hp VP GS/SX inboard. Rated RPM is 4200 to 4600.
Current prop is factory 14.5 by 19 pitch factory 3 blade aluminum, with very minor imperfections that I filed clean.

I've run the boat up to 4300 RPM WOT with driver and one teen. 1/2 tank of fuel. Negligible extra weight.

Estimated non-gps speed was in the low/mid 40's.

I pull two teens on a double tube regularly, and usually have 2 more, plus driver, in the boat.

I can get on plane rather quickly (maybe 5 to 7 seconds?), but boat will only hit 3300 rpm WOT until I'm on plane, and then I
back it off to 3000 to keep speed about 20mph and stay on plane without dropping off. I keep my trim set just above bottom,
then pick it up just a hair after I'm on plane, gaining about 200rpm.

I want to get a 4-blade aluminum. From what I've read, a 14.5 by 18 would put me in the same RPM range.

But I'm thinking that with the extra load while pulling tube and passengers, I might be better off by going with
a 14.5 by 17. I'm thinking that because of the low WOT RPM with that load, so that train of thought might be wrong?

This is not a heavy boat. I do not run it for speed at all. I'm either pulling a double tube, or just getting across the lake
while on plane, to get to a fishing spot and shut down. I don't cruise for miles and miles. Top speed or mpg is irrelevant.

I was going to do more testing at full speed, with a GPS, but just dinged my prop up good tonight, so will be ordering
one this coming week. I have another 14.5 by 19 3 blade, but it has some minor nicks, so it will be my spare.

Thanks for any input!

 

[craze1cars]

Re: 4 blade for Volvo Penta 3.0 liter GS SX Drive; go from 19 3 blade to 18 or 17 4 b

I agree with your analysis 100%. An 18 pitch would get you to about the same RPMs you are now. However IMO this is not where you want to be.

For watersports and better overall performance you will be better to prop closer to 4,600 RPM, instead of your current 4,300 rpm.

In order to do that, a 17 pitch 4 blade would be more likely to be a better choice.

 

[insttech1]

Re: 4 blade for Volvo Penta 3.0 liter GS SX Drive; go from 19 3 blade to 18 or 17 4 b

Thanks for the input.
The 17 seems to be more readily available, so it's a bit cheaper to boot, and available from iboats

 

[insttech1]

Re: 4 blade for Volvo Penta 3.0 liter GS SX Drive; go from 19 3 blade to 18 or 17 4 b

Seems there's a new twist to this: I just contacted Regal to get my boat hull paint code, and I got a copy of the boat OEM spec' sheet, and it says the original prop was 15.5 by 19.....not 14.5.

I think going even larger in diameter would slow my RPM even more? I also looked up the next size engine replacement props here on iboats (for the 4.3 liter, not the 3.0), and even they show a 14.5" diameter prop on them, with a 4 blade...

I also now have factory recommended spec's of 24mph at 3000 RPM, and 44mph at 4400 RPM. And I was including the trailer weight with the boat; the hull and engine only, without fuel, only weigh 2000lbs. But with fuel and 5 people, I'm adding about 900lbs.

Any additional thoughts?

Thanks!!

 

[craze1cars]

Re: 4 blade for Volvo Penta 3.0 liter GS SX Drive; go from 19 3 blade to 18 or 17 4 b

Factory prop specs and mid-speed RPM/speed specs from the mfr are meaningless IMO...

Prop changes need to be made from known performance data based on YOUR boat and YOUR prop, not a theoretical boat that you do not drive. You have done that analysis and I think you've done it well.

There are an awful lot of people who appear on this board because they've just brought home a brand new boat from the dealer, went for a ride, and it performed like garbage becuase the factory and/or dealer did not put the correct prop on for the owner's intended usage/load/elevation/desires...

So I say throw the mfr original specs in the trash, stick with your own analysis, and buy a 17 pitch 4 blade.