Experiment #1

[Mark Anderson]

The rounded top surfaces on the Bud were to kill lift when thatthing is going sideways in the corners. It basically takes the wings off the sponsons.

I have watched full size tunnel boats try to circle race. They don't likeit. A corner is a series of hooks at each bouy.

Rod is not telling you to put spoon shaped stumble blocks on your boat. A spoon shape will pull the boat down as Jerry saw on his 1/4 scale. We had a 3.5 in our district that would not get on plane because of spoon shaped stumble blocks and round corners on the sponsons. A few whacks with a hammer and chisel and some strips added to the sponson edges to sharpen them and the boat went on to win the district championship.

The coke bottle shape of an aircraft fuselage is a hugh aerodynamic drag reducer. Do some research there are alot of examples of this shape. All shapes are effected by the speed at which they are traveling through the air/water. Air and water are similar but very different because of there different density

Rod brought up this experiment to get people thinking. Then I am sure he would explain his thoughts on it's relevence to model boats. I have been lucky enough to have the chance to spend alot of time sitting at the lake talking to Rod about boat design and what makes these silly things not work. When someone tries to share their knowledge you will usually get alot more out of the conversation if you ask pertinant questions than if you make smart remarks.

Mike, Rod eluded to the boat building aspect of learning how these things work, and I agree with both of you. I don't try to build tunnel boats any more but because I spent several years thinking that I could. I have a lot better understanding of the hows and whys which brings me to a better set up quicker.

It's just like the arf revolution in model aircraft. No body knows how to build anymore and the average modeler has very little understanding of what makes things work. They still complain about costs and the fast guys that do understand things having an advantage, but they don't want to put in the effort to learn.

Enough random thoughts

Rod, lead them down the dock but not too far or you will get wet.

Mark

 

[TomMoorehouse]

i have always said I need to pay my dues to become really good. That is why I have been designing my own riggers for years..... and cause i'm a cheap skate and i can save a few bucks

 

[Eric Perez]

Just a thought..
A good way to learn about tunnel hull design is to build a boat.. You can darn well bet that both Rod (wood builder) and Tommy (Glass and Wood) darn well mastered their craft by building boats.

I was digging up a video for Gener (code name TL65+) h34r:   and there is some old footage of me in my first few years of racing. The boats were slow (i thought they were fast) and looking back i can tell you that in the video i can remember specific points that light bubs started coming on. The footage is old (i had short hair) but the lesions in building were quite profound.

Grim

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I know what you mean. Most of the points touched on this topic are not going to do squat for the folks that buy finished fiberglass hulls. The wood builders on the other hand have the ability to adapt and try just about anything that they can imagine. The bottom of my cat hull has gone through like 3 re-designs since it was first finished -every time it gets faster and is more stable than the last. My next mod will be to step the bottom into three sections to see what happens...

 

[MikeP]

"I know of one little design item right now just waiting to be found.It absolutely solves one of the tunnel boats biggest faults".

This is a line from Rod's post, does anybody know what what one of Tunnel boats biggest faults is?

MikeP

 

[Dale0]

Thats what i was thinking, what is the biggest fault. I have always wanted to build a tunnel, or rigger that had NO curved surfaces on the bottom. I am pretty sure thats what Rod is getting at with the spoon experment. Curved surfaces suck!

 

[user 115]

Tunnels blow over!!!

 

[Grimracer]

Tunnels blow over!!!

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If you could adjust the rate to be equal regarding the speed that the air passes through the tunnel... now you have something..

As air passes through the tunnel its speeds up as it exits.. for the most part the tunnel is really not packing air. The air speeds up as it exits and this just adds to the force the back of the sponsons see regarding Bs equation. Aero probs here adding to the big ugly B thing..

I would think that if you pitched up the back of the tunnel near the transom that you might be able to slow the air back down getting rid of some of the low pressure that is taking place in the back of the boat.

grim

 

[Eric Perez]

Tunnels blow over!!!

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Yeah, tunnels prefer to be run on "nervous" water. Throw wind and chop in the equation and you'll be lucky to finish a race! The newer tunnel designs run a little flat so they are not as sensitive to the wind as previous designs that held the nose up high.

I've thought of designing a running surface that would be flat -but would be shallower in the back than in the middle of the spoon. This would allow the boat to remain very flat, yet still allow the hull to ride on the rear-most section of the spoon. Basically it would lower the AOA of the hull by building in a degree or two into the spoon as opposed to having the entire hull riding nose high.

You need to cut drag by reducing the wetted area as much as possible, yet retain enough contact to turn well and not blow-over. A compromise no doubt, maybe there is a combination of compromises that yield better results than others...

