On Feb 15, 5:47 pm, Keith Hughes wrote:
R Swarts wrote:

Well, yes, but HP is really irrelevant other than as a comparison
between similarly configured devices. What you're really concerned with
is *Force*, calculated as F=ma (i.e. force = mass x acceleration). No
matter what system you employ for propulsion, it boils down to the mass
of water displaced per unit time.


Keith, I think you're missing a factor here. "Displaced" implies a
DISTANCE per unit time. That's where horsepower (Or any other POWER
unit) matters. 550 FOOT - (Pounds-Force) (Per SECOND) means that you
could "Gear Down" (or use other force multiplication arrangements) and
lift 550 pounds at 1 foot per second, or instead lift 55 pounds at 10
feet per second. Right?

A large diameter 4-blade prop on a 60 foot boat with a small pitch and
a 20 HP diesel with a reduction gear can put a (lot) of FORCE on that
boat and move it at 5 or 6 knots. I've seen an old 20 Hp Mercury
outboard push a 3-point Hydro at over 40 MPH.

We don't know enough here (yet) to say what the efficiency of an
inboard pump would be in moving a small boat against it's frictional
resistance at a certain speed. We all know, from experience, that
moving a small boat like the Hobie mentioned at the beginning of this
thread takes VERY little force at very small speeds. A gentle push by
hand moves it right away from the dock. Most of us have moved a 10
meter or larger boat a few feet by leaning a little on a dockline.

What we're missing is some approximation of the efficiency of a well-
designed pump in converting electrical power to mechanical power to
move a boat. I think we'd have to do some research and talk to some
mechanical engineers who understand pumps! I have a friend who
recently built a small Hydroelectric plant in New York, who did his
own calculations and is using a large (Thing formerly sold as a pump)
as a turbine, with excellent efficiency. He's running 2 typical homes
on it.. It's a BIG pump and he's at the bottom of a 85 foot
waterfall...

Someone somewhere knows a lot more about propelling a boat with an
inboard pump than I do!