Does Pitch/Prop vary the gas mileage?

[Alumacraft1]

All,
I have a "new to me" Alumacraft Trophy 180 powered with a 125 hp Mercury (2-stroke)Both 1996. It came with a dinged up prop and skeg (which I just had both of them fixed). The original prop was an 19 pitch aluminum (generic, could barely read the numbers). While I was rigging I picked up the same diameter Stainless Laser II prop in a 20 pitch. Figured I could pick up a couple miles an hour etc. I used the stainless until I just got this one fixed a couple of weeks ago. However, now the boat is being used for tubing/waterskiing etc, so I put the lower pitch Aluminum Back on. This weekend, I was fishing leech lake with the family and threw the Stainless back on. Anyway, long story short I ran around with 5 of us approximately 12 miles with the stainless and used up 3/4 tank of gas. I then went back to the aluminum the next day and made the same running around and only used 1/3 tank of gas. Is this prop over pitched for my rig, or am I crazy? Details: Stainless runs 38 mph at 5,000 rpm wot. Aluminum runs 38 mph at wot of 5800 wot. No difference in top end, but the aluminum seems to have much better hole shot. 13-1.4" prop size.
Any info is appreciated.

Justin

[REW]

Justin,
You are really Lugging the motor with the heavier pitched prop.

If you are by yourself in the boat, I expect that you would really like the 20 pitch stainless. Then, you could be running about 43 at wot and your mileage would likely be better than the lower pitch prop.

It sounds like you need to pick up another 18 pitch prop that is not beat up for your heavy load running.

A 125 with a boat your size normally will run either a 17 or an 18 pitch prop for all round operation.

As I said at the beginning of the post - with a light load - for example an empty boat with one or two folks in it - the 20 pitch prop could be the optimum prop for your rig.

Good luck
REW

[Alumacraft1]

REW,
the recommended RPM's from Mercury on this motor is 5250 max at wot. Do I drop down a pitch from the current 19 aluminum to go with a stainless? I probably won't drop the pitch on an aluminum since the rpm's are already running a little high(5800 with current 19 pitch aluminum), but what should I run in a stainless? any ideas are much appreciated.

J

[Shellback]

Most often you go down in pitch when switching to stainless.

[REW]

Alumacraft,
In your particular case, I would suggest that you work with your dealer and first try a stainless in the same pitch as an aluminum prop that works well.

Depending on the prop design, the rpms of equal pitch stainless and aluminum props will be quite similar in your HP range.

Good luck
REW

[vc135]

I experienced the same thing the last 2 days test running props. in one hour tubing burned up a 1/4 tank of gas with SS prop and today went 4 hours on a 1/4 tank of gas with original Aluminum. I did notice the prop i was testing had vents on it and the SS was running 4000-4200 at 25 MPH (tubing speed) and my Original only runs at 3000-3200 at 25 mph Tubing speed. this is a yamaha 150. The lower RPMs way better on gas the aluminum was a 17 pitch and the SS I tested was an 18 pitch. Both are 14 1/4" I wonder if the prop vents had something to do with the extra RPMs being as a slower speed always venting?? Top end was 2 mph faster only with the SS at 46 and the original at 44. Have a 17 pitch on the way to test with NO vents and see what happens

[yarcraft91]

OK- I'll bite.

Justin's outboard has a WOT max rpm rating of 5250. He said he was turning 5800 rpm with his aluminum prop (on that motor, that would be 25% prop slip!!). Doesn't that motor have a rev limiter? Shouldn't a rev limiter have kicked in well below 5800 rpm, like maybe 5500? Is it possible his tach readings are incorrect?

Something doesn't sound quite right here.

[moose-hunter]

Originally Posted by yarcraft91

OK- I'll bite.

Justin's outboard has a WOT max rpm rating of 5250. He said he was turning 5800 rpm with his aluminum prop (on that motor, that would be 25% prop slip!!). Doesn't that motor have a rev limiter? Shouldn't a rev limiter have kicked in well below 5800 rpm, like maybe 5500? Is it possible his tach readings are incorrect?

Something doesn't sound quite right here.

You are correct... The numbers seem a little "off". I'd think revving that much over redline would do some serious damage to the powerhead.

[jopes]

did you by chance go from a 19 pitch 3 blade to a 20 pitch 4 blade?