Duo prop question for a 2005 larson 228 w/5.0GXI

[Big boomer]

I bought this boat from a dealer which seems to know very little about duo prop set ups. Ill start at the beginning, the boat is a 2005 larson 228 with a 5.0 gxi motor w/270 prop HP and its a volvo penta duo prop leg. Gear ratio 1.61 (is what i was told buy same dealer) It was traded in and it had f5 props on it! Took it out for a test drive and the max RPM was 3500 and it was a dog out of the hole! top speed 30 mph, So next we put F4 on it and now out of the hole is about 30% better and pull about 3900 RPM top speed 35mph, Should i just keep going down until i achieve my 4600-5000 recommended RPM's?? Over loading the motor is no good for it! Does anyone have a similar set up that wants to share some info? I am thinking about going to F2, which if i understand that each size i should gain 300-400 rpm then it should put me into my recommended RPM range! Any input? What are the pitch sizes on F props? I read that a F7 is 26 and go down 2 for every F so thus a F5 is 25 and F 4 is 22 F3 is 20 and!F2 is 18!! Can anyone confirm this ??
Add info
2200 altitude
Motor has been checked and good
Leg has been checked as well
Boat is clean and like new
280 hrs
Please help!

 

[Maclin]

Re: Duo prop question for a 2005 larson 228 w/5.0GXI

The performance numbers you are getting are confusing, I sense you know that, and that it is a source of frustration as you seem to know way more about it than the dealer :blue:

Not sure how much help I can give except to back up your feelings that something is not quite right and needs to be addressed.

It sounds more like an engine performance problem to me after running your numbers thru a prop calc-er. I believe an F4 is about 21-22 pitch as your list mentions. Your other pitch numbers for those props are accurate in my estimation.

So, 35mph at 3900 rpm with a 21 pitch (est.) and 12% slip (est.) shows a 1.95 ratio. The 1.61 ratio stated by the dealer is (beyond) suspect, pretty sure a duoprop of that model has no ratio available like that. The lowest number I have seen listed is a 1.78 for 7.4l engines. The only three ratios I have seen available are 2.3, 1.95 and 1.78. Again, yours calcs to a 1.95 with estimated slip% and prop pitch. The 1.95 with a 5.0-5.7 V8 is THE most commmon out there on your size of boat.

I have a 22 foot cuddy with a 570/DP, which is a duoprop with 1.95 ratio and carbed 275hp 5.7. With B5's, which is 22-23 pitch, it jumps out of the hole and hits 46-48 at 4600 rpm. I am not sure how the boats compare but it just seems like yours should have hit about the same numbers as mine with the F5's or F4's on it. I can run B7's and get 50+ or so but we are usually loaded with kids and tow a lot, and the B7's are really for cruising with not as many people on board.

I would not buy/try any more propsets until the ratio is ascertained for certain, and the engine given a thorough health check to determine the true state of tune.

Here is a link to a free prop calc-er...
Propeller Calculator / Prop Calculator

 

[Big boomer]

Re: Duo prop question for a 2005 larson 228 w/5.0GXI

I just went and checked for myself and the serial number plate in the side of the leg says gear ratio 1.68. I don't get it! Could this be a leg from something else and was just put on in order to sell it i wonder?

 

[Big boomer]

Re: Duo prop question for a 2005 larson 228 w/5.0GXI

Did some more investigating and the gear ratio of 1.68 was produce between 2003 and 2008. I think if i was to gondown to a f2 prop which i estimat to be around a 18-19" i should pull full RPM and if mymfiguresvate right i should still get between 40-45 mph. What ya think?

 

[Maclin]

Re: Duo prop question for a 2005 larson 228 w/5.0GXI

Going to an F2, that is usually reserved for houseboats But if you truly have a 1.68 ratio then prop calc shows 38mph at 4500 with a 17p and 10% slip, 41mph at 18 pitch and same slip%. And that is IF the engine can rev to the top rpm.

 

[Maclin]

Re: Duo prop question for a 2005 larson 228 w/5.0GXI

Regarding the tag, it seems to me that the duoprop lower has more gear reduction in it than the single prop, so maybe just the lower was swapped to go to duoprop from single. That would explain the 1.68 tag but really closer to the 1.95 real ratio. But then we are back to the engine not putting out all it is supposed to. I am still struggling with the 35mph at 3900 with the F4's, if the ratio is 1.68 then that is 27% slip, very odd for a duoprop.

Have you verified the tach and actual speed?

 

[Big boomer]

Re: Duo prop question for a 2005 larson 228 w/5.0GXI

Rpm no but speed yes with a GPS. I am going to try putting f3 on it, i have a funny feeling the gauges dont read very accurate and my RPM is a little higher than what it shows.

 

[doyall]

Re: Duo prop question for a 2005 larson 228 w/5.0GXI

... i have a funny feeling the gauges dont read very accurate ... .

Are they Faria gauges? I have had nothing but problems with my Faria tach. Had it refurbed over the winter and within 10 hours it was malfunctioning again.

For comparison, my 2007 Azure AZ220 sportdeck with 5.0L GXi and 1.95 ratio DPS-A is rated at 5000 RPM max and will get there (per tach when working correct) with an F5, lightly loaded. Claimed dry weight is 4300#. Reached 48 - 50 MPH per GPS, depending on whether up or down river. Put a D3 on and at 5000 RPM got 40 - 42 MPH.