BoatBanter.com - View Single Post

Keith Hughes

wrote:
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?

I'm not saying that HP is not a valid unit of measure, just that
comparing HP for two different propulsion types, with what could be
vastly different efficiencies, is of little practical value.
"Displaced" was probably not the clearest of terms to use. I was not
referring to 'volume' displaced by the boat through the water, rather
the 'mass' of water accelerated (assuming a fixed exit velocity), per
unit time, by the propulsion device (prop or what have you).

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.

In both cases, however, the only FORCE available comes from accelerating
a mass of water.

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.

The boats' frictional resistance is a fixed quantity, and therefore
irrelevant for comparison of propulsion types.

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.

However efficient the 'pump' is, it still has the added frictional
losses associated with the system (suction line, pump casing, discharge
line) that the outboard prop does not. Also, keep in mind that a Bilge
pump is *not* designed for this application, so it's a safe bet that the
resulting efficiency will be sub-optimal at best. OTOH, the outboard
*is* designed for the application.

Clearly this type of pumping system *can* work for propulsion, and for
any given pump you just have to look at the pumping curve (for the
suction and discharge head, which wouldn't be too difficult to
calculate). From that, you can calculate the resulting mass flowrate,
and from that the force applied. The bigger problem I see is
characterizing the "flow", if you will (mass of water accelerated, and
the acceleration applied), of the outboard prop versus energy input. A
curve which will almost certainly vary significantly with boat speed.

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.

Excellent "results" don't necessarily imply 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...

Next time I find myself with an 85' waterfall in my backyard, I'll have
to give it a try. 'Course if you have a spare one laying around that you
want to part with cheap... ;-)

Keith Hughes