T-Pain Is On A Boat: Re: [Electric Boats] Re: Question about a prop for a Columbia 9.6m

Let us be a bit more rigorous before making pronouncements. "Brake" horsepower has nothing to do with stopping the engine.

"Brake horsepower (bhp) is the measure of an engine's horsepower before the loss in power caused by the gearbox, alternator, differential, water pump, and other auxiliary components such as power steering pump, muffled exhaust system, etc. Brake refers to a device which was used to load an engine and hold it at a desired RPM."

I remember my days in engineering school over 50 years ago when we had voltmeters and ammeters on the supply side of an electric motor and a drum brake and a spring scale attached to the brake to keep it from turning and a tachometer attached to the shaft. By applying the brake we would load the motor, note the readings on the meters and the scale and thereby calculate the horsepower and efficiency of the motor.

Happy sailing,

Ned

On Dec 4, 2012, at 11:16 PM, Kerry Thomas wrote:

Hi

Ice HP is given as brake HP. Actually the power required to "brake" stop,
the engine at a certain rpm.

It does not translate directly to electric HP, which is the input power x
efficiency. (80 to 90% for a direct electric transmission).

You could do the same sums for a small diesel by using the fuel energy input
at the desired boat speed x 36% (Average under operating conditions) ) . As
said below the manufacturer will often have fuel consumption HP curves.
Unfortunately still usually given as brake HP, whereas we really want to
know what HP the prop sees.

The electric motor seems to need less nominal HP is because it is capable of
higher torque at lower rpm than an ICE engine and needs less ancillaries.
An efficient small diesel also requires energy sapping transmission (To get
the revs down enough for the prop) and ancillaries like water pumps, fans,
alternator ( for starting batteries, house loads and engine management) etc.
10% losses is probably optimistic.

At the displacement speeds we are, mostly, interested in, big slow speed
props are more efficient.
An electric motor can do this as a direct drive.

Matching propeller, boat and engine optimally is still not an exact
science. Requiring tank or real life testing or thousands of dollars of
iteration time on a computer.

You need to fix at least one variable or you will drive yourself nuts.

The next best thing is to work backwards from the fuel consumption, prop
size, rpm and performance of an ICE engine in that boat, at the top speed
you want, to quantify hull resistance, and size the electric motor to
deliver the same.
Then you can work out the effects of increasing propeller diameter, pitch
and/or decreasing blade numbers from there. Dave Gerr, in the propeller
handbook has comparable numbers for differing props.
Remember the diesel was probably oversized in the first place to give
reliable performance in adverse conditions.
Where an undersized diesel will stall an electric motor can be simply fed
more power, within limits, of course.

Regards

Captain Kerry Thomas
kerryjthomas@gmail.com

-----Original Message-----
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
On Behalf Of Mike Gunning
Sent: Wednesday, 5 December 2012 11:43 a.m.
To: electricboats@yahoogroups.com
Subject: [Electric Boats] Re: Question about a prop for a Columbia 9.6m

I am not sure a concise document can be created that will allow everyone to
understand propulsion. A wike might be valuable. I rely on a propulsion
engineer. Rules of thumb are normally pretty accurate. I like Reid's rule
of thumb as it seems to be what we also experience comparing 10hp gas to
7.5hp diesel to 5hp electric for comparing systems. Also we know that the
ICE has maintenance and RPM issues that as it moves off of its peak
performance it will likely provide less power. But how does that explain
how an A4 at 30hp can be replaced by an electric at 5kw (6.7hp)? The
question seems to go back to the propeller.
Everyone who owns an ICE know the hp that is given to it by the builder.
The builder rates it as "brake horsepower" which is before the transmission
(gear), without alternator, without water pump, without muffled exhaust,
etc. I have read papers estimating these components rob 10% or more of the
power. It is not the power that is actually is at the shaft, "shaft
horsepower". Therefore the electric motor with very little power robbing
components, the 5kw motor at the 5000/746 = 6.7 hp is closer to the hp to
push the boat. But what does that mean to the boater if he thinks 6.7 hp is
all the same?
Shaft hp is not always the best indication of the performance because it is
the propeller that actually does the work as Larry points out. I remember
how my college Instructor/captain maintained that the propeller in the water
functions as the transmission and that it is a gear and not a transmission
attached to the engine. This makes sense if you think of the propeller and
the water being a hydraulic transmission.
Mike

