Sometimes a horse is just a horse

I manufacture electric outboards. I have read several places on this forum suspicions that electric outboard motor manufacturers claim that electric Hp are larger than gasoline Hp. Not so. A Hp is a Hp and a Watt is a Watt. That said, I claim that our motor is approximately equivalent to gasoline motors that produce 3x the input power of our motors. How can this be?

All electric drives have low efficiency at low power output and efficiency climbs as the power output increases --> if its a well designed system. So you should use your electric system close to full power to maintain the best motor efficiency. For example my measurements show that the EPcarry has a peak 47% efficiency its maximum 220 watts. It's made for dinks so that power point makes sense, and anything above 180 watts is in the groove. The smallest Torqeedo has similar peak efficiency at a higher 500 watts. If you have a Torqeedo you'll want to use it at close to that power level so use it on larger dinks and daysailers. Gasoline motors don't want to be used at full power for cruising so you'll need a larger engine rating for the same cruise speed. So that's one element. But it's not able to account for the difference.

Another element is the propeller and gearing design. Most outboards and auxiliary inboards are way under-propped; not for the engine but for the boat. For displacement boats there is a direct correlation between propeller disk area and overall efficiency- larger prop diameter, higher efficiency. Higher pitch also equals higher efficiency. Good e-motor manufacturers have figured this out. Ray electric was the first to launch a big-propped high pitch electric right after the war and their efficiency was not far off of the new electrics; way back then! But almost no gasoline motor manufacturers have figured that out (except for Seagull). Gasoline engines are characterized by the industry by Hp measured at the prop shaft. So there's no common basis in claims for them to swing over to a larger higher pitch prop. That change would also require them to increase the gear ratio to match engine torque-power needs. So they haven't. Their props are way to small, too low in pitch and have way too much surface area to be efficient (too much vortex creation and too much back-pressure). Comparing an EPcarry prop to a BF2.3 prop- you're gaining at least 200% in efficiency. More like 300% comparing to a trolling motor prop by my measurements.

Add it up and arguably good electrics can be lower rated than gasoline units for cruise comparisons. And if its a well designed system 3:1 is generally accurate.

If you are concerned about overcoming heavy sea states and high winds, any reputable electric manufacture will tell you that you will slow down. While electric has high torque in all conditions, you simply don't have the same headroom provided by the gasoline motor's over-specification. In extreme situations you may also lose steerage. We tell our customers this up front and steer them away from ordering an EPcarry for a boat that is too large if they anticipate a need to maneuver in a tight marina with a 30 ft mast in 25 knots of wind.

Every time you change technology, there are pluses and minuses. New technology is never the same as the old. Some accommodation has to be made if you want the advantage.

The market is accustomed to horsepower right now, and I've never had a horse on my boat. Over time, people will become just as comfortable with electric systems because well engineered systems just make more sense. But we have no collective experience with it yet.

It's happening- just not done yet.

Joe

 

Joe, I had 3 horses (looking at my boat , not on my boat) , I had to put down in December last year my Sadler at 33, good age for an horse. We have 2 left over, our youngest one is 25 and is as strong as hell. What I try to say is, you did not mention what electric motor are we talking about. A serial motor? A induction motor like used in a lawnmower, a brushed motor? a brush less motor? an out runner? All of them have different behaviours. I assume you have it over a brush less e motor and I agree fully with you. But moving 75 kg one meter in 1 second (746 watt) is the same for all, also for
735.50 kilograms force meter/second (kgf m/s). I love e-motors (brushless out-runners) and I enjoy every minute of having them. But I agree there are a lot of people who are not familiar with electrics and electronics and shy away from them. Or don't understand the advantages. Pity. Bert

 

If you compare a 1 kW electric motor to a 5kW gas motor your claims could be somewhat justified. However, if we compare a 5kW electric motor to a 1kW gas motor, the numbers reverse and, by your comparison system, gas motors are better.

 

Really no need for another thread.

Your claim of "Because the rpm x torque vs thrust x speed characteristics of electrics are so different from gas. " is 100% bogus. Let me be clear, it is a lie.

You want to compare small motor with prop optimized for tiny boat and a bigger gas engine with prop optimized for higher speed.

It is a nonsense comparison and I bet if the wind picks up (windage resistance goes up) the 2hp Honda will leave your 0.3hp e-motor in dust.

Deal with reality. Sell your strengths but don't insist on snakeoiling it.
Your motor can be thrown inside a car without stinking it. Your motor is near silent. Your motor is handy to carry and costs near nothing to operate.

BUT it is a 0.3 hp outboard with a prop optimized for low speeds.

