Thread:

Prop Design Variables

So I have a bad habit of writing long winded posts. I'm going to try and avoid that as best I can by writing about one thought at a time. Here goes. Roughly a year ago I'm walking through the pits at an NJBA race. As I'm checking out one of the faster flats I can't help but notice a bunch of props on the table next to it. So I strike up a conversation with the owner/driver (who's name out of respect I will not mention) about this array of props. If my memory serves me right, they were all Menkens. However under the boat resided a shiney new Grose prop. The unique thing is that the owner stated the Grose prop only outperformed his best prop by hundreths of a second, not tenths. So what can I take from this? That he had done his due diligence, bought and tried many props, and found his Cinderella prop. Now that story is rather unique, because most I hear are more like Vinces, that the Grose prop picked up more than hundeths. Stay tuned.....

Same speed with 800 less rpm definately sounds like more pitch. Good story... Now I know there's probably a handfull of you in here thinking what the hell is this Todd guy doing talking about props? He's essentially a rookie with limitied experiance. Agree'd. I'll say a few things though. This place has been boring me for the most part lately. I enjoy a good technical thread where you have the ability to share thoughts and experiances to further your knowledge. I like talking to people, getting input, and drawing my own conclusions. That said, I'll press forward.

I know of one other racer that was running an older Precision prop not to long ago. It was on a blown alcohol flat running the 8 second bracket. He too described the same characteristics as I did. Hard off the line, bat outta hell to half track, then flatline. Just cranking R's down the rest of the track not gaining any speed. We do believe that just as in my case he did not have enough gear in the box. Not wanting to sell the Precisions/Braaksma's short I need to experiment with gears and see if that changes anything. I have a feeling it will make some improvements, but I can't see it turning the prop into a top end monster. Time will tell. Until my next installment, I know you guys like pictures so I'll leave you with a few. Here's a couple I snapped the other year of one of Harolds newer Radiece runner bottom props. Enjoy...

Flat bottom props:

A lot of this is a simple re-hash of information you can find here on the boards, however I feel comfortable repeating it because over the years I have found it to be quite accurate.

There are two basic styles or categories of propellers for a flat. The runner bottom prop and the flat bottom prop.

A true flat bottom has more wetted surface than a runner bottom therefore it will need a propeller with a lot more bite to get the boat moving. So your flat bottom prop will have a significant amount of cup from the leading edge to the trailing edge, and around 6 pitch changes. Each pitch running from the hub to the tip. To get the best performance out of a flat, a flat bottom prop is designed to leave hard and have a fair amount of lift.

The runner bottom prop is designed for a boat that is generally lighter and is going to free up from the water a lot more. It has a smaller amount of cupping, and usually around 3 pitch changes. A runner bottom prop will tend to slip more on the hit, but will also tend to pull the entire quarter mile. This means higher rpm's on the burn, but it will also be more hooked up at the end of the track meaning more mph.

If you put your runner bottom prop on your true flat you will expect to loose that traction and lift resulting in a slower ET and mph.

If you put a flat bottom prop on your runner bottom you can expect to leave very hard, and pull very hard to about the 1/8th mile. At the 1/8th the boat will simply stop pulling, also you can expect the mph gain from the 1/8th to the ¼ to be very little. Yes I have experienced this. Something to think about is a flat bottom prop on a runner that is used as a lake hot rod makes a good combination. Lake racing is usually far less than a ¼ mile.

A runner bottom prop on a runner bottom will hook up slower but will continue to hook up and pull the entire ¼ mile. You will see the best ¼ mile ET’s and mph.

A good tip from Harold to quickly discern between the two is to lay them over on a flat surface (nut side up). With the blade laying flat on the table a runner prop will have around 3/8” gap under the leading edge. The true flat bottom prop will have around ¾” or more.

The first photo is a Menken's and is a runner prop. In the next photo the two props on the right are both Menken's, one a runner and one a flat.

My prop I have on my tunnel is a cut down Precision 10 1/2x15 Flat bottom prop. A really good CFW prop with 22's ET very hard but flattened out about 1000 ft. Put 25's & 29's in it & it pulled hard through the 1/4 mile. The prop seemed to come alive with more prop speed. The smaller diameter let me pull the taller gear. A friend of mine hurt a set of 22's & put 15's in his runner bottom. I let him try my prop on his boat. It flattened out by half track. 18's were a little better. The prop doesn't free up the runner as well as my tunnel. Less drag I guess. I will see some of you guy @ Chowchilla on Sat. I will be taking video. " No Boat" RACERICK

Originally Posted by 1bdhondo

Wait for it ........wait for it ......... Wait for ittttt.........

Bite me Ray! Nice to see good people following this. Craig I was trying to cover new ground guess it hasn't been clear enough yet. I'll make that point soon. Thank you for all your input, I value your opinions, Todd.

Ray keep sitting on the sidelines with your pom poms and put a sock in it!