Potencia En Embarcaciones Planeadoras

PLANING POWERBOATS WITH

MIDSHIP INTERCEPTOR AND AFT STABILIZER

The purpose of this presentation is to show an alternative for a more efficient
powerboat than the commonly used V-bottom boat.
The result of the investigations shows that speed can be increased by 25% with
the same power or that the power can be decreased by 40%.
The solution is to put an interceptor amidships to createhigh lift on a small surface
and in that way to reduce the frictional drag.

An interceptor is a small vertical plate that is pushed down under the planing
surface. This creates a high lift pressure in front of the interceptor.
An interceptor at the trailing edge of a foil has been used since 1971 in another
context.

Vaxholm den 30 September 2008
Jürgen Sass
www.sassdesign.net

Aplaning powerboat has a great area of friction. At
low speeds, when there is only static lift, this hull also
has an unnecessarily great wave drag.
A normal V-bottom is efficient at just one speed with
one load and one centre of pressure. Also, it is
difficult to achieve the optimum centre of pressure
while maintaining longitudinal stability.

To reduce the frictional area and get control overthe
centre of pressure the stepped boat can be chosen.
But this alternative has high drag at low speed due
the suction aft of the step. In the same way as the
normal V-bottom boat, the stepped boat has just one
load and one speed at which it is efficient.

A boat with a midship interceptor has the smallest
frictional area in the higher speed region. The aft body
can be designed in such waythat the drag at low
speeds also is low.
At high speed the drag will be about the same as on a
boat with foils, but a boat with a midship interceptor is
much simpler and sturdier.

The distribution of pressure on a flat surface has a
pronounced peak behind the leading edge and
practically no pressure at the trailing edge. The centre
of pressure is normally located one fourth of thewetted length from the leading edge.
The surface with an interceptor at the trailing edge
has the highest pressure at the trailing edge. The
centre of pressure is located aft of the middle of the
(short) wetted length.
The lifting force from the planing surface and from the
interceptor are additive. Together they produce about
80% more lift than usual.

The lift force on a bottom with anadjustable midship
interceptor and an aft trim surface, also with an
interceptor, can be controlled by trimming the hull so
that a minimum of drag is created over a great range
of load and speed.

During the winter 2006 and spring 2007 some simple
tests was accomplished. These tests confirmed the
presumptions presented above.
Only two of the best models are shown here. The
boats are doublechined and have an optimised beam
over the lower chine to create a minimum drag and a
soft ride in waves. By making the planing surface as
narrow as possible the deadrise can be reduced to
increase the lifting power and retain the same soft
ride as that of the deep V-bottom.

The test results are here shown in dimensionless
figures. These figures can be recalculated into all
speeds andloads. Above that the results can be
compared with those of other investigations.
According to these tests a small boat with a total
weight of 300 kg will be most efficient at 20 knots and
a boat of five metric tons at 32 knots.
The calculated air drag is the same for both boats. As
shown in the figure, the air drag is a significant part of
the total drag on an easily driven boat.

Theresults from these tests were so convincing that
the next step was to carry out a full-scale test.
In October 2007 a reconstruction of a boat was
carried out. The original hull is a Whisper 55, an
efficient boat for rowing or for use with a small engine.
The bottom was lowered a few centimetres amidships
to create a marked knuckle at the adjustable
interceptor. Aft of that knuckle is a...