3 blade vs 4 blade

[darrklim2]

I have a 1985 30 hp evinrude. How much difference would it make to go from a 3 blade 10 inch X 13 pitch to a 4 blade 10 inch X 13 pitch prop? I am looking to lower my rpm's. I don't know what my current rpm's are exactly. (I don't have a tach) But it sounds like the rpm's are high.

 

[Silvertip]

Re: 3 blade vs 4 blade

Why would you want to lower your rpms if you don't know what they are? For all you know they are perfect with what you have. Lowering rpms may then cause the engine to lug whch will eventually destroy it. There is no simple answer to your question. It's tough enough comparing two different style 3-blade props from the same manufacturer. So trying to figure out what will happen going from a three blade (aluminum???) from one manufacturer to a four blade (Stainless????) from another is a real crap shoot. On one hand, it may actually speed up the engine -- on the other it may slow it down, but then it may stay the same. Without maximum speed, observed wide open throttle rpm and prop names (current and new), its anyones guess.

 

[darrklim2]

Re: 3 blade vs 4 blade

Why would you want to lower your rpms if you don't know what they are? For all you know they are perfect with what you have. There is no simple answer to your question.

So trying to figure out what will happen going from a three blade (aluminum???) from one manufacturer to a four blade (Stainless????) from another is a real crap shoot.

Without maximum speed, observed wide open throttle rpm and prop names (current and new), its anyones guess.

The reason I want to lower my rpm's is because to me it sounds like it is about to blow up the rpm's are so high when at full throttle. I realize sounding like and actually what it is might be two different things but I am working on getting a tachometer to find out. So for now I was just trying to see if anyone would have ideas on what would happen between the 3 and 4 blade so that way when I find out the true rpm's I can get the correct prop. I realize there is no simple answer but I was hoping someone had somewhat of an idea.

The props are both aluminum. As far as I can tell they are both OMC props.

 

[RubberFrog]

Re: 3 blade vs 4 blade

Get a tach.

 

[Silvertip]

Re: 3 blade vs 4 blade

You will probably lose some top end speed. You will probably help hole shot. You will probably lower rpms slightly, maybe none. Best I can do.

 

[darrklim2]

Re: 3 blade vs 4 blade

Alright I got a tach today but I don't know which wires to hook it up to on the motor? I have heard somewhere on here about a gray wire but I don't see a gray wire anywhere on my motor.

 

[darrklim2]

Re: 3 blade vs 4 blade

I have two threads going right now that have led to the same point from different starting positions. I don't know which one to continue with.

the other one is called " 85/86 evinrude 30 hp need a tach "

 

[darrklim2]

Re: 3 blade vs 4 blade

Well, I got the tach hooked up last night and took the boat out.

My rpm's

Second hole from the bottom - 5900
Third hole from the bottom - 6150 see I was right.(Guess there is a first time for everything. LOL)
Fourth hole from the bottom - 6100

Does this mean that I should go up to a 10 x15 from my 10 x 13 prop?

Am I correct in that generally there is a 200 rpm drop per pitch?

 

[darrklim2]

Re: 3 blade vs 4 blade

bump.... Not many people in the props forum today?

 

[Bry21317]

Re: 3 blade vs 4 blade

What are your RPMS supposed to be for that Motor? I am not sure, but I would think you would want to drop it down some.

Yes normally, rule of thumb is when you go up 2 inches in pitch your drop 200 rpms or vice versa when you go down in pitch.

So with a 15 pitch you should drop down to about 5900 in that Third hole or just over it.

 

[darrklim2]

Re: 3 blade vs 4 blade

From what I have found and from what imported F R said, my recommended rpm's are 5200 to 5800 full throttle.

 

[MikDee]

Re: 3 blade vs 4 blade

Yes, generally 200rpm change in rpm per each change in pitch, which is usually in 2" increments. Try the 4 blade prop, it maybe just right, hole shot will amaze you, and you will gain extra power, but maybe a bit less speed, and RPM

 

[darrklim2]

Re: 3 blade vs 4 blade

Yes, generally 200rpm change in rpm per each change in pitch, which is usually in 2" increments. Try the 4 blade prop, it maybe just right, hole shot will amaze you, and you will gain extra power, but maybe a bit less speed, and RPM

The only thing is I can't afford to throw money at this. I really need to get it right the fist time if I can or get the prop from somewhere that will let me exchange it if it doesn't work. But I do appreciate the comment.

Also I have found a 4 blade 10.5 inch X 14 pitch that I am thinking would probably cover both areas??????

 

[MikDee]

Re: 3 blade vs 4 blade

I would think that particular 4blade may take your RPM down a bit too much, especially trimmed down, and running more weight in the boat, but it's your money.

 

[darrklim2]

Re: 3 blade vs 4 blade

I would think that particular 4blade may take your RPM down a bit too much, especially trimmed down, and running more weight in the boat, but it's your money.

Are you sure I am looking to come down at least 350 rpm's thats how much I am over maximum recommended. I really don't know anything other than what I have learned on here so any info from you is appreciated. Can you tell me what about that prop would make you think that it would be too much for my boat?

When I tested the rpm's last night I had everything in the boat that I will ever have. Including another passenger.

I am deffinitely not saying your wrong I would just like to understand the reason why this would be too much

Thanks

 

[MikDee]

Re: 3 blade vs 4 blade

Reasons not to,
1)your going up in diameter, rpm loss
2)your going up in pitch, rpm loss
3)your going with a 4th blade rpm loss
I can't tell you how much rpm you'll lose, but you have 3 factors working against you here, what happened to that 10X13 4blade you mentioned in your first post? Why do you want a 4 blade anyway, you'll gain some in the hole shot, & power, but top speed would be better with the right 3 blade, IMO

 

[darrklim2]

Re: 3 blade vs 4 blade

Reasons not to,
1)your going up in diameter, rpm loss
2)your going up in pitch, rpm loss
3)your going with a 4th blade rpm loss
I can't tell you how much rpm you'll lose, but you have 3 factors working against you here, what happened to that 10X13 4blade you mentioned in your first post? Why do you want a 4 blade anyway, you'll gain some in the hole shot, & power, but top speed would be better with the right 3 blade, IMO

Honestly I am not sure that I want a 4 blade, I started this forum because I had found a 4 blade on ebay so I was just trying to find out if that would fix my problem. I am not too worried about loosing some speed because the way it is now my boat is very unstable at full throttle anyway. I need to slow to about 4500 rpm's before I feel comfortable making any resonably sharp turns. And I don't mean a 90 degree turn but maybe a 45 degree turn.

 

[darrklim2]

Re: 3 blade vs 4 blade

sorry wouldn't be a 45 degree turn either my math is a little off it would be a 135 degree turn. A 45 degree turn would be much sharper than a 90 oops

 

[MikDee]

Re: 3 blade vs 4 blade

Honestly I am not sure that I want a 4 blade, I started this forum because I had found a 4 blade on ebay so I was just trying to find out if that would fix my problem. I am not too worried about loosing some speed because the way it is now my boat is very unstable at full throttle anyway. I need to slow to about 4500 rpm's before I feel comfortable making any resonably sharp turns. And I don't mean a 90 degree turn but maybe a 45 degree turn.

Then it would probably be fine for you.

 

[darrklim2]

Re: 3 blade vs 4 blade

Well, I have good news. I got my new prop yesterday its a 10 X 15 and my rpms are now 5500. I still don't know my speed because I don't have a speedometer but it is noticable faster. So I am happy with this. Thanks to everyone for all of your help.