Originally Posted by PSea
LAMO. Now torque plays a role in choosing the appropriate HP? Pick one side of your mouth and stick to it. Exactly why every objection of yours has been a complete joke. Thanks for the laughs!
Hey!!! Welcome back.
Let me explain the physics behind it since you don't seem to understand the difference:
When we rate a motor
(electric or IC), that doesn't mean the full HP is used or even available all the time (it's not). It's typically rated by the peak continuous HP the motor
can put out reliably. (You could pump more amps into an electric motor or bolt a turbo on an IC motor to force more HP but without modifications, you can seriously shorten the life if not destroy it within seconds.)
Electric motors can produce peak torque pretty much regardless of RPM
IC motors have a torque curve and produce peak torque in a narrow RPM
range. When outside that RPM range (such as when at idle), they produce significantly less torque.
While the motors have a rated HP, there is also the HP that is being produced at any point in time. That can be calculated as:
HP = Torque * RPM (using the instantaneous torque and RPM)
Coming from a standing start, this strongly favors Electric as they are at peak Torque from the instant power is applied to the prop. Depending on how the IC motor is optimized, it may only be generating 1/3 of it's peak torque at idle. It then takes a few seconds to spin up into an RPM range where it is putting out peak torque. As a result, motors of the same HP rating, the electric motor will initially accelerate faster because there is more HP initially available.
BUT, as the diesel spins up to a higher RPM both the RPM and torque are increasing which means the HP builds faster until it catches up to the electric motor and the total HP is the same when it reaches peak.
Remember, the electric motor as it spins up, is only increasing RPM not torque so the HP builds in a very linear fashion. The IC motor gains hp in more of a cubic fashion where it's slow at first and then very fast towards the end. (If you've ever tried to start out in 2nd gear
with a stick, you can see this as acceleration is horrible from idle to maybe 1500 rpm and then suddenly you will feel a surge of acceleration as the engine starts building torque in addition to RPM.)
So how does this differ between a boat and a car:
As previously mentioned, for a small car, depending on aerodynamics, it may only take say 60hp to maintain freeway speeds (taking into account a headwind and climbing a moderate grade) but if you use a 60hp engine with only 1/3 of peak torque, and 1/3 of the RPM at peak HP, the engine is only generating maybe 7hp off the line. An electric motor with max torque is going to generate around 3 times the HP at the same RPM. It gets a little complicated as you have to take into account the clutch
actuation with a IC motor and you can rev the IC engine to counter act some of the limitation but the electric motor has far more power off the line. So in the end the electric motor has roughly twice the power off the line.
A typical modern compact car will have somewhere on the order of 90-150hp IC engine. Let's say 120hp, and that means 14hp off the line. Since you can get the similar off the line power from a 60hp electric motor, the 60hp electric motor is plenty for both accelerating from a stop and for cruising down the freeway. It serves both key use cases.
As a result for the use case of a small passenger car, an electric motor has a big advantage over an IC motor (ignoring fuel storage).
With a displacement
cruising sailboat, sailboat, the situation is a bit different.
Let's say you need 20hp to maintain your desired cruising speed under a headwind and waves.
- Once up to speed, both the IC and electric motors are putting out the same HP, so as long as both are using appropriate gear
ratios, the same torque and RPM are applied to the prop shaft and can turn the same prop at the same speed.
- Assuming the IC motor has been properly selected, cruise
speed has the motor operating close to peak torque when at cruise
- The electric motor is also operating at the peak torque and once gear ratios are accounted for, Torque and RPM of the prop shaft are the same.
So for maintaining cruise speed, there is no significant difference between IC or electric motors in terms of the power they produce.
For acceleration there is still a benefit but most cruisers simply don't place a lot of value on acceleration. Once you get out of the marina, whether it takes 5 seconds or 20 seconds to get up to 6kts, really isn't that important.
- Also unlike a car where the car must accelerate if the drive wheels turn (assuming you don't spin the tires), propellers always start out spinning faster than the theoretical distance they should drive the boat. There is always some slippage before the boat starts moving forward. What this means is it's much easier to get the propeller
moving and within a second or so, the IC motor is moving up the torque curve more quickly as the motor doesn't have to wait for the boat to accelerate, just the prop. By the time the prop starts developing a real strong bite on the water
, the IC motor has already climbed a good ways up the torque curve and can produce a significant percentage of peak HP.
- While there is that rare save a botched docking
maneuver moment, it's really very rare to use more than 10-20% throttle when docking
. Usually, if you hear someone firewall the throttle, the situation has already turned to poo.
So while you do get a bit of a boost accelerating with an electric motor, it's not something that drives the HP selection process. The end result is it's the cruising speed HP requirements that drive the motor HP selection on a displacement
cruising boat and that HP is the same regardless of electric or IC.
It doesn't matter what speed you select. If you only want to cruise at 3kts in dead calm conditions, you might be able to drop the requirement to 5hp, but otherwise it's pretty much the same. I've actually played with this scenario as I have a 5hp dingy outboard
that is stored on a spring loaded outboard
bracket. Flat out it will get the big boat (34' cat) up to around 3kts. Docking, it needs more like 50% throttle but most of the issue is the prop is set up for a small higher speed planning dingy and the bracket doesn't get the prop deep in the water
, so it doesn't do a good job transferring the power to the water. A dedicated drivetrain would be much better.
I hope that helps you to understand the difference. I've glossed over a few technical details that don't really change the outcome but add a lot of complication (example: how power transfer with a IC motor spinning at low RPM with a clutch
differs from an electric motor that starts from zero RPM with the clutch fully engaged).
This is where the frustration with "magic electric HP" comes from. People take the advantages electric motors have in an automotive use case and mis-apply them to a displacement boat use case.
If you think there is a different use case for a displacement cruising boat, let's discuss it.