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

the First shock to readers will be that as far as props are concerned they do not know hp. The prop recognizes only the two components of horsepower torque and rpm. That torque and rpm occurs only at the prop shaft. For the prop everything else is "relative" and only an approximation.
Torque and Rpm are the two basic factors of mechanical HP and in most cases they are not even considered in electrical hp discussions. Any discussion of hp without either rpm or torque is totally meaningless as far as a prop is concerned. Even mechanical comparison of HP quantity without rpm is meaningless. For example:
Here are two common engines from the same company with the same gear ratio so the ratings for hp will have been measured using the same methods, equipment, and ratings systems. both are " 18 HP" yet they require two different props because one is 18@3000 rpm and the other is 18 at 3600 rpm
18@3000 14x11 propeller
18@3600 12x10 propeller
So the number for hp alone is insufficient information for prop caparison, and that is without all the other complexities of prop selection
Consider various definitions of electrical hp, the easiest is the volt amp relationship. There is no direct number to determine either torque or rpm for an electric motor which is where the prop selection process breaks down. If you really want to size a propeller then a torque/ rpm curve is needed.
without that curve you are comparing apples to orangesIt can be modified in many ways to allow for heat,battery life etc but when the smoke clears (pun intended) torque rpm are the prop drivers.

larry

--- In electricboats@yahoogroups.com, "Carel Ruysink" <c.ruysink@...> wrote:
>
> It was close but indeed not correct.
> For brakehorsepower you connect a calibrated brake to the engine and then you measure how much "brake" you have to apply to attain a certain rpm at open throttle.
> From brake you can read power and torque at that rpm.
>
> There are much more kinds of horsepower depending on the way you measure it and where.
> On the shaft (shp), after the gearbox, with or without appendages (waterpump, alternator eg) and much more.
>
> Carel.
>
> ----- Original Message -----
> From: Ned Farinholt
> To: electricboats@yahoogroups.com
> Sent: Wednesday, December 05, 2012 1:52 PM
> Subject: 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@...
>
> -----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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