sponson tubes

[Grimracer]

Aircraft guide wires are not round..

Take a look at just about any engineering book and learn the benefits of proper air flow.

The rules do change the closer we get to the sound barrier but of course that does not come into play for us. Having said this we do, each time we run a boat, play with the speed of sound in water.

Grim

 

[Paul Traufler]

Maybe I see things different, but I was thinking not from a aero drag but rather a stiffness/weight path. Oval shaped booms COULD be made to give more strength in one axis but I really think our boats see many forces in several different directions so there may be no gain to this approach. Round tubes can take loads in any direction and hold up fine, most any other form will have higher/lower strengths that COULD be used if you understand the load path. As some others have said, water forces are the real deal and need to be worked before the areo side is worked. Recall water is 750+ times more dense than air.

 

[Charles Perdue]

Our record holding 110 mph rigger is "streamlined". It is more to look good and have controlled lift than to reduce drag. Running an exposed pipe as well as adding various blisters and imperfections didn't change the speed. Changing the rudder design added 6 mph at 100 mph. Changing the sponson design added 3 mph. Shortening the rudder 1/8 inch got the peak speed from 108 to 110 mph. Water drag is the problem, not air drag.

Lohring Miller

Mr.Lohring, several times you have mentioned your test of airfoiled shaped sponson brackets on your SAW rigger.

One test does not make it very scientific, also the sponson brackets of your rigger are very short, the rear bracket is close enough to the front bracket to be running in the turbulence of the front bracket, either one of these would effect any test results.

I think that more test need to be run on other types of riggers, especially on a boat of the JAE type with long sponson brackets spread farther apart before it could be stated as fact that there is no speed difference between the two.

You may have to buy Mike a steak dinner.....

Charles

 

[lohring]

The drag of a cylinder is well understood. See Hoerner, Fluid Dynamic Drag. The drag coefficient is very close to 1 in the size and speed range we are talking about. That means the total drag from 1/2 inch diameter boom tubes is around 1 pound per foot of length at 100 mph. A streamlined body of the same width might have a drag as low as .05 pounds per foot of length at 100 mph. However, that drag is very small compared to the drag of objects in water. A cylinder with the same drag per foot in water at 100 mph would be .0006 inches in diameter. Of course it wouldn't be strong enough. Boat speeds have increased as more powerful engines have been developed and hull designs have kept more of the boat out of the water. A good history of hull design over the years is at http://www.lesliefield.com/other_history/speed_boat_developments_from_the_past_into_the_future.htm#[18]

Lohring Miller

 

[Andy Brown]

Our record holding 110 mph rigger is "streamlined". It is more to look good and have controlled lift than to reduce drag. Running an exposed pipe as well as adding various blisters and imperfections didn't change the speed. Changing the rudder design added 6 mph at 100 mph. Changing the sponson design added 3 mph. Shortening the rudder 1/8 inch got the peak speed from 108 to 110 mph. Water drag is the problem, not air drag.

Lohring Miller

"Water drag is the problem, not air drag."

Lohring is right. With the Horsepower we have airplanes would go 250+ mph.

Even big bulky ducted fans with an O.S. MAX .91 on 30% nitro will do 200 mph.

Not that aero isn't important or can be neglected, but it is currently the smallest portion of our drag.

 

[Charles Perdue]

Andy, I completely agree that the hydrodynamic drag is the larger part of the equation but as the speeds of our toy boats increase, the aerodynamic drag cannot be ignored. As our speeds increase the boat runs lighter with less wet surface in contact with the water allowing the aero drag to become more important.

Lohring, I read your reply and the link that you provided and I did not read anything that said that there would not be a measurable performance difference in the two tube profiles.

That is really a nice looking gas SAW boat that you and Mike built. You said in a prior thread that you had problems turning the boat at speed. Just curious as to why you did not widen the sponson stance to help this problem since there is supposedly no difference in aero drag of the sponson brackets.

Over and out.....I will be quiet now......This is all in fun.....

Charles

 

[Jack ODonnell]

Our record holding 110 mph rigger is "streamlined". It is more to look good and have controlled lift than to reduce drag. Running an exposed pipe as well as adding various blisters and imperfections didn't change the speed. Changing the rudder design added 6 mph at 100 mph. Changing the sponson design added 3 mph. Shortening the rudder 1/8 inch got the peak speed from 108 to 110 mph. Water drag is the problem, not air drag.

Lohring Miller

"Water drag is the problem, not air drag."

Lohring is right. With the Horsepower we have airplanes would go 250+ mph.

Even big bulky ducted fans with an O.S. MAX .91 on 30% nitro will do 200 mph.

Not that aero isn't important or can be neglected, but it is currently the smallest portion of our drag.

Andy, aero drag is what is used to control the speed in tether cars and speed airplanes.

 

[Andy Brown]

Charles I did not say that aero could be "ignored". My statement does imply that we must be aware of aero.

I did write:

"Not that aero isn't important or can be neglected, but it is currently the smallest portion of our drag."

Yes Jack, It is obvious that aero is almost everything in cars and planes, but those cars and planes are going Much faster with Much less horsepower. Current Tethered cars are going over 200 mph with a 10cc engine on 80/20 fuel. Speed planes are also at 200 mph with 2.5 cc engines on 80/20.

Tethered boats are around 135 mph with 10cc engines on 80/20 and they have no rudder or turnfin..and very thin Tool steel props.

With an R/C boat with a rudder in the water we'd be hard pressed to run 80 mph with any one of those engine/fuel combinations.

Yes, we must keep an eye on aero and my boat designs of the last 25 years makes it apparent that I do that within the reality of production race boats.

 

[Anders_Martinelle]

I can only add that as long you don`t run over 100mph the drag from the tubes and boat arn`t mutch to think of. And that`s from the windtunnel tests we did almost 10 years ago.

I can give you a hint, complete SG eagle 80 at 100mph = 1,4hp.

So at normal "race" speed............not mutch is winddrag.

Anders

 

[dwilfong]

I think the thing is if you want to turn at 90+ MPH on the water you have to have enough in the water to take the load. You can't turn a boat on a race course with other boats inches from you with a rudder and turn fin that is not in the water.

On a SAW boat you could fly just above the water with just the prop in the water. Now that would be the trick.

A gyro on a front canard and rear wing would be the ticket. the prop should give the back the lift needed to a point.

David

 

[TUNLBOTR]

In World Of Outlaw sprint cars they started "dimpling" the leading edge of the wings to reduce drag and aid airflow, same priciple as golf balls use. Would this lower the drag coe?

 

[Paul Traufler]

What the "dimples" do is make the airflow go turbulent, it is VERY hard to keep a flow in laminar flow. So the thinking is since you cannot keep the flow laminar (reduced drag flow) why not “trip” the flow and make it turbulent as soon as possible and use an airfoil that will work best with the turbulent flow. In MOST cases that means a “fatter” airfoil would work better in the turbulent flows. But as I keep saying the air drag on the boats is NOT the big ticket item, water drag is the big dog on the block.