80 MPH 21 hydro

[Ron Zaker Jr]

Booms?

Are you saying symetrical,semi symetrical,or flat bottom,or flat bottom inverted?

 

[Grimracer]

Although round booms are thought of as much drag on our boat i could likely bet that they make little difference at the speeds we are going.. going OVER MACH..now you best have your aero right. Under MACH and parabolic shapes are best.

Booms are not the place to be looking "IF" we continue down the "Hydro" definition as we know it. :huh: Sorry just my job..

Grim

 

[Preston_Hall]

Booms?
Are you saying symetrical,semi symetrical,or flat bottom,or flat bottom inverted?

67872[/snapback]

​

My first guess would be symmetrical. This should only remove drag if the incidence is correct.

Booms are not the place to be looking "IF" we continue down the "Hydro" definition as we know it.  Sorry just my job.. 

Hydro=multiple wetted surfaces. You are correct. But I would squeeze the rules and count the rudder, prop, and turn fin in the event of no "wood" being in contact.

 

[Anders_Martinelle]

Booms are not the BIG problem!!!

Mike and Tim are right.

Aerodynamic drag is not so mutch compered to other things.........

As windtunnel tests showed a SG 21 at 79mph requied 371 watts in drag.....

That is the same as 0,5hp...

Anders

 

[Preston_Hall]

Here we go

Streamlined booms

Maybe the booms aren't the big problem but there is definitely room for improvement.

 

[Preston_Hall]

If you want to see a CS float

then SCOLL DOWN. :lol: :lol: :lol:

 

[Ron Zaker Jr]

I really understand that the booms are a small piece of the puzzle.I think flat bottom inverted would keep the nose pulling down.Wouldn't the semis or symetrical tend to fly more if the nose came up?

Anders

When the Eagle was in the tunnel,where any changes made with cowlings,pipes,etc ? Very curious.

 

[Preston_Hall]

I think flat bottom inverted would keep the nose pulling down.

Not sure I understand. :huh:

Wouldn't the semis or symetrical tend to fly more if the nose came up?

I would think so. So I never tried. Too difficult to keep neutral.

 

[Ron Zaker Jr]

I think flat bottom inverted would keep the nose pulling down.

Not sure I understand. :huh:

Wouldn't the semis or symetrical tend to fly more if the nose came up?

I would think so. So I never tried. Too difficult to keep neutral.

67886[/snapback]

​

Preston,

Flat bottom wings always want to climb,when you fly flat bottom wing airplanes inverted,they want to to go towards the ground.That was my thought process.

 

[Preston_Hall]

Got it. I think. Did you know the CS has a flat bottom roughly from the engine back? If so, it's already flat and I don't get it. If not, I get it.

But the flat seems to float instead of pulling down. Take into consideration the bernoulli stuff, the bottom of the boat, the water surface and you should see a lower pressure under the boat.

 

[Anders_Martinelle]

Ron, we tested w or w/o pipe and w or w/o cowling and the changes were so small so it was not a big deal ( in the speed range up to 100mph)

Over 100mph it have some effect.....

Anders

 

[Grimracer]

Got it. I think. Did you know the CS has a flat bottom roughly from the engine back? If so, it's already flat and I don't get it. If not, I get it.
But the flat seems to float instead of pulling down. Take into consideration the bernoulli stuff, the bottom of the boat, the water surface and you should see a lower pressure under the boat.

67893[/snapback]

​

What is the operating AOA of this area? this might explane..

grim

 

[Ron Zaker Jr]

Anders,

Thanks for the info and it blows my mind.I have tested with others on the water and the differences with and without a cowl were very evident.No cowl, my particular hull went way to the "loose" side,cowl back on, back to normal.Iwill say it did not go faster it just got loose,very strange.

 

[Preston_Hall]

Got it. I think. Did you know the CS has a flat bottom roughly from the engine back? If so, it's already flat and I don't get it. If not, I get it.
But the flat seems to float instead of pulling down. Take into consideration the bernoulli stuff, the bottom of the boat, the water surface and you should see a lower pressure under the boat.

67893[/snapback]

​

What is the operating AOA of this area? this might explane..

grim

67899[/snapback]

​

I would have guess flat - 0. At least is is on the board if you expect the back to lift slightly.

 

[Anders_Martinelle]

Ron, that is a nother thing how the boat react with or without a cowl.

You are right.

Some boats will react alot and other not so mutch.....

What i could see in the windtunnel was the airdrag not the lift or downforce.

I had straped it down for the tests on a trolly, not fun if your boat went around in the windtunnel :lol: :lol: .

Anders

 

[Ron Zaker Jr]

Ron, that is a nother thing how the boat react with or without a cowl.You are right.

Some boats will react alot and other not so mutch.....

