--- In electricboats@yahoogroups.com, Roland Kruijer <martine_roland@...> wrote:
>
>
> Hello group,
>
> My name is Roland Kruijer and I am a memeber since about two years now.
>
> I own a little electric boat like this one: http://www.electraboat.com/
>
> I changed the motor in the boat for this one: jpg P7051547. I have attached the specifications of this motor in pdf.
> That motor has a controller: jpg PA101651
>
> It all works ok but I think the motor can do better.
> The boat has two 12V 210 aH batteries in series so 24V 210 aH. The motor uses a v-belt to transfer the power to the propellor axle.
> On the motor axle it has a 100 mm pulley.
> On the propellor axle I have a 74 mm pulley which has a 8.2" X 12 pitch propellor.
> Oh, I know that the hull speed can be about 8 - 9 kph. The boat weighs about 400 kg including me.
>
> It seems the motor doesn't get to 1100 rpm, allthough I don't have a rpm meter. Does this happen because the motor can't get to this rpm because of the pulleysizes? Or is the resistance from the propellor(water) too big to get to this rpm?
>
> I have experienced a bit with different pulley size but I'd like to know if there's a way to calculate the best performance possible with this motor. I want some more speed/torque from this engine in relation to the propellor / waterresistance
>
> What would be the best proportion of pulleys for this given motor?
> I am also thinking about placing an extra battery to run the motor on 36 V. Would that give the boat the extra power I want?
>
> Ok guys, I know my boat is not as big as most boats I see here in the group, but maybe someone can help me figure out what is the best way to go!
>
> Thanks so far,
>
> Roland Kruijer
> The Netherlands
>
Hi Roland. There's a simple answer to your question but not a simple solution, but then again, they have software for everything these days.
Performance with any motor depends on a lot of factors, and I guess you can use a wide range of motors to do the job. But performance depends on eight or ten critical factors:
1) Look at the motor's perf. charts to find its limitations: Its rpm at 24 volts; its max. continuous (or "cruise") torque rating; calculations and judgments should only focus on the "plateau" section of the chart's efficiency curve. Now you're ready to configure your prop and pulleys, whose job is not only to balance everything smoothly but to eliminate over-amping (a.k.a. overtorquing, overheating, straining).
2) Download David Rasberry's free "E-Boat Model Spreadsheet" so you don't have to get into a complicated mish-mash of math. (Other members of this group can probably advise you on its accuracy or authenticity.) What you do with it is keep changing the (a) prop diameter, (b) maximum cruising rpm, and (c) reduction ratio (1:1 would be nice but prop torque is more important). Try determining the torque requirement based on weight and hull resistance, and acceleration. The goal is achieving desired speed with minimum horsepower. But for every change in the spreadsheet, you need to check its impact on hp-torque-voltage-rpm, with the goal of gobbling the least amperage, while exploiting the motor's potential.
Look at the pitch result on the spreadsheet. One clue to efficiency is keeping the diameter number lower than the pitch: 10x12 is better than 12x7. Another one, of course, is opting for lower prop speed. Diameter changes have the biggest impact.
Does this help or am I being redundant?
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