2006 Etec 300 hour service

[Bushwacker]

Originally Posted by Vezo, Part II

. . . In the efficiency category is there a breaking point at which say a 15.25 x 15 turns 5300 revs., but a 15.25 x 16 turns 5500 revs, is the 16 "less efficient" because you're burning more fuel? . . .

First of all, I mean MPG when I'm talking efficiency, since RANGE is what really counts when you only have a 34 gallon tank! (Some guys mean prop slippage, actual mph/theoretical mph, when they say efficiency, but that's a theoretical number that doesn't mean much in the real world.)

It's hard to predict the effects of small differences in pitch and diameter on 2 different propellers if they're different brands or styles because there are many other variables that affect speed vs. rpm, WOT rpm, and stern lift, etc. How deep the lower unit is in the water, boat weight and distribution, trim tab settings, blade area, and how high the engine is trimmed are just a few of the variables that can make a big difference. The number of blades and amount and location of blade cupping, and blade rake all determine if the prop is a "bow or stern lifter", and as you've noticed, those characteristics determine min planing speed, which can be MUCH more important than pure WOT speed or MPG! Although very experienced guys like Prop Gods can usually get pretty close, the only way to find the best prop is to test several different ones under similar load and wind conditions! I like to test with full fuel and max weight, simulated with coolers and life well full of sea water, bimini top up, and trim tabs fully retracted.

For example, before I tried Capt. Terry's 14.5 x 15p Cyclone on my boat, we used a length of string on both it and my 15 x 15p PT ELE prop to measure the circumference of a single blade to get a rough idea of how the blade area compared. Turns out that the blade circumference/area was exactly the same, so the Cyclone has pretty large blades, even though it's a half inch smaller in diameter than the PT prop. Now since the blade areas and pitch are the same, you'd expect the smaller Cyclone to spin faster than the PT prop, right? WRONG!! The smaller prop was actually harder to turn, only turning about 5250-5300 @ WOT vs 5400 for the bigger PT prop, AND it was 2-3 mph faster than the PT, so the Cyclone acted like it had more pitch! A closer look at the Cyclone blades revealed a VERY heavy cup on the trailing edge, which was over 1/8" thick, compared to ~ 1/16" on the PT prop, so I think the heavy cup is why it's "effective" pitch was higher than advertised! Since the mpg on the digital gage was noticeably higher with the Cyclone than PT, I decided to add a small jack plate and raise the motor almost 3" to get the AV plate up above solid water! Although I can't trim the motor past 40% with the jack plate (used to be ~50%), the Cyclone now turns up to 5400-5450 @ WOT with max load and I picked up a couple more mph, so after about 10 years, I'm finally dialed in about right!

Originally Posted by Vezo, Part II

. . . If I understand my Seafari, the Viper is a bow lifting prop and seems to take a lot of energy to get on plane, and falls off plane at almost 3000 revs . . .

Since ALL OB SeaCrafts were designed/balanced for motors of about 300 lbs, when you hang a modern motor that's over 100 lbs heavier (your 20" 150 weighs 419 lbs), you would expect it to behave better with a stern lifting prop, which is why I recommend 4B props when heavier motors are installed, as the extra blade will add stern lift if it's designed right. I went from a 300 lb motor on the transom to a 427 lb motor mounted 30" further aft on a bracket, and the difference was dramatic! Min planing speed with a 3B prop went from 12 mph to 22-23 mph! Switching to a 4B prop got it down to ~ 15 mph, and adding a fin to the AV plate got it down to 12 mph. Since the Viper has less blade area, it won't have as much low speed thrust, and with less stern lift, it'll definitely take longer to plane. Either of my 4B props will jump on plane quickly at only ~ 45% throttle . . . you'd better warn folks to hang on when you nail the throttle!

Originally Posted by Vezo, Part II

. . . What are the chances of number of splines being the same as my 1999 Johnson 175? Maybe for my next trip I will borrow the three blade SS from the MA, and see how that feels, as a comparison. . . .

Your E-TEC is identical to the 99. We've run my props on Sandy's boat, and it's an older motor, maybe a '94 or 95? What size and brand/model prop do you have on the MA? I'd definitely try in on the Seafari . . . the more you test, the more you'll learn! Too bad we didn't have time to try a couple of my props on your boat when I was over there chasing scallops!