The way of water.

[ozzie-crawl]

Maybe be a strange and to most boring topic but has me intrigued.
I have seen on rc boats the prop wash at a certain speed all of a sudden flattens out.
What I am wondering is this due to the speed of the boat and hence the water coming of the prop hitting the air causing it to flatten or is it the props just being more efficient at higher rpm and condensing the thrust cone.
If it is at a set speed then knowing it could be like a built in gps (flattens at say 45mph)
But then I think the water coming of the props is probably moving much faster than the boat.
Anyone out there a fluid engineer ???

[Cooper]

I believe it has to do with terminal velocity (so to speak) in the amount of water the prop is displacing. At low speed the prop tries to move more water, at a more efficient speed of movement through the water surface the prop will be displacing less water but matching the pitch distance travel more than at slower speeds. If I'm understanding you correct, I would think that could be a somewhat relative speed to what prop is used. Might be that every prop is different. But I'm still not sure if I'm understanding your question, fully.

[ray schrauwen]

Pete has a good point but, I think it has to do with how the prop is going into the water. At lower speeds more blade surface is in the water at higher speeds the hull and prop are more out of the water.

The theory goes that the thrust (and rooster tail) should be more backwards than up so the power is pushing the boat forward instead of forcing the nose down...??? Maybe, I think I'm close...

[Cooper]

Now that I'm thinking of what you said , thrust being more backwards, that could possibly explain it, at slower speed the thrust is pushing on the water closer to the boat, hitting it and some splashing upward (that's what the roost is). At higher speed the cone would be longer so to speak and farther away from boat resulting in less deflection. I was watching a boat go up river , it was a outboard jet drive, the propulsion jet of water was almost arcing and then splashing up about 30 feet behind boat. Not to efficient but when he took off it seemed to be deflected more up than backwards do to the forward motion and water. I guess it would be like spraying a garden hose into a wall vs, just spraying it into air? Hitting wall would have lots of deflection and more thrust. Into the air little deflection and less thrust?

[Eodman]

Think Grasshopper!

[Cooper]

Think Grasshopper!

Lol,,,,, I think I've melted all the ear wax and now its starting to burn!!! Lol.

[viper1]

I do understand your question fully, because I've been wondering as well.
Cooper I think ozzie is referring to the rooster-tail shape, took this shots from a YouTube clip to explain, same boat, same pass, acceleration to full speed:



I think it is just a combination of during acceleration the prop has more slip and throws the water straight up and starts to 'carve' more through the water at high speeds, together with hull lift at higher speed (prop runs shallower).


EDIT: OOPS!! By the time I had this posted including photo fabrication & upload there are 4 other replies.. Too slow Joe! :-)
Well, what they said..

[ozzie-crawl]

Spot on what I was wondering guys. Vipers pics say it all. Was not thinking as pointed out there is only the 1 blade in the water when up at speed.

[keithbradley]

Once the boat reaches top speed, the prop is essentially pushing the boat forward more, and pushing the water back/up less. It's a drastic overall increase in efficiency.

[ozzie-crawl]

Once the boat reaches top speed, the prop is essentially pushing the boat forward more, and pushing the water back/up less. It's a drastic overall increase in efficiency.

So what you and others are saying it is not a given speed this happens at but depending on boat and prop ?
I have no others to compare only when running my own and I am usually watching the hull attitude but do notice the prop spray changes and was very curious.

[keithbradley]

Yeah, it's essentially a component of the boat getting up on the water and reaching near top speed where prop slip is at a minimum.

[Gary]

I have some prop wash for ya' !!!LOL !!

http://www.youtube.com/watch?v=mvi9KIth-kg

[ozzie-crawl]

Yeah, it's essentially a component of the boat getting up on the water and reaching near top speed where prop slip is at a minimum.

Cool thanks guys it has put my over analytical mind at rest lol.

[ozzie-crawl]

I have some prop wash for ya' !!!LOL !!
http://www.youtube.com/watch?v=mvi9KIth-kg

Good to see the saftey crew at the end jump in the water to check if your boat was ok ;-)

[daveives]

I think that the prop is changing from capitating to non-cavitating.
On the full scale boat in my avatar, the top speed is 50mph but at about 40mph
it feels like someone has hit the boat with a sledge hammer as the boat changes
from slow acceleration to a faster acceleration which then tapers off as speed peaks.
This boat has a prop fully submerged such that the prop tip is just below the plane
of the hull flat bottom. Actuator disk theory from Wikipedia indicates that the water
speed in the disk swept by the prop is 1.5 times the boat speed, and that the water
speed downstream in the jet-like portion of the wake is twice the boat speed.
But that is just an approximate theory for a fully submerged prop. In the real world,
FE boats run at roughly eighty to ninety percent of a "zero slip" speed calculated
from the prop pitch and RPM.