Prop pitch question

[ken]

Hi, looking for advice regarding prop pitch. I have a 2000 Supersport with the 315 horse GT40 engine. I currently have a 14" x 18" OJ mounted. There are times when we use the boat to cruise down the Cal Sag Channel towards Navy Pier here in Chicago rather than using it for water sports. Total travel time by the time we go through all the no wake zones is around 2 hours 1 way. My question is, would I decrease fuel usage by using a 14" x 20" prop instead of the 18" pitch. We would normally cruise at around 30 / 35 MPH going, and on the return, in the dark I would be going much slower. WOT throttle RPM shouldn't be an issue, and because of that I assume it would not create undue strain on the drive train. Does anyone have any practical experience with something like this ? Could you share your opinion ?

Thanks
Ken

[s_kelley2000]

I don't know what the actual improvement in fuel usage would be but your thinking is correct. A higher pitch prop will be slower out of the hole (especially when weighted down) but it will cruise at a lower RPM and have a higher top speed. I don't think you would have much fun trying to pull a boarder or skier out of the hole with a 14x20 but if you only plan to use it for cruising then it might work well for you. The 14 x 18 is a pretty efficient and well rounded prop for that boat if you are not adding too much ballast.

[ken]

Shawn thanks for the input. We do this cruise 3 or 4 times a year. I would swap the prop each time. The boat IS generally weighted down with crew and coolers, but top speed,hole shot and towing wouldn't be part of the mix.

[nyryan2001]

Ken-

I am not thinking the difference in 200-400rpms in that low 30s mph even over 4hr round trip cruise is going to be anywhere worth the $400+ for the new prop or the PITA to swap props.

when you look at GPH/MPG fuel consumption charts by RPMs, 200-400rpms are relatively close... as as they approach your most econmical cruise, likely around 28mph... you will see 4.3, 4.6 etc etc.

I would estimate 200rpms lower per 1" increase of pitch at cruise , 2-3mph faster at WOT. roughly 100-200rpms per lower at cruise per .050 increase of cup, 1-2mph faster.

The only reason I could see to do that wouldnt have to do with gas savings, it'd have to do with engine/exhaust sound. Some boats inadvertently are tuned in their exhaust say in the 3200-3700rpm range..which could be your perfect cruising speeds 27-34. Over a 4hr trip, with a booming V8 pipe screaming at you that could get tiresome. Some boats arent "comfy" sound wise to be on at 3400rpms+ for long periods.

[ken]

Nyryan, thanks for the insight. Given those stats I agree with you. My intention was to find a good used prop rather than a new one, but even at a reduced price it would take a long time to recoup the $$, given the piddly decrease in consumption. I guess I'll just have to find other ways to trim the budget. Thanks for the interest guys !

Ken