2 blade or 3 blade prop

[Bob1957]

I have a 1974 Mercury 200, 20hp outboard with what I assume is the original 2 blade prop. My boat is a 14 foot deep v aluminum fishing boat with a capacity of 950 pounds. Currently the hole shot is a little sluggish but top end speed is outstanding, actually a little scary at times. I primarily fish small lakes and medium size rivers with two passengers and am normally loaded to about three quarters of boat capacity.
What can I expect to happen, performance wise, by switching to a three blade prop? Is there an advantage to running a three blade versus a two blade?

Thanks,
Bob

 

[Crownie2]

Re: 2 blade or 3 blade prop

Bob - I guess there is no one out there with any real experience on your question.

I don't have any either but I can say that a three blade should help carry a load better - which may not be something you need.

Back when two blades were common they were used on lighter weight boats with flatter hulls and I think three blades were just not really needed - and therefore not much development work was done on them.

Bob

 

[Bob1957]

Re: 2 blade or 3 blade prop

Thanks for the reply Bob, ironic isn't it LOL, I remember reading somewhere that a three blade would produce less vibration than a two. I guess that somewhat makes sense if you consider that it would be easier to unbalance a two blade, but if both are perfectly balanced then .....? I'm not really looking for any speed boost, I definately have enough of that. I am considering installing a live well, an extra battery, electric anchor hoist and probably some other goodies which would most likely take me right up to capacity. Have looked at a new three blade for the motor and am not sure that I want to spend the $100 just for experimentation. If I do I'll make sure to post the results here, ii's hard to believe this hasn't been hashed over before on here.

Bob

 

[Texasmark]

Re: 2 blade or 3 blade prop

Well guys, when I joined this forum last year, I remembered all the Merc's that came from the factory with 2 bladed props and the racing props we used were only 2 blade...(note racing props on light fast boats running at like a 1:1 ratio with the powerhead). So I was making waves in that direction and got put into the 21st century very quickly by Kenny (Walleyhed) and others.

Only reason I can think of (that Merc shipped engines with them for years) was the fact that the 2 blade was the most efficient (from a propulsion engineering standpoint)......but as you said they vibrated the most (other than a single blade) due to the distance between the thrust points as the blade turns. We never raced a prop with more than 2 blades.....back in the '50's. That was then.

Also I recall having 2 props. One 2 blade that came with the engine and another 3 blade used for water skiing. I called the former my "speed" prop and the latter my "power" prop.

Today, the 3 blade has become the prop of choice (I think) because of it's balance between efficiency and vibration, and for dazzling speed and good hole shooting they port them (drill holes in front of the blades).

For heavier loads the 4 and 5's have been added and each installation has to be judged on it's own merit as to whether the multi blade is better than the 3 because you get both pro and con feedback from owner/users.

What does all this mean to you? The 3 has more blade area and I have found that that helps you at the lower speeds where prop slip is usually high. It also provides more hole shot thrust for the same reason. The drawback is that that extra blade could cut into your top end, but that probably isn't a problem for you as you mentioned that you don't like to run at WOT.

Now all you have to do is determine what pitch 3 blade to buy. Probably the best answer is the same pitch as your current 2 blade......that's where I'd start. It will reduce your low end slippage resulting in better hole and midrange performance, and should run pretty close to what you have now at WOT. Beins you are running a 20 hp engine, I wouldn't worry about high performance SS and all that stuff.....if it's available at that hp level.

My 2c
Mark

 

[Bob1957]

Re: 2 blade or 3 blade prop

Thanks Mark, I took the boat out for a run and noticed that when running at top speed I can let back on the throttle a considerable amount before the boat begins to slow down. Am starting to think that with my current setup I may be overspeeding the motor, which leads me to rethink going to the three blade prop. As far as I can tell iboats sells two props for my motor, an 11" pitch 2 blade and a 9" pitch 3 blade. Haven't checked anywhere else yet. I definitely need to find a tach and get a handle on what is going on currently. With the hull design of my boat WOT is pretty scary right now, I have never had a big enough motor on it to fully plane it out till now. I can't imagine dropping a max rating 35hp on it.

 

[Texasmark]

Re: 2 blade or 3 blade prop

Michigan Wheel Co. has been making props for 100 years and back in the '50-60's if it wasn't alum it was a MWC prop. Might look at their lineup.

On backing off the throttle, a lot of engines are "noted" for being able to back off and keep speed up. It cuts the fuel, but doesn't cut the ignition timing advance. Since the load on the engine is usually less at WOT than coming out of the hole and all, they require less fuel (are more efficient).

So I wouldn't necessarily add that feature to your list of thoughts.

Also, unless you have instruments, you have a hard time determining speed differences the faster you go. 20 to 35 mph is easy. Above that you have to change like 15-20 mph before you notice a difference (very big difference).

About 3000 rpms most engines are really quiet. 4500 is about the dividing line (for me anyway). At 6000 they scream. A tach would certainly help.

No doubt in those two props your hole and low end would be day and night. But you just threw away the top end you don't use. d

Mark