Propping down, how many sizes?

[Ckale]

1) Boat is feeling sluggish, will not plane, or get to max rpms.
2)Volvo penta, B5 pitch, aluminum
3) Volvo penta 290DP outdrive, B5 Duoprops
4)WOT rpms experienced were at most 3000, should be around 4300 I believe, Speed: slow :joyous:
5)Volvo Penta, 431 V6's, with 290DP outdrives, year 1990, approx 190hp each i believe
6) 1990 Carver 27 Santego, weight 8400lbs +fuel/gear

The boat is propped for elevation around 1000ft i believe, I am using it at 3700ft, wondering how many pitch sizes down I should go to achieve best efficiency and performance. I am guessing i should go down to a b3. Also wondering how B# sizes correlate to actual pitch #'s. Thanks!

 

[Maclin]

Re: Propping down, how many sizes?

Solas makes these propsets now, here is a link to their B series, in their "Select one" dropdown they have a reference to the pitch in inches for each B propset they have.

Solas Duo Prop 280/290DP Type-B Aluminum Propeller Set.

Has it always acted like this? Or Is this boat new to you? Is the bottom clean? When was last full/good tuneup? Gas ok? Carbs ok?

I would want to make sure the engines are in tune, and that there is not some other issue or combination of issues contributing to the sluggishness besides altitude and prop pitch.

Having said that, each B series "step" is about 1.5-2 inches of pitch. Your estimate of a B3 is probably close if engines and all are running as good as it gets.

 

[Ckale]

Re: Propping down, how many sizes?

Thank you for the link, that is very helpful!!!

The boat has always acted like this, we bought it two years ago, it came from a lower elevation. (around 1000ft), we are now using it at 3700ft. The bottom is clean, engines are tuned up and running well. Just cleaned carbs last year, and gas is good.

We also wanted to ensure this was not due to engines being out of tune, but we have made sure they are in good running condition. They only have 175 hours on them.

I also am thinking B3's would be about right given the amount of RPM needed to increase.

 

[Don S]

Re: Propping down, how many sizes?

You are only going to gain 200 to 250 rpm per prop size. You should be seeing between 4400 and 4800 rpm at WOT.

Before you start looking for props, make sure the tach is reading correctly to begin with, are you trimming the drives for best rpm and speed? Do you have the correct gear ratios on the drives?

 

[Ckale]

Re: Propping down, how many sizes?

Tach seems to be reading correctly. Yes trimming for best rpm and speed. Gear ratio is 2.3 to 1 I believe, which is pretty common.

With the B5's the boat would not plane out, it was close but could not get it. If the boat were able to plane, this should help to get more rpms also since there is less load on the motors. Correct? Either prop (B4 or B3) should give it the ability to plane, but which one would yield the correct WOT and cruising rpms once planed?

 

[Don S]

Re: Propping down, how many sizes?

Tach seems to be reading correctly. Yes trimming for best rpm and speed. Gear ratio is 2.3 to 1 I believe, which is pretty common.

Check, don't assume anything.

 

[Ckale]

Re: Propping down, how many sizes?

Gear ratio is 2.3 to 1. Considering there are two engines with two seperate tachs reading the same, they are correct. Back to my question please.

 

[Don S]

Re: Propping down, how many sizes?

You now have a B5 set and getting 3000 rpm at WOT. Assuming you can get the full 250 rpm drop per size change, and you are shooting for middle of the road in your WOT rpm range of 44-4800 rpm you need to come down 6 sizes. So B5 minus 6 equals a B-???????

 

[emilsr]

Re: Propping down, how many sizes?

As Don pointed out, the numbers just don't add up. The difference between 1,000' and 3,700' elevation is going to be one prop size (if that). Sounds like there's more going on here than just a prop issue.

 

[Maclin]

Re: Propping down, how many sizes?

Sorry guys, I transposed the 3700 feet into the RPM figure thinking he was attaining 3700 rpm's...! That made me think he was closer to just propping being a help. 3000 rpm is way too low for just a prop change especially with the 2.3 ratio.