Sponsons and Tub Tips

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Wasted wages

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Jan 2, 2006
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Actually, leading edges would be more
accurate...

I've noticed recently that people are building rounded leading edges on the tub and sponsons of their riggers.
More like the leading edge of an airfoil, instead of the sharp tips I'm used to seeing.

Is this by design or does it really make that much difference? With all the talk about aerodynamics and hydrodynamics playing such a big part in the way a rigger is basically flying across the water, I can't help but think the shape of the leading edge has to play some part with airflow and lift over and under the hull and sponsons..

Discuss..... !
 

JRXHydros

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I like to build rc aircrafts & know something of air flow. The air must be directed. The airstream hits the wing. Where it divides to go up to the top of the wing and the bottom of the wing is called the stagnation point. The airstream directional motion going up is pulled back down by the low pressure created on the top of the wing. The bubble of low pressure and the momentum that directed the air molecules up are balancing in a favorable pressure gradient caused by the airfoil shape. Since a partial vacuum is on top of the wing and higher pressure is on the bottom of the wing there is lift. If the wing did not sharpen at the back a kind of suction at the back of the wing would create turbulence causing induced drag. Think of the back of a truck trailer. It can pull a guy on a bike along but the airflow is bumpy. That suction on the truck robs engine power.
 

JRXHydros

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The thicker the front,the more drag you get,but you get more lift.
 

Brad Christy

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Jun 24, 2002
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Actually, leading edges would be more
accurate...

I've noticed recently that people are building rounded leading edges on the tub and sponsons of their riggers.
More like the leading edge of an airfoil, instead of the sharp tips I'm used to seeing.

Is this by design or does it really make that much difference? With all the talk about aerodynamics and hydrodynamics playing such a big part in the way a rigger is basically flying across the water, I can't help but think the shape of the leading edge has to play some part with airflow and lift over and under the hull and sponsons..

Discuss..... !
Frank,

Sharp leading edges causes a body to "hunt". The rounded leading edge allows an airfoil to maintain stability at a wider range of attitudes. This was discovered years ago, which is why airfoils all have blunt leading edges. Look at land speed record motorcycles. While the cushion of air created by the blunt leading edge "lubricates" the boundary surface, it also reduces this tendency to "hunt".

Thanks. Brad.
Titan Racing Components
BlackJack Hydros
Model Machine and Precision LLC
 

Wasted wages

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Jan 2, 2006
Messages
655
Ok, so I understand the basics of flight... lift over gravity and thrust over drag. gives you the potential for flight.
And with AoA changes,, the pattern of airflow over the wing reduces pressure above the wing and away you go.

I guess where it gets blurry for me is that on a hydro, the sponsons and recurve of the tub, the air flow is not a constant. sometimes it is in aerodynamic lift and others it's stuck to the water in ( I think hydrodynamic, but not sure that's right) effect.
Plus there are so many different airfoil shapes, all requiring different amounts of AoA to be effective. Flat bottom wings, semi semetricial, fully semetricial...So what category does a riggers fall into ?

And another thing is frontal area of a sponson,,
versus the rest of it, width, trailing edges, angles of the outer sides..what does the shape of the rest of the sponson play ?

I guess what I wonder about is the cool,sexy lines of a rigger tub and sponsons...and then we stick two half inch boom tubes out of the sides of the thing at right angles...so can the rounded LE of the hull and sponsons be that big of a deal ?
 

Wasted wages

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655
Why couldn't the front boom become the LE of the sponson and tub shaped as an airfoil ?

there's an idea...
 

Charles Perdue

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Just a hint...straight sided, square airplane shapes were discontinued about 100 years ago.

Build and show us what you are describing. Get back to us in about 10 years.
 

LohringMiller

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Jan 25, 2017
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Water is 800 times as dense as air. At our speeds, air drag is insignificant compared to water drag. However water spray contacting surfaces does cause significant drag. Air lift, especially from surfaces close to the surface is very important. The planing surfaces of the sponsons generate significant lift from air as well as the water. Videos often show that the sponsons of SAW hydros only contact the water surface intermittently. Narrow fronts of the sponsons reduce this lift. Today's very fast electric hydros don't need this extra lift.

Lohring Miller

Joergs rigger 3.jpeg
 

JRXHydros

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561
Just a hint...straight sided, square airplane shapes were discontinued about 100 years ago.

Build and show us what you are describing. Get back to us in about 10 years.
Well the post is about Sponsons & Tub Tips if your reading & Brad Christy explained it very well.
 

Brad Christy

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Jun 24, 2002
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Joel,

As Lohring explained, the aero drag isn't really a problem. It's the hunting. A sharp leading edge will cause it to flutter from lift to downforce (downward lift), even without any real change in attitude. When the tub sees enough upward lift, the boat will let go of the water. To be honest, I don't really know what causes hunting. I just know that it happens with sharp leading edges and doesn't with blunt ones.

Thanks. Brad.
Titan Racing Components
BlackJack Hydros
Model Machine and Precision LLC
 

Charles Perdue

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Oct 8, 2005
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Joel, I was responding to Frank and I was not disagreeing with Brad, but adding to it. There is a lot more involved than just the front and tips. The entire hull designs, top, bottom, front, back and sides are affected by aerodynamics

Think about this for a minute....at what speeds and how much are our toy boat hulls, deep vees, outboards, sport hulls, ect affected by aerodynamics?

At what speeds do the loose running hulls begin to wobble and lift off???? This is the speed at which aerodynamics begin to have a large effect, if not why are they wobbling and lifting off? It is much lower speed and greater effect than most people think.

My opinion is that not enough thought is given to the aerodynamic design of our toy boat shapes in giving them speed and stability.

A triangle (the front to back shape of most sponsons) is one of the most aerodynamically unstable
shapes there is.
Charles
 
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Grimracer

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parabolas are more efficient at sub sonic speeds.. when contesting model rockets (your typical Estes type).. you would see major differences between pointed and round leading structures.. ROUND.. is much more gooder!

Note.. those rockets flew between 50 and 90 MPH.. hum.... neat eh!

Grim
 

JRXHydros

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All good information if want to play with all that stuff,I just focus on balance & Flat plane. I saw years back a 21 2x4 outrigger beat & win over nicer looking outriggers & had good balance. My 2 cents.
 

Grimracer

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Joel, hope you are well.. the balance you speak of comes in part from design. Aero does indeed play a part.. but not as we typically see aerodynamics in performance.

ROCK ON!

Grim
 

Charles Perdue

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With everything else being equal, a boat with good aerodynamics will have a faster top speed and be more stable.......it just depends on if you want to put in the effort and time to learn.

Some may want to make the effort.........some may not.
Myself, I really enjoy learning new things.

I AM A ROCKING...

Charles
 

JRXHydros

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Apr 5, 2008
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561
I
Joel, hope you are well.. the balance you speak of comes in part from design. Aero does indeed play a part.. but not as we typically see aerodynamics in performance.

ROCK ON!

Grim
I’m doing good,got 5 more outriggers to build,but I’m doing good. Hope your doing good in this covid crap. Yeah designs....well I’m sticking with what holds records.
 

dwilfong

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Jul 1, 2008
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Sharp sponsons work good. Tub not so much. The sponsons need to control the boat front end. The tub needs to be neutral on the front end. If you have to many surfaces trying to control the boat it never finds a balance be tween them. Sharp on the sponsons is good as you use the aero to control the front end. There is a top angle of attack on a sponsons. So the sharp front helps keep it on track. The tub does not need to control the front end. So rounded is better to keep it out of play. Just my 2 cents for what is worth.
 
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