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About Jimboat

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    powerboat design, build, race.
  1. Jimboat

    Jack plate

    here is an article on Jack Plates that may help...
  2. Jimboat

    Speedometer pickup question

    Epoxy the screws into place...they'll stay put.
  3. Jimboat

    Do STV�s Blow over?

    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.
  4. Jimboat

    Do STV�s Blow over?

    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.
  5. Jimboat

    Do STV�s Blow over?

    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.