Props, 3 blade vs 4 blade

[rcwass]

I went to my local marine service and asked about the purchase of a 4 blade prop for my 1990 70HP Johnson 2 Stroke. The service manager told me not to put a 4 blade on this motor. He said a 2 stroke needs to run a higher RPM's and a 4 blade would run at lower RPM's and bog the motor down eventually doing damage to the motor. He said 4 blade props are great for 4 stroke outboards or in/out drives but not 2 stroke outboards.
Any comments on this?
Thanks,
Dick

[Rabidfish]

Your engine is rated (and recommended) to run to 6000 rpms. As long as the prop lets the engine do this, then you'll be fine.

You may have to drop down in pitch to accommodate the 4 blade. The added blade does put more load on the prop(for the same pitch)

Also, the 3 blade will be more efficient, but you may not notice it.

[starcrafttom]

dick i ran a 4 blade compprop on my 70 hp johnson on my old boat, a 20 starcraft. I really loved it. I was able to cruise on plan at lower rpm. my top end was higher and the boat was firmer in the water. it did not slide out on tight turns. trolled in a straighter line and trolled slower to. also if you are boating in colder water and troll with your main you can thin your oil mix just a little to get the motor to idle at trolling speed all day. to mush oil and they will not troll at all. that was a great motor and I got a lot of use out of it.

[rcwass]

Another thing I should mention is I went to Michigan Wheel Corps, a prop manufacturer, website and went to thier "Propellor Search", searching for a replacement 4 blade prop. I supplied the following data: Outboard, Johnson, 70HP, 3 Cylinder, 1990, 22' boat, aluminum prop, thru hub exhaust. My current prop is a 3 blade, 13 1/4 x 17. Michigan Wheels website suggested a 14 x 12, 4 blade prop. That's a huge change in pitch. I say the Michigan Wheel brand on the Boat US website and don't know of other brands. This pitch change seems quite drastic. Any comments?

[tpbrady]

I run a 4 blade Solas the same pitch as my 3 blade stock prop on an ETEC 90. The difference in RPM was less than 100 WOT (5100 vs 5200). The difference in performance was more pronounced. When accelerating from 3800 rpm up to 4600, you can actually feel the thrust (push back in the seat) whereas on the 3 blade, I didn't feel anything. All in all I'm pleased with the 4 blade.

[dogon dory]

Sorry, Folks - Post Deleted By Author

[rcwass]

I was able to put my boat in the water for the first time last Friday. With all the heavy rains there has been loads of debris in the river so I've waited for the waters to clear. I ran my 22' C-Dory Cruiser, 70HP 2 Stroke 1990 Johnson at WOT. I got 4700 RPM and after playing with the trim the best I got was 5000 RPM. I still have the original prop on my boat, a 13.25 X 17 Prop. I'm using a lot of gas. I'm told that I should switch to a 13 pitch prop, a 13.75 X 13 to be exact. i normally run my motor at 4000 RPM which means with a 4700 RPM WOT I'm running that motor hard and using more gas. Is this right and is this change a good change?
Thanks for all of your responses.
Dick

[Sea Wolf]

rcwass-

You'll never really know until you try it!

However, since you're contemplating going from a 17 inch prop to a 13, and each inch of pitch ='s approximately 200 rpm (oversimplified rule of thumb). 4 inches of prop pitch should equal 800 rpm, and 4700 + 800 ='s 5500, which is about right, since your motor is rated for a max rpm range of 5000-6000 rpm.

As pitch comes down, diameter is raised to get more bite, because the lower pitched props are used for heavier boats and/or loads. A 1/2 inch increase in diameter seems reasonable and is in the stanbdard range for a 13 inch diameter prop.

Sooner or later you're going to have to put it to the test and see how it performs.

No one can exactly predict what will work and exactly what will be the best or ideal solution. Too many variables exist between motors, boats, loading, atmospheric performance conditions, and even your own preferences between top speed, hole shot, cruising speed and mileage importance.

And you'll never really know if there just isn't another slightly different prop that might be just a bit better.

But go ahead and give it a try!

If it performs well to your satisfaction, you're good to go.

If you want to find the perfect prop, find a dealer with a big inventory of alternative props on the shelf that will let you try them out and make extensive measurements until you're satisfied.

Good Luck!

Joe.

[Adeline]

Hi Dick, I've got an '89 22' Cruiser w/ Johnson 70. My motor was supplied with a 13.25 x 17 also. I went to a 13.75 x 15 and got my rpms up to 5600 or so. As time passed I carried more and more stuff and my rpms dropped so I finally went to a 14 x 13 and am propped for about 5700. I recently picked up a Michigan 4 blade 14 x 10 (link) I'm getting about the same 5700 rpms with it. It was on sale cheap and I'm hoping it will eliminate the occasional "ventilation" I get in rough, frothy water such as the Columbia River Bar. Go here (link) and do a search and you'll find that the resident experts agree that our motors need to be propped for 5500-6000. The closer to 6000 the better, in their collective opinion. You're overpropped and your motor is being lugged with a max of only 5000.

[petemos]

Anyone install a Piranha 4 Blade Prop on a Suzuki? The instructions say to not use the Thrust Washer, just the flatwasher. Well the prop assembly has 4 different washer types, none called a Thrust washer in the service manual. From the prop nut i have a washer, spacer, propeller, stopper, and another spacer. Anyone know which one is the Thrust Washer?

If I ever get the prop installed, I bought a 16 and 18 pitch, I llet you know the results. I'm now running a 3x14x17.

Thanks Jim

[Sea Wolf]

[petemos]

Joe, thanks for the informaton. Now that I understand the perpose of the Thrust Washer, it should be real easy to see which one it is. It's got to be the washer they call the "stopper".

Thanks Again.

Jim