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Do STV�s Blow over?

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THANKS NICK...

for sharing your experiences!!! YES cool pic!!!

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Brad.

Yup, she has 1 mph on me,  (I got 96).  

But thats only cause she got the good water,  and the fuel tank was almost empty,  and she's not SKEEERED like me.

We still have not held the pedal all the way down.

Maybe next time.

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Lemme bring this one back to the top with another question.

I know Roark built the STV bottom, but did he design it as well?  If so, what modeling did they do to figure out what to build.

Also, does anyone know how much aerodynamic lift these boats generate at what speeds?  I've got a friend who is building a new boat company that says he believes it's less than 5%.  On my boat, that's less than 75# of aerodynamic lift.  Seems pretty low.  I know they also build hydrodynamics lift, but I'm looking for the aero side.

Thanks.

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Good questions KEITH...lets get AEROmarine to address..

CALLING JIM :D

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Anthony - great article!

Analysis and proper design of a high performance tunnel hull is a tricky balance of aero and hydrodynamic forces, that change at every different speed on every hull configuration.  I have done some examination of the STV design...(see article STV Euro Ski 19 Performance Analysis).  I use the AeroMarine Research® "Tunnel Boat Design Program©" to do the analysis, since it has features that make "tuning" the analysis easy for top speed, acceleration and/or stability simulation.

One of the questions raised (STV_Keith) was... "does anyone know how much aerodynamic lift these boats generate at what speeds?"  The STV is quite good at generating aerodynamic lift.  The lift is generated by the ground effect of the aerofoil (wing section and tunnel roof) operating in close proximity to the water surface.  The aerodynamic lift generated is about 60lb @ 40 mph; 311 @ 87 mph; 413 lb @ 100 mph.  This is probably as much as 28% of total lift at this speed.  The angle of attack at high speeds is optimum at 1.6 to 1.9 degrees.  

Another point made was the change of lift characteristics as the STV accelerates through its speed range.  The location of acting forces is constantly changing in a tunnel boat.  (This is why you can't really "balance" your tunnel hull while it's still on the trailer).  At operating speeds, it isn't likely that there is any real Ground (Water) Effect that sucks it to the water.  The phenomenon that is observed is actually the change in dynamic center of gravity (Dynamic CG) as the balance of aerodynamic and hydrodynamic forces change.  The observation of "tail lift" at certain speeds, is actually the result of the CGDynamic moving as the balance of forces change.  On the STV, the CGDynamic is located approximately +9 ft (fore of transom) at 40mph; it moves aft to +3 ft at 87 mph, and it moves back to +4 ft at 100 mph.   (The aerodynamic center of the STV is approximately +10ft throughout the speed range).  This shifting of CGDynamic is what makes the tunnel so "interesting" (fun) to drive.

Euroski referred to the "hump zone".  Since a tunnel hull cannot be inherently aerodynamically stable, I use a measure of stability that references the dynamic CG of the hull, using in part, the aerodynamic center of pressure from the "wing" or aerofoil of the hull  (referenced fore (+) of the transom).  The dynamic CG and the center of pressure (XPRESS) will change throughout the range of operating velocities.  (To maximize design stability at operating velocity, dynamic CG should be ahead of the aerodynamic center of the wing (aerofoil) section, XPRESS).   Well, this hull design (like most tunnels of this size) sees its inherent instability (refered to as the "porpoising hump") at about 55 mph, when the aerodynamic forces start becoming more important.  In this case, we can define the stability measure as the change in the location of the XPRESS.   From analysis of the STV (same as most all tunnels, by the way) we can see that XPRESS is constantly moving foreward (getting less stable) - but as long as this change is at a reasonably slow and steady (constant) rate, the drivers attention and skill is able to compensate.  (Those of you that drive tunnels or ModVP hulls will surely know the feeling of this "compensation" and driving through the "hump zone".  This hull maintains a remarkably consistent rate of change in XPRESS, indicating that it will have a very stable "feel" to it throughout it's velocity range.  Nevertheless, the "hump zone" or "transition zone" is always very apparent in all ModVP hulls, often observed as "porpoising" if not controlled with trim or driven through quickly.

Here is another Point..

Anthony showed a picture of Helmut's STV setup, and his NO ROOSTER TAIL!  It is rare to see a boat set up as perfectly as this one.  The "spray" of rooster tail shows very little loss of energy through the lower unit and propeller setup. (see my article in September 2004 HotBoat magazine - "What a Drag" for more explanations).  His hull setup is perfect for the speed he is going, and the engine trim angle is perfect to mainain hull stability and minimize power lost through generation of rooster tail.  What a great rig!

