What makes the tunnel hull work?
Part II: Drag & Thrust
by Jim Russell, AeroMarine Research
This is a multi-part article on the engineering basics of what makes a tunnel hull work.
Last week, we looked at the fluid dynamic forces involved in making a tunnel boat work. Recall that several requirements must be satisfied for an object (boat) to maintain a steady, stable, straight-line velocity.
(1) Lift = Weight. (Discussed in Part 1)
(2) Drag = Thrust. The drag experienced because 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 center-of-gravity (CG) is eliminated.
This week we will look more closely at the second part of the picture - the drag and thrust relationship.
2 Drag & Thrust
The propeller is the source of all available thrust and must be sufficient to overcome the drag created under running conditions. The drag of a tunnel hull is made up of both aerodynamic or 'air' drag (drag from the tunnel and deck surfaces as well as appendage air drags such as driver, cockpit area, motor, etc.) and hydrodynamic or 'water drag' (from the planing sponsons and motor appendages under water). Although the water drag is responsible for most of the trouble, particular attention to the air drag will pay off at high speeds.
Up until rather recently in the history of power boating, one could say quite safely that the 'air-drag' of a hull was really not very significant. Today however, with the super-fast high-horsepower racing tunnel boats now easily exceeding the speed of many light aircraft, careful attention to the smallest details of hull design is necessary to reduce the drags and squeeze every bit of speed and stability out of the boat. Better performance can be achieved even on lower-speed recreational or commercial Tunnels when particular attention is given to aerodynamic drag reduction.
Figure 2-1 - Induced Drag
As with the tunnel lift, the Air Drag increases as the square of the velocity and as the angle-of-attack increases. So, air drag can become a rather significant problem at high speeds and high angles of attack. This air drag originates in three (3) forms:
(1) Skin Friction is the drag created by the passage of the fluid (air in this case) over the exposed surface area (the tunnel and deck surfaces).
(2) Induced Drag is the drag created due to the lift generated by our aerodynamic 'wing' (the tunnel roof and deck surfaces). This portion of the drag is there to remind us that we really don't get all that nice lift for free!
(3) Profile Drag is the one that can be both designed into a hull and improved upon with an existing hull design. This part is reduced by fairing around the cockpit, shrouds or appendages.
While we are on the topic of appendage drag, it is noteworthy to point out that such 'appendages' as the driver and/or passengers, windscreen, outboard motor area, and even race-boat cockpit fairings do create air-appendage drags. It is well worthwhile keeping these areas both as small and as clean as possible.
Water Drag is generated in two separate, and for the most part, unrelated areas. These are drags caused by the motor appendages (skeg, torpedo and propeller) under the water, and the sponson planing areas on the surface of the water.
As with aerodynamic drag, drag generated by hydrodynamic surfaces can be categorized into three separate contributors.
(1) Frictional Drag is a function purely of the surface condition (which we assume perfectly smooth and flat) and the 'viscous-force-ratio' - in our case, how fast the planing surface is going with respect to its length. This viscous-force-ratio is called the Reynolds Number, and is used to simplify comparisons of different sized objects, at different speeds. The total surface area exposed to these viscous forces (i.e. the wetted surface) tells us just how much friction drag we will have.
(2) Induced Drag is caused by the lift generated by the planing surfaces. The lift generated by the difference in pressure between the lower surface and top surface of the sponson bottoms 'induces' trailing vortices at each edge of the surface (i.e. the inside of the sponson or keel, and the outside chine). A higher Aspect ratio running 'pad', will in general reduce the induced drag.
(3) Profile Drag or 'form drag' accounts for the types of drags that do not normally depend on the 'Reynolds Number' or hull velocity. These drags are generally constant for a given hull configuration and include the effects of hull discontinuities, hull curvature, and drag caused by interruptions to the smooth flow of the water along the surfaces or 'separation drag'.
Appendage drag or motor drag is difficult to calculate in a simple manner, since there are so many different designs of lower units in use today, and since every boat has the drive unit set up just a little bit differently.
Figure 2-2 - Powerboat drive Appendage drag
The 'Skeg' is really just a kind of wing, flying through the water - sideways. A 'flat' shaped skeg may not seem like a 'wing', but careful managing of its shape can improve lower unit performance. Thinner is better, and be sure that the front edge is well rounded, that the trailing edge is feather sharp and that the shape throughout is exactly symmetrical (same shape on both sides of the skeg). The 'Torpedo', or the part of the lower unit that houses the transmission, etc., is actually just a projectile moving through the water generating drag. The Propeller is a multi-aerofoil component generating lift and drag in all different directions. (One of these directions we call thrust, and we use it to 'propel' our boat). As far as additional drag on the hull however, the propeller contributes very little. The total drag of the motor then, is determined in a based on the velocity of the hull, and the 'type' of motor lower unit. (The AR "Tunnel Boat Design Program© " software does a complex analysis of the lower unit forces based on each unique configuration).
You can see that "balancing" all the 'pros' and 'cons' of lift and drags is the key to high performance powerboat design.
Click here to continue to Part III - Dynamic Force Balances.
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