I was thinking plant life not barnacles. The HP rating of electric motors is based on 100 percent duty cycle. When you think 10 HP you can stay at that rate all day. Only stay at 2.5 percent of that for short bursts or risk overheating the motor. We are talking close to locked rotor here.
The reason many will gear reduce the motor output is efficiency is increased a distance from locked rotor.
Once again, prop selection on fuel engines is the need for slippage to keep from stalling the motor and the need to get the engine up to designed RPM. Because e motors have a constant torque(the power equation factor used to drive the prop) e motor drive systems are designed with top speed efficiency in mind, not low speed engine requirements.
Your top speed ie..pitch vs. rpm is going to be your design requirements. Include the HP requirements needed to drive your hull at hull speed fighting any head wind you may encounter and you system will be engineered.
Shakedown with small bank of batteries for battery life then increase the bank for max distance you will need fighting a headwind.
HP is only used for hull speed calc, not for prop selection. Two blade props will give most efficiency so design with distance in mind. Prop slippage will always be a factor that create losses so keep slippage as low as you can and calculate prop pitch and RPM to meet design hull speed with slippage in mind.
On Mar 8, 2019 8:25 AM, "tvinypsi@gmail.com [electricboats]" <electricboats@yahoogroups.com> wrote:
Thank you for the thoughts, Kevin. Much appreciated! I've seen at least one other boater's suggestion that the prop calculators are intended for ICE driven boats and not electric, although the Vic Prop calculator seems to be designed the goal of attaining as close to hull speed as the engine/motor seems able. The torque it assumes is the maximum as calculated for the user-entered horsepower. I don't think any by highest RPM attainable by the user-described system is considered in its calculation.
With this in mind, the problem with using the Vic Prop calculator is knowing what figure for HP should be entered. Thunderstruck states their 10Kw motor will generate up to 10HP continuously, but offers a peak of 28HP. What would be the correct figure to use? I haven't been able to figure it out.But then there's also the question of what ratio of reduction to use. A big wheel is most efficient, but if constrained in diameter, I've read, increase the RPM or increase the pitch at the expense of efficiency.Not being an engineer of any kind, it's all quite a lot to get my head around!On the prop size/kort nozzle concepts ... My prop diameter is constrained by being in the aperture of a full-keel boat. In my readings I have indeed run across kort nozzles and thought about the possibility of such a retrofit. We're in the Great Lakes, so beyond the material and structural design questions that I'm not entirely competent to answer, the potential for fouling is a serious consideration. A couple of different types of mussels are particularly pesky. So, I doubt I'll attempt this. Great suggestion though .. sort of like an aft-facing bow thruster!
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Posted by: Kevin Pemberton <pembertonkevin@gmail.com>
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