Prop Recommendations

[Landon116]

I have an '01 Johnson 70 HP on a 17' Fiberglass CC (Hydra Sports) maybe 1800-2000 lbs wet fully loaded that at WOT hits ~5000 RPM with its current prop at about 29 MPH. The current prop is a Power Tech 4 Blade SS 14" Dia. x 15 pitch. This prop is heavy! After doing some research, this prop is intended for V-4 engines stated by the manufacturer. I think this is where my RPM's are going as the engine is low hours and I have fine-tuned every adjustment, it runs very good. I am looking for top speed as we rarely do any water-sports, just running the river for long distances and running the ocean to the reefs. The only need for hole-shot in my case is getting moving in the inlet when the tide is ripping, but not a big concern. I'm looking to move to an aluminum 3-blade.

The WOT max by the manual is 5,000-6,000 and I barely getting it to 5,000 as it is. Any recommendations for going more top-speed?

Thanks

 

[ahicks]

The 4 blade is about low speed torque and ability to hold a boat on plane at lower speeds than a 3 blade. The downside is there is more resistance due to the 4th blade. More resistance = less speed. If your boat is more about top speed than it is about torque, you have the wrong prop on it. Get a 3 blade. Maybe start with a 17p and go from there.

BTW, the 70 hp uses a lower unit that's very similar to some V-4's. Some call the 70 a commercial engine for that reason. Kinda like a Mercury Big Foot. They're designed to run larger diameter props - which is also counterproductive to your quest for speed. Larger diameter = more torque = less speed.....

Point being, you have a couple of things working against you when it comes to making speed. Welcome to the world of compromises that boating offers! -Al

 

[dingbat]

How do you know the engine is running well when your missing 700-800 rpm?

New to you boat?

Seems like an awfully big prop for that motor. I’m running a 15 x15 3-blade on a much bigger boat with a 200 outboard.

Go to one of the many prop selectors on the web. Punch in the requested data and see what it spits out. That will get you in the ball park.

 

[Landon116]

The 4 blade is about low speed torque and ability to hold a boat on plane at lower speeds than a 3 blade. The downside is there is more resistance due to the 4th blade. More resistance = less speed. If your boat is more about top speed than it is about torque, you have the wrong prop on it. Get a 3 blade. Maybe start with a 17p and go from there.

BTW, the 70 hp uses a lower unit that's very similar to some V-4's. Some call the 70 a commercial engine for that reason. Kinda like a Mercury Big Foot. They're designed to run larger diameter props - which is also counterproductive to your quest for speed. Larger diameter = more torque = less speed.....

Point being, you have a couple of things working against you when it comes to making speed. Welcome to the world of compromises that boating offers! -Al

Thats exactly the info I needed, thanks :thumb:

 

[Landon116]

How do you know the engine is running well when your missing 700-800 rpm?

New to you boat?

Seems like an awfully big prop for that motor. I’m running a 15 x15 3-blade on a much bigger boat with a 200 outboard.

Go to one of the many prop selectors on the web. Punch in the requested data and see what it spits out. That will get you in the ball park.

Youre right- It runs well all throughout the rpm range, idling, and so forth (from where I started with this engine a year ago you would think its a different engine, I've had it for just over a year and the P.O had all the adjustments wrong both timing and carbs so it ran terrible). Just minus that tail end of the rpm range, the more I look into it seems that its too much prop! And I've used a prop selector and it spits out props in the 13.25x15p range for a 3-blade. Thanks!

 

[racerone]

Sorry-----There is nothing " commercial " about the 70 HP motor.------And 2001 was the last year they marketed these 2 stroke 3 cylinder carbureted motors.

 

[dingbat]

And I've used a prop selector and it spits out props in the 13.25x15p range for a 3-blade. Thanks!

Big difference between a 14x15, 4-blade and a 13 x 15, 3-blade

 

[Faztbullet]

BTW, the 70 hp uses a lower unit that's very similar to some V-4's. Some call the 70 a commercial engine for that reason. ​

All 3 cylinder Johnrude use the 3-5/8 diameter gearcase except the short shaft 75hp which had the Niro gearcase.They have been this size since 1969

 

[ahicks]

All 3 cylinder Johnrude use the 3-5/8 diameter gearcase except the short shaft 75hp which had the Niro gearcase.They have been this size since 1969

My bad, must be thinking of another motor.

 

[Landon116]

The 4 blade is about low speed torque and ability to hold a boat on plane at lower speeds than a 3 blade. The downside is there is more resistance due to the 4th blade. More resistance = less speed. If your boat is more about top speed than it is about torque, you have the wrong prop on it. Get a 3 blade. Maybe start with a 17p and go from there.

BTW, the 70 hp uses a lower unit that's very similar to some V-4's. Some call the 70 a commercial engine for that reason. Kinda like a Mercury Big Foot. They're designed to run larger diameter props - which is also counterproductive to your quest for speed. Larger diameter = more torque = less speed.....

Point being, you have a couple of things working against you when it comes to making speed. Welcome to the world of compromises that boating offers! -Al

Picked up a 13.25x17p 3 blade aluminum prop last week and hit the water today. Gained 5 mph and 500 rpm! I know I can dial things in a bit more and get a little bit more but Im happy for now. This puts my max WOT rpm at 5500, top speed @ 35 mph with it trimmed right. Thanks for the recommendation!

 

[dazk14]

Since that motor will make peak HP at ~5500rpm, that's pretty ideal for top end. What brand/model prop did you install?
Power tech props are nice, but they do sell each model to a rather broad audience and 14" diameter 4 blade, is simply too much wheel for that motor and one reason your rpm went up with more pitch!