What i could see in the windtunnel was the airdrag not the lift or downforce.

I had straped it down for the tests on a trolly, not fun if your boat went around in the windtunnel :lol:   :lol: .

Anders

67910[/snapback]

​

Okay I see,very different test,thanks.

 

[Brad Christy]

Guys,

I am not an engineer of any kind, but I have spent some time in wind tunnels and have seen "vapor trail" tests. I'm here to tell you that the airstream spoilage behind a round cross section is enormous. That "Foil Sim" was rediculous. There seemed to be nothing I could do to cause any sort of spoilage at all.

What happens with a round cross section is this: As the air approaches the tube, it for the most part splits. Some goes over the tube, some goes under, and some "stalls" right at the point of perpendicularity to the tube. This is actually a good thing because it "lubricates" this area to the oncoming airstream. All is well and good at this piont. The trouble starts after the air passes the midpoint of the tube and starts to be drawn around the tube by the Bernulli effect (sp). The air that went over the tube slams into the air that went under it, some distance behind the tube, and turbulance is produced. It also creates an area of low pressure directly behind the tube. This is what induces what those engineer types call "drag". To make matters worse, in our appplication, there is an identical tube immediately following it that must travel through the turbulance. The turbulance prevents the air from stalling at the mid-section of the tube like it did before, but the low pressure area is still there. Even though the low pressure area is decreased due to the lack of a consistant airstream, the increases pressure on the leading side more than makes up for it.

If you change the cross section to an airfoil, the air is no longer drawn around the trailing side nearly as strongly, therefore there is almost no decrease in pressure trailing the cross section. There is also a drastic decrease in trailing turbulance so the second tube is allowed almost the same factors as the first.

Look at the record speed setting bicycles. They have an air foil shell that is round on front and comes to a point, like a tear-drop shape, in the back. Also look at the general profile of a submarine. They have a blunt, round nose and a sharp tail. If you look at most fish, they have similar profiles.

In our application, we would use a symmetrical airfoil, because we want neither lift nor downforce. We would just have to adjust them, just like you do any turnfin with a significant curl or bend below the waterline. I'm not concerned about the airfoil causing lift if the boat changes angle (bounces of a wave) because the tub is going to take over long before the air foils are.

Just to show what kind of influece these foils could have, I remember a friend telling me about something he saw in the tether cars. These cars are as streamlined as they can be. The problem is that there is a 3/16" steel cable attached to the side of the car, and this causes a ton of drag. There is also a "buckle" where the cable actually attaches to the car, and this is located outside the streamlined body of the car, and this too induces alot of drag. There was a guy who showed up to an even with an air foil that attached to the buckle after the cable was secured to the car that was about 2 1/2" long, just enough to cover the buckle. While this was not a legal item, they let him run the car, just for giggles. Two back to back runs, with no changes other than one with the air foil, one without. I don't remember the exact speeds, but at somewhere in the range of 200 MPH, there was a 10-15 MPH loss without the airfoil. Just think what changing a round cross section to an air foil over a 6-8" length, FOUR PLACES, could do.

Thanks. Brad.

Titan Racing Components

 

[Brad Christy]

Guys,

I don't think "wings" are going to help either. We already have everything out of the water we can. I don't think we can do without a turnfin on a heat racing boat, and I'm almost positive we need a rudder. Except these items, there is almost nothing left in the water. Anything you hang on the side of a boat to create lift is going to cause aerodynamic drag. I think you will give up much more to aerodynamic drag than you will gain by reducing hydrodynamic drag.

Also, I think the overall shape of the tub is pretty much set, unless you are going to quit building with wood. Most guys don't have the capabilities to build boats that look like submarines. Terry Keely's riggers look cool as hell, but I don't see them outrunning the wood boats by 10 MPH. Nothing personal, Terry.....

Thanks. Brad.

Titan Racing Components

 

[Brad Christy]

Guys,

One more thing:

I see several discounting an idea because the gains are "minimal". Let's not forget that this thread is focused on the 21 class. The overal power at our disposal is minimal at best. Even though the output of the average 21 engine has nearly doubled in the last couple years, a 21 is still not a powerhouse by any stretch of the imagination.

Thanks. Brad.

Titan Racing Components

 

[Anders_Martinelle]

Hmmmm, i don`t discounting it but the gain might not be more then 2-4mph........

I know that airdrag takes 0,5hp when a 21 boat goes 79mph but were is the other 2,5hp you should have in a 21 engine???

So you are right Brad that if you can gain 2mph from drag, 2mph form a more efficient prop, 2mph form a better rudder and 2mph from a better fin you have 8mph.........

So yes, small "thing" will add up in the long run....

But were goes the other 2,5hp??? :unsure: :unsure:

Anders