outboard motor thrust
kt
many pounds of thrust is produced by the average
"off the shelf" 25Hp outboard motor.
Thanks for any feedback,
Ken Trull.
Marcus G Bell
It depends on what prop you use and what the gear ratio is, what the
RPM of the engine is, and what the speed through the water is.
Maybe if you told us why you are asking, we could answer your question
more fully.
The electric trolling motors are rated in thrust, and are built for
thrust at low water speed. It is said that about 26-28 pounds of
thrust from an electric trolling is equivalent to about 1 HP from a
gas outboard. This is based on the typical prop fitted to the gas
outboard, which by the way is usually propped for a bit of speed and
not as much low-speed thrust.
Are you pushing a houseboat, pulling stumps, or something where the
actual "thrust" matters?
I recently went through the exercise of wanting to know the actual
thrust, as I was installing a roller thrust bearing in the lower unit
of a 10 HP outboard which formerly had a flange bushing. So, for
"moving thrust", that is the thrust that the outboard actually
experiences while the boat is moving, the conversion is simple:
power = thrust X velocity , or thrust = power/velocity
To put it in units commonly used, 1 HP / 1 MPH = 375 lbs force.
So, if my outboard is pushing my boat at 10 MPH and using all its 10
HP (reasonable numbers), it is pushing on the boat with a thrust of
375 lbs. WOW. Certain things to consider: as boat speed slows, prop
"efficiency" drops because of a lot of factors, even if you are
adjusting the prop pitch and blade area to achieve proper engine
RPM. At 0 MPH your efficiency is 0, technically speaking, but you
still generate static thrust. Your thrust will have practical
limitations because you can't make the prop bigger than the lower unit
can accomodate, and you generally can't change the gear ratio.
So, if you're looking to see how strong your transom or thrust
bearings need to be to withstand the thrust, you err on the side of
high thrust. If you're asking "will this outboard provide enough
thrust to move this boat" you should err on the side of low thrust,
perhaps 15 lbs/HP.
--
--
Marcus. ( be...@mail.med.upenn.edu )
kt
Marcus G Bell wrote in message <6s6n9v$8qu$1...@netnews.upenn.edu>...
>kt (k...@obltcidr.com) wrote:
>It depends on what prop you use and what the gear ratio is, what the
>RPM of the engine is, and what the speed through the water is.
>
>Maybe if you told us why you are asking, we could answer your question
>more fully.
Thanks for the info.
I was just reading an advertisement for an electric outboard
that produces 101lbs. of thrust and was wondering how
that would compare to a gas engine which is usually
rated in HP.
Ken
Marcus G Bell
> Marcus G Bell wrote in message <6s6n9v$8qu$1...@netnews.upenn.edu>...
> >It depends on what prop you use and what the gear ratio is, what the
> >RPM of the engine is, and what the speed through the water is.
> >
> >Maybe if you told us why you are asking, we could answer your question
> >more fully.
kt (k...@obltcidr.com) wrote:
> Thanks for the info. I was just reading an advertisement for an
> electric outboard that produces 101lbs. of thrust and was wondering
> how that would compare to a gas engine which is usually rated in HP.
> Ken
Ahh. I see. Earlier you said
"Just for reference, I'd like to know APPROXIMATELY how many pounds of
thrust is produced by the average "off the shelf" 25Hp outboard
motor."
I'll answer this more fully now: based on the figures obtained in
reviews of real-world off-the-shelf 10 HP gas outboard motors, I'd say
the 25 will produce about 700 lbs static thrust.
The electric outboard with 101 lbs thrust may do that at 0 MPH, but we
have no way of knowing what it will do at 5 MPH. Or whether the prop
along with any reduction gearing would be capable of producing ANY
thrust at 5 MPH, such that 5 MPH would be unobtainable.
Amperage times volage, or wattage, is a better estimator of electric
motor "horsepower". DC motors are pretty efficient, so you can
estimate based on 85% efficiency and be pretty close. This will tell
you what size gas outboard the electric motor will approximate,
speedwise, if you are able to play games with the prop. Otherwise,
most commercially available electric outboards, or "trolling motors",
are designed for pushing the boat at a few knots tops, even the
biggest motors.
Doubtful that a 101 # trolling motor would give the speed of a 3 HP
gas outboard. It will be a lot quieter, however.
Del Cecchi
my boat at (about) 40 mph is putting out 115*550/(1.48*40) or 1068 lb thrust.
more or less :-)
add or subtract windage per your personal preferance
--
Del Cecchi
cecchi@rchland
Del Cecchi
watts. So if the motor draws 40 amps at 36 volts, that is about 2 hp.
Del Cecchi
Marcus G Bell wrote:
>
> > Marcus G Bell wrote in message <6s6n9v$8qu$1...@netnews.upenn.edu>...
> > >It depends on what prop you use and what the gear ratio is, what the
> > >RPM of the engine is, and what the speed through the water is.
> > >
> > >Maybe if you told us why you are asking, we could answer your
Paul Kamen
> of thrust is produced by the average "off the shelf" 25Hp outboard
> motor.
250 pounds of thrust at zero speed, or about 10 lb./HP is the rule of
thumb.
A "high thrust device" like a bow thruster or tugboat populsion system (or
an electric ouboard) will be more like 25 lb/HP, and an optimized system
can be up around 50 lb./HP. But this is all at zero speed, where
propulsive efficiency is zero so there's no theoretical upper limit to the
amount of thrust that even a small amount of power can produce. As soon
as you actually start to move then different rules apply.
--
fish...@netcom.com
http://www.well.com/~pk/fishmeal.html
-"Call me Fishmeal"-
Paul Kamen
>A horsepower is 550 ft-lb/sec, and a mph is 1.48 ft/sec, so
>the 115 merc pushing my boat at (about) 40 mph is putting out
>115*550/(1.48*40) or 1068 lb thrust.
Um, only if it's a land vehicle. There's that pesky issue of "propulsive
efficiency," which just about cuts the thrust in half on most real-world
boat propulsion systems.
John Weiss
rated RPM at the time...
Actually, only the full throttle and RPM need be verified. Windage will
only subtract from the water speed.
---------------------
John R. Weiss
Seattle, WA
Remove *NOSPAM* from address for e-mail reply
Del Cecchi wrote in message <6s76jl$reo$1...@news.rchland.ibm.com>...
>A horsepower is 550 ft-lb/sec, and a mph is 1.48 ft/sec, so the 115 merc
pushing
>my boat at (about) 40 mph is putting out 115*550/(1.48*40) or 1068 lb
thrust.
>