B mode - same as short gear on manuals?

it uses engine braking to slow the car down, same as would happen when the battery is full and can;t take any more charge

 

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The short answer is NO. Your trying to compare a totally manual system with a computer controlled system. That's like comparing apples to oranges.
B mode is telling the car's ECU that you want to utilize engine braking to slow the car. The ECU's decide the best and most efficient way to accomplish this. I'm pretty sure the USA version of the OM manual states that tow capability is zero.

But it's you car, abuse it as you see fit....

Sorry I just looked it up and found a website (NOT OEM) that stated 1000 pound @ sea level and reduced limit by 10% for every 1000 meters of elevation gain your attempting.
IMHO; towing with a car that's known to blow head gaskets, without towing anything and the small brakes on that car is unsafe and a nonstarter for me, but to each own. Just don't run into me if you can't stop.....

 

I don't think so.

I'd qualify that to going down long downhills, long enough they'll fully fill the battery charge display.

 

It's designed to feel that way, so a driver from a conventional car can say "ok, this feels like something I'm familiar with." What happens under the hood is a lot different though.

A short gear in a manual gives you both torquier propulsion when you press the go pedal and stronger slowing when you let off.

The Prius B mode doesn't change the propulsion behavior at all. It does give you stronger slowing when you let off the pedal (this just by changing a number stored in the ECU meaning "how strongly to slow when the human lets off the pedal").

It also changes another number that influences how the car apportions that slowing between regen and engine braking. The car knows how to do both things whether you're in B mode or not, but normally in D it will stick to regen as long as it can, and only substitute engine braking as it nears 80% battery capacity and doesn't want to charge anymore.

In B it will start phasing out regen and phasing in engine braking earlier, so the battery does not get charged as hard. That's why it's described as for use on long downhills. If the downhill is long enough, the battery's going to get fully charged by the end anyway, so may as well give it a little break and charge it more slowly on the way down, twirling away some excess energy with engine braking.

On shorter downhills, or in other conditions, it doesn't buy you anything; the extra engine braking just loses energy you could have reclaimed.

It doesn't increase either the propulsion or braking capacity in any way that would matter to pulling a trailer.

 

Yes, but my newer 2021 still runs away with only 2 passengers. so around 450 pounds within the car. I still have to pump the brakes to slow down the car - that's without something on a trailer hitch pushing me forward.
For reference purposes, I tow a two axle ski boat that has trailer brakes in my GMC truck. Sometimes that's difficult to manage, coming back from the lake at only 3500 feet elevation Narrow one lane mountain roads. There was smoke coming from the trailer brakes on multiple occasions. I have to rebuild them every 3-4 years and have to buy new drums, because they usually have stress fractures or blue spots.

 

My generation 3 moved my FLOE 5x8 trailer aluminum and plastic with my residential Cub cadet zero turn a weed eater and maybe a push mower or something in the back of it and could pretty much drive around like I wanted without any consequence slam on the brakes and everything in the trailer will be moving around that's about it No real funny business I've moved 30 sheets of however thick it was plywood from home Depot in the same trailer to the house about 8 mi You can't drive like a complete idiot but pretty uneventful I wouldn't want to drive this way to Tennessee from North Carolina but.

 

Everything I've found stated 1000 lbs max @ sea level; but Enjoy......

 

It uses the engine a a dead spinning weight (seems like it switches the fuel to the motor off for some of it) and regeneration to get rid of the energy when you lift off the accelerator (e.g go down hill). I used to tow a boat (aprox. 250-300kgs) with only me and my luggage behind my Gen3 2011 Prius with no problems here in mountainous New Zealand. Put about 120 000kms issue free on it and sold it when I wanted to build a Toyota hybrid Alphard 4wd campervan during Covid lockdown. You should be ok if you go gently on the Prius and drive it within it's capabilities. I changed the transmission fluid every 50 000kms as I deemed it working harder than normal and coolant at 100 000kms. Nothing difference in strain on the engine+drivetrain between towing about 300kgs by yourself and having just the car with four 100+kg adults and their luggage. Go easy and you will be just fine.

 

it spends the extra energy spinning the engine without fuel, and uses that drag to slow the car

 

A motor and a generator are essentially the same thing. When you put electric power in to get motion out, it's a motor. When you put motion in to get electric power out, it's a generator.

The big traction motor, MG2, is doing the same thing in regen braking as in engine braking. Forward motion from the car is being turned into electrical power out.

That power has to go somewhere. If the battery isn't full, it can be sent there, which is regen braking. Otherwise it can be used (with the help of MG1) to twirl the engine as a big vacuum pump (the throttle is pretty much closed then), and that's engine braking.

Engine braking isn't "using some of the stored energy" to do anything. It just isn't adding to the stored energy. The power that the forward motion is being converted into just gets thrown away by twirling the engine, instead of stored.