Propping Correctly and how to test props

[Bushwacker]

Due to popular requests for a sticky post on this subject, I decided to start a new thread with the link to the post from the Verado site which contains a ton of useful info. There are a couple other items also required for a good prop test:

1. Make sure your tach is accurate! If you're running a factory digital tach that gets it's signal from the engine computer you're good, but if you're running an analog tach, you should know that they are notoriously inaccurate! Even factory analog tachs can be off by several hundred rpm! I bought a new OMC tach when I bought my new 115 Evinrude in 1975, and it turned out to read high by about 200 rpm, which led me to buy the wrong prop! Fortunately my dealer checked it, discovered the problem and exchanged it for an accurate tach. Based on comments from the techs on the E-TEC forum, this situation is still true today!

2. Make sure you test with at least as heavy a load as you ever expect to run, if not the maximum possible load! When I first got my new motor, I found a prop that gave great gas mileage and ran almost 50 mph, but one day I had 6 divers on the boat and discovered that it would only turn 4600 rpm @ WOT and took forever to get on plane! Ever since then I always test with full fuel, and live well, plus a couple coolers filled with sea water for ballast!

3. Check your min-planing speed! This is often neglected but it's especially important on an 18 or 20, because although they're great riding hulls for their size, they are relatively light and will start going airborne at about 20 kts when seas get up to about 3'! In heavy traffic or rough sea conditions, it's really nice to be able to hang on plane at 12-15 kts and not lose your fillings!

If you have an E-TEC, you should know that BRP has published the specific narrow OPTIMUM WOT rpm range for each motor, such as 5300-5500 as opposed to a 5000-6000 rpm "Recommended Operating Range". Propping to the OPTIMUM range will give you the best overall performance. BRP discontinued model year designations starting in 2013, but uses the last 3 letters in the model number to indicate any significant changes. For 2013 and later motors designated by letters in the model number, you can find the Optimum RPM charts in the General Information section of the Owners Group forum. In general, ALL engines will perform better, last longer, and build up fewer carbon deposits if they rev up the correct/Optimum rpm @ WOT. Although 2 strokes won't have the risk of crankshaft failure mentioned on the Verado if over-propped, they will build up more carbon on the pistons, rings and exhaust ports. And just like your car or truck engine doesn't like to be lugged by running in too high a gear, ALL marine engines are more likely to encounter damaging detonation or pre-ignition (pinging) if over loaded with a prop that has with too much pitch!

[Bigshrimpin]

Here are a few links that I found helpful.

When you have your engine height set properly

Engine Height Setup: http://www.propgods.com/forum/defaul...=posts&m=11462


Different props can make a BIG difference in speed and handling.

2 examples.

http://www.boatingmag.com/boats/choosing-right-prop

and

https://www.mercurymarine.com/en/us/...units=imperial
https://www.mercurymarine.com/en/us/...units=imperial
https://www.mercurymarine.com/en/us/...units=imperial

Other links

Prop Calc http://www.go-fast.com/Prop_Slip_Calculator.htm

Speeds Predictorhttp://www.go-fast.com/boat_speed_predictions.htm

[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!"