Prop slippage

[TCHedOff]

First, let me thank Glen Quarles for explaining to me that the gear ratio to be inputted on Mark Sholund's prop chart calculator should be 1 (DUH!!!!) Sometimes your asked a question that just gives you a brain fart 'cause your suprised by a question & end up overthinking what you are really being asked! :lol: :lol: and afterwards you go I KNEW that....

Anyway, I used Mark's tool and put in 25k for RPM on a standard 1445 prop w/2.478 pitch to see MPH at 30% slippage (Mark's recommendation today) and you get 41.1 MPH. (Great) So I started playing w/variables.... hmm for every 1000 RPm you pick up @ 1 MPH. (Great to know - want RPM).

Then I noticed at the bottom it suggests 15% slippage for a hydro not 30% (hmmm lets try that!) - (should have asked Mark today about the discrepancy!) at 25k on the 1445 prop - you get 51.9 MPH :blink: . Jeez!!!!!!! Slippage is an important factor! <_<

So I do a search on "slippage" here.... and after much reading, see it comes up in passing a few times but not as a devoted topic... so I'm bringing it up as a can o' worms.

What factors influence prop & how can it be reduced? To add fuel, here's a quote from Andy Brown:

"I'm not as sharp as some of the Professors of "Propology", but having been a model boater for over thirty years and having had a keen interest in props since day one, I've had the hands on experience of working with and seeing the effects of changes to thousands of props.

The key to the slippage is the "MOVED" water and learning how to use it to in fact make it USEFULL.

The difference between a good prop and a great prop are what they do with the "MOVED" water.

Slippage will always appear to be present if the average pitch is compared to the actual forward movement.

But this is a totally incorrect way to look at the potential forward movement of a prop."

To which a newbie like me goes "HUH?"

 

[moparbarn]

i think andy is looking at how the prop directs the water it is 'pushing'. not all the water moved by a prop is translated into foward movement. some of it spills off the edge/side of the prop, not contributing much to foward movement. how you shape a prop, and where the water is directed can affect A LOT more than just foward movement. slippage is determined by a combination of hull design, prop size/shape/pitch/diameter, rpm, & water density. may not be the same day to day, due to the water & rpm/tuning. there are some #'s, generally accepted as being close, that you can plug into programs. i don't think they can be accurate for different props, 'cuz different props slip at different rates, on the same hull, on the same day, at the same pond. rcu has some good prop stickies on their boat forum. screamandfly.com also has some excellent prop info, although a full scale boat site, the info is directly related/transferrable to our hulls. you can use all the programs you want, they do provide some useful info, but the only real way to tell is go to the pond & time laps. the fastest straight line prop may not be the fastest heat racing prop. and, the prop that runs best in the turns may not be, either. has to be the "complete pacgkage", only way to tell is to time laps, imho.

 

[Charles Perdue]

i think andy is looking at how the prop directs the water it is 'pushing'. not all the water moved by a prop is translated into foward movement. some of it spills off the edge/side of the prop, not contributing much to foward movement. how you shape a prop, and where the water is directed can affect A LOT more than just foward movement. slippage is determined by a combination of hull design, prop size/shape/pitch/diameter, rpm, & water density. may not be the same day to day, due to the water & rpm/tuning. there are some #'s, generally accepted as being close, that you can plug into programs. i don't think they can be accurate for different props, 'cuz different props slip at different rates, on the same hull, on the same day, at the same pond. rcu has some good prop stickies on their boat forum. screamandfly.com also has some excellent prop info, although a full scale boat site, the info is directly related/transferrable to our hulls. you can use all the programs you want, they do provide some useful info, but the only real way to tell is go to the pond & time laps. the fastest straight line prop may not be the fastest heat racing prop. and, the prop that runs best in the turns may not be, either. has to be the "complete pacgkage", only way to tell is to time laps, imho.

Some other variables that aren't often address is the props forward speed through the water and the speed of the prop blades as they travel through the water affecting the percentage of the slippage.

The water has mass, and has resistance to being moved out of the way. The faster the prop speed and/or forward speed through the water, the greater the resistance of the water to being moved out of the way of the prop blades, and the less slippage because of the less time and the greater force necessary to displace this water rather than screw its way through it, and this translates to less slippage of the prop.

The other side of this, is the increased rate of drag on the hull with speed. As the hull speed increases this adds a greater load on the prop and increases the percentage of the slippage.

Its all fun and games.

 

[LH7018]

The real potential lies in the L.E. pitch. This is where it all starts then pitch progression, blade area and diameter, hull drag, cup, etc. will affect what happens next and the perceived "slippage" or inefficiency.