Good info on drag .

[Tom Foley]

Some food for thought about a subject we all run into with our models in many different forms . Neat old video .

 

[Kris Flynn]

Nice!

 

[harvknox]

I wonder if anyone has done any studies on our typical wedge rudders with their sharp nose and flat rear?

 

[Marty Davis]

Are our sponson boom tubes creating a lot of drag?

 

[John Finch]

Are our sponson boom tubes creating a lot of drag?

Marty,

I experimented with wing shaped boom tubes. The problem is when the bow comes up a bit in rough water or crossing a wake the airfoils become lifting wings because the angle of attack becomes positive. The boat then blows off the water. To experiment further I put a small carbon fiber wing between the sponsons of my sport gas hydro and controlled the angle of the wing with a third channel servo. Photo attached. The wing was way to sensitive! a few degrees positive and the boat lifted off the water. With 30 degrees negative the front sponson stuck so hard the prop would blow out. I found the wing very sensitive in controlling the attitude of the boat. When the AOA was just right for optimum speed all it took was hitting a ripple on the water to launch the boat. At least with round booms they don't care what AOA the hull is at because they don't cause lift, just drag.

 

[Bob Blazer]

Here is what you can try. You can try I am too busy and let us know how it turns out. Build a flexible air foil that just hangs (pivots) on the boom tubes and will rotate. The air foil will then be neutral as far as lift goes and will cut down on the drag. So in John F. experiment there would be no increase in lift at all. If the boat is on dry land the air foils would be pointing up to the sky, But as you increase the air speed over the airfoil they would conform to the direction you are going. I have spent some time hanging out of the car window. A vacuum cleaner or air compressor probably would be better. Let us know.

Bob

 

[lohring]

As I've posted before, water is 800 times as dense as air so the same object in water has 800 times the drag at the same speed. I got a free lunch by winning a bet on streamlining sponson tubes. We couldn't measure any difference in boat speed at around 100 mph even though a leaf blower drag test showed a lot lower drag for the streamlined tube. The sponson tube airfoils were great for generating down force when we needed it to prevent blow offs. On the other hand we gained 6 mph with a thinner, smaller rudder design. Just cutting 1/8" off that rudder gave 2 mph at around 108 mph. The key is to reduce contact with water and use aerodynamic lift to help. The drag penalty for lift in air is tiny compared to water drag.

Lohring Miller

 

[anthony_marquart]

I have tried to look a tapered turn fins in the past. The one I have to test is a fin that is tapered in .015 steps to the trailing edge. The problem with this fin is that the tapers go all the way up the fin, and not just below the water line. This creates a very flimsy fin.. I have commented on the airfoil shaped rudder before and quickly told that the tapered fin works best in our application as we need the drag on the transom to keep the boat from getting out of shape..

I think that if we could find a way to just break the surface tension on these TF and rudder surfaces it would reduce drag considerably..

Very much interested in this topic.. I think there is more to be gained with reducing water drag than air.. but when we get the water drag reduced we should also look at the air..

 

[brad nichols]

Great info. on drag! So do we go to the old Prather stainless rudder and then what is the drag on the water pickup?

Brad

 

[Marty Davis]

John:

I agree totally with the airfoils becoming huge lifting forces when the nose comes up.

There is no reason to make the airfoil pivot on the sponson tube thus making it follow the plane of the boat even when the nose comes up. Just make it a tube over a tube so that the foil is not fixed.

I was told by the Late Tom Grannis that the hydrodynamic drag is 10X more than the air drag. A great experiment for those who want to see the effect of water drag is to take the turn fin off and run the boat. You will see a HUGE gain in MPH. Most of the current SAW runners are running without turn fins at all.

This topic is of HIGH importance for those running record trials and less for those racing. Optimizing the driver and the hull, engine, prop combo is usually enough to be super competitive in heat racing.

 

[John Finch]

As I've posted before, water is 800 times as dense as air so the same object in water has 800 times the drag at the same speed. I got a free lunch by winning a bet on streamlining sponson tubes. We couldn't measure any difference in boat speed at around 100 mph even though a leaf blower drag test showed a lot lower drag for the streamlined tube. The sponson tube airfoils were great for generating down force when we needed it to prevent blow offs. On the other hand we gained 6 mph with a thinner, smaller rudder design. Just cutting 1/8" off that rudder gave 2 mph at around 108 mph. The key is to reduce contact with water and use aerodynamic lift to help. The drag penalty for lift in air is tiny compared to water drag.

Lohring Miller

Lohring,

I found out the same thing as you for SAW. I airfoiled the tubes and didn't find any noticable difference at 90 to 100 mph. BUT when I went from a rudder with stock width to a thinned rudder my twin rigger went from 102 mph at record trials to 108 mph. Same depth just thinner. Exactly what you noted. I think a lot has to do with how fast the boat is running. At heat racing speeds I don't see much difference in thinning the rudders. I use the thickness of the rudder to compensate for propwalk so the right side of the rudder is parallel to the tub.

John

 

[anthony_marquart]

man o man,.. really making me want to go out and run boats!!!!

I have enough trouble just not having to retrieve a dead boat!!!

Charlotte guys:

Lets try to get together and do some of this type of testing..

 

[John Finch]

You are exactly right Marty. For SAW I find a 5mph gain with no fin. The secret is to make the sponsons wide enough to float the boat in the SAW yet narrow enough to fall into the water to make a turn at slow speed without bogging down. Which leads to....How wide the sponsons are has more effect on drag that aerodynamics. When everyone was going to narrower sponsons a decade ago Kenley and I were doing 8.5 second laps with wider sponsons at oval record trials. We made them so wide at oval trials that there was an air cushion between the water and the sponson with no water coming off the sponsons at all. Of course you need glass water to do that so you can't do that for heat racing, but it proves the point on drag. Don Pinckert is the one who tipped me off on how to float the boat. Anything touching the water is major drag!

 

[anthony_marquart]

I think we all know that a boat that runs loose on the water will be fast, but harder to keep under control.. The trick I think is in the balancing and set up,.. free from water but not so light as to blow of is a challenge.. and that ratio is not static, it varies with speed,. so the faster you go the setup has to change.. Just my opinion,..

 

[John Finch]

Anthony,

You are right there. People sometimes expect to see record speeds from the SAW and Oval record holders at heat races. Not going to happen. Completely different ballgame. Heat racing is all about completing 6 laps in all water conditions with moving obsticals cutting in front of you or throwing water just where you want your boat to be. That means setting up a boat that can run reasonably fast but also able to slow down when needed and accellerate when needed. Be able to cross wakes and survive poor driving decisions. Bottom line for me is being able to have fun driving the boat and not having to worry about what kind of crazy thing it might do. If you lose a few mph to gain that kind of control I am all for it.

 

[John Finch]

I like Martys idea about the tube over a tube. Anyone up for trying that? I am experimenting with electric monos right now.

John

 

[aaron alberico]

Great topic!

John i might try it with new 21 SAW rigger im building at the moment!;-)....

regards Aaron

 

[dwilfong]

tried it a few years ago and it worked good.

thy where fixed and it made the front of the boat with the shingled ride surface VERY light.

Now with them free floating that may be the trick.

 

[Tim_Duggan]

Did you know that a round tube that is spinning is more aero efficient than one that is fixed. That wouldn't be too difficult to retrofit on an existing boat.

 

[wayne middlemiss]

Great topic!

John i might try it with new 21 SAW rigger im building at the moment!;-)....

regards Aaron

Aaron get Sam pedden to get some photos of the stick boat , it's setup with foils over the boom tubes .it would be 25-30 year old boat now.