Prop position relative to piston

[Flyin Rat Sass]

With a surface drive setup, is there any impact(no pun intended) on the performance having the prop blades hitting the water with the piston at TDC and BDC or at mid-stroke?and how much difference is made up by flex cable twist?

 

[ClayGlover]

LOL, I was just going to ask the same exact question! Weird!

I personally can't tell just by looking, but I don't have a gun to be sure. Larry Lahue showed me that trick and said you'll gain some speed. Larry runs some seriously quick crapshooters with Stu Barr and knows his stuff. He once told me that by slight (hopefully) margin of error, anyone's prepared props have one blade that typically has just slightly more pitch than the other(s). That's the blade you want to just begin to enter the water at the beginning of the powerstroke (immediately after TDC). Says you should try it with both blades and see which gets that little additional speed, then mark it. Amount of shaft twist? You got me... I guess without knowing that, you can't really know where the sweet spot is can you? So it's probably moot unless you can experiment and find the right place to set it, then mark it if it actually exists.

Maybe one of the masters can chime in.

Using math on paper, I'd say the answer is probably no difference, but in real-life? I don't know, but I do it anyway. Thats a simple thing to do, and if it does add a little something, why not do it, right?

Good question Rat.

 

[Scott Schneider]

NO The flex cable absorbs any rotational load that might be in phaze with a power stroke of engine. Has been tried, tested and was a bust. Scott

 

[ClayGlover]

There you go. That makes sense

Scott, now that you killed that bird, how about while using a straight shaft and joints, eliminating the twist?

 

[Lake S.P.O.R.T.]

Think of it this way....30,000 revolutions per minute is 500 revolutions per second. Really don't think it is going to matter given the forward speed of the boat and also factoring in prop slip.

Mark

 

[gene gager]

:huh: don't know answer to your question, but for some unknown reason I put a dot of yellow paint on flex cable and drive nut so they always go in the same position, don't ask me why. think I will bring it up to tdc with blade just cutting the water and find out. I do know a 5 bladed prop on a four cylinder (displacment hull) sail boat is much much quiter and smother than a 4 blade prop. why would anybody run a 25" 5 blade on a sailboat? to keep the shaft turning , while under sail, when the motor is off,, to run the generator, to cool the beer. pretty simple. gene gager