Thanks for a cool article, Anthony.  Mr. Summerford is one of the great tunnel hull and ModVP hull designers!

[p.s. - Techno - your picture looks as though your hull is running at about 4 degrees to me.]

(Check out "Secrets of Tunnel Boat Design" book

and "Tunnel Boat Design" software on AeroMarine Research's web site.

post-2-1066879013.gif

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What a wonderful answer to a technical question...28% aero lift @ 100 is a bit higher than the 5% someone posted...I would say the STV is pretty stable and you would have to force it to blow over..if in fact you could...I can see situations where blow over could occur but common sense should eliminate such a situation...I will continue to show a lot of respect for anything running over 100...be safe

and thanks to Jimboat

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JIMboat...don't thank me I am just the messenger.

I want to thank you...that was an awesome responce and we would welcome more of them.

That should keep us thinking for some time and thanks for the kudos on the STV design

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I do think it should be noted that Aeromarine's info is VERY accurate for the Euro, which is what he tested, but the different deck designs will effect the Aerodynamics of the other hulls. Dont ask me how much, but i'm sure it will.

BJH

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Speaking of the program finaly decided to bite the bullet. Still need a stamp but figure that for the price and since I still have to rig should tell me where to put the OB fuel tank batteries and such.

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KEITH,

THANKS to RT he obtailed this insighful info on the PCS deck design.

The PCS deck creates a negative vacuum right at the front cockpit cowl which induces lift right at the cockpit, thus making it harder to fly flat. Very Tru right!

The RR deck design, with the torpedo taper on the rear deck actually creates the NEG pressure at the tail of the boat, adding stern lift making it easier to make the hull fly-flat.

So I would say the RR design would be better then PCS...just cause it's easier to fly flatter and make more speed.. Just a thought!

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Anthony, what's Jim's feelings on the STV at the upper limits?

Is there a point where the dynamic CG moves too far to the rear, or the aerodynamics run out of favor ???

GREAT article....I'm gonna look real smart the next time that question comes up :D

Thank YOU :)

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Trikki10106 - Thanks for the compliments on the article.  [There's lots more where that came from...check out the list of Free Downloads of articles on my website].

There is always a point where the dynamic CG moves either too quickly, or too far away from the AC (aerodynamic center).  That point will bring onset of instability [often called 'blowover'].  I explain some of this in the article I wrote on the STV Euro Ski 19 Performance Analysis).  The 'Secrets of Tunnel Boat Design'® book explains the rest.

This onset of instability can often be seen when analyzing the movement of the dynamic CofG through the velocity range.  When the location of the XCGDynamic starts changing quickly, or dramatically (such as moving aft then fore) then we're approaching the point of instability.

Having said this, the Euro exhibits very stable characteristics, and when set up properly will make the feel of the hull one of stability, and will contribute to a more confident ride, with good control, by the driver.  Excellent balance of aerodynamic and hydrodynamic design.

post-2-1078801192.jpg

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I am guess the flat spot is the "hump" at 80 mph where the hull fights dowwnforce vs, lift and we get that porpoise. Right now I can bang about 120 w/ a warmed over 280. Ya got Helmet breeching 124 and Raceman hitting 134. I was trying to figure out what point we are all driving "timebombs" or approaching instability on the top end

Thanks much again Jim, I got alot more reading ta do

Rick :D

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LakeRacer - you're right about reading the performance graph.  The TBDP software does a great job of showing where the "hump" zone will be.  It really helps us to design the "hump" to a velocity range that is 'least disruptive' to the type of driving expected to be done.  There are some design things that can be done to 'move' the transition zone somewhat, and to make it easier to drive through.  All part of the game.

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Jim's Graph is so true to real form as the C/G is always moving and as a rule the the bow needs to be a little bow hi to get over the hump...so you can see the  graph shows the C/G furthest back at that 80 mph hump zone...then as the arir is burped out and the transom satrts to lift the C/G is almost right in the middle of the boat at WOT....that's waht makes the STV so stable and safe at speed... THANKS JIM :)

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Great read. Worth bringing back to the top. Nice to see see only one blow over recorded as of the time of the artical. I got slammed on S&F for asking these questions a while back. interesting to see the over 400 #'s of lift at over 100 mph

Lance

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Lance there has been more than one STV go over. There is a ModVp down south that has gone over twice I think. Ted at speedmaster marine blew one over in a race or testing back in the modVp race days, and of course there was Hank that went over.

All in all they are very safe boats, I have practically tried to blow one over and could not get it to go up.

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Lance, thanks for bringing this back to the top, this is the first time I have seen this thread.

Jimboat, that was some fantastic information, thank you.

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