I had a chance to put this into practice today. Wind was blowing about 15, with about 1' of chop. So I set my boat at 40amps going downwind, cross-current (so shouldn't be a factor) and got to 4.4 knots. then I turned the boat 180 and sure enough - amp draw increased to 47amps. But...boatspeed dropped to 2.2 kts. I did not do multiple runs, but did go long enough in each direction that the boat speed had equalized, etc.
Probably something interesting in those numbers about prop efficiency, but beyond my ken to figure out. You can imagine that an arbitrarily large prop and unlimited battery would probably hold better upwind speed.
Anyway - interesting to put the theory to practice.
-Keith
--- In electricboats@
>
> Keith, Actually voltage and rpm are constant at WOT. Amps are whatever it takes to turn the shaft at that rpm. Add drag to slow the boat and it takes more work (more amps) to keep the prop turning the same speed.
>
> Here's an experiment anyone can make. Tie the boat to the dock and open the throttle - note amp draw. Now let it run free and check the amps.
>
> On an average boat you will find about 10 to 15% less amp draw at speed - not too much difference. The reason is that the prop on a displacement boat slips a lot no matter how fast you are going. Slip is 100% tied to the dock (this is the bollard pull test) but is still probably 50% when running free.
>
> If you had such a tail wind to push the boat at the zero slip speed of the prop at max rpm the amp draw would be about zero - just like running the motor with the prop out of the water.
>
> At WOT my launch draws 8 amps out of the water, 100 amps the moment I floor it from a standing start and levels out at about 88 amps with the boat going 9 mph. I'd estimate a fierce headwind might add 5 amps to the WOT draw. Admittedly this is not a sailboat with the tremendous windage of all that rigging. If you a re powering into a lot of wind you are probably encountering good sized waves on the nose - they add a lot of drag too.
>
> Denny
>
>
> ----- Original Message -----
> From: aweekdaysailor
> To: electricboats@
> Sent: Monday, June 22, 2009 8:39 PM
> Subject: [Electric Boats] Re: motor overheating
>
>
>
>
>
> I don't think it works like that. At WOT you have perfectly balanced the thrust from the prop (X) to the resistance of the hull and have speed (Z).
>
> With a headwind, you have added another thrust vector (Y) in opposition to the prop. Net thrust is thus reduced to X-Y, resistance is the same, and speed is now some high fraction of Z - amp draw remains the same. The prop is still happily spinning away but that doesn't mean you'll draw more amps. That would require the boat trying to "solve" for boat speed - it could care less - it solves for voltage (=RPM) and simply slows down because the net-net is that resistance has increased.
>
> *I* can solve for boat speed...because I can over-throttle :)
>
> You can over-heat diesels by running them WOT as well - they are usually set to around 3000 RPM with stern warnings not to exceed 2500RPM continuous. But it's there...if you need it...
>
> (I'll re-pitch if I hear convincing arguments otherwise)
>
> -Keith
>
> --- In electricboats@
> >
> > Colin,
> >
> > I appreciate your brilliant clarity
> > 90% hull speed, 100amps, WOT in CALM conditions;
> > 90% (ok, maybe a little less) hull speed, 150amps (wacky guess), WOT in STRONG HEADWINDS. Yes, that is my understanding also, but only with a large diameter prop. You get full voltage (better efficiency) in each case, just different amperage (power required to do the job).
> >
> > My comment about shallow pitch was for a fast turning direct drive large diameter (24+ inch) propeller. I don't think such an animal exists though. Pity - a 900 or 1600 RPM (or whatever you motor RPM rating) 24x2(?) prop going 6knots would be an all-weather sailboat prop. With that shallow a pitch, there may be zero regen capability though, and huge drag when not motor-sailing.
> >
> > I suspect you are right about slow turning, large diameter, steep pitch props being more efficient than a fast turning, large diameter, severely shallow pitch prop. Moving a large diameter item through the water quickly would waste frictional energy (water & prop friction). Of course, reduction gears waste frictional energy too. That would be an interesting study: which wastes more under varying conditions. Where do the efficiency curves intersect? With what diameters and RPMs and pitches and hull speeds would it have been more efficient to go with a reduction gear instead of direct drive, or visa versa?
> >
> > See, I make a mess of things, and you straighten them out.
> >
> > Mark Stafford
> >
> >
> > --- In electricboats@
> > >
> > > Thanks Mark
> > >
> > > I think I'm finally getting my head around this issue, I always wondered why people on the forum made such an issue of getting the prop size, pitch RPM equation to match continuous amps at WOT.
> > >
> > > I could understand not wanting to be under the continuous rating as you would not be using all the available power, but being over the rating I thought would be a good thing. I realize from Dennis and your comments that it's not quite that simple.
> > >
> > > If I understand your explanation correctly if a boat was set up to do say 80-90% of hull speed at a continuous rating of 100 amps in calm conditions with WOT. Then WOT into say 30 knots of wind it would still try to do the same RPM and therefore draw much more amps thereby automatically using the extra temporary power available.
> > >
> > > Sorry if this seems overly obvious to you guys but my understanding lies more with boats rather than electrics, so I appreciate your responses.
> > >
> > > One thing that I didn't follow was your comment on "severely shallow pitch" I thought large slow turning props with high pitch was the way to go.
> > > Thanks again for your thoughts.
> > >
> > > Colin Girvan
> > >
> > > BC Canada
> > >
> > > --- On Mon, 6/22/09, Mark Stafford <mstafford@> wrote:
> > >
> > > From: Mark Stafford <mstafford@>
> > > Subject: [Electric Boats] Re: motor overheating
> > > To: electricboats@
> > > Received: Monday, June 22, 2009, 4:28 PM
> > >
> > >
> > >
> > >
> > >
> > >
> > >
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> > >
> > >
> > > Colin,
> > >
> > >
> > >
> > > Practically speaking (with prop diameter limitations) , you are right about electric vs ICE horsepower. However, if you can afford giant propellers and reduction gears (or severely shallow pitch), six knots against a headwind vs six knots calm wind would just require different amperages, as the propeller/motor would be spinning at nearly the same rpm (voltage) in either case. You still get the efficiency of higher voltage, AND the emergency power reserve of over-amping the motor to your hearts content.
> > >
> > >
> > >
> > > Mark Stafford
> > >
> > >
> > >
> > > --- In electricboats@ yahoogroups. com, Colin Girvan <colingirvan@ ...> wrote:
> > >
> > > >
> > >
> > > > Good point Dennis, I hadn't thought about actually losing power at cruising speed. So I guess when people quote the advantage of an electric motors ability to put out 2 or 3 times its rated horsepower for short periods that it's actually a bit of a fallacy as this is not really usable power.
> > >
> > > >
> > >
> > > > I appreciate your comment about trying to exceed hull speed, but my thoughts were not so much aimed at speed but more in the situation of fighting headwinds into an anchorage or channel. There have been several posts in the past (namely Arjan Bok and I think the other was Keith) that have given me pause.
> > >
> > > >
> > >
> > > > It seems that the general consensus that an ICE engine can be replaced with an electric motor one third the size might be good for calm conditions but in adverse situations it might be more like 1:2 ore less.
> > >
> > > >
> > >
> > > > Colin Girvan
> > >
> > > >
> > >
> > > > BC Canada
> > >
> > > >
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> > >
> > >
> > >
> > > ____________
> > > Looking for the perfect gift? Give the gift of Flickr!
> > >
> > > http://www.flickr.
> > >
> > > [Non-text portions of this message have been removed]
> > >
> >
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> [Non-text portions of this message have been removed]
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