Running low-tunnel pressure and having wide riding surfaces is probably not the way to go.

To much riding surface and/or too much tunnel pressure will yield a boat that is very sensitive to wind and water condittions and will blow-over at the very first sign of instability.

One of the best designs I've seen is to angle the top of the hull to create a wedge. This tends to put a speed sensitive down-force on the hull to help counter act the increase tunnel pressure and lift caused by the running surfaces.

The other idea is to conver the top of the hull into a wing section to create lift at the CG of the hull, thus helping balance the hull at higher speeds.

Also running a crowned deck can help stabilize the model as well, by creating down-force that is not so dependent on the attide of th the hull.

I think someone mentioned adding long strips to the running surfaces to make them thinner. At higher speeds the hull would ride on the pads and thus lower the effect of water conditions by reducing the size of the contact patch. Not sure how the tunnel would react to this, maybe if they are thin it would work.

Has anyone thought of terminating the tunnel "early" before it gets to the transom? Either with a wind channel or by extending the spoon further back?

 

[Slideblues]

Mark A hit tha nail on tha head for the coke bottle shape, its bout reducing drag,

topside in the airstream. It also makes for a neutral handeling, better for varying conditions.

Mikie found tha vid!!!! Much cool, I have use of a prof editing machine,so can go frame by frame with clairity if need be.

Ditto on the wood construction, it allows you to try out things that others cant.

The more I learn the more I can respect the O'l Masters for there tips, some things I am sure none have thought of trying. Im just more interested in top end than handeling, but most of the members run oval so thats a good subject too.

As far as the humor I can put that aside if need be, I'll blow a gasket if I wait to long, But can try to hold it in :lol:

Waiting for the wisdom,

Gene

As far as questions....last six inches of sponson bottom ride surface and relative to add on pad, or cut into to make a pad, Is a good one fer my app.

 

[Precision Boats]

Guys,

You all are a little wet! :huh: I watch different tunnel hulls run in the .21 outboard class, wood, fiber glass, wof, etc it does not matter. Then I watch my Lynx run HUMMMMMMMM.......... Looks like someone did some homework. Even better listen to the motors on the .21 tunnels then listen to a Lynx's tunnel motor.... More RPM's are noticable!!

Where is it written that all tunnels have to follow actual Formula 1 designs? Remember you can't scale up water, wind, or air. They are all constants!!

The fastest tunnel will be the one that doesn't run on the water. It won't be a boat at all. It will be an simple airplane that only flys on the water surface that looks like a tunnel hull. Figure out how to do that and your there. My engineer colloge teacher always told me "Don't over think it!!!!"

Josh

Precision Boats

P.S.

"Wake Up America!!"

Ole Man Says - "Run a Lynx or chase a Lynx? - Such a simple question!"

 

[Grimracer]

:blink:

Grimracer.

 

[waterbug]

You have to think of it as a GIANT Spoon...turn it upside down over your tunnel less cowl, it creates lift , put that lift on or just behind the CG and you have a very fast hull...

I know that did not help either...!!!!!!!

I know the LYNX and HORNET look like the full size hulls more and more..... for what ever thats worth....

 

[Geraghty]

Tunnels blow over!!!

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BINGO!!

 

[TomMoorehouse]

has anyone tried a tunnel with an upside down airfoil section? I suggested that a while back.

 

[Geraghty]

Guys,
You all are a little wet!  :huh:   I watch different  tunnel hulls run in the .21 outboard class, wood, fiber glass, wof, etc it does not matter. Then I watch my Lynx run HUMMMMMMMM.......... Looks like someone did some homework. Even better listen to the motors on the .21 tunnels then listen to a Lynx's tunnel motor....  More RPM's are noticable!! 

Where is it written that all tunnels have to follow actual Formula 1 designs?  Remember you can't scale up water, wind, or air.  They are all constants!!

The fastest tunnel will be the one that doesn't run on the water.  It won't be a boat at all.  It will be an simple airplane that only flys on the water surface that looks like a tunnel hull.  Figure out how to do that and your there.  My engineer colloge teacher always told me "Don't over think it!!!!"

Josh

Precision Boats

P.S.

"Wake Up America!!"

Ole Man Says - "Run a Lynx or chase a Lynx? - Such a simple question!"

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The list of names of the boaters that consistantly win boat races changes very little but the list of the manufacturers of the boats they run do.

Put a decent boat in a great boaters hands and you will have a consistant winner.

Now I am not saying that the Lynx isn't a good boat but there are a lot of good boats out there.

