I haven’t seen many posts about 4 blade SS props on lighter aluminum boats, so I figure I will give a little feedback about my experience.
I have an older 1988 Alumacraft Trophy 190 that I recently repowered with a brand new Etec 135HO. This boat only weighs about 2300lbs (boat, motor, equipment, 1 person)
As a comparison between props, the first prop that was on the motor, which the dealer said was a “perfect setup”, was a 14.5x19 Evinrude aluminum prop. Well, it wasn’t. It was only after I took the boat out for the first time that I found out the motor was only able to achieve 42mph @ 4700rpms (+8 prop slip). I was also experiencing some porposing, and had a couple of prop blowouts when turning hard.
Now, some of this I blame on the boat, as the older Alumacraft trophy boats were wider and heavier in the middle of the boat than the back of the boat (91”mid, 84” rear) (dead rise= 12” mid, 10” rear), and the rest I blame in on the fact that the dealer set the motor way too low, which they said it “supposed to be that low”.
I then decided to take a look at other prop options. Even though I am sure a 17p Viper prop would be a perfect match for top speed, I really wanted a prop that will give me the best handling, especially for rough water, regardless of top speed. I decided on the RX4, as it is a bow lifting prop and should never blow out on me.
As insane as this sounds, that same dealer recommended a 15x20 RX4 for a “great match” for my boat. But, after a bit of online research, I found the RX4 seems to have more of a negative slip in lighter boat applications. I also felt the boat should top out at about 45-46mph. This is what made me decide to go with a 15”x16p RX4.
My first reaction upon receiving the prop was how HUGE it is. I am usually used to working with regular 13” aluminum props and smaller, so for a small lake boat this thing really stands out.
I also liked the idea that I can set the motor all the way up to the bottom hole, which would have the least amount of drag and allows the boat to ride in even shallower water.
Upon closer inspection there are a few things I was disappointed in, especially in regards to my application, mainly the weight of it and the thickness of the blades.
The blades seems to be as thick as the blades on the aluminum prop and you almost need two hands to carry it due to the weight.
I do understand that the RX4 is really for offshore boat applications where you have very heavy boats with very high horsepower outboards attached to it, but I still thought SS blades were supposed to be more efficient (thinner).
And now the testing…
The anti-cavitation looks too be a good amount above the water during a mid-trim 25mph test.
Even with purchasing the lowest pitch RX4 my motor was only able to achieve 5200rpms, and that is with the trim ALL THE WAY UP.
My final top speed I achieved 44mph @ 5200rpms. (-4 prop slip)
Even with being a little disappointed with the top end I cannot express how happy I am with the handling.
Not only is the porposing completely absent now, but even with the trim low, I still get a nice bow lift. I honestly thought I would never be able to keep that heavy mid part of the boat (mostly) out of the water.
The boat is also now slicing over the waves instead of pounding over them. And, instead of slamming hard back down there is more of a easy descent, like what you would feel in a roller coaster.
Lastly, the blowout problem. When people say a 4 blade prop is like having 4 wheel drive, they are not lying. Believe it or not I can start off with the trim all the way up and it still grabs. I have also towed very large tubes, and even with hard turns, where the water “tornados” behind the boat, it still grabs.
Overall, I am very impressed with the RX4. I have never experienced this much of a dramatic change in boat handling before just by changing the prop. And, even though most people would say a 15 pitch RX4 would be perfect, even if they would sell that pitch, I would rather stay with my 16 pitch and have it Labed in order to get rid of the extra thickness in the blades that I do not need in my application
06-25-2019 06:00 - edited 06-25-2019 06:05
Thank you for that detailed report. I am sure that it will help others on this site and future readers who join up.
You may want to get with your dealer and have someone go for a ride with you with the laptop and the Evinrude diagnostic software to check the accuracy of your tachometer.
It is hard to tell from your pictures, but you may want to see how the boat performs by lowering the motor by one hole. It may or may not make a difference.
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.
So I had purchased the diagnostic tool just so I can have an idea how the motor is working and to adjust the trim function.
So apparently the tachometer is reading accurate..
I also believe dropping the motor will be counterintuitive, as I need more rpms not less rpms, and I am not having issues with blowouts.
Seeing as how I do not want to give up the RX4 I have decided to have it Labbed by BBlades during the off-season. They will thin out the blades some, balance and true the entire prop, and take a little bit of cupping off of the inner rear part of the blades. This should get me the couple hundred rpms that I need and I will have less drag.