Propping Correctly and how to test props

[Bushwacker]

Thanks Tim, that's great additional info!

To add some more data to illustrate the impact of getting the motor height correct and the difference between propellers, I'll summarize my own experience here, some of which I've posted at various times.

For years I've been running a 4B PowerTech ELE4 15x15 prop which turns about 5400-5450 rpm @ WOT pretty much regardless of load, right in the middle of the 5300-5500 optimum range for my motor. It supposedly uses the same blade profile as the OMC SST prop that worked very well on my old motor and it's been an excellent prop with great hole shot and stern lift; it will run about 39 mph @ WOT with virtually any load, and will cruise at about 3.75 mpg @ 3700-3800 and about 25 kts. Since Capt. Terry and I have the same basic boat and motor we've exchanged props and I was able to try his 4B 14.5 x 15P BRP Cyclone prop. Terry and I compared blade area on the PT & Cyclone props by using a piece of string to measure the circumference of a single blade and found them to be identical, so although the Cyclone is 1/2" smaller in diameter, it has very large blades! I was amazed to find that this smaller prop was harder to turn and acted like it had more pitch than the bigger PT prop and would spin about 100-150 rpm less than the PT prop in back to back testing with the exact same load! Top speed was about the same as was hole shot and low speed planing ability, but the Cyclone showed noticeably higher mpg on my I-Command gage. A closer inspection of the props showed one significant difference . . . the trailing edge of the Cyclone is about 1/8" thick and very heavily cupped, which probably explains why it's pitch appears to be higher than advertised! The BRP Rebel, which is a 3B stern lifting prop with lots of blade area, much like the Merc Mirage 3+, also appears to behave the same way. I haven't tried it because my dealer advised that the small block 2.6L motors typically can't turn it fast enough on a boat like mine, so the Rebel is normally only used on very light boats with the 2.6L motors or with the bigger 3.3/3.4L big block motors.

I was intrigued by the apparent better efficiency of the Cyclone and I knew my AV plate was way too low, despite the motor being mounted as high as possible on the bracket. So I installed a small Vance Mfg. JPL4300 manual jackplate with 3" setback and 4" of lift capability. I was finally able to get the AV plate up out of the water by cranking it all the way up to where AV plate is now about 6 1/8" above the keel! I didn't really see much change with the PT prop in either speed or WOT rpm, although I may be getting slightly better mpg. Tried the Cyclone prop again and the improvement was dramatic! It now spins up to the same 5400-5450 rpm as the PT, but now it's running 42-43 mph at WOT and a solid 4.0 mpg @ 3700-3800/25 kt cruise, so I gained ~3 mph at the top end and about 0.25 mpg better gas mileage at cruise! That's like running a motor with about 25 more hp!

So my bottom line message is: "Get that AV plate up out of the water before you start screwing around with props!"