45' solar catamaran

This is not correct. Electric motors are rated at power input. Whether they are for continuos maximum power or not depends on the construction of it and its rating. Electric motors can not output several times their rated power. Rated power is the maximum power. There can be a continuous rated power that is less than the maximum, which is a different thing.

This is a generic statement that is probably based on the usual sales hype. There are different types of electric motors that are designed for high torque, high speed, constant speed, etc. That is the same as ICE designs. Further, gear boxes can match the necessary torque by reducing or multiplying the speed.

Once more, power is the same regardless on source. Your friend being impressed is not really substantiated data. If this is related to your claim of larger propellers being more efficient, are you saying an trolling motor has a propeller of significantly larger diameter than an ICE outboard of similar power?

 

Haha! gonzo, I'm here to lean about how to fix two wood part together in a boat because I know very little about this.
But, clearly I know a thing or two about battery and motor.
So...

Absolutely not. The maximum power output of any electric motor is just before copper/insulation melt from to much current.
You can take a forklift motor or e-bike hub motor, by example, and feed it with more voltage and current to output 5-10 time is rated power.
The Nissan Leaf motor is another good example as it was rated at 80 kW in 2011, then rated 110 kW few years later and then 160 kW today. Some guys tuned it with another inverter and output way more than his rated power.
Or even, the ME1507 motor I was planning to put in the Cat is rated 14 kW, but can output over 50 kW peak.

Sure, but who want a 200 lbs dinghy motor? For the same output torque, an ICE need to be bigger.
I mean, take in example a 6hp ICE outboard motor. They are relatively lightweight because they use a small torque/high rpm motor by design. That imply the use of a low diameter/high pitch propeller.
For the same weight, a 6hp (4.5 kW) electric outboard motor can output WAY more torque at lower RPM. Then it's possible to use a larger, more efficient prop.
At high RPM, prop cavitation can start to be an important factor of inefficiency.
So, shaft power is one thing, but power who effectively propel the boat is another.

 

That is silly. Running a motor with voltage above the rating makes no sense as a comparison. I could tell you that I put a supercharger and nitro on my ICE engine and it produces more power than your electric one, even though the factory sticker says 6HP.

Even outboards from the 1920's did not weight 200 lbs for 6HP output. Also, you are not counting the weight of batteries. You keep on insisting that power from an electric source is more powerful than the same power from a different source. That is nonsense. I am at a loss to try to explain to you that shaft power is what drives the propeller. What do you think transmits the power to the propeller? Cavitation is caused by low pressure at the forward face of the propeller and has nothing to do with RPM, but with blade loading. However, supercavitating propellers are more efficient at high speeds than standard propellers. Surface drives run ventilated propellers which are also more efficient at high speeds.

 

Isn't exactly my point. Or is it?
Let take the 6hp outboard motor as example.

ICE: 6hp at motor shaft, minus bearings, seals, bushings, bevel gears, water pump, oil so you end with 5 hp at 3000rpm to a small prop because torque is low. Propeller effective thrust = low.
If you put a big prop, then the torque will not be enough, the rpm will drop, so hp will drop.

Electric pod: 6hp at motor shaft, minus seal, so 5.9 hp at 1500rpm to a large prop. Propeller effective thrust = high.

I think it's how many marine electric motor supplier end to compare some kW to a HP number who isn't simply X = X/0.746.
And it's exactly what I experimented with my small 1.5 kW outboard motor.

I tried to read about propeller hydrodynamic, propeller tests and learn as much as possible about all this in order to choose the best propeller for our Solar Catamaran.
Please let me know if you have good references or thread about propeller efficiency to share.

 

I am exited about your project. Just letting you know that an outboard motors HP is rated at the prop.

 

Oh! Really. Cool, one more thing learn today.
Thanks
I should focus on electric motor

 

No. 6HP at the shaft means 6HP at the propeller. The propeller needs to be matched. Mismatching a propeller and then making claims of inefficiency is cheating.

