How to figure efficiency in a test boat?
Dan Bollinger
want to compare standard designs plus an idea of mine. I have a 12' test
boat, with 6.5Hp engine and jury-rigged drive train with a variable pitch
pulley on the engine. I can measure boat speed relative to the water,
propeller rpm, engine rpm. I also have the performance curves for the
engine. I'm interested only in the prop's efficiency, not the system. I
could also do a tow test for the boat with a spring scale. But, how to
calculate or graph the efficiency curve?
Bobh
Shouldn't be too hard to incorporate a spring into the drive
(weaken it and measure its deflection ;-)
Frictional resistance varies as the square of velocity and
inducted drag is proportionate to hull's pitch, which can be
normalized by adjusting CG. but how absolutely much of each
depends on the boat.
The graph could be then a vertical scale that is the product of
forward (boat velocity X thrust from deflection ) divided by /
(power applied by engine to prop) This is called propulsion
efficiency. And a horizontal scale of the boat velocity.
Dan Bollinger <danbol...@home.com> wrote in message
news:J00a5.47066$dF.19...@news1.rdc1.il.home.com...
brian whatcott
<danbol...@home.com> wrote:
>I want to know what the efficiency is for a certain propeller design. I
>want to compare standard designs plus an idea of mine. I have a 12' test
>boat, with 6.5Hp engine and jury-rigged drive train with a variable pitch
>pulley on the engine. I can measure boat speed relative to the water,
>propeller rpm, engine rpm. I also have the performance curves for the
>engine. I'm interested only in the prop's efficiency, not the system. I
>could also do a tow test for the boat with a spring scale. But, how to
>calculate or graph the efficiency curve?
>
>
The sort of efficiency which interests you is the mechanical
efficiency represented by the ratio of power input to the prop
shaft divided by [a function of] the speed achieved by the hull
in still air, no water current.
You say you can measure the hull water speed directly, but measuring
the shaft power to the propellor is more difficult.
You can measure the shaft RPM, but you do not have the shaft torque
data available. This would be desirable, I think.
Perhaps the engine is on flexible mounts.
Perhaps you could fix a long lever to the engine.
Perhaps you could apply a force meter at this lever to centralize the
torque reaction of the shaft, for which a spring balance might serve.
This approach might help you reduce the data, to resolve issues that
might arise, like these:
a more efficient prop drives the boat slower than a less efficient
one, because it is optimal for a slower shaft speed.
a more efficient prop drives the boat at the same speed as a less
efficient prop, because it takes less HP from the engine at the same
shaft speed.
Needless to say, uncareful handling of such data will drive you crazy.
Good luck!
Brian
Brian Whatcott Altus OK
O mani padme hum.
Chris Pollard
: I want to know what the efficiency is for a certain propeller design. I
: want to compare standard designs plus an idea of mine. I have a 12' test
: boat, with 6.5Hp engine and jury-rigged drive train with a variable pitch
: pulley on the engine. I can measure boat speed relative to the water,
: propeller rpm, engine rpm. I also have the performance curves for the
: engine. I'm interested only in the prop's efficiency, not the system. I
: could also do a tow test for the boat with a spring scale. But, how to
: calculate or graph the efficiency curve?
prop is going to be used with this engine then you'll just need to know
what is the fastest speed with lowest fuel consumption. Most efficient
propeller is fastest speed at certain rpm.
Bobh
Chris Pollard <cpol...@teal.sni.net> wrote in message
news:XMba5.31$tA2....@wdc-read-01.qwest.net...
apparently not committed to this engine's hp/rpm curve. As
Whatcott indicated you are going to have to measure your hp since
engine chart hp are going to be reduced substantial by losses in
you variable pulley drive whose efficiency is probably only
around 80%. Another problem is distortion to the inflow to the
propeller by the hull/drive and hull-prop interactions
particularly if these are differential between you prop and the
comparison conventional. Formal prop tests try to arrive at an
'open water' number which is approximated by a long horizontal
prop drive-shaft coming from a drive in a streamlined body. A
good screw prop at its best speed should give you 65-78%
Strongly recommend if a radical design and if otherwise suitable
you do this all in small scale -toy electric motors -aluminum
flashing -Bud cans and hot glue unless high accuracy or joyriding
is crucial. Most marine testing is done in scale.
DB TECH
The only way to measure propeller efficiency is to know the real
SHP of the engine at said RPM, and the actual HP driving the
boat, which is derived from the "propeller thrust" and the "boat
speed" through the water. (assuming no wind)
Driving HP= T X V over 550
Where:
T=prop thrust in lbs.
V=velocity in FPS
Propeller efficiency= Driving HP over SHP.
Dave Brownell
DB Technologies
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