Diameter question

[Gary H NC]

Looking at the Solas Amita props.They offer two different diameters in the 17 pitch for my motor.
I have enough clearance for the larger size than my original 14.5 prop.
Any benefits in stepping up in size?
Only about 15 bucks difference in price too.

 

[Silvertip]

Re: Diameter question

Sure theres a difference. It will take more power to twist it and speed will likely be the same or less because of it. 17P is 17P meaning the boat will move forward 17 inches with each rev of the prop -- in theory. You may lose speed because of the extra power required that you may not have. People get way too concerned about diameter of a prop when it really is a function of design. Your engine/outdrive is designed and geared to work with a specific diameter and range of prop pitches. Just because you can physically fit a larger diameter prop on the engine does not mean it is acceptable. Hub diameter and the potential for interference or exhaust bypassing the center of the hub is also an issue. If you are talking a quarter inch or so that's one thing. If you are talking upwards of an inch that's quite another. Larger diameter means more blade area. More blade area means it takes more power to twist that prop. If you look at the same model of props from the same manufacturer you will notice that as pitch goes up, diameter comes down a bit. Conversely as pitch comes down, diameter goes up a bit. It is all about "design" so diameter should be a concern ONLY in that it is correct for your application -- not simply that "it fits". You gave us no performance numbers (speed and WOT rpm), no engine data, no boat data, and no idea what you are trying correct. So without that data all we can tell you is that you may already have the best prop.

 

[Bondo]

Re: Diameter question

Sure theres a difference.
People get way too concerned about diameter of a prop when it really is a function of design.

Ayuh,.... I agree with Silvertip,...
But,....

If you're seeing 2 Identical props in 2 diameters,...
It's being used as another Tool in tuning props...
Much like Cupping, or amounts of Cupping...

With some hulls you get on the ragged edge of where ya wanta be, or the next prop up in pitch, puts it Too low under,...
Adding Cup the the smaller 1, or adding diameter, can lead to the Right prop...

Hope that muddies the water enough for ya,....

 

[Tail_Gunner]

Re: Diameter question

Here is where i have to disagree with some very skilled members....The most important aspect oof diameter is that the prop barrel mates flush with the outdrive...Any mismatch will create a disturbance in water water flow which ends up as slip..

To that end...This is strickly a opinion and your milage will vary..err may...

 

[Gary H NC]

Re: Diameter question

Thanks guys....makes sense to me...lol!

I have the 14.5 SS 19 pitch prop and the aluminum is 14 4/5th 17 pitch.....the other option was 15.5 diameter in the 17 pitch.

I will go with the 14 4/5th. Trying to get the rpms up a bit.

Just too many dang choices....

I have a used 17 pitch aluminum a guy gave me to test with but it has a ton of nicks on the blades.
Wondering if i should test with it first or just buy a 17 aluminum.

 

[hwsiii]

Re: Diameter question

Gary, if it was me, I would run a test with that old prop, you have nothing to lose and if you have the numbers for RPM and speed and put them here it will help people here help you.


H

 

[Gary H NC]

Re: Diameter question

Gary, if it was me, I would run a test with that old prop, you have nothing to lose and if you have the numbers for RPM and speed and put them here it will help people here help you.


H

Sounds like a good idea.It was a free prop so i think i will give it a test anyway to see where it gets the rpms.

I will try to find some numbers on it so i can figure out what brand it is.

It will be a couple weeks before i splash it but wanted buy the Solas while Iboats had them on sale.
Still i better test a 17 first...Thanks for the input...

With my current 19 pitch Stiletto SS i can only turn 5100 rpm.

 

[Bondo]

Re: Diameter question

Here is where i have to disagree with some very skilled members....The most important aspect oof diameter is that the prop barrel mates flush with the outdrive...Any mismatch will create a disturbance in water water flow which ends up as slip..

To that end...This is strickly a opinion and your milage will vary..err may...

Ayuh,... Actually, There's No Disagreement there...

I know myself,+ probably Silvertip, were thinking that matching the barrel, or body of the prop to the gearcase was a Given...
And was Just discussing the virtues of Diameter, when All else is the Same....

Probably should have clarified that I guess...

 

[hwsiii]

Re: Diameter question

Gary, I talk more about blade area and blade geometry than diameter. I have found in the past that normally these matter more than small changes in diameter and pitch. In heavier boats you normally want more blade surface area as they have the ability to produce more thrust than props with less surface area, but in light boats you normally want less surface area because you are just wasting the extra horse power needed to turn a prop with more surface area when your boat doesn't require that much thrust. The problem with having a prop with more blade area than is needed for a particular boat and motor is that it produces more thrust at lower speeds and RPM and thus it eats more HP and produces less Effective pitch, and since it does this it will NOT let you attain the maximum RPM and speed as well, and the same is true when you do NOT have enough blade area for a heavy boat, as the prop does not produce enough thrust and thus the prop slip is very high and your speed goes down, assuming you have the correct rake and cup for either bow lift or stern lift as needed.
The same thing is true about blade geometry, which includes blade surface area, those are the reasons that you can't just exchange one 17" pitch prop for another one and expect them to perform at the same speed and thrust level. Especially when you are changing from a SS prop to an aluminum one.
Without you having numbers showing the RPM and GPS speed for the props you have now so we can figure out how much prop slip you have it makes it very hard to tell what the consequences are with that change. SS props have much thinner blades than aluminum as well as they are usually progressive pitch and have more cup on the blades and can be run higher in the water column by moving the motor up, which means there is less drag from the lower unit in the water and thus more speed normally.


You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057


H

 

[Gary H NC]

Re: Diameter question

Thanks!
I will have the GPS and both props when i hit the lake in a couple weeks.
Would love to do it this weekend but cool and rain...