A fast tunnel boat is an aired out tunnel boat.Everybody understands that."When in doubt,air it out"

The Lynx is just the latest tunnel boat to appear towards the top of the Hit Parade list of good boats.That is a direct function of the people that are running them not just the boat.

I started this thread to generate interest in tunnel design and encourage some of you to start building boats and learn something about tunnel boats other than just how to spend $$$$.That is what the For Sale forum is for.

 

[Geraghty]

has anyone tried a tunnel with an upside down airfoil section? I suggested that a while back.

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Yes we have.[Tommy and I,several times]It doesn't work.

You will have the most evil porpoising tunnel boat on the planet.

 

[TomMoorehouse]

has anyone tried a tunnel with an upside down airfoil section? I suggested that a while back.

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Yes we have.[Tommy and I,several times]It doesn't work.

You will have the most evil porpoising tunnel boat on the planet.

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oh well..... just an idea.

 

[Grimracer]

When you think about it the addition of a set of stumble blocks is little like adding an airfoil to the bottom of a tunnel boat. The blocks cause the air passing through the tunnel to speed up in front thus helping to pin the front of the boat.. this is just ONE of the reasons you see them on all my tunnel boat designs.

I have had people say that the stumble blocks are causing the boat to blow off... im not buying it.. i will be honest with you. (that was a Tuttle quote). Most say this as they are looking for a scapegoat for not taking the time to tune the hull. Or trying to make up for a building mistake they have made.

Not trying to sound negative here its just that hours and hours of testing does not lie..

Give it a try and run 20 gallons of fuel through a 21 OB in 5 months and see what you get.. I have done just that..

Lots of fun and you can learn a TON doing it..

Keep building and testing..

grimracer

 

[Geraghty]

Several years ago now Tommy Lee was commisioned by a very well healed full scale tunnel boat owner to build a scale model of a tunnel boat to be tested in the wind tunnel at the Huntsville space center research lab.He wanted to build the perfect tunnel boat and was willing to pay for the research to accomplish that end.

After it was all said and done and a foot tall stack of data assembled by a $100.00 and hour aerodynamicist the conclusion was the "tunnel boat" design is an aerodynamic nightmare.

The pursuit of the perfect aero designed tunnel is a waste of time and $$$$.

Aerodynamic "drag" under 100 mph is negligible.Now aerodynamic "forces" used for boat attitude control is significant.

Think about what I just said,drag and control are two separate areas of consideration.

You want to design a boat as neutral as possible using pressure differentials as your control parameters.

The only areodynamic benefit Tommy ever came up with on a tunnel boat was the large fillet where the cowl meets the deck like on the LeeCraft XTR.21.That works and works very well on helping control blowovers.

The purpose of this exercise is to get you,who want to,"to think".

There are "NO OPTIMUMS",no"PERFECT NUMBERS" or "MAXIMUM DESIGN PARAMETERS"!There is only designing for an application.That is the reason you will never see a Lynx go 70 mph with a .21 motor.

Forget about the spoon effect on the air for the time being.Just focus on the spoon effect on the water surfaces for right now.

Also forget about trying to draw a parallel between full scale tunnels and model tunnels.Ther are like trying to compare apples to oragnes.About the only things they have in common is they float.

As it turns out "blowovers" on model boats are as much about "water control" as they are about "aero control".The spoon effect not only works on sponson bottoms but also on work the sponson chines.

For the few of you out there that give a sxxt,get out your spoon,go to your sink and feel what is going on.The bigger the spoon the better because the forces for you to feel are greater.

Hint:Atmospheric pressure on the concave surface of the spoon is "pushing" the spoon into the water stream.

There is experiments #2 and #3 yet to come for those who are interested.

 

[tunnel290]

Guys,
You all are a little wet!  :huh:   I watch different  tunnel hulls run in the .21 outboard class, wood, fiber glass, wof, etc it does not matter. Then I watch my Lynx run HUMMMMMMMM.......... Looks like someone did some homework. Even better listen to the motors on the .21 tunnels then listen to a Lynx's tunnel motor....  More RPM's are noticable!! 

Where is it written that all tunnels have to follow actual Formula 1 designs?  Remember you can't scale up water, wind, or air.  They are all constants!!

The fastest tunnel will be the one that doesn't run on the water.  It won't be a boat at all.  It will be an simple airplane that only flys on the water surface that looks like a tunnel hull.  Figure out how to do that and your there.  My engineer colloge teacher always told me "Don't over think it!!!!"

Josh

I think it is more like buy a Hopper 290 or chase one!!!!!!!!!!!  

Precision Boats

P.S.

"Wake Up America!!"

Ole Man Says - "Run a Lynx or chase a Lynx? - Such a simple question!"

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