Rigger Sponson Design Q: Trailing Edge

[Meniscus]

Here's a question for you designers out there and folks with experience on this subject. I believe I already know the answer, but am looking for everyone's input.

Why don't we see different sponson trailing edge shapes on a rigger. If we have a straight trailing edge, then I would imagine it would be the most even and efficient design. However, it may be beneficial to have an inverted "U" for reduced surface area when encountering deflection due to race conditions.

Take a look at my quick sketch below demonstrating what I'm referring to and add your input. Thanks!

SponsonTrailingEdge.jpg

[Ub Hauled]

Are you talking about the trailing edge that gets in touch with the water or the on that stays in the air? Water I guess...
If in the water, you don't want the sponsons sinking every time it hits a wake, it'll make your rigger run wet, you need to keep her in the air as much as possible and the "square" edges would provide the best solution as far as aerodynamics and hydrodynamics.
if in the air, In my book, trailing edges 1 and three are out... they would work like suction cups adding drag... much like they would in the water...
number 2 maybe and number 4 not sure yet, possibly...
I am basing my answers in thoughts only I have not used nor seen any of the above used as a water side trailing edge yet.

[Simon.O.]

Ben, you have asked a ripper of a question here.
As you know I run saw type runs so can not comment too much on race water applications. As an idea of how calm I like my water I will do two passes and let the water settle again before take off again. Where I run has a long narrow stretch and it is best I let it calm out between double passes.
Right that is that said.

Now onto the trailing edge. As my riggers are set light in the front they tend to pack air and have very minimal water to break off the rears.
I have gone with the traditional square edge because...............well it seems to be the done thing and works.

My only rationale for keeping it this way as opposed to running with a more geometric shape as you have sketched is that I want to keep the edge as short as possible (width wise) and for the water to break off in a uniform manner.
How the water coming off makes a difference I can not see.
I may even make some test pieces to go on the back of the small rigger sponson and "see" if they make a difference.
I have no gps anymore and even when I had a little garmin it was too big to go inside.

I will follow this thread to see if some clever bugger who knows more about fluid dynamics can give a good answer.

[egneg]

I think it would be difficult to mount hardware as all that I have seen are made to mount to a flat squared off surface. Fishing boats that were once used in the Chesapeake Bay had rounded sterns but that was to help with stability when they were not moving.

[m4a1usr]

The answer is pretty simple. Shapes or contours allow for additional localized pressure to develop. These are parasitic in nature. They tend to add drag unless coupled with structures taking advantage of hydrodynamics. Sharp edges, while simple, cut to the chase. Monolithic designs tend to be the best in tradeoffs.

John

[Meniscus]

Great input guys. Egneg, I don't know what hardware you are referring to or why it would matter.

OK, here's a twist that I wanted to ask after initial responses:

What if you had the inverted V shape, such as #1 in the diagram, and placed rails on the outside edges? It wouldn't need very large and also wouldn't need to be deep right at the trailing edge.

I would think that this would act like a tunnel hull, packing the air under the sponson, but also spilling more evenly and straight at higher speeds. At the same time, you'd have all the benefits of a ride pad edge and perhaps allowing for race conditions deflection.

Ideas? I'll try uploading a sketch when I get to a scanner later today.

[Diegoboy]

What about a saw tooth trailing edge??

[Simon.O.]

, and placed rails on the outside edges? It wouldn't need very large and also wouldn't need to be deep right at the trailing edge.

I would think that this would act like a tunnel hull, packing the air under the sponson, .

Ben, I recall seeing something similar to this on rrr a couple of years ago. It was a rigger made by a Eastern European chap and it was bloody fast. I recall well that there was a tunnel hull thing going on with his sponsons.

I lost all the pics of that one when my last 'pooter died.

With my saw style riggers I am sure I do not want any more lift but for the circuit guys I can see a gain in having a sponson that will keep its lift in rough/race water, or just as importantly regain its lift very quickly if its comes off the pace during a turn or avoidance manouvere.

[Meniscus]

Simon, the point is, if you can have less surface area in contact with the water for the same lift, why not try less drag? That's what I was thinking regarding my concept.

[Meniscus]

I guess my last post seemed a little cynical and that was not the intent. I would like to see more input on this subject from everyone.

Thanks!

[Ub Hauled]

There is an active topic on International Waters in regards of the "shingles" on a sponson...

I have read a few articles on the tunnel version of the sponsons, mostly from Europe...
I think I even read one from here, not sure though...
There were mixed reviews, some say that it worked like a charm and other say it adds too much lift.. I guess it depends on what experience you had prior to implementation...
I would think that it adds lift due to the compressed air. With the flat bottoms, like JAG's riggers for instance, if there was too much pressure build up the air would simply not stay under the ski... but once your have a tunnel there is no way out, do I make sense?

[Gerwin Brommer]

Very interesting subject !

I don't have any knowledge about riggers.

But my brains say :

In fact ; all types will work. As long as both sponsons are the same.(in case of SAW)
In case of oval racing : two different shapes might help turning/handling.

In all cases : the conditions in/on the water never are constant.
Ideal conditions ; no wind and no waves.
real life : boat bounces. When the boat hits the surface : what edge design would be optimal ?

When I look at all 5 edges-designs ; look at the lenght of the edges !!!!
N0. "original" has the less amount of lenght. Does that mean : less drag ??

I think design "drawing left" is most used because it is the most "constant factor" ??

cl # 4 =THAT

[Meniscus]

Jan, I'm following what your saying and the answer is I don't know either. Could you provide a link to those threads? Thanks!

Gerwin, what do you mean by drawing left? I think you mean the inverted V furthest to the left in the diagram, but I'm not sure. I should have numbered the examples. LOL

From a reputable source:

"You will find that full-scale hydros have sponson trailing edges perpendicular to the boat's axis of movement. This maximizes the lift the sponson produces and works great on surfaces that are close to parallel with the water, i.e. - having little to no dihedral.

The maximum hydrodynamic lift occurs close to the trailing edge of a sponson; 'angling' that edge will reduce the lift and move its center inboard or outboard, whichever direction the angled trailing edge has. This can upset the boat's balance, not good. In short, any angle to the trailing edges of sponsons is probably a bad thing.

You will find some full-scale monos with angled trailing edges on their steps. This is done to balance the total wetted surface and the lift as the hull rises up out of the water on step, because there is a large change in wetted area on a vee hull. But on flat or nearly flat ride surfaces, a straight trailing edge is preferred."

This still leaves me wondering if the inverted V when used in conjunction with side rails to keep the air from spilling over the side edges of the sponsons might add some advantage. I think I understand that it may not be as efficient in theory, but does it lead to less drag or any other characteristics that could be beneficial?

[Gerwin Brommer]

Ben.

Is the use of rails on sponsons "legal" ?
If you don't do races ; give it a try ! I can imagine it sure would reduce drag and create lift.

[Meniscus]

I certainly don't know about the legality, but from some of the designs I've seen, I don't think that it would be of concern. After all, you're not introducing another riding surface/point or an extra turn fin. Under many of the rules, I think it would be treated the same way as a fin on top to keep the model straight at speed.