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
--- On Sun, 6/21/09, dennis wolfe <dwolfe@dropsheet.
From: dennis wolfe <dwolfe@dropsheet.
Subject: Re: [Electric Boats] Re: motor overheating
To: electricboats@
Received: Sunday, June 21, 2009, 9:12 PM
Colin,
I didn't.
I think there is a downside in that if the 100a motor is loaded to draw, say, 200a at full voltage then the cruising voltage is going to have to drop down a good bit to get the cruising amperage under 100.
48v x 100a = 4.8 kw for the conventionally propped boat.
A wild guess of 30v for the over-propped boat; 30v x 100a = 3.0 kw, a lot less hp. The motor efficiency would probably be less too, further degrading performance.
To avoid this compromise you would need a variable pitch prop or variable speed prop. A lot of complexity for a few minutes at a few mph faster. Remember, doubling the power of a displacement boat already at hull speed will not make it go much faster.
Denny Wolfe
www.wolfEboats. com
----- Original Message -----
From: Colin Girvan
To: electricboats@ yahoogroups. com
Sent: Sunday, June 21, 2009 12:42 PM
Subject: Re: [Electric Boats] Re: motor overheating
Just a quick question, does anyone set up there motor via prop size, pitch and RPM to reach full temporary amperage at full throttle rather than full continuous amps. The reason I ask is that, for example the mars motor is rated at 6hp continuous and 15hp max but if you limit the power loading to the continuous rating all you'll ever see is the 6hp.
I realize going beyond this limit would mean monitoring temp and probably trying to cool the motor some way but both of these are good measures regardless of how hard you push the motor. It just seems to me there would be times even for a minute or two that it would be nice to be able to draw that extra power.
Colin Girvan
BC Canada
--- On Sun, 6/21/09, jameslambden <james@toolboat. com> wrote:
From: jameslambden <james@toolboat. com>
Subject: [Electric Boats] Re: motor overheating
To: electricboats@ yahoogroups. com
Received: Sunday, June 21, 2009, 3:10 AM
The amount of current that an electric motor demands is dependent on load. As a propellor increases in rpm, the power required increases exponentially.
Once you have installed your electric motor in your boat, you then run your motor at full throttle. If the motor draws more than the maximum continuous power rating, you need to choose a smaller propellor or a different gear ratio.
If you do not change the propellor or the gear ratio, the motor will develop maximum amperage at a lower voltage and will be less efficient. It will still work though. It will be working harder and you will be compromising efficiency and range.
THe most important thing is to have an ammeter on the system and not exceed the maximum amperage of the motor or controller. The maximum amperage of the motor or the controller is dependent on how well either is heat sinked.
The Sevcon millipak controller can run continously at about 95 amps and the Mars motor can run continuously at up to 120 amps. All depending on ambient temperature and heat sinking. Providing cool air will help. Mounting the controller to a heat sink with thermal paste and providing a fan for the controller will help too. Keeping the controller away from the motor so they do not share the same heat buildup helps too.
James
www.propulsionmarin e.com
--- In electricboats@ yahoogroups. com, "constancedraper" <drmark.draper@ ...> wrote:
>
> What are the circumstances under which the motor overheats? How fast, how long,etc.
>
> md
>
> --- In electricboats@ yahoogroups. com, "rbrt.beebe" <rbrt.beebe@ > wrote:
> >
> > I installed the Thunderstruck sailboat kit in my Hunter 27.
> > I'm having problems with the motor overheating.
> > I've adjusted the packing gland so as to have the proper drip for cooling and the prop shaft is turning freely.
> > I would appreciate any other considerations/ advice to resolve this condition.
> > Thanks,Bob
> >
>
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