Low RPM at WOT, looking to upgrade prop

[Shark_bait99]

Hello,
I am currently running a 2004 Four Winns Sundowner 225 cuddy with a Volvo 5.7 gi-e, sx-m 1.51, with a ss volvo 14.75 x 19 prop.

At WOT she runs at 4200 Rpm and 43 mph(gps).. The manual suggests the operating range should be between 4600-5000 rpm.

I would like to improve the hole shot as much as possible for tow sports. We are in freshwater at 4000ft elevation so that may have some effect on the lower rpm?

To get the rpm into the correct range and also improve the hole shot I am thinking of a high five prop. The mercury prop selector suggests a 13.5 x17 prop but this is based on the 1.43 ratio, as the 1.51 is not coming up in the options. I can save $300-400 by ordering online instead of my local dealer, but trade ins will not be possible.

Does his sound like the correct prop? Any other suggestions would be appreciated. Should I be looking into why I have a low rpm at wot, or just change out the prop to get in the range.

Thanks!

 

[Sea Rider]

Re: Low RPM at WOT, looking to upgrade prop

Have you tried boat/engine combo at sea level ? That should put you to at least min wot rpm factory stated for that engine. Elevation reduces top rpm, less air, that's normal. The higher you go, the worst it gets. Must compensate with carb jet and prop pitches. Before buying a prop, check with engine manufacturer if they have a high altitude jet kit if your engine happens to be carbed.

Happy Boating

 

[craze1cars]

Re: Low RPM at WOT, looking to upgrade prop

Volvo Gi-e's are fuel injected. There is no carb to rejet and nothing to adjust. The FI auto-compensates for altitude. Plus 4000 isn't all that high really, but it does cost you 12% of your HP (assuming you have a 300 hp motor). So you're basically running a 264 hp motor that's probably propped like a 300 hp motor, and that is just enough loss that you'd lose about the same 12% in RPM. So at sea level I'd expect your current prop to spin around 4800 rpm, so for 4000 feet, pitching down a little bit would certainly make sense.

You're doing the right thing by prop shopping. So if you're down 12% in RPM and HP, it follows common sense that you need to pitch down by...12% indeed. So that moves you from your current 19 to a 17.

17 pitch High 5 sounds like a very reasonable guess. The only way to know for sure it to test drive. Prop purchases are ALWAYS a risk. But High 5's are easy to sell for very little loss of $$...

You might save money and get great hole-shot out of something like a Solas HR Titan 4 blade in a 17 pitch also. Fantastic prop for the money, simply less expensive to purchase, and will get you more top speed than a High 5.

Regardless of what you pick, the combination of adding a blade or two and raising your RPMS with a lower pitch will probably shock you with the improvement in holeshot you'll experience. And I bet your top speed stays very similar.

 

[Shark_bait99]

Re: Low RPM at WOT, looking to upgrade prop

Thanks for the advice. There is no where around here anywhere near sea level, so I can't test it. I checked out the pricing on the solas prop and even if I did need to buy a second, I would still end up paying less than my dealer wants for one high five!
I was out today with a full load and quite embarrassed that we couldn't plane. once we moved everyone forward and i sat on the bow we did plane, but as soon as everyone sat down we she went nose up and back to 13 mph.... :facepalm:

 

[craze1cars]

Re: Low RPM at WOT, looking to upgrade prop

Be aware that there is a CHANCE you will actually need to go down to a 15 pitch. That will absolutely get you on plane real easy, and it might put you real close to 5000 rpm at WOT which would be beneficial if you regularly run heavy loads.

Again the only way to know is to try. But you can sell your current 19 pitch prop, since it is of absolutely no value to you if you never run sea level.

 

[Shark_bait99]

Re: Low RPM at WOT, looking to upgrade prop

I noticed yesterday that the rpm on my perfect pass stargazer was almost 1000 rpm different from my oem tach. At WOT and full trim down, I was getting 39 mph with rpm of 5200 on the perfect pass and 3800 on the tach. Trimmed up a little, the speed increased to 42 mph with 4200 on the tach and 4800 on the perfect pass. At one point however the pp rpm showed that it matched the 4200 of the tach, so I dont know what is going on. At speeds above 20 mph the pp was consistently 1000 rpm above the oem tach.

I called pp and they asked me to check that my settings were correct for the boat (V8 vs V6) which I am pretty sure are correct, but I will confirm tonight. I also read somewhere in the forums that cleaning the contacts on the back of the oem tach may improve its accuracy. PP says that they are reading the rpm off of the same line, so if there is a discrepancy to trust the PP.

If the oem tach is not accuate, moving straight to a 15 pitch prop would send my rpms through the roof. Better to stick with a 17 pitch? I found a dealing in town that will let me demo props, so I think I will have to pay a little extra so that I can try a few options.

 

[craze1cars]

Re: Low RPM at WOT, looking to upgrade prop

All advice given is meaningless if we don't know your actual RPM. Fix & confirm tach first, then prop shop. Agree it is most likely that Perfectpass is correct due to being digital, but only if it is set properly for your engine.

But based on prop slip calculators, your Perfectpass is absolutely wrong right now.

19 pitch, 4800 rpm, 42 mph, 1.51 ratio = 27% slip.
5200 rpm and 39 mph = 37% slip.
Both these slip figures are unrealistic for a planed boat.

Factory tach 3800/39 mph = 14% slip.
4200/42 mph = 16% slip.
Both of these are realistic numbers.

Prop slip calculator confirms that the latter numbers you provided from the analog tach are much closer to reality.

 

[Shark_bait99]

Re: Low RPM at WOT, looking to upgrade prop

I rolled the dice and went straight to the 15 pitch hr titan 4 blade prop from iboats, as adding a blade already necessitates a 1" pitch decrease which would put me at 18" to keep the rpm the same. I did not want to keep the rpm the same as it was low, so I figured 1 extra inch to 17 was not going to give me enough rpm increase.

Btw the perfect pass was set to the correct engine, but for some reason the setting needed to be inverted (which it shouldn't for this engine) but nonetheless it got the pp matching the tach almost exactly.

Something was amiss initially when I hit the water, but once that problem disappeared, the new prop performed marvellously! Plane time was reduced to 8 seconds from 12 and the top speed was only reduced by 1-2 mph. I have not tested with a full load, but the huge improvement in acceleration gives me a lot of hope that I will be able to plane fully loaded.

Back to the little hiccup when I first hit the water. She accelerated well initially, however, at around 30 mph and 3200 rpm, she began to momentarily lose a little power then regain it repeatedly, in a light jerking sort of motion. The rpms would drop slightly (-250) then regain, but she would not accelerate past this point. Trimming up did not help. I initially suspected ventilation or cavitation could be causing this? Then I thought it might be the efi in the engine not recognizing the different prop and power needs? Of something blocking the fuel line???
But then it disappeared. I slowed to idle, turned upwind, and she accelerated right to My new WOT rpm of 4750, right in the range (4600-5000). I could not repeat the problem although I was not out for long, just a short water test. Other possibilities from searches indicate the prop could be slipping on the shaft or hub? Any ideas? Maybe just needed a second to get settled in?

I am going to take the prop off and make some marks to see if anything is moving where it should not, but I am not sure where to go frame there?

Thanks!