Stepped hull geometry is complex, and
generates better Lift/Drag ratios due to higher Aspect ratio for similar
Steps can generate required wetted surface with improved
Lift and Drag characteristics.
Stepped hulls operate with a larger effective
angle of attack due to the inherent step angle 'built-in' to the
Wetted surfaces of each stepped area have a
controlled location of forces, and usually enhanced Lift efficiency due
to local 'angle of attack' and higher local aspect ratio.
Multiple Steps can improve performance through greater velocity range.
Stepped Vee hull
Typical stepped hull wetted surface,
water channel proof testing.
AR® has developed advanced analysis techniques that accurately calculate
the performance effects of single or multiple steps in tunnel hulls of vee
Making a Step Work
A stepped hull can
be visualized as a Vee or flat bottom with longitudinal "offsets". When
functioning, the area immediately aft of each step is void of water - an
air or air/water mixture. At low speeds, the entire hull bottom is in the
water, but at higher speeds only a portion of each step is wet, so
theoretically we should get less hull drag.
The stepped hull
creates 2 (or more) planing surfaces that helps maintain a constant (trim)
angle of attack for more efficient lifting. The stepped hull has less
bottom surface in contact with the water creating less drag and allows for
The 'fore' and 'aft' planing surfaces created by the
'step' helps maintain a constant trim angle Stepped hulls have two main
1) Steps can maintain near optimum angle of attack
throughout a wider speed range; and,
2) Steps can reduce the amount
of wetted surface that is not near the leading edge (and would otherwise
produce less efficient lift).
Jim Russell applies these advancements in
newest versions of AR's TBDP©/VBDP© performance
The design of an effectively performing step will most
always achieve optimum 'benefits' (more than the 'losses') at only one
velocity. A step design is generally only good for a single (trim) angle
of attack with a single center of gravity (LCofG). This is why it is
complicated to find a step design that can "help" the performance
throughout the entire speed range of a performance boat.
planing surfaces of the stepped hulls operate with a larger effective
angle of attack due to the inherent 'Step Angle' that is 'built-in' to the
planing surfaces. Even though the hull 'Trim' angle is often less, the
higher total angle of attack can generate improved CLW (more efficient
Lift). The result is a lesser (total) wetted surface requirement and less
Location of LW (planing surfaces hydrodynamic lift) is
normally located further forward in a stepped hull design, as compared to
a non-stepped hull. This can sometimes have a desirable effect on
XCGDynamic (Dynamic Center of Forces).
Here's How It Works
hydrodynamic Lift generated by planing surfaces is influenced, in part, by
the surface area, aspect ratio, and trim angle of the wetted surface(s).
In the case of a stepped hull design (say 2 steps), there can be 3
separated wetted surfaces. If effectively designed, these 3 separate
lifting surfaces can be more efficient than the 1 single surface that a
non-stepped hull could deliver.
step design imposes a set angle of attack for the planing surface area aft
of the step. This 'step angle' incrementally increases the total
'trim angle' that the planing surfaces see (hull trim angle + step angle =
total 'planing trim angle'). Higher trim angle generates more lift.
At the same time, the 'Aspect Ratio'
(width/length ratio of the planing surface) for each of the 3 lifting
surfaces of the 2-step hull are greater and so contribute to more
efficient lift (more lift, less drag) than the shape of a longer, single
(non-stepped) planing surface.
Finally, the imposed 'step angle' helps
maintain a minimum angle of attack for the surfaces that, if properly
designed, can optimize the lifting efficiency (L/D) of each surface.
Any change in trim angle affects each of the 3 planing surfaces - which
also changes the wetted surface and AR too, and thus affects the Lift &
Drag generated by each of the 3 surfaces. So the performance
analysis of step design is quite complex. Overall, however, there
are benefits of the application of steps to a hull design [TBDP©
do all of the engineering performance analysis to optimize effective multi
Issues with Steps
The most significant issue with step design is the engineering challenge
of properly locating an efficient step on the hull. The length of planing
surface behind the step (i.e.: the location of the step) and depth of the
step have an impact on the performance of the setup. To design the step
improperly can actually decrease performance. The issue of multiple steps
makes the challenge even trickier.
Some designers stay away from
including steps in designs because of degradation in performance at low
and moderate speed range. At lower speeds the steps are entirely
immersed, so each step actually adds drag to the hull. At moderate speeds
during the transition to full planing, air must get back behind the steps
or the boat will suffer the penalty of continuing high drag. So the step
is really only ideally functional at its single design velocity, and thus,
it can potentially generate a penalty at all other velocities. Use of
ventilating steps by design can cause the hull to "trip" on the irregular
chines causing a dangerous stability problem with serious handling
Although the geometry of stepped hull
lifting surfaces is extrememly complex, the multiple lifting surfaces can
generate more efficient Lift and Drag due to higher Aspect Ratio of multiple
surfaces for same wetted surface. Most importantly, the location of Dynamic
CG of stepped surfaces is shown to be foreward of non-stepped surfaces,
improving Dynamic Stability in key regions of velocity range. Instead of a
single longer non-stepped wetted length, stepped planing surfaces are
shorter (thus higher Aspect Ratio for similar planing width, b) separated by
aerated (non-wetted) surfaces. While complex to calculate, these multiple
surfaces can be more efficient and can move CGDynamic further forward,
improving Dynamic Stability.
Step Height and
A properly designed step can help maintain a
constant trim angle (WAngle), reducing cyclic changing trim angles under
operating conditions. A designer can even design the step height and step
angle to optimize the planing trim angle of the planing surface(s). A step
angle that is too large can impose a step angle that is too high, causing
additional drag and, more importantly, resulting in very low hull trim
angles (even WAngle = 0), exposing other surfaces to increased wetting and
extra drag under many conditions. Step height should be considered so that
step angle does not result in very low operating WAngles. A good guideline
is a step angle of less than 1/2 of optimum trim angle (example, if
optimum trim angle = 5 degrees, then max step angle could be set to less
than 2.5 degrees).
There are additional challenges with multiple steps.
1) If the steps
are located too close to each other, the water attaching to the second
step is "contaminated" by aerated low-density water from the first step,
so the aft step does not produce the high lift forces desired.
Where do we locate the center of weight (LCofG) so that the weight is
balanced across the steps? Remember, the running trim angle can change
throughout the speed range and this makes a huge difference in the
lift-force distribution on your steps. It takes only a small change in
the relative locations of the CofG to change your boat from a stable,
efficient boat to one that porpoises at several velocities.
performance design software helps to optimize step design and
placement, and provides performance analysis of any hull with or without