Propeller Modifications

[Rich Z]

was reading an old thread on prop mods for various hull sizes/types. One thing that kept coming up is "prop needs to be cut down" e.g. 50mm to 47mm

What does that mean? Reduce the diameter at the tip? Reduce the entire blade outside curve by 3mm? A little help from the experts.... thanks,

Rich

 

[ClayGlover]

Here's a curveball, how do the prop guys reduce diameter while maintaining symmetry on each blade? In other words, how do you accurately measure distance from any given point of one of the blade's leading edge to the stubshaft's centerline. You can reduce the total prop diameter by cutting down just one blade, which obviously isn't desirable. How do you ensure the same amount of each blade has been removed along the entire profile? Measuring with a depth mic from the hub to the leading edge and comparing seems inaccurate.

Maybe I'll catch someone in a good mood and they'll share some tips. thanks

ps- I was thinking that a prop mold (epoxy putty mandrel) could be use to help create mirror-image blade profiles. Did I answer my own question, or is there a quicker way? :blink:

 

[izitbrokeyet?]

I use a threaded protractor from one of my fathers old engineering ink sets.

 

[shoboat]

Rich,

I use a digital caliper,and just measure until I get them right. When you do

600 to 700 propellers a year, you kinda get good at it. :lol: It is not really that

hard to do,like anything else, practice makes perfect. You will be surprised how

much 0.25mm affects the performance of a boat. Clay, I get the blades very

close and then I measure them to get them just about perfect. As far as the

blade thickness,I think the real advantage is a nice sharp leading edge and the

proper amount of cup on the trailing edge. Balance is the most critical part of

the propeller,and it will change when you polish your propellers up,so you then

will have to go re-balance the propeller while you are polishing it. The real fun

thing is to do all of this in under an hour, so you can move onto the other 50

waiting for you. :lol: :lol: :lol:

Have Fun,

Mark Sholund

 

[ClayGlover]

Mark, you just use the calipers to measure prop diameter at a 90 degree angle to the shaft down to .25 mm accuracy??? I guess perfect symmetry along the whole blade from toungue to tips isn't as critical as the "optimum" diameter at the tips themselves? Makes sense I guess, at speed the tips are doing most of the work with surfave drives. Kind of like trying to get each blade the exact thickness across the entire face... moot?

 

[Brad Christy]

Rich,

What I try to do is to cut down the diameter, but try to maintain the overall original blade shape. This will often get into the "tounge" area of the prop, and may cause some other effects, which may or may not be desirable. After you cut the prop down, you must then thin the leading edge back down to what it originally was, as well.

Clay,

You touched on something that is every bit as important as a well balanced prop, but few actually go to the effort of doing. Even if both blades are the same "length" from the C/L of the hub, non-matched blade shape can cause significant performance loss.

I use what we in the machine trades call "impression wax", which is actually a very low melting temp thermoplastic. It takes some time, but after you get a prop shaft melted into the wax, you can then make an impression of one blade and match the other on to it. This is the first thing I do after making the basic "re-sizing". Then I thin the blades down, hand sand all remaining casting marks out and THEN I balance the prop. I can put as much as ten hours into a single prop, depending on what I want to do to it. This is why I don't do props for other people.

Thanks. Brad.

Titan Racing Components

BlackJack Hydros

 

[Rich Z]

Thanks for the good info. As usual you guys are great!

Rich

 

[LOU]

Heres a great artical to check out

http://www.geocities.com/jwohlt5362/Propguide1.html

LOU

 

[Norm Doerr]

Figure your profiles with a prop guage and dial calipers. I tape the blade with thick masking tape. Trim it with a razor blade then you can stick the tape on the opposite blade and you have a mirror image. I even got lucky once and managed to get a perfect balence too.

 

[Slideblues]

Mark S brought up a good point, check your balance after you polish or do your final finish (satin or shiney) which ever you perfer, It does change

Gene

 

[Propjockey]

I cheat. Just order from Shoboat and tell him what I want done to it.