I'm not a physicist, but I do have a PhD in hydrodynamics so let me see if I
can shed a little light on the subject of boat drag without getting too
complicated.  In fact, I'm going to do this in an over-simplified manner to
make it more easily understood.  So don't go snipping at it if you recognize
that it is not complete.

The simple question is why a heavier boat requires more power to travel at
the same speed as a lighter one.  The simple answer is that a heavier boat
lies deeper in the water.

There are basically 3 different forces a boat has to overcome with respect
to the interaction between the boat and the water.

There is friction force due to the shear stress of the water flowing over
the hull.  The friction drag is a function of the Reynolds number (a ratio
of velocity to viscosity).  The force increases with the square of the
velocity and directly proportional to the wetted surface area of the
hull(s).  Friction is the one value that can actually be reasonably
accurately calculated, but it is also likely the smallest of the 3 forces.
A boat lying deeper in the water clearly has more wetted surface area.  Most
everyone can also attest to the costs of a fouled hull on fuel burn due to
more friction drag of the hull.

The next force is a pressure drag - often called form drag force since it is
strongly dependent on the shape or form of the hull.  Simply put it is the
pressure drag caused by pushing the water aside.  It too is dependent on the
Reynolds number and is proportional to the square of the velocity and
directly proportional to the projected area of the hull(s).  The projected
area is the area you would see looking at the boat bow on - of course this
only counts for the amount submerged.  The drag coefficient is easily
estimated to a reasonable value but very difficult to calculate precisely
without experimentation.  Again, the deeper a boat lay in the water the
greater the projected area.

The final drag force is that required to produce the various wakes the boat
will make while moving through the water.  The interference of the bow wake
with the stern wake is what ultimately limits the speed of a
full-displacement boat.  It is dependent of the Froude number - the ratio of
velocity to that of a surface wave.  Experimental evidence on wave drag
demonstrates that it is not a smoothly increasing line but one with subtle
changes in slope (wiggles) that sometimes even flatten out in spots.  It is
this drag characteristic that often demonstrates more than one 'sweet'
operational spot for a given hull.  Finally, the deeper a boat lay in the
water the greater propensity it has for producing waves.

All three forces are increased by the boat being heavier and deeper in the
water, although the calculations are far from straight forward.  In fact it
is even difficult to obtain values experimentally with models since 2 of the
forces scale to the Reynolds number and the other scales to the Froude
number.  It is not possible to hold both in scale at the same time so the
forces have to be isolated even in scale models.

Bill Fleenor

Double-Wide (still under construction)

Bill,

Since you so clearly identified pressure drag
would you take a crack at identifying
the bow wake, stern wake and the interference between them.
You treat them as a given, please identify their origin, in your
simplified
and clear manner.

Also is there any meaning to having the catamaran hulls far enough
apart so that the bow waves do
not interfere with each other.

Chuck

-----Original Message-----
From: wefleenor@ucdavis.edu
To: power-catamaran@lists.samurai.com
Sent: Mon, 4 Sep 2006 8:33 PM
Subject: [PCW] a question of weight

The next force is a pressure drag - often called form drag force since
it is
strongly dependent on the shape or form of the hull.  Simply put it is
the
pressure drag caused by pushing the water aside.

The final drag force is that required to produce the various wakes the
boat
will make while moving through the water.  The interference of the bow
wake
with the stern wake is what ultimately limits the speed of a
full-displacement boat.

Bill Fleenor

Check out AOL.com today. Breaking news, video search, pictures, email
and IM. All on demand. Always Free.