TWL: Fwd: Re: Thrust vs. Horsepower (was Dinghy/motor question)
I think I've been reading a lot of "techno-babble" on this subject. I
and most boaters are not interested in some esoteric formula to
convert electric trolling motor performance to gasoline outboard
performance. What we need is a reasonable approximation to a question
like: " When I take my dinghy across the bay (ca 1 mile) with a 2 hp
Yamaha and it does so in about 12 minutes (5 knots), then how many
lbs thrust MinnKota (or the likes) do I need to get the same
performance on the same dinghy on the same bay in the same weather?"
Who can I ask that from?
George of Scaramouche
George Geist wrote:
I think I've been reading a lot of "techno-babble" on this subject. I
and most boaters are not interested in some esoteric formula to
convert electric trolling motor performance to gasoline outboard
performance. What we need is a reasonable approximation to a question
like: " When I take my dinghy across the bay (ca 1 mile) with a 2 hp
Yamaha and it does so in about 12 minutes (5 knots), then how many
lbs thrust MinnKota (or the likes) do I need to get the same
performance on the same dinghy on the same bay in the same weather?"
Who can I ask that from?
George of Scaramouche
Maybe this will help answer your question. If you go to
http://www.johnsonoutdoors.com/motors/minnkota/faqminn.html#hdt
you will find an FAQ on Minn Kota trolling motors. The excerpt that I
believe is most useful reads as follows;
"How do I convert thrust into horsepower?
Many people think that the more pounds of thrust they are getting, the
faster the boat will go. This is not exactly true. With trolling motors
and outboard motors, boat speed (and acceleration) depends on horsepower
and prop pitch. By changing prop pitch, you can trade acceleration for
top speed. Our trolling motors have props designed for maximum
acceleration. When you hit the power, the boat responds immediately.
This also allows you to hold the boat in high wind conditions. Top speed
with our motors and props is about 5mph. Small boats will reach this
speed with most of our motors. Larger boats require our largest motors
to approach this speed. Comparing our motors to gasoline outboards is
difficult because most outboard props are designed for much higher
speed."
In other words the trolling motor prop is optomized to povide maximum
thrust at low speed and most stock outboards are optomized for higher
speed, so a lower power trolling motor may give your dink comparable
performance to a higher horse power outboard unless you optomize the
prop on the outboard for acceleration.
Another significant difference between gas outboards and electric
trolling motors is the torque versus speed curve. The gas engine
produces more torque as engine speed increases. The electric motor
produces constant torque regardless of engine speed. This means that the
propeller can operate efficiently over a wider speed range.
I think the bottom line is, if you are satisfied with going 5 mph (thats
4.3 knt) an approporately sized trolling motor will probably work.
P.S. If you are buying one, make sure you get one designed for salt
water use, i.e the Minn Kota "Riptide," or a really good deal. O, and I
have no affiliation with any of the companies or products mentioned.
Mike Schooley
Trailerable Trawler Dreamer
George,
I didn't realize that a conversion from hp (which is what gasoline outboard
power output is measured in) to KW (which is what electric motor power
output is commonly measured in) was "esoteric techno babble". I'll try to
simplify my answers a bit more from now on so as not to confuse anyone. My
apologies.
If you want an electric motor to have roughly the same power output as a
gasoline motor, a good place to start is with comparing their respective
power outputs, hence you want to compare like units to like units?
Now "thrust" is a little more difficult to compare. I haven't seen "thrust"
values published for outboards, so without those numbers what are you going
to compare? And thrust is dependent on a number of factors such as the prop
size and pitch, the gearing in the outboard transmission, and the torque of
the engine. That being said, and to answer your question below, if a 2 hp
outboard produces 100 lbs of thrust to move your dinghy at a certain speed
over a certain distance, then the MinnKota will need to produce about 100
lbs of thrust as well (assuming that the weight of the dinghy is similar).
Now what I think you're really asking is "what size MinnKota motor will give
comparable performance to a 2 hp outboard?" I don't know. But I'd start by
looking at their respective power outputs (that hp to KW thing again). Then
I'd probably talk to a MinnKota dealer...
Bob Deering
Im a bit confused about all this after reading Calders
boat book. He says something like packing gland should
not leak at all. However, the previous owner of my
boat told me that I want a few drips a minute and
loosen it before heading to sea. Anyone care to
comment on what setup they have?
Also, Calder said not to tighten the locking nut too
much, or it would cause heat, and eventually damage
something. I would think that you would want that
sucker cranked down hard so that nothing comes loose.
?? I cant imagine that you want a lock nut loose,
allowing somewhat free play of the box on the shaft?
Someone care to enlighten me?
So, my impression is that when you are in port, head
down with your wrenches, tighten the thing so it does
not drip. Then when you head to sea, loosen some so
that water can circulate. Is this what others do?
I also want to make sure I have all this. The stuffing
box is the "unit" or "holder" for the packing gland?
And the gland sits inside the box and is composed of
some material that compresses and keeps water out.
correct or incorrect? ???
Obvious newbie,
Dan
37 Roughwater
"Wen I Dream"
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