No. If have the maximum power of either engine at 1500 HP then the propellers would be identical. You are picking a high RPM for an ICE at an arbitrary number to suit your argument. A propeller effective thrust is not only based on diameter and RPM. There are plenty of textbooks that cover the subject. Also, the kW of an electric motor are the input, not the output. Further, there are several winding styles for electric motors. Each has different power and torque curves. You need to understand that and also whether they are AC, DC, 1 or 3 phase, stepper or not, etc. They are all different, the rest is sales hype.

 

It's one of my point.
The output rpm for the same hp cannot be the same. Why? simply a question of design and weight.
A 6hp ICE turning at 6000 rpm output 5 lbs-ft of torque. Then, a small transfer shafts, small bearing and small bevel gears.
An electric 6hp pod drive can output 21 lbs-ft of torque at 1500 rpm despite being lightweight.
A 6hp ICE can output 21 lbs-ft of torque at 1500 rpm, but engine will be big, shafts and bearings will be large and the bevel gear will need to be huge. That make no sense by design.
It's why small propeller are need for ICE and why larger propeller have nice fit with electric motor.

It's exactly what I'm looking for to have best prop/system efficiency for my Cat project.

So if a motor need 10kW, but I lock the rotor it's a 10 kW motor? No, the shaft don't turn, so it's a 0 kW motor.
Another example, you should explain me how electric motor manufacturers calculate motor efficiency if they only rate input power

Clearly understand this part. At the moment I only think about using the most popular motor type use in electric vehicle: Brushless PMAC motor.
Details here: Electric inboard motor, my journey to find proper motor https://www.boatdesign.net/threads/electric-inboard-motor-my-journey-to-find-proper-motor.69574/

 

Those are just unfounded claims. The bearings on a 6HP outboard are about 1 1/2 inches in diameter at most. Also, the rating for RPM is 5000. Power = Torque x RPM. You can use gearing to adjust the torque and RPM to suit the application on an ICE or an electric motor. Except for small boats, electric motors often use gears too. It is interesting that your first sentence calls the shaft and gear small, but then call the shaft and gears huge to suit your argument. Bottom line, output power is the same regardless of source. You can't change the laws of physics by expressing your opinion. Further, you conveniently not include the weight of batteries on the argument for electric motors. The overall weight to energy of an electric system is higher than an ICE.

 

It's simply because I compare 6hp (1500rpm 21 lbs-ft) engine to a 6hp (6000 rpm 5 lbs-ft) engine. Maybe read another time my post?

Yeap, yeap and yeap. I'm fully ok with that and don't claim other way.
My point since the beginning is about effective propulsive power, not only power.
Like: Look dude, I put 10 kW in the water but my boat don't move fast. Man, that is a 10 kW water heater

Absolutely and it's because we are talking about power here, not energy.
Trust me, I can build a 5 lbs battery able to output 10 kW of power. Simply it will be for a very short period of time.
In the case of our solar Cat it will be the opposite. A very large battery who output low power for long period of time.

 

A data point. The 6m barge in this video has 5 x 300W solar panels on it's roof, but only 2 are connected. The motor is a 40 hp outboard, with an electric motor replacing the petrol part. 600W is a little less than 1 hp. The boat, with 3 people on board is doing 4 knots by gps. I doubt a 1 hp outboard would be as efficient.



Another picture of the boat at https://static1.squarespace.com/sta...8a284c728567/1710268701235/CPPBB Feb 2024.pdf

 

What kind of 4cyl diesels are alredy installed ?

 

Two VW 4 Cyl. SDI 60-4 diesel engines. Automated transmission.

 

...and the weight of the cables, connectors, controller, charger, BMS, additional fire and safety gear, etc.

 

Weight of electric stuff especially high current/capacity batteries and e.g. leaf motor isn't really big . Modern batteries are quite impresive weight/ power density/current . Most challenging part is a salt water.