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What makes the tunnel hull work?  This is a multi-part article on the 
engineering basics of what makes the tunnel hull work.  This week, we'll 
look at the principles of operation and Lift/Weight balance.

What makes the tunnel hull work?

To understand this, we must examine the fluid dynamic forces involved.  
Several requirements must be satisfied for an object to maintain a 
steady straight&#64979;line velocity.  
(1)	Lift = Weight.  The weight for the hull must be exactly supported by 
forces such as lift from the hydrodynamic planing surfaces and 
aerodynamic lift.  
(2)	Drag = Thrust.  The drag experienced as a result of a velocity and 
all the lift mechanisms must be overcome by the available thrust.  
(3)	Pitch = Null.  All of these various forces acting, must act so that 
the tendency to pitch about the centre&#64979;of&#64979;gravity (CG) is 
eliminated.  

So for a tunnel boat (as for any boat) these forces must all balance out 
&#64979; and the design of the hull can be thus generalized into the 
three areas of hull lift, hull drag and dynamic stability.  Let us look 
mare closely at these areas of lift and drag.  

Part 1  Lift and Weight
As we have noted, the hull weight (including engine, driver, fuel, etc.) 
must be EXACTLY equaled by the lift forces generated.  This is true for 
any boat (or aeroplane, too) in stable flight.  The tunnel hull must 
however always be in 'stable flight', and so this balance is especially 
critical.  Too much lift and we take off like an aeroplane - too little 
and we sink! 
 
There is lift generated in basically two ways.  The planing sponson 
bottoms create 'hydrodynamic' or water-ift (lift due to forces on and 
reactions with, the water surface).  The aerodynamic lift is generated 
by the relative air flow over the tunnel and deck surfaces (lift due to 
forces on and reactions with the air, itself).  There are additional 
sources of lift in the tunnel hull rig, such as the slight lift 
generated by surface piercing propellers, for example, but the 
contributions of forces like these to the whole force 'picture', are 
insignificant.  

It is important to note that the relative significance of these forces 
changes as the speed of the hull increases.  Generally, under about 50 
mph, the aerodynamic lift accounts for less than 10% of the total lift, 
the sponsons supporting nearly all of the weight of the boat.  At the 
speeds now attainable by conventional racing tunnels, the tunnel lift 
can account for well over 80% of the total lift.  This tells us then, 
that the sponson lift is reduced accordingly, which gives dramatic 
improvements in the performance of the boat, as we'll see later.

Next week well look at the second part of the picture - the drag and 
thrust relationship.

Let us know any ideas you have, requests for articles, questions or 
comments on our TBDPNews.

/Jimboat
AeroMarine Research
www.aeromarineresearch.com

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>>>>>> Tunnel Boat Performance News >>>>>>>>>>>>>>

Let us know any ideas you have, requests for articles, questions or comments on our TBDPNews
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GO TO: www.aeromarineresearch.com

Secrets of Tunnel Boat Design book
History of Tunnel Boat Design book
History of Propellers e-book
Tunnel Boat Design Program for Win98 software
PropWorks2 software for propeller selection and powerboat speed prediction

GO TO: www.aeromarineresearch.com
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>>>>>> Tunnel Boat Performance News >>>>>>>>>>>>>>

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