"Richard Casady" wrote in message
...
On Fri, 4 Jul 2008 15:19:27 -0700, "Calif Bill"
wrote:


"Richard Casady" wrote in message
. ..
On Fri, 4 Jul 2008 08:15:19 -0400, "Floyd"
wrote:

I've read that the jet boat's impeller system is about 30% less
efficient
than a similarly powered outboard. Does that mean that a jet boat will
get
beat by an outboard, or just use more gas?

The jet will have a lower top speed. It will burn more fuel at any and
all lower speeds.

Casady

It will burn more fuel. May or may not be faster. There are different
type
pumps. There are axial flow, low pressure pumps that are slower speed,
but
handle white water better at reloading up after losing intake water, and
there are the high pressure pumps that are faster. Kodiak and Hamilton
are
examples of the first, Berkeley and American Turbine are examples of the
second. Jet ski pumps are probably the most inefficient of all the pump
designs. Small engine and high RPM's trying to move lots of water through
a
small impeller pump. The newer Hamilton 212's etc are about 95%
efficiency
of props.

Our Turbocraft is axial flow, and in fifty years has never sucked air
into the intake. Weeds once. Once the ski tow rope. Had to turn the
engine and pump backwards, with a pipe wrench on the driveshaft, to
get it out. The pump is a licenced copy of a [New Zealand] Hamilton.

Casady

Mine is a Kodiak 3 stage that is a licensed copy of an older hamilton. I
have sucked weeds several times and sticks a couple times. Does not take
much of a stick stuck in the impeller to cause cavitation. Makes me think
a lot of prop boats with small dings in the prop are effecting performance
huge amounts.