 

Ps.
Meaning of terms over-propped and under-propped http://www.ybw.com/forums/showthread.php?369643-Meaning-of-terms-over-propped-and-under-propped

 

I decided to consult google and see what they have to say. To my surprise it is totally different:
ONE REPLY.
Here's an easy way to end the argument. If Horsepower is a number that requires torque as part of the equation....you're simply saying that without torque, you have no horsepower. On the other hand, torque doesn't require anything.
Which immediately tells me that torque is the foundation.
Horsepower doesn't get your car going fast, because it's STILL torque being multiplied and divided.
ANOTHER REPLY
Torque is the actual force that moves an object around a pivot.
In this case, your crankshaft.
Horsepower, was designed as an advertising gimmick, to help sell cars.
There's many ways to calculate it, to the point where you can have 10 different ratings for one object.
Torque is the actual force that moves an object around a pivot.
But the torque would still be the same number.
Making horsepower totally pointless for anything except marketing and bragging.
All anyone does is talk about horsepower, but very few people realise exactly how worthless the number actually is.
Keep one major thing in mind, horsepower is not it's own force. All it is, is Torque times rpm divided by 5252. It's a calculated number, not an actual unit of it's own.
If you took torque, multiplied it by rpm, and divided it by 500, you could call it monkey power.
So, you have an actual force, and a made up unit. What do you think is more important, or "better"?
Vipassana · 8 years ago
Lot of uninformed people on here....You dont' "need" horsepower, because it doesn't exist. You do need torque though, because that's real, and that's what moves a object.

My apology, that is not my opinion, but it was part of a Mr Vipassana

THAT IS A VERY DIFFERENT OPINION THAN YOURS

I will take your hp story with a pinch of salt. I love my torque on brushless motors.
Bert

 

HP and Watts are units of power. I think that as long as you compare same units, they are useful.

 

Bert finding silly statements from internet discussion forums doesn't mean much. Torque is like force but for rotating movement. Yes you could say that force F is what pushes car or boat. But ANY force/torque can be turned into ANY OTHER force/torque by gearing or leverage.

Power is work done in set amount of time. Saying that torque only counts is taking time from the equation.
So: a 1hp engine can be geared to push 30 tonne truck up a steep hill. Keep in mind that extreme torque on the shaft doesn't care whether it is a result of gearing down or a power source with such characteristics (low rpm, very high torque).

Now with 10hp (even if the motors rpm is 100x higher of the earlier 1hp one, and thus 1/10th the torque at motor) the same truck can be geared to go up same hill in 1/10th of the time.

 

Btw it is not an opinion

 

Here is a riddle for you. Look at the video below and compare it to the power and torque chart. If it is the torque that makes the car move (in a way that power numbers are less relevant) then shouldn't the Ferrari accelerate the fastest at the revs where peak torque is produced?

It seems that the driver does shift at 9k rpm and it stays nicely between 7k and 9k rpm, near the peak power. I have seen this argument of torque numbers ruling power on many discussions but it is false. Just think about it a bit.

 

It is not a coincidence or marketing ploy that all professional machinery state rated power as class defier- be it an agri tractor, ship engine, boat's power requirement, heavy truck etc.
Equally farming machinery, say a plough or tilling rake, will state required power to pull said device.

To say that power doesn't exist is exatly like saying power doesn't exist from e-motor either but current is all that matters (excluding voltage).
You cannot say that 100A motor is more powerful than 50A motor without knowing the respective voltages.

 

This horse has been beaten to death over and over again on this forum and others. The bottom line is that you have to generate the electrical power somehow. The most practical way to do this on a boat of any size is with a diesel engine. Adding an electrical motor just adds to the complexity and maintenance of the boat without providing any benefit that I have been able to determine. JMHO

 

As long you don't beat my 2 older horses to death, I am happy. You don't have to read any of the threads or reply, if you feel there is nothing to learn what others has to say. I am enjoying opinions from others and came to the conclusion that I love now my torque friendly motors more and more. It is not even difficult to make a 500 Ampere very powerful e-motor with lots of torque using Lithium batteries. I am not able to make a diesel engine, but am able and have proven to myself that with the limited equipment I have, I could make a very powerful e-motor. Thanks guys , it was great reading what others has to say. Bert

 

No. The car will accelerate fastest where the most power is produced, and power is torque X rpm.

 

Naturally you are right. That is exactly my point. You can have two tractor pulling a plough one with 400 Nm torque @ 1000 rpm and one with 400Nm @ 2000rpm. The latter will have double the hp and double the pull.

Let me clarify if it wasn't clear: "shouldn't the Ferrari accelerate the fastest at the revs where peak torque is produced If the claims* quoted in BertKu's post were true?"

*see here: Sometimes a horse is just a horse https://www.boatdesign.net/threads/sometimes-a-horse-is-just-a-horse.59359/#post-817108