--- In electricboats@yahoogroups.com, "mkriley48" <mkriley48@...> wrote:
>
> I agree!! There seems to be a dearth of logic. also boat shape, hull
speed, and whether the prop is out in the open or tucked behind a deadwood
affects prop selection. A faq or wiki would be helpful.
> A hp at the prop at the same rpm is the same whether it is; gas diesel or
hamsters.
> mike
>
> --- In electricboats@yahoogroups.com, "Reid" <axius@> wrote:
> >
> > This is the strangest thread we have ever read. There are too many
> > assumption needed to even comment, but here goes; Before you can start
comparing 2 blade to 3 blade, diesel to gas to electric, we need to know the
the gear reduction of each.
> > We need to assume one is going to use the same diameter and pitch
whether 2 blade or 3 blade. Then we can START looking at theory and finally
compare to actual results.
> > What exactly do you mean by a prop being a little "too powerful?" Too
much pitch, too large a diameter or too much cup?
> > When you speak of a "smaller prop" do you mean diameter, pitch, cup or
all 3?
> > The last time we checked 1hp=746W, not 760.
> > In our experience of building aux power packs; 10hp gas powered,
> > requires 7.5hp diesel or 5hp electric for the same loads.
> > Additionally a lot of consideration on a sail boat needs to be give to
the drag of the prop when not under power. Again, we must make the
assumption that the prop folds?
> > With your Yanmar, is would be interesting to accurately measure your
fuel burn rate and then look up the hp on the manufactures power curve
diagrams.
> >
> > --- In electricboats@yahoogroups.com, "Mike Gunning" <mike@> wrote:
> > >
> > > A question was asked on the Columbia Yahoo site and I thought I would
share my answer with this group. The question was about the boat with a
10hp Volvo with a 2 blade prop and changing it to a 3 blade prop and
possible issues. The technical issues of propellers are not as well
understood by most of us. I would like to see a technical discussion of
these issues and a formula that is easy to apply. This is especially
important to the DIY as the system units are engineered.
> > > Mike Electric Yacht of Southern California
> > >
> > > Jeff,
> > > I agree with Jim in his assessment that the Volvo 10hp is a little
light for the boat. The two blade prop is likely the only way the motor can
get to its power curve.
> > > We have a 15hp Yanmar 2QM15 on our boat (the factory offered this
motor) plus our electric motor. With the factory 2QM15 a two blade prop was
recommended. We installed a 3 blade 14X12 prop and the prop is likely a
little too powerful but it does get us to 6 kts with hull speed being 6.57.
We know the true shaft hp used when we run the electric with the computer
display. We artificially limit the electric because of the small diesel
thrush power inhibitor.
> > >
> > > This is what we see calculated at shaft hp (shp) and which is not
brake hp (bhp). Bhp is a bench test without the losses from transmission and
alternator and other loads on the motor and at peak power rpm. These numbers
are spot on (formula is (amp X voltage)/760 = hp):
> > >
> > > 4 knots at 40amps 48v is 2.25shp
> > > 5 knots at 80amps 48v is 5.05shp
> > > Our computer projections calculates the requirement for 6 knots
> > > as
> > > 6 knots at 150amps 48v is 9.47shp
> > >
> > > Your Volvo is likely in good enough shape will likely put out about
7.5 shp, less than the 11.25 shp that our Yanmar does.
> > >
> > > I would think that the two blade prop is right for your boat. If
> > > you want to go with a smaller three blade prop, you should work with a
good prop guy and follow their recommendations. I can tell you that the prop
is a very important part of the propulsion equation. Get an expert as I know
from direct experience when we built a system with the wrong prop
information it does not perform. We re-engineer and re-program the system
once we get the correct data and it works as projected.
> > > Mike
> > > sv Electric Fluke Columbia 9.6
> > > Electric Yachts of Southern California
> > >